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LocandinaA4 per Marisa
Nappi et al. The Journal of Headache and Pain 2013, 14:66 http://www.thejournalofheadacheandpain.com/content/14/1/66 REVIEW ARTICLE Open Access Hormonal contraception in women with migraine: is progestogen-only contraception a better choice? Rossella E Nappi1,2,5*, Gabriele S Merki-Feld3, Erica Terreno1,2, Alice Pellegrinelli1,2 and Michele Viana4 Abstract A significant number of women with migraine has to face the choice of reliable hormonal contraception during their fertile life. Combined hormonal contraceptives (CHCs) may be used in the majority of women with headache and migraine. However, they carry a small, but significant vascular risk, especially in migraine with aura (MA) and, eventually in migraine without aura (MO) with additional risk factors for stroke (smoking, hypertension, diabetes, hyperlipidemia and thrombophilia, age over 35 years). Guidelines recommend progestogen-only contraception as an alternative safer option because it does not seem to be associated with an increased risk of venous thromboembolism (VTE) and ischemic stroke. Potentially, the maintenance of stable estrogen level by the administration of progestins in ovulation inhibiting dosages may have a positive influence of nociceptive threshold in women with migraine. Preliminary evidences based on headache diaries in migraineurs suggest that the progestin-only pill containing desogestrel 75 μg has a positive effect on the course of both MA and MO in the majority of women, reducing the number of days with migraine, the number of analgesics and the intensity of associated symptoms. Further prospective trials have to be performed to confirm that progestogen-only contraception may be a better option for the management of both migraine and birth control. Differences between MA and MO should also be taken into account in further studies. Keywords: Migraine with aura (MA); Migraine without aura (MO); Combined hormonal contraceptives (CHCs); Combined oral contraceptives (COCs); Progestogen-only contraception; Desogestrel-only pill; Venous thromboembolism (VTE); Stroke Introduction Migraine is a disabling headache, characterized by moderate to severe head pain, usually accompanied by nausea, photophobia, phonophobia and osmophobia (migraine without aura, MO). In about 30% of patients migraine attacks are preceded by transient focal neurologic symptoms which are called aura (migraine with aura, MA). Migraine has a high socio-economical impact. In fact during migraine attacks most migraineurs reported severe impairment or the need of the bed rest and almost 40% of migraine patients have five or more headache days monthly [1]. The Global Burden of Disease survey 2010 * Correspondence: [email protected] 1 Research Center for Reproductive Medicine, Gynecological Endocrinology and Menopause, IRCCS S. Matteo Foundation, Pavia, Italy 2 Department of Clinical, Surgical, Diagnostic and Paediatric Sciences, University of Pavia, Pavia, Italy Full list of author information is available at the end of the article (GBD) recently published showed that migraine is the seventh highest cause of disability in the world [2]. Review In the last few years significant advance in the field of reversible hormonal methods has been achieved in order to maximize the benefits and to minimize the risks. We believe it is relevant for clinical practice to briefly review in here potential vascular risks according to the category of migraine, with and without aura, and to the type of hormonal contraceptive option. Epidemiology of migraine and combined hormonal contraceptive (CHC) use Migraine affects about 18% of women and 6% of men in USA and Western Europe [3,4] and its cumulative lifetime prevalence is 43% in women and 18% in men [5]. It is then © 2013 Nappi et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Nappi et al. The Journal of Headache and Pain 2013, 14:66 http://www.thejournalofheadacheandpain.com/content/14/1/66 mostly a female disorder, that it is active in particular during the fertile period of the women’ life with a peak of prevalence in their 20s and 30s [6]. The reproductive life is characterized by the need of reliable and convenient methods of contraception. Among the several forms of contraceptives available, hormonal contraceptives are the most popular reversible method, both in USA and Europe and the “Pill” is the most used [7-9]. Low dose [20 to 30 μg ethynil estradiol (EE2) per day] combined hormonal contraceptives (CHCs) have become the method of choice and the availability of new progestins (third- and fourthgeneration) has allowed to achieve non-contraceptive benefits in comparison to older progestins (second – generation) [10]. CHCs are available in several regimens and routes of administration (oral, transdermal vaginal) in the attempt to improve tolerability, adherence and convenience of use [11]. Moreover, two new CHCs containing natural estradiol (E2), instead of EE2, have been introduced to increase safety and future developments are ongoing [12]. Interestingly, many gynecological conditions that are comorbid with migraine can be treated with CHCs. This enhances the likelihood of their use in migraine population [13]. The prescription of CHCs may have different effects on migraine with not univocal results because of many methodological limitations (diverse hormonal combinations, variable research settings, retrospective and/or cross-sectional designs, lack of a clear phenotyping of the headache according to IHS criteria, inadequate duration of observation) [14-16]. Historically, combined oral contraceptive (COCs) is the category best studied in migraineurs with an aggravation of migraine reported in 18-50% of cases, an improvement in 3-35% and no change in 39-65% [17]. A more recent crosssectional study on a large population found that migraine is significantly associated with COCs assumption. Yet because of the design of the study it is not possible to define a causal relationship between exposure and disease [18]. Analysis on the different effect of the COCs on the two forms of migraine revealed that MA worsen more (56.4%) than MO (25.3%) [19]. Furthermore, women can present MA for the first time during the initiation of COCs [20]. During the last decade, a specific “window” of vulnerability triggered by the 7 days free hormone interval has been identified and the definition of hormonallyassociated headaches (exogenous hormone-induced headache and estrogen-withdrawal headache) encompasses several patho-physiological mechanisms which are likely to explain nociceptive threshold in women [21,22]. Strategies to minimize estrogen withdrawal at the time of expected bleeding or to stabilize circulating estrogen at lower concentration include, respectively, 1) to use transdermal E2 during the free interval of hormonal contraception [22] or to shorten the interval from 7 to 4 and even to 2 days [23,24]; 2) to administer low dose COCs in Page 2 of 6 extended/flexible regimens [25] or to use extended vaginal contraception [26]. Very recently, Mac Gregor [27] reviewed the effects of currently available contraceptive methods in the context of the risks and benefits for women with migraine and non-migraine headaches and concluded that for the majority of women with headache and migraine, the choice of contraception is unrestricted. Indeed, the contraceptive method is unlikely to have an impact on headache, whereas migraine deserves accurate diagnosis and recognition of the impact of different methods on such condition. Vascular risks associated with CHCs and migraine MA, and to a lesser extent also MO, may increase vascular risk, especially the risk for ischemic stroke in younger women [27-30]. Moreover, evidences that need to be corroborated by further studies suggest an association between MA and cardiac events, intracerebral hemorrhage, retinal vasculopathy and mortality [31]. Even though the association between migraine and stroke appears to be independent of other cardiovascular risk factors [32], the presence of some risk factors, such as smoking and/or COCs use or their combination, further increase risk [33]. MA is associated with a twofold increased risk of ischemic stroke but the absolute risk associated with CHC use is very low in healthy young women with no additional risk factors and mostly related to the estrogen dose [34]. In spite of the considerable advances in terms of safety and tolerability of CHCs in migraine sufferers [13], their use is still questioned especially in women with additional risk factors for stroke, including, smoking, hypertension, diabetes, hyperlipidemia and thrombophilia, age over 35 years [35]. New evidences [36-38] have warned clinicians on the use of CHCs and the risk of venous thromboembolism (VTE) which is likely to be dependent on the type of the progestin [RR 1.6–2.4 by using third- and fourth-generation CHCs (namely, desogestrel, gestodene norgestimate and drospirenone, respectively) in comparison with those containing LNG (second-generation)] and the total estrogenicity of CHCs [39,40]. Even new routes (trandermal patch and vaginal ring) seem to be associated with an increased VTE risk, but data are contradictory [41-43]. Indeed, according to a statement very recently released [44] many factors contribute to VTE risk (e.g. age, duration of use, weight, family history) which makes epidemiological studies vulnerable to bias and confounders. In addition, the decision-making process should take into account non only the small VTE risk (absolute risk depending on the background prevalence rate between 2 to 8 per 10,000 users per year [45]) of the contraceptive method but also other elements such as efficacy, tolerability, additional health benefits, and acceptability which have to be discussed with the individual woman. In any case, it Nappi et al. The Journal of Headache and Pain 2013, 14:66 http://www.thejournalofheadacheandpain.com/content/14/1/66 is essential to follow the appropriate guidelines by avoiding the prescription of CHCs to women at elevated risk for VTE. In the context of migraine and CHCs use, it is very important to remember that the World Health Organization Medical Eligibility Criteria for Contraceptive Use stated that MA at any age is an absolute contraindication to the use of COCs (WHO Category 4) [46]. The US/ WHO MEC is more restrictive than the UK/WHO MEC as regard to MO, rating CHCs as a category 4 for any migraineur over age 35 [47]. That being so, the personal risk assessment should guide the prescription of CHCs in selected conditions. When there is the sole need of contraception, without the added benefits of a peculiar hormonal compound and/or combination, CHCs with the lower vascular risk or alternative methods for birth control should be considered. Progestogen-only contraception: a class on its own The progestin component of hormonal contraceptives accounts for most of their contraceptive effects (inhibition of ovulation, suppression of endometrial activity, thickening of cervical mucus). Progestin-only methods includes pills (the pill most used in Europe contains low doses of desogestrel), injectables [depot Medroxyprogesteroneacetate (DMPA)], implants (the most recent long-acting reversible contraception contains Etonogestrel single-rod implant for at least 3 years), and intrauterine devices (levonorgestrel for at least 5 years) [48]. By providing effective and reversible contraception, progestin-only contraception has many noncontraceptive health benefits including improvement in dysmenorrhea, menorrhagia, premenstrual syndrome, and anemia [49]. Indeed, there is a general reduction of the amount of menstrual bleeding but cycle control may be erratic, a feature that may influence acceptability [50-52]. Progestin-only methods are appropriate for women who cannot or should not take CHCs because they have some contraindications to estrogen use and therefore display a higher risk of VTE [35,36]. The progestogen-only contraception is a safe alternative to CHCs and the avoidance of the estrogen component has many advantages not only for breastfeeding women but also for women with vascular diseases or risk factors for stroke [46,47]. The use of progestin-only contraception is not associated with an increased risk of VTE compared with non-users of hormonal contraception [53]. In addition, progestin-only pills, injectables, or implants are not associated with increased risk of ischemic stroke according to a recent metanalysis (OR 0.96; 95% CI: 0.701.31) [54]. Since the 1-year prevalence rates for migraine in women are 11% for MO and 5% for MA, respectively [55], there is potentially a high number of women in whom CHCs may be contraindicated according to WHO guidelines and progestogen-only contraception may be safely used [35,36]. Page 3 of 6 Evidence of progestogen-only contraception in women with migraine Given the evidence that progestogen-only contraception is a safer option for women with migraine, the main question is whether such contraceptive choice may influence the course of both MA and MO and offer a better management of the disease. Indeed, even though the excess risk of death for a woman taking modern CHCs is 1 in 100,000, which is much lower than the risk of everyday activities such as cycling [56], there is a biological plausibility that in women with migraine should be wiser to use an estrogen-free containing contraception to avoid any potential vascular risk. Two recent very large epidemiologic studies [36,57] reported the association between CHC and progestogen-only methods and cardiovascular risk, thrombo-embolic risk and stroke. Whereas no increased risk for deep venous thormbosis, myocardial infarction and thrombotic stroke was found for the progestogen-only methods, the risk were two-sixfold elevated in CHC users. The role of progesterone/progestins in the pathophysiology of migraine has been overshadowed by Somerville’s early observations that it was the prevention of estrogen but not of progesterone withdrawal in the late phase of the cycle to be able to prevent the occurrence of migraine attacks [58,59]. Indeed, at variance with the influence of estrogens upon the cerebral structures implicated in the pathophysiology of migraine [60], cyclic variations in progestin levels were not related to migrainous headaches, but they rather seem to be protective. Progesterone apparently attenuates trigemino-vascular nociception [61] and its receptors are localized in areas of the central nervous system, which are involved in neuronal excitability and neurotransmitter synthesis release and transport [62]. It has been shown that progesterone can antagonize neuronal estrogenic effects by downregulating estrogen receptors [63]. Whereas estrogen peak decrease the threshold for cortical spreading depression (CSD), the neurobiological event underlying MA, estrogen withdrawal increased the susceptibility to CSD in an animal model [64]. Therefore, the maintenance of low estrogen levels and the avoidance of estrogen withdrawal by the administration of progestins in ovulation inhibiting dosages might decrease cortical excitability. Indeed, progestogen-only contraception has a continuous administration, without the hormone-free interval, and does not induce withdrawal stabilizing circulating estrogens, but some fluctuations according to different preparations may still occur [65]. Clinical data are scarce and no comparative studies with progestogen-only contraceptives and placebo or COCs are available in the literature [27]. Diagnoses are often inaccurate, without distinction between headache and migraine, and headache is reported in contraceptive progestin implant users as a potential cause of discontinuation [66]. Similarly, there is an increase in Nappi et al. The Journal of Headache and Pain 2013, 14:66 http://www.thejournalofheadacheandpain.com/content/14/1/66 headache, but not migraine, reported over time with both norethisterone enanthate and, especially with depot medroxyprogesterone acetate [67]. Anecdotally, migraine is more likely to improve in women who achieve amenorrhea [68]. In a large, cross-sectional, population-based study in Norway of 13944 women, a significant association between CHC and headaches, but no significant association between progestin-only pills and migraine (OR 1.3, 95% CI: 0.9–1.8) was found but the number of users was small [18]. To date two diary-based studies pilot studies on the effect of desogestrel 75 μg on migraine have been published [69,70]. Such oral daily pill inhibits ovulation and the dose allows the ovary to synthesize stable amounts of estrogen which are relevant for wellbeing and bone density [71]. The first study included thirty women with MA [69]. The use of desogestrel 75 μg resulted in a significant reduction in MA attacks and in the duration of aura symptoms, already after three months of observation. Interestingly, the beneficial effect of desogestrel 75 μg on visual and other neurological symptoms of aura was significantly present only in those women in whom MA onset was related to previous COCs treatment. These findings suggest that the reduction in estrogen levels may be relevant to the amelioration of MA, but do not exclude a direct effect of the progestin on CSD. The second study on the effect of desogestrel 75 μg included women with MA (n°=6) and with MO (n°=32) and evaluated migraine days, pain score and pain medication [70]. An improvement of each parameter was observed during 3 months use of desogestrel 75 μg in comparison to a three months pretreatment interval. A subanalyses of the effect on 32 women with MO revealed significant improvements in number of migraine days, pain medication and pain intensity. The mean number of migraine attacks at baseline was higher in comparison to that in the study of Nappi et al. [69], indicating that also very severe migraineurs might profit from such a progestin-only contraception. Chronic migraineurs often develop medication overuse headaches with severe limitation of their quality of life. The reduction of pain medication by progestin-only contraception is an interesting approach and it should be studied further. Indeed, there is a broad variation in the intensity of improvement with a reduction in migraine frequency ranging from 20% and 100% [70]. No indicators to identify those women who will profit from desogestrel 75 μg could be ascertained. On the other hand, there were few dropouts of women experiencing more migraine after starting contraception with this progestin in both studies, indicating that progestins can also deteriorate migraine in few cases. A very recent study investigating the changes of quality of life in migraineurs 3 months after initiation of the progestagen-only pill desogestrel 75 μg demonstrates a highly significant reduction in Midas Score and Midas Page 4 of 6 grades [72]. However, clinical experience with desogestrel 75 μg in migraineurs further shows that during the initial 4 weeks migraine frequency can raise slightly before headaches improve. This information has to be mentioned and discussed during counseling. In summary, the potential advantages of using progestogen-only contraception in women with migraine are the following: 1) Continuous use 2) Absence of estrogen peak 3) No influence on threshold for cortical spreading depression (CDS) 4) No evidence of increase in cardiovascular, stroke and thrombo-embolic risk 5) No data on progestins inducing migraine Conclusions In conclusion, contraceptive counseling in migraine should take into account the risk-benefit profile of the individual woman before prescribing CHCs. In order to reduce potential vascular risks, recommendations should be a main point of guidance for prescribers. Progestogen-only contraception is a safer option in MA and eventually in MO with additional risk factors. Given preliminary evidences of a positive effect of the progestin-only pill desogestrel 75 μg on MA and MO in many, but not all women, further prospective trials have to be performed to confirm that progestogen-only contraception may be a better option for the management of both migraine and birth control. Differences between MA and MO should also be taken into account in further studies. Competing interests Dr. Rossella E. Nappi has had financial relationships (lecturer, member of advisory boards, and/or consultant) with Bayer Pharma, Eli Lilly, Gedeon Richter, HRA Pharma, MSD, Novo Nordisk, Pfizer Inc., Shionogi Limited, Teva/Theramex. Dr. Gabriele S. Merki-Feld has had financial relationships (lecturer, member of advisory boards) with Bayer Pharma and MSD. Authors’ contributions Conception and design: REN, GSM. Acquisition of data: ET, AP. Analysis and interpretation of data: REN, MV, GSM. Drafting the article: REN, MV, GSM. Revising it for intellectual content: REN, MV, GSM. Final approval of the completed article: REN, REN, GSM, ET, AP, MV. All authors read and approved the final manuscript. Author details 1 Research Center for Reproductive Medicine, Gynecological Endocrinology and Menopause, IRCCS S. Matteo Foundation, Pavia, Italy. 2Department of Clinical, Surgical, Diagnostic and Paediatric Sciences, University of Pavia, Pavia, Italy. 3Clinic for Reproductive Endocrinology, University Hospital Zürich, Zürich, Switzerland. 4Headache Science Center - National Neurological Institute C. Mondino, Pavia, Italy. 5Research Center for Reproductive Medicine, Unit of Obstetrics and Gynecology, IRCCS Policlinico ‘San Matteo’, Piazzale Golgi 2, 27100, Pavia, Italy. Received: 9 June 2013 Accepted: 28 July 2013 Published: 1 August 2013 Nappi et al. The Journal of Headache and Pain 2013, 14:66 http://www.thejournalofheadacheandpain.com/content/14/1/66 References 1. Bigal ME, Lipton RB (2009) The epidemiology, burden, and comorbidities of migraine. 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Eur J Contracep Reprod Health Care, in press doi:10.1186/1129-2377-14-66 Cite this article as: Nappi et al.: Hormonal contraception in women with migraine: is progestogen-only contraception a better choice? The Journal of Headache and Pain 2013 14:66. Page 6 of 6 Submit your manuscript to a journal and benefit from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the field 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 REVIEW ARTICLE Open Access Peripheral vascular dysfunction in migraine: a review Simona Sacco1*, Patrizia Ripa1, Davide Grassi2, Francesca Pistoia1, Raffaele Ornello1, Antonio Carolei1 and Tobias Kurth3,4 Abstract Numerous studies have indicated an increased risk of vascular disease among migraineurs. Alterations in endothelial and arterial function, which predispose to atherosclerosis and cardiovascular diseases, have been suggested as an important link between migraine and vascular disease. However, the available evidence is inconsistent. We aimed to review and summarize the published evidence about the peripheral vascular dysfunction of migraineurs. We systematically searched in BIOSIS, the Cochrane database, Embase, Google scholar, ISI Web of Science, and Medline to identify articles, published up to April 2013, evaluating the endothelial and arterial function of migraineurs. Several lines of evidence for vascular dysfunction were reported in migraineurs. Findings regarding endothelial function are particularly controversial since studies variously indicated the presence of endothelial dysfunction in migraineurs, the absence of any difference in endothelial function between migraineurs and non-migraineurs, and even an enhanced endothelial function in migraineurs. Reports on arterial function are more consistent and suggest that functional properties of large arteries are altered in migraineurs. Peripheral vascular function, particularly arterial function, is a promising non-invasive indicator of the vascular health of subjects with migraine. However, further targeted research is needed to understand whether altered arterial function explains the increased risk of vascular disease among patients with migraine. Keywords: Migraine; Migraine with aura; Arterial stiffness; Endothelial function; Flow mediated dilation; Pulse wave velocity; Augmentation index; Stroke; Coronary artery disease; Cardiovascular disease; Risk factors Introduction Numerous studies have indicated that migraineurs have an increased risk of vascular diseases. The association between migraine and ischemic stroke was the earliest to be recognized [1-3]. Thereafter, migraine has been associated also with myocardial infarction, hemorrhagic stroke, retinal vasculopathy, cardiovascular mortality, incidental brain lesions, including infarct-like lesions, and in some instances also with peripheral artery disease (Table 1) [4-7]. Mechanisms which may explain these * Correspondence: [email protected] 1 Department of Neurology and Regional Headache Center, University of L’Aquila, Piazzale Salvatore Tommasi 1, L’Aquila 67100, Italy Full list of author information is available at the end of the article associations have been discussed previously [5,8-10]. One of the plausible mechanisms predisposing individuals to atherosclerosis and vascular diseases is endothelial and arterial dysfunction [11]. Consequently, several studies have evaluated the vascular function of migraineurs outside the brain, providing equivocal results. The aim of the present study was to review the published evidence linking peripheral arterial function with migraine. Background information about arterial and endothelial function and its measurements The endothelium, i.e. the inner layer of cells of blood vessels, has physiologically favorable and atheroprotective © 2013 Sacco et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Table 1 Evidence referring to the association between migraine and the risk of vascular disease Ischemic stroke Numerous studies demonstrating an association with any migraine [e1-e22]; Definite association with migraine with aura [e1,e3-e4,e7-e8,e11-e12,e15-e16,e19-e21]; No definite association with migraine without aura [e1,e3,e7,e11-e12,e16,e19-e20]; Association with migraine with aura confirmed by three meta-analyses [e23-e25]. Transient ischemic attack The risk seems to be increased in migraineurs, although this issue has not been extensively investigated due to a challenging overlap of symptoms with migraine aura [e6,e19]. Hemorrhagic stroke Several studies addressing the topic and providing inconsistent results [e5,e8-e10,e26-e29]; A meta-analysis of those studies indicating a small but significant association [e30]; No definite conclusion regarding migraine type. Cardiac events Two large studies indicating an association with any migraine in men and women and with migraine with aura in women (data not available for men) [e1,e2]; Conflicting results provided by other available studies [e4,e31-e33]; No association with any migraine in meta-analysis of data but few studies available [e23]; No analysis according to migraine type due to lack of data. A recent study reporting an association between migraine (any migraine, migraine with aura and migraine without aura) and myocardial infarction [e4]. Vascular death A meta-analysis and a large study supporting an association with migraine with aura [e23]; No association with any migraine according to meta-analysis of data [e23]. Other vascular diseases Studies indicating a possible association with any migraine and retinal disease [e34,e35] and peripheral artery disease [e36-e37]. Brain lesions Page 2 of 10 cells capacity. This leads to “endothelial dysfunction” with a reduction in endothelium-dependent vasodilation and the induction of a specific state of “endothelial activation,” characterized by a proinflammatory, proliferative, and procoagulatory milieu which favors all stages of atherogenesis, pathological inflammatory processes, and vascular disease [14]. Endothelial dysfunction represents an early step in the development of atherosclerosis [15]. Endothelial function can be investigated by invasive and non-invasive tests [16]. Invasive tests are based upon intra-arterial infusion of vasoactive agents. Among the several non-invasive methods of measuring endothelial function, flow-mediated dilation (FMD), laser Doppler flowmetry, and digital pulse amplitude tonometry (PAT) are the most widely adopted [16-18]. Figure 1 shows details about measurement of FMD [16-19]. Measures of arterial elasticity are also being investigated as non-invasive means of gauging vascular health [14]; the most commonly considered parameters are pulse wave velocity (PWV), augmentation index (AIx) and local arterial distensibility in the carotid artery or in the aorta. While FMD addresses functional status of the endothelium, PWV, AI, and local arterial distensibility are better related to structural changes in the vessel wall. PWV is an indicator of segmental arterial stiffness. Carotid-femoral PWV is the gold standard of assessment (Figure 2), but it is difficult to measure, so in clinical practice it can be replaced by the measurement of brachial-ankle PWV (although this measurement includes some muscular arteries and not only the elastic ones). The AIx (Figure 3) is an index related to the stiffness of the systemic arterial tree. An increase in arterial stiffness leads to a more proximal point of reflection, thus altering the wave profile as measured by the instruments. Local arterial distensibility is assessed by measuring the minimum and maximum diameter of central arteries – such as the carotid artery or aorta – by ultrasound or MRI with simultaneous recording of the blood pressure in the arterial district from which the diameters are obtained. Migraine has been associated with white-matter hyperintensities and infarct-like lesions [e38-e42]; Association of migraine with white matter hyperintensities confirmed in two meta-analyses [e41,e43]; no definite association with infarct-like lesions [e43] References are listed in the online supplement. effects [12]. It works as both a receptor and effector organ and responds to each physical or chemical stimulus with the release of substances, including nitric oxide (NO), with which the endothelium maintains vasomotor balance and vascular-tissue homeostasis [13]. The established cardiovascular risk factors cause oxidative stress initiating a chronic inflammatory process which alters the endothelial Search strategy and selection criteria Data for this review were obtained through searches in BIOSIS, the Cochrane database, Embase, Google scholar, ISI Web of Science, and Medline from their first availability up to April 2013. The search terms included “migraine” OR “headache” AND (“endothelial function”, “endothelial injury”, “arterial stiffness”, “arterial distensibility”, “vascular resistance”, “vascular reactivity”, “flow-mediated dilation”, “forearm blood flow”, “arterial pulse wave velocity”, “augmentation index”). We also searched reference lists of identified articles and papers quoting identified articles. We reviewed only studies published in English. Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Page 3 of 10 Figure 1 Flow-mediated dilation. Flow-mediated dilation (FMD) measures endothelial function by inducing a temporary ischemia in a brachial artery and observing the amount of vasodilation following the stressor event. After a baseline measurement of the artery diameter via an ultrasound probe, a sphygmomanometer blood pressure cuff is positioned on the right forearm 2 cm below the elbow and inflated to 250 mmHg to produce ischemia in the forearm. The cuff is deflated after some minutes, usually 5, thus causing a reactive hyperemia which in turn produces a shear stress stimulus that induces the endothelium to release nitric oxide as a vasodilator. FMD is considered as diameter after reactive hyperemia - basal diameter/basal diameter × 100. The figure shows the diameter (upper part) and shear rate (lower part) of brachial artery during FMD before (grey-shaded part on the left), during (black-shaded part), and after (grey-shaded part on the right) ischemia. Figure 2 Carotid-Femoral Pulse Wave Velocity. Increased arterial stiffness leads to increased velocity of the pulse wave generated in the arteries by the contraction of the left ventricle. Pulse wave velocity (PWV) consists in measuring pulse wave profiles by tonometry at two distant locations (carotid and femoral) and measuring the delay in the onset of the wave between those two locations. PWV is calculated as the distance traveled by the wave divided by the time taken to travel that distance. Surface distance between the two recording sites and simultaneously recorded electrocardiograms are used to calculate wave transit time. The figure shows tonometric (white lines) recordings of the carotid (above) and femoral (below) artery waves according with simultaneous electrocardiographic (yellow lines) ‘R’ wave of the electrocardiogram as a timing reference. Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Figure 3 Augmentation Index. The interaction between the incident pulse wave and the reflected pulse wave, which is generated by the arterial resistance, is expressed by the augmentation index, which is the amount of pulse wave induced only by arterial resistance. In the figure, the upper arrow indicates the first systolic peak (incident wave), while the lower arrow indicates the second systolic peak (reflected wave). The ratio between the reflected wave/incident wave × 100 is the augmentation index. Review Arterial and endothelial function in migraineurs Several lines of evidence for vascular dysfunction have been identified in migraineurs and there is increasing evidence to suggest that in migraineurs the vascular system is impaired not only within the brain. Endothelial function in migraineurs The many studies assessing endothelial function in migraineurs have used heterogeneous techniques and indicators as shown in Tables 2, 3 and 4. Most of them assessed endothelial function by measuring FMD (Table 2) [20-30], some used PAT (Table 3) [31,32], some measured peripheral vasodilation in response to pharmacological stimuli (Table 3) [20,30,33-35], and others assessed endothelial progenitor cells (EPCs) (Table 4) [23,36,37]. Findings were not consistent across the studies. Several among the available studies did not reveal any alteration in the endothelial function of migraineurs. Six of them did not find any difference in FMD between migraineurs and control subjects (Table 2) [20-23,25,26] and a further study confirmed the lack of any alteration in endothelial function assessed by PAT ratio (Table 3) [32]. Three studies assessing peripheral vasodilation after pharmacological stimuli found comparable responses in subjects with Page 4 of 10 and without migraine (Table 3) [20,33,35]. However, other studies supported the presence of endothelial dysfunction in migraineurs. Four studies reported decreased FMD in migraineurs [24,27,29,30]; in one of those studies, FMD correlated with dysfunction of the autonomic nervous system (Table 2) [24]. Another study, using PAT, reported that subjects with chronic migraine had worse endothelial function (Table 3) [31]. A further study found an abnormal vascular response to dilating pharmacological stimuli (Table 3) [34]. The possible underlying mechanisms of endothelial dysfunction in those studies were unclear. Since the impairment of FMD was demonstrated also in individuals with recent onset of migraine it is unlikely that the disturbance was a consequence of longstanding and repeated exposure to the vasoconstrictor agents used for migraine treatment [27]. In addition, the results of one study suggested that the impaired vascular reactivity in migraineurs is entirely attributable to a reduced response of vascular smooth muscle cells to NO while the endothelial response to NO appeared intact [34]. Some additional studies supported the presence of an endothelial dysfunction in migraineurs by documenting a decreased number of EPCs [23,36] or a higher number of more mature EPCs (which could be associated with potential endothelial damage) in migraineurs (Table 4) [37]. EPCs maintain the integrity of damaged endothelium and are considered a marker of endothelial function [38]. At variance with other studies, Vernieri et al., found that subjects with migraine with aura had higher FMD than both control subjects and migraine without aura subjects indicating an excessive arterial response to hyperemia. It is suggested that this is probably an effect of an increased sensitivity to endothelium-derived NO or of increased release of NO induced by shear stress (Table 2) [28]. In agreement, Yetkin et al., found that migraineurs have an increased nitrate-mediated response showing a major role of NO supersensitivity in migraine pathophysiology (Table 3) [30]. Several studies support the presence of biochemical changes in the endothelial activation of migraineurs. These biochemical changes may precede structural damage. In particular, alterations in the NO pathway have been described [30,39,40] as well as increased levels of calcitonin gene-related peptide (CGRP), vascular endothelial growth factors (VEGF), von Willebrand factor (vWF) activity, tissue plasminogen activator antigen, C-reactive protein and endothelin-1 [21,23,40]. Recently, a study involving 40 non-hypertensive subjects with migraine without aura randomized to treatment with enalapril 5 mg twice a day for 3 months or to placebo, showed that active treatment was associated with an increase in FMD values [41]. Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Page 5 of 10 Table 2 Studies on endothelial function using flow-mediated dilation in migraineurs Study (First author, year) De Hoon, 2006 [20] Study design Patients included (n) % women Exclusion criteria Migraine diagnosis CC Migraine: 10 60 CVD, HYP, DM, dyslipidemia, CS ICHD-I No differences in FMD between migraineurs and controls 89 CVD and vascular RF, DM, CS, alcoholism, active gastrointestinal disease, gout, epilepsy, recent infection, renal failure, pregnancy or lactation, regular use FANS or antimigrainous drugs and OC ICHD-II No differences in FMD between migraineurs and controls 80 CVD, obesity, HYP, dyslipidemia, pregnancy or lactation and regular use of vasoactive drugs ICHD-II No differences in FMD between migraineurs and controls 98 CAD, inflammatory disease, obesity, HYP, DM, dyslipidemia, CS, infectious disease, severe systemic disease, ovarium pathology, pregnancy or lactation, use of vasoactive drugs ICHD-II No differences in FMD between migraineurs and controls 75 Age ≥50, vascular RF, CS, use of vasoactive drugs 88 None ICHD-II No differences in FMD between migraineurs and controls 100 Use of any daily drugs ICHD-I No differences in FMD between migraineurs and controls 78 Age >50, CVD, HYP, obesity, DM, dyslipidemia, pregnancy or lactation, use of vasoactive drugs (except OC) Controls: 10 Hamed, 2010 [21] CC Migraine: 38 (MwA: 14; MwoA: 24) Controls: 35 Perko, 2011 [22] CC MwA: 20 MwoA: 20 Results Controls: 20 Rodriguez-Osorio, 2012 [23] CC Migraine : 47 (MwA:14; MwoA: 33) Controls: 23 Rossato, 2011 [24] CC Migraine: 20 Controls: 20 Silva, 2007 [25] CC Migraine: 50 (MwA: 25; MwoA: 25) Not reported Controls: 25 Thomsen, 1996 [26] CC MwoA: 12 Controls: 12 Vanmolkot, 2007 [27] CC Migraine: 50 Controls: 50 Vernieri, 2010 [28] CC Migraine:21 Validated questionnaire Decreased FMD in migraineurs Decreased FMD in migraineurs None ICHD-II FMD increased from controls to MwoA to MwA patients 80 CAD, HYP, obesity, DM, infectious disease, ovarium pathology ICHD-I Decreased FMD in migraineurs 89 HYP, CAD, DM, infectious disease ICHD-I Decreased FMD in migraineurs (MwA: 11; MwoA: 10) Controls: 13 Yetkin, 2006 [29] CC Migraine: 45 Yetkin, 2007 [30] CC Migraine: 24 Controls: 45 Controls: 26 CC case–control; MwA migraine with aura; MwoA migraine without aura; CVD cardiovascular disease; HYP arterial hypertension; DM diabetes mellitus; CS cigarette smoking; RF risk factors; FANS non steroid anti-inflammatory drugs; OC oral contraceptives; CAD coronary artery disease; ICHD International classification headache disorders; FMD flow-mediated dilation. Arterial function in migraineurs Compared with studies on endothelial function, reports on arterial function measured with PWV, AIx, and local arterial distensibility are more consistent and suggest that functional properties of large arteries are altered in migraineurs [27,42-46] (Table 5). Four studies addressed PWV in migraineurs (Table 5) [27,42-44]. Ikeda et al. evaluated peripheral PWV [42] while the other authors assessed central PWV [27,43,44]. Three studies on PWV [27,42,43] found higher values of this parameter in migraineurs with respect to nonmigraineurs. However, in one of the studies PWV did not correlate with the presence of migraine after adjustment for age and mean arterial pressure [27]. The inclusion in the study of patients with a short history of migraine and with less use of vasoconstrictive agents may explain the difference [27]. Schillaci et al., also addressed PWV according to migraine type and reported that subjects with migraine either with or without aura had increased PWV with respect to controls while migraine with aura subjects had higher aortic PWV than those without aura [43]. A more recent populationbased study, involving a larger number of migraineurs, has challenged the suggestion of possible impairment Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Page 6 of 10 Table 3 Studies on endothelial function by arterial tonometry or vasodilation in response to pharmacological stimuli in migraineurs Study (First author, year) Study design Patients included (n) % women Exclusion criteria Migraine diagnosis Results CM: 21 71 Age ≥50, CVD, inflammatory disease, obesity, HYP, DM, dyslipidemia, CS, active cancer, ovarium pathology, pregnancy or lactation, regular use of vasoactive drugs ICHD-II Smaller RHI in migraineurs 100 CVD, obesity, HYP, DM, pregnancy, use of drugs (statins, anticoagulants or antiplatelet and intake of triptans within the previous 24 h) ICHD-II No differences in PAT ratio between migraineurs and controls 60 CVD, HYP, DM, dyslipidemia, CS ICHD-I No differences between migraineurs and controls in vasodilation response to serotonin, sodium nitroprusside, and CGRP 78 None ICHD-I No differences between migraineurs and controls in vasodilation response after local heating and iontophoretic administration of acetylcholine, sodium nitroprusside, and CGRP 58 HYP, DM, hypercholesterolemia, CVD, CS ICHD-I Reduced response to endothelium-dependent vasodilation and production of cGMP in migraineurs; no difference in production of NO 75 Age >50, CVD, HYP, obesity, DM, CS, dyslipidemia, pregnancy or lactation, use of vasoactive drugs (except OC) ICHD-II No differences between migraineurs and controls to sodium nitroprusside, substance P, and NG-monomethyl-L-arginine 89 HYP, CAD, DM, infectious disease ICHD-I Higher nitrate-mediated dilation in migraineurs Arterial tonometry Jiménez-Caballero, 2013 [31] CC Controls: 21 Liman, 2012 [32] CC MwA: 29 Controls: 30 Vasodilation in response to pharmacological stimuli De Hoon, 2006 [20] CC Migraine: 10 Controls: 10 Edvinsson, 2008 [33] CC MwoA: 9 Controls: 9 Napoli, 2009 [34] CC MwoA: 12 Controls: 12 Vanmolkot, 2010 [35] CC Migraine:16 Controls: 16 Yetkin, 2007 [30] CC Migraine: 24 Controls: 26 CC case–control; CM chronic migraine; MwA migraine with aura; MwoA migraine without aura; CVD cardiovascular disease; HYP arterial hypertension; DM,diabtes mellitus; CS cigarette smoking; OC oral contraceptive; ICHD International classification headache disorders; RHI reactive hyperhemia index; PAT peripheral arterial tonometry; CGRP Calcitonin Gene Related Peptide; cGMP cyclic guanosine monophosphate; NO nitric oxide. of arterial function in migraineurs [44]. In fact, Stam et al. reported no differences in PWV among subjects with migraine with aura, migraine without aura, and controls [44]. In this latter study subjects with known cardiovascular risk factors were included and this may explain the differences in the results [44]. Higher values of AIx were found in migraineurs by five studies [27,31,32,43,45], which variously measured AIx in the radial artery [45], in the aorta [27,43] or by finger tonometry [31,32]. One of those studies involved only women with migraine with aura [32] and one involved only subjects with chronic migraine [31]. Schillaci et al. reported increased AI in migraine with and without aura with respect to controls and no differences between the two groups of migraineurs [43]. All the studies about AIx had a case–control design, except for a populationbased one [45] which involved cases and controls older than those of the other studies. Two studies assessed static arterial parameters [27,46]. De Hoon et al. found that migraineurs had larger temporal artery diameter compared with control subjects but a decreased distensibility and buffering capacity of the brachial artery in the absence of significant alterations in the common carotid and femoral arteries Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Page 7 of 10 Table 4 Studies on endothelial function by endothelial progenitor cells in migraineurs Study (First author, year) Study design Patients included (n) Lee, 2008 [36] CC Migraine: 92 % Exclusion women criteria Migraine Results diagnosis 70 CVD, diabetic retinopathy ICHD-II Decreased number and migratory capacity and higher senescence levels of endothelial progenitor cells in migraineurs versus controls 73 CVD, inflammatory disease, cancer or treatment with antimitogen agents, pregnancy in the last year ICHD-I Higher number of activated endothelial progenitor cells in migraineurs 98 CAD, inflammatory disease, obesity, HYP, DM, dyslipidemia, CS, infectious disease, severe systemic disease; ovarium pathology, pregnancy or lactation, use of vasoactive drugs ICHD-II Lower endothelial progenitor cell counts in migraineurs (MwoA: 67; MwA: 25) Controls: 37 Oterino, 2013 [37] CC CM: 51 EM: 48 Controls: 35 Rodriguez-Osorio, 2012 [23] CC Migraine : 47 (MwA:14; MwoA: 33) Controls: 23 CC case–control; CM chronic migraine; EM episodic migraine; MwA migraine with aura; MwoA migraine without aura; CVD cardiovascular disease; CAD coronary artery disease; HYP arterial hypertension; DM diabetes mellitus; CS cigarette smoking; ICHD International classification headache disorders. [46]. Vanmolkot et al. found that migraineurs had a decreased diameter and compliance of superficial muscular arteries [27]. Discussion The results of the systematic review suggest the presence of a peripheral vascular dysfunction in migraineurs. However, while several studies support an alteration of arterial function among subjects with migraine, findings on the endothelial function are less clear. Endothelial function in migraineurs has sometimes been reported as impaired and sometimes as altered, while some studies found no difference compared with endothelial function in controls. Differences among available studies may be explained by various inclusion/ exclusion criteria and the generally small number of participants. Migraine, and particularly migraine with aura, has been associated with an elevated cardiovascular profile. The exclusion of subjects with cardiovascular risk factors from some of the studies may at least partly explain the described differences. Duration of migraine, severity of attacks and the active or inactive state of the disease may also have played a role in the betweenstudy differences in results. To clarify the status of endothelial function in migraineurs further, studies are required that need to involve large number of subjects of properly selected individuals, applying rigorous inclusion and exclusion criteria, and including subjects with a wide range of migraine conditions in terms of frequency, duration, and severity of the attacks. In addition, further studies should also adopt a standardized and reliable methodology to address endothelial function. Regarding arterial function, most of the available evidence supports greater stiffness or impaired compliance of the arterial system in migraineurs. The mechanisms underlying the association between migraine and altered arterial function are likely to be complex and may involve both structural and functional changes in the arterial wall. Since all the studies had a cross-sectional design, a cause-effect relationship between migraine and increased arterial stiffness cannot be established. However, migraine usually starts in early adulthood, at an age in which arterial wall pathologies are rare. In the general population alteration in arterial function has been associated with the presence of vascular risk factors and is considered a marker of vascular disease associated with risk of future vascular events [47,48]. Since most studies addressing arterial function in migraineurs excluded subjects with comorbid vascular risk factors, other mechanisms can be hypothesized. The possible hypotheses include alterations in vessel wall structure in migraineurs as supported by the presence of impaired serum elastase activity [49], higher sympathetic tone [50,51], use of drugs such as triptans and ergot derivatives, or it may represent a primary alteration linked to the presence of migraine itself [52]. At the moment it is unknown whether the duration of migraine and the frequency and severity of the attacks has an impact on arterial stiffness. Moreover, gender differences may also play a role in determining arterial stiffness in migraineurs with females being more susceptible to changes [43]. In addition, possible clinical implications of arterial dysfunction in migraineurs have to be clarified. In the general population, arterial dysfunction has been linked to an increased risk of vascular disease through an atherothrombotic mechanism [47,48]. However, in migraineurs Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Page 8 of 10 Table 5 Studies on arterial function in migraineurs Study (First author, year) De Hoon, 2003 [46] Study design Patients included (n) CC MwA: 11 % Exclusion women criteria Migraine diagnosis CVD, inflammatory disease, HYP, DM, dyslipidemia ICHD-I Diameter and compliance parameters of brachial, carotid, femoral, and temporal arteries Smaller diameter and distension of brachial artery and larger right temporal artery diameter in migraineurs; no differences in carotid and femoral arteries 73 CVD, vascular RF ICHD-II Brachial PWV Higher PWV in migraineurs 71 Age ≥50, CVD, inflammatory disease, obesity, HYP, DM, dyslipidemia ICHD-II Radial AIx Higher AIx in migraineurs 100 CVD, obesity, arterial HYP, DM ICHD-II Peripheral AIx Higher Aix in migraineurs 93 Stroke 85 CVD, inflammatory disease, HYP, DM, dyslipidemia ICHD-II Aortic PWV and aortic AIx Higher PWV and AIx in migraineurs, especially in MwA 75 None ICHD-II Carotid and femoral PWV No differences between migraineurs and controls 78 None ICHD-I Diameter and compliance parameters of brachial and femoral arteries, aortic AIx and aortic PWV Smaller brachial artery diameter and compliance and smaller femoral artery diameter in migraineurs; higher aortic AIx in migraineurs; higher PWV in migraineurs not confirmed after adjustment for age and mean arterial pressure MwoA: 39 CC MwA:22 Results 76 Controls: 50 Ikeda, 2011 [42] Parameters assessed MwoA:89 Controls:110 Jiménez-Caballero, 2013 [31] CC CM: 21 Liman, 2012 [32] CC MwA: 29 Nagai, 2007 [45] PB Group A:134 Controls: 21 Controls: 30 Validated Radial AIx questionnaire (5% migraineurs) Higher AIx in migraineurs Group B:138 (17% migraineurs) Schillaci, 2010 [43] CC MwA:17 MwoA: 43 Controls:60 Stam, 2013 [44] PB MwA: 123 MwoA: 167 Controls: 542 Vanmolkot, 2007 [27] CC Migraine: 50 Controls: 50 CC case–control; PB population-based; CM chronic migraine; MwA, migraine with aura; MwoA, migraine without aura; CVD, cardiovascular disease; HYP, arterial hypertension; DM, diabetes mellitus; RF, risk factors; ICHD, International classification headache disorders; AIx, augmentation index; PWV, pulse wave velocity. the evidence of an atherothrombotic mechanism leading to vascular events is not supported and much evidence points against this possibility. [53,54] In fact, intima-media thickness (IMT), which is a surrogate marker of subclinical atherosclerosis has not been consistently linked to migraine status [20,24,27,55,56]. In addition, despite little information being available on the frequency of the different subtypes of ischemic stroke in migraineurs, the available studies do not support an increased atherosclerotic load [57,58]. Also the cardiac events observed in migraineurs did not seem to be attributable to atherothrombosis and alternative mechanisms have been hypothesized [59-61]. Future studies should also clarify whether the vascular dysfunction may represent a marker able to identify those migraineurs who are at higher risk of presenting vascular events and who might be the target of possible preventive strategies. Conclusion The study of systemic vascular function is a promising tool for non-invasively investigating the vascular health of subjects with migraine. At the moment, evidence is too scarce to support the clinical application of the techniques and further research studies are needed to better clarify all the pending issues. Sacco et al. The Journal of Headache and Pain 2013, 14:80 http://www.thejournalofheadacheandpain.com/content/14/1/80 Competing interests The authors declare that they have no competing interests. Authors’ contributions All authors of this manuscript (SS, PR, DG, RO, FP, AC, TK) have made substantial contributions to conception and design of the review, have been involved in drafting the manuscript and revising it critically for important intellectual content and have given final approval of the version to be published. Author details 1 Department of Neurology and Regional Headache Center, University of L’Aquila, Piazzale Salvatore Tommasi 1, L’Aquila 67100, Italy. 2Department of Life, Health, and Eviromental Sciences, University of L’Aquila, L’Aquila 67100, Italy. 3Inserm Research Center for Epidemiology and Biostatistics, Team Neuroepidemiology, Bordeaux F-33000, France. 4University of Bordeaux, Bordeaux F-33000, France. Received: 5 August 2013 Accepted: 20 September 2013 Published: 1 October 2013 References 1. Connor RC (1962) Complicated migraine. A study of permanent neurological and visual defects caused by migraine. Lancet 2:1072–1075 2. Carolei A, Marini C, Di Napoli M, Di Gianfilippo G, Santalucia P, Baldassarre M, De Matteis G, di Orio F (1997) High stroke incidence in the prospective community-based L’Aquila registry (1994–1998). First year’s results. Stroke 28:2500–2506 3. 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The Journal of Headache and Pain 2013, 14:71 http://www.thejournalofheadacheandpain.com/content/14/1/71 REVIEW ARTICLE Open Access TRPA1 and other TRP channels in migraine Silvia Benemei*, Francesco De Cesaris, Camilla Fusi, Eleonora Rossi, Chiara Lupi and Pierangelo Geppetti Abstract Ever since their identification, interest in the role of transient receptor potential (TRP) channels in health and disease has steadily increased. Robust evidence has underlined the role of TRP channels expressed in a subset of primary sensory neurons of the trigeminal ganglion to promote, by neuronal excitation, nociceptive responses, allodynia and hyperalgesia. In particular, the TRP vanilloid 1 (TRPV1) and the TRP ankyrin 1 (TRPA1) are expressed in nociceptive neurons, which also express the sensory neuropeptides, tachykinins, and calcitonin gene-related peptide (CGRP), which mediate neurogenic inflammatory responses. Of interest, CGRP released from the trigeminovascular network of neurons is currently recognized as a main contributing mechanism of migraine attack. The ability of TRPA1 to sense and to be activated by an unprecedented series of exogenous and endogenous reactive molecules has now been extensively documented. Several of the TRPA1 activators are also known as triggers of migraine attack. Thus, TRP channels, and particularly TRPA1, may be proposed as novel pathways in migraine pathophysiology and as possible new targets for its treatment. Keywords: Migraine; Transient receptor potential ankyrin 1 (TRPA1); Transient receptor potential vanilloid 1 (TRPV1); Calcitonin gene-related peptide (CGRP); Neurogenic inflammation; ThermoTRP; Headache; Pain Review TRP channels The observation that the stimulation of a specific receptor, and the consequent, associated transient inward current [1,2] are necessary to vision in Drosophila melanogaster has been the primal evidence of the transient receptor potential (TRP) family of channels, which currently encompasses more than 50 different channels [3]. TRP channels represent a heterogeneous system oriented towards environment perception, and participating in sensing visual, gustatory, olfactive, auditive, mechanical, thermal, and osmotic stimuli. TRP channels consist of six transmembrane domains (S1-S6) with both the NH2 and COOH termini localized into the cytosol. The COOH region is highly conserved among TRPs, whereas the NH2 region usually contains different numbers of ankyrin repeats, the 33-residue motifs with a conserved backbone and variable residues that mediate proteinprotein interactions [4]. The entry of cations through homo- or heterotetramers occurs via the pore formed by loops between the S5 and S6 domains. TRPs are * Correspondence: [email protected] Headache Center and Clinical Pharmacology Unit, Department of Health Sciences, Careggi University Hospital, University of Florence, viale Pieraccini 6, Florence 50139, Italy commonly described as nonselective Ca2+-permeable channels, however, their Ca2+/Na+ permeability ratio may vary widely between different members of the TRP family [5]. TRP channel gating is operated by both the direct action on the channel of a plethora of exogenous and endogenous physicochemical stimuli, and changes in the intracellular machinery, including activation of G-protein coupled receptor (GPCR) or tyrosine kinase receptor [6]. In mammals, the TRP family consists of 28 proteins grouped into 6 subfamilies according to sequence identity, namely TRP canonical (TRPC), TRP vanilloid (TRPV), TRP melastatin (TRPM), TRP polycystin (TRPP), TRP mucolipin (TRPML), and TRP ankyrin (TRPA) [7,8]. The mammalian TRPC subfamily enlists 7 members (TRPC1-7), activated by the stimulation of GPCR and receptor tyrosine kinases [9], although TRPC1 seems to be directly activated by membrane stretch [10]. The TRPM has 8 members (TRPM1-8), and TRPM8 is activated by menthol and low temperatures (< 25°C). Of the three TRPML members, TRPML1 is widely expressed and has been described as an H+-sensor of endosomes/lysosomes, where it probably prevents overacidification [11]. The TRPP family can be subdivided into PKD1like (TRPP1-like) and PKD2-like (TRPP2-like) proteins, © 2013 Benemei et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Benemei et al. The Journal of Headache and Pain 2013, 14:71 http://www.thejournalofheadacheandpain.com/content/14/1/71 which couple to act as a signaling complex at the plasma membrane plasma membrane [12]. Various diseases have been attributed to mutations of TRP channels. However, only a few TRP channelopathies, commonly known as “TRPpathies”, have been conclusively identified so far. TRPpathies include neurological disorders, renal diseases, and complex skeletal dysplasias [13]. TRPA1 and other thermo-TRPs Primary sensory neurons express different TRP channels, including four of the six members of the TRPV subfamily. TRPV1, TRPV2, TRPV3, and TRPV4 channels sense warm-hot temperatures and are chemosensors for a large series of naturally occurring and synthetic ligands. TRPV1, the receptor of the vanilloid compound capsaicin (which promoted the labeling of the group with the V), is responsive to high proton concentrations (pH 5–6) [14,15], anandamide [16] and various lipid derivatives [17]. Camphor and hypotonic solutions are non-selective activators of the TRPV3 and TRPV4, respectively [18,19]. The synthetic compound 4α-phorbol 12,13-didecanoate (4α-PDD), low pH, citrate, endocannabinoids, and arachidonic acid metabolites may also gate TRPV4 [20,21]. Activators of TRPV2 are not well identified, although the uricosuric agent probenecid can activate the channel [22]. Additional TRPs expressed in nociceptors are TRPA1 and TRPM8. Finally, there is also evidence that TRPM3, rather uniquely activated by pregnenolone sulfate, seems to be also expressed in primary sensory neurons [23]. A large NH2-terminal domain with 17 predicted ankyrin repeat domains characterizes TRPA1, the sole member of the TRPA subfamily. TRPA1, first cloned from human fetal lung fibroblasts, is widely expressed in mammals, where it has been found in hair cells, pancreas, heart, brain, keratinocytes [24], urinary bladder [25], prostate [26], arteries [27], enterochromaffin cells [28], odontoblasts and dental pulp [29,30], synovial fibroblasts [31], and epithelial and smooth muscle cells of the airways and lung [32]. A large amount of evidence shows that TRPA1 plays a key role in the detection of pungent or irritant compounds, including principles contained in different spicy foods, such as allyl isothiocyanate (mustard oil) in horseradish [33], allicin and diallyldisulfide in garlic [34], and cinnamaldehyde in cinnamon [35]. Gingerol (in ginger), eugenol (in cloves), methyl salicylate (in wintergreen), carvacrol (in oregano), thymol (in thyme and oregano) [36], are also able to gate TRPA1. In addition, environmental irritants and industry pollutants, such as acetaldehyde, formalin, hydrogen peroxide, hypochlorite, isocyanates, ozone, carbon dioxide, ultraviolet light, and acrolein (a highly reactive α,β-unsatured aldehyde present in tear gas, cigarette smoke, smoke from burning vegetation, and vehicle exhaust), have been recognized as TRPA1 activators [37-45]. The dispute regarding the role of TRPA1 Page 2 of 8 as a sensor of mechanical stimuli and noxious cold (< 17°C) remains unresolved [36]. Electrophilic molecules have been found to activate TRPA1 via a unique mechanism, mediated by a Michael addition with specific cysteine and lysine residues identified in both the rat and human channel [46,47]. Finally, TRPA1-expressing neurons also express other TRP channels, in particular TRPV1, and, even more importantly, the sensory neuropeptides substance P (SP), neurokinin A (NKA), and calcitonin gene-related peptide (CGRP). SP/NKA and CGRP release from peripheral endings of nociceptors promoted by TRPV1 or TRPA1 activation produces a series of responses collectively described as neurogenic inflammation [48]. TRPA1, pain and neurogenic inflammation It is generally recognized that C and Aδ-fibre sensory neurons convey pain signals, while neurons with largersize fibres mediate touch sensation. Thermo-TRPs, which under normal conditions are expressed in C and Aδ-fibre neurons, have been proposed to contribute to transmission and modulation of nociceptive signals. This conclusion originates from empirical observation that TRPV1 or TRPA1 agonists, derived from foods and spices, are able to cause, in a dose-dependent fashion, a range from appreciated hot feelings to unpleasant pain sensation, as in the case of capsaicin, the selective TRPV1 agonist contained in hot peppers, and piperine, another TRPV1 agonist present in black pepper, or allyl isothiocyanate, the TRPA1 agonist contained in mustard or wasabi [5,49]. Another finding that enlists thermo-TRPs as major pain controlling mechanisms is the clinical use of topical (cutaneous) capsaicin application that, by defunctionalizing sensory nerve terminals, alleviates several pain conditions, including post-herpetic neuralgias or pain associated with diabetic neuropathy [50]. Generation of deleted mice and, more importantly, identification and preclinical development of selective antagonists for thermo-TRPs, have greatly increased our knowledge of the role of these channels in the regulation of acute nociceptive responses and the development of allodynia and hyperalgesia. While TRPV1-deleted mice exhibit decreased hyperalgesia to elevated temperatures [51], TRPA1-deletion abrogated the two classical nociceptive phases to formalin [38]. However, gene deletion may not completely recapitulate the effects of channel blockade, as compensatory mechanism may counterbalance the function of the absent gene-protein. Thus, findings produced by using selective thermo-TRP antagonists may better unveil the role of these channels in models of pain diseases. For detailed information on other thermo-TRPs, the reader is referred to previous review articles [5-8] whereas we will focus on the increasing evidence that supports the role of TRPA1 in models of both inflammatory and Benemei et al. The Journal of Headache and Pain 2013, 14:71 http://www.thejournalofheadacheandpain.com/content/14/1/71 neuropathic pain. Mechanical hyperalgesia [52] and ongoing neuronal discharge [53,54] evoked by complete Freund’s adjuvant (CFA) and tumour necrosis factor-α (TNFα), and mechanical hyperalgesia induced by low doses of monosodium iodoacetate (MIA) [55] are inhibited by TRPA1 receptor antagonists in rodents. Convergent findings indicate that TRPA1 contributes to carrageenanevoked inflammatory hyperalgesia [56,57]. Carrageenan administration, among a number of lipid derivatives, notably increases metabolites of 12-lipoxygenases, particularly hepoxilins A3 (HXA3) and HXB3, whose hyperalgesic/allodynic effect is abrogated by TRPA1 antagonism [58]. It has also been found that TRPA1 mediates ongoing nociception in chronic pancreatitis [59], and that both TRPV1 and TRPA1 initiate key pathways to transform acute into chronic inflammation and hyperalgesia in pancreatitis [60]. The clinical observation that an antioxidant afforded protection in patients with pancreatitis [61] indirectly supports the role of the oxidative stress sensor, TRPA1, in this condition. The underlying mechanisms that from neural tissue injury produce the chronic allodynia and hyperalgesia typical of neuropathic pain are largely unknown. However, recent reports have pointed to the role of TRPA1 in different models of neuropathic pain. Several metabolic pathways in glycolysis or lipid peroxidation produce methylglyoxal (MG), which appears in the plasma in diabetic patients as hyperglycemia strongly enhances MG accumulation. MG has been recently described to react reversibly with cysteine residues, and probably due to this property stimulates TRPA1, thus representing a likely candidate metabolite to promote neuropathic pain in metabolic disorders [62]. Chemotherapeutic induced peripheral neuropathy (CIPN) is a scarcely understood and poorly treated condition, which, characterized by spontaneous pain, and mechanical and cold allodynia and hyperalgesia, causes significant discomfort, and often therapy discontinuation. CIPN may outlast the time period of chemotherapeutic drug administration for weeks or months [63]. Alteration of several ion channels has been advocated to explain this painful condition, but a univocal consensus has not emerged. Recently, in mouse models of CIPN produced by a single administration of oxaliplatin, paclitaxel or bortezomib, TRPA1 has been shown to play a major role in the development and maintenance of cold and mechanical [64-66]. In a therapeutic perspective, it is of relevance that treatment with a TRPA1 antagonist just before and shortly after (about 6 hours) the administration of bortezomib or oxaliplatin totally prevented the development and maintenance (for 10–15 days) of mechanical and cold hypersensitivity [66]. This finding suggests that TRPA1 is key in initiating CIPN and promoting the transition from an acute to a chronic condition. Page 3 of 8 In addition, TRPA1 antagonists could represent novel therapeutic strategies of CIPN. In this context, it could be better understood that the unexpected attenuation by etodolac, a nonsteroidal anti-inflammatory drug of mechanical allodynia in a mouse model of neuropathic pain, might be due to its ability to inhibit TRPA1 [67]. In addition, to contribute to mechanical hyperalgesia, TRPA1 seems to be involved in the development of cold allodynia [68]. Cold allodynia was reduced in models of neuropathic (peripheral nerve injury) and inflammatory (CFA) pain [69,70] or in models of CIPN, which typically exhibit this type of hypersensitivity [64-66]. Interestingly, a familial episodic pain syndrome has been attributed to a gain of function mutation of TRPA1 TRPA1 [71]. Coincidence between TRPA1 and neuropeptide expression has been suggested [72,73], although evidence for coexistence of isolectin B4-positive and non-peptidergic neurons with TRPA1 has also been reported [74,75]. Notwithstanding, exposure of tissues containing either peripheral or central sensory nerve terminals to TRPA1 agonists invariably results in a calcium-dependent release of SP/NKA and CGRP. Thus, TRPA1 stimulation has been proven to increase sensory neuropeptide release from oesophagus and urinary bladder, meninges, or dorsal spinal cord [76-78]. The outcome of such a release in peripheral tissues encompasses the series of responses commonly referred to as ‘neurogenic inflammation’ [48] (Figure 1). Although implication in transmission of nociceptive signals has been proposed, the pathophysiological outcome of the central release of sensory neuropeptides within the dorsal spinal cord or brain stem is less clear (Figure 1). TRPA1, TRPV1, and migraine One of the first findings that, although indirectly, suggested the role of TRP channels in cluster headache and migraine was represented by the protective effect of the topical, desensitizing application of capsaicin to the patient nasal mucosa [79,80]. Capsaicin treatment couples the unique ability of the drug to first activate, and, subsequently, upon repeated administration, desensitize both the afferent pathway that from channel activation conveys nociceptive signals, and the ‘efferent’ function that results in sensory neuropeptide release [48]. Within the fifth cranial nerve, capsaicin desensitization causes the defunctionalization of peripheral and possibly central nerve endings, thus preventing SP/NKA-dependent plasma protein extravasation within the dura mater, CGRPdependent dilatation of meningeal arterioles, and inhibition of afferent nociceptive impulses. Further support to the contribution of TRPV1 to migraine mechanism derived from the observation that ethanol, a known trigger of migraine attacks, activates CGRP release from sensory neurons, and promotes CGRP-dependent menin- Benemei et al. The Journal of Headache and Pain 2013, 14:71 http://www.thejournalofheadacheandpain.com/content/14/1/71 Page 4 of 8 Figure 1 Schematic representation of the probable mechanisms of the action of antimigraine remedies, either currently used, or proven effective in clinical trials (gray boxes) or of novel medicines (empty box), regarding their ability to modulate the release of calcitonin gene related peptide (CGRP) or the activation of its receptor (CGRP-R). (1) Non steroidal antiinflammatory drugs (NSAIDs) block prostaglandin synthesis and the ensuing nociceptor sensitization and CGRP release evoked by prostaglandin receptor (PG-R) activation. (2) Triptans, by activating neuronal 5HT1D receptors, inhibit CGRP release. (3) CGRP-R antagonists inhibit the action of CGRP on effector cells. (4) Antagonists of transient receptor potential channels (TRPs), including TRP ankyrin 1 (TRPA1), block the ability of a series of stimulants (for TRPA1, cigarette smoke, acrolein, nitric oxide, umbellulone, and others) to release CGRP. All medicines may act at both peripheral and central endings of trigeminal nociceptors. Receptor/channel activation may trigger/facilitate (+) or inhibit (−) CGRP release. geal vasodilatation by reducing the threshold temperature for channel activation, a phenomenon that eventually results in TRPV1 activation [81,82]. TRPV4 is stimulated by hypoosmotic stimuli that cause plasma membrane stretch and mechanical distension. Although this effect could, in principle, be implicated in the throbbing pain often described by migraine patients during their headache attacks, no evidence has yet been reported in support of TRPV4 in any model of head pain. In contrast, a series of observational findings has recently been obtained regarding a possible association between TRPA1 and migraine. In this respect, it is of interest that a number of compounds, recently identified as TRPA1 agonists, including cigarette smoke, ammonium chloride, formaldehyde, chlorine, garlic and others [34,38,41,78,83] are known triggers of migraine attacks in susceptible individuals [84-89]. Nitric oxide (NO) donor drugs, including nitroglycerine, are known inducers of migraine attacks and have been extensively used in migraine provocation studies in humans [90]. Although their pro-migraine action has been attributed to their intrinsic vasodilatatory action [91], the temporal mismatch between vasodilatation and the onset of migraine-like attacks argues against a close association between the two phenomena. In fact, at the time of the maximum vasodilatation, a mild to moderate headache develops both in migraineurs (more) and in healthy subjects (less), while delayed migraine-like attacks, present only in migraine patients, are observed only 5–6 hours after nitroglycerine administration [92,93]. Although not confirmed in vitro [94]. NO may release CGRP from trigemino-vascular neurons [95]. More recently, NO has been revealed to target TRPA1 by nitrosylation of channel cysteine residues [96], that seem to differ from those targeted by other reactive molecules [97], and this novel molecular mechanism could contribute to the nociceptive response evoked by NO [98]. It is possible that the S-nitrosylation process [99], produced by NO, contributes to channel sensitization to eventually (hours after the exposure to NO) amplify CGRP release by other agents, thus leading to exaggerated neurogenic inflammation and potentiation of pain responses. The environmental pollution agent, acrolein, is produced by the combustion of organic material which causes its accidental inhalation, which, however, occurs also with cigarette smoking [100]. Several components of cigarette smoke, such as acrolein, crotonaldehyde [78], acetaldehyde [37] and nicotine [101], are TRPA1 agonists. Recently, acrolein application to the rat nasal mucosa has been shown to produce ipsilateral meningeal vasodilatation by a TRPA1- and CGRP-dependent mechanism [102], thus offering a mechanistic explanation for the association between the exposure to cigarette smoke and migraine attack appearance or worsening [103,104]. Herbalism has been instrumental for the development of pharmacology and also to a better understanding of pathophysiological mechanisms. These principles also Benemei et al. The Journal of Headache and Pain 2013, 14:71 http://www.thejournalofheadacheandpain.com/content/14/1/71 apply to the migraine field. Umbellularia californica (California bay laurel) is also known as the ‘headache tree’ because of the ability of its scent to trigger headache attacks in susceptible individuals [105]. A case of cluster headache-like attacks preceded by cold sensations perceived in the ipsilateral nostril following inhalation of Umbellularia californica scent has recently been described in a cluster headache patient whose attacks had ceased 10 years before [106]. The irritant monoterpene ketone, umbellulone, one of the most abundant reactive molecules of Umbellularia californica, was found to activate the human recombinant and the constitutive rat/mouse TRPA1 in trigeminal ganglia (TG) neurons, and via this mechanism to produce nociceptive behaviour and the release of CGRP from TG or meningeal tissue in rats [77]. In addition, similar to acrolein, umbellulone application to the rat nasal mucosa evoked ipsilateral TRPA1- and CGRP-dependent meningeal vasodilatation [77]. Ligustilide, an electrophilic volatile dihydrophthalide of dietary and medicinal relevance, has been found to produce a moderate activation of TRPA1, but also to inhibit allyl isothiocyanate-evoked stimulation of TRPA1 [107]. This newly identified target of ligustilide offers a novel mechanistic explanation for the use of the compound in traditional medicine to treat pain diseases, including headaches. Finally, although several hypotheses have been advanced, the mechanism of the analgesic action of acetaminophen (paracetamol) in different pain conditions is far from clear. The reactive metabolite of acetaminophen, N-acetyl-p-benzo-quinoneimine (NAPQI), is able to activate the TRPA1 channel and thereby evoke a moderate and reversible neurogenic inflammatory response, which, in susceptible individuals, may contribute to emphasizing inflammation in peripheral tissues [76]. However, NAPQI may also be produced by cytochrome activity within the spinal cord, and NAPQI action at the spinal level results in channel desensitization [108]. Inhibition of central TRPA1 has been advocated as the mechanism, which may be responsible the hitherto unexplained analgesic and possibly antimigraine action of its parent molecule [108]. Importantly, this novel spinal mechanism could be of general relevance also for other TRPA1 agonists which share with NAPQI the ability of desensitizing the channel. A host of endogenous inflammatory mediators possibly released during migraine attacks can activate and sensitize peripheral and central sensory neurons, including trigeminal neurons. Sensitization of first-order neurons is involved in the perception of headache throbbing pain [109], while sensitization of second‐order neurons contributes to cephalic allodynia and muscle tenderness [110,111]. Recently, it has been shown that innocuous brush and heat stimuli induce larger activation in the thalamus of patients who exhibit allodynia during mi- Page 5 of 8 graine, as compared to pain‐free state, and that topical application of inflammatory molecules on the rat meninges sensitizes thalamic trigeminovascular neurons [112]. Each component of the nociceptive pathways could contribute differently to sensitization of the neural tissues. TRP channels could be sensitized by different compounds and via different mechanisms [5,36,66,81,113]. Although there is no specific information on the role of these channels to peripheral nociceptor sensitization or central sensitization in migraine, it is possible that TRP channels, and in particular TRPV1 and TRPA1, contribute to this key mechanism. Conclusions The still largely unfinished puzzle of the migraine mechanism is being completed by novel unexpected pieces of information, which compose a clearer picture. The first, corner of the picture, known for decades, is that cyclooxygenase (namely cyclooxygenase 2) inhibition has a beneficial effect on migraine attack. The second corner, predicted by scientists and appreciated by clinicians and patients, is that targeting 5-HT1 receptors is also highly effective. The third corner is that a series of clinical trials show that CGRP antagonists afford a protection similar to that of triptans. To complete at least the frame, a fourth corner, which should reconcile the other three in an intelligible sequence of events, is needed. While prostaglandins sensitize nociceptors and eventually promote CGRP release, triptans inhibit such release, thus producing an indirect anti-migraine effect, and CGRP antagonists abrogate the final common pathway of migraine mechanism. The pathway, which, sensitized by prostaglandins and inhibited by serotonin receptor stimulation, results in trigeminal neuron activation and the promigraine release of CGRP could represent the fourth corner of the picture. Emerging information on TRP channels, and particularly TRPA1, which, targeted by migraine triggers, contribute, by activating the trigeminal CGRPdependent pathway, to the genesis of pain and the accompanying symptoms of the attack, seems to be of paramount importance to solve what still remains the enigma of the migraine mechanism. Competing interests The authors declare that they have no competing interests. Authors’ contribution FDC, CF, ER, and CL searched for the literature. SB and PG wrote and revised the text. All authors read and approved the final manuscript. Acknowledgements We acknowledge Regione Toscana (Regional Health Research Program 2009, P.G.). Received: 22 May 2013 Accepted: 10 August 2013 Published: 13 August 2013 Benemei et al. 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The Journal of Headache and Pain 2013, 14:61 http://www.thejournalofheadacheandpain.com/content/14/1/61 REVIEW ARTICLE Open Access Genes and primary headaches: discovering new potential therapeutic targets Innocenzo Rainero1*, Elisa Rubino1, Koen Paemeleire2, Annalisa Gai1, Alessandro Vacca1, Paola De Martino1, Salvatore Gentile1, Paola Sarchielli3 and Lorenzo Pinessi1 Abstract Genetic studies have clearly shown that primary headaches (migraine, tension-type headache and cluster headache) are multifactorial disorders characterized by a complex interaction between different genes and environmental factors. Genetic association studies have highlighted a potential role in the etiopathogenesis of these disorders for several genes related to vascular, neuronal and neuroendocrine functions. A potential role as a therapeutic target is now emerging for some of these genes. The main purpose of this review is to describe new advances in our knowledge regarding the role of MTHFR, KCNK18, TRPV1, TRPV3 and HCRTR genes in primary headache disorders. Involvement of these genes in primary headaches, as well as their potential role in the therapy of these disorders, will be discussed. Keywords: Primary headaches; Genes; MHTFR; KCNK18; HCRTR1; HCRTR2 Introduction Primary headache disorders, according to the International Classification of Headache Disorders 2nd edition (ICHD-II), include migraine, tension-type headache, cluster headache, and other primary headaches [1]. Primary headaches represent a common and major health problem worldwide and significantly impair patients’ quality of life [2,3]. These disorders may affect individuals from childhood and are most troublesome in the productive years of life, thus generating an economic burden for both society and healthcare systems [4,5]. Recently, Global Burden of Disease (GBD) studies have rated primary headaches among the top ten disorders causing significant disability [6]. In recent years, genetic studies have provided substantial evidence supporting the notion that primary headaches are complex, multifactorial disorders. Population, family and twin studies have shown that migraine, tension-type headache and cluster headache have a significant heritable component [7,8]. Different genetic factors may, therefore, be involved in the generation of a specific “headache threshold”. In rare primary headache subtypes, such as Familial Hemiplegic Migraine (FHM), single gene mutations co-segregate with disease phenotype [9-11]. In the more common forms of primary headaches, as in other complex diseases, the phenotype is thought to be caused by an interaction of multiple genetic variants, each of them having a small to medium effect, with different environmental factors. Due to the complexity of these disorders, the isolation of different genetic factors involved in primary headaches has proven to be difficult. Genetic association studies have provided evidence that genes involved in vascular, neuronal and endocrine functions may have a significant role in primary headaches [12,13]. The purpose of this review is to highlight recent discoveries, in particular about methylenetetrahydrofolate reductase (MTHFR), potassium channel, subfamily K member 18 (KCNK18), transient related potential vanilloid type 1 (TRPV1), transient related potential vanilloid type 3 (TRPV3), hypocretin (orexin) receptor 1 (HCRTR1) and hypocretin (orexin) receptor 2 (HCRTR2) genes which are involved in different subtypes of primary headaches and that, in the near future, might be of relevance as novel therapeutic targets. * Correspondence: [email protected] 1 Headache Center, Neurology I, Department of Neuroscience, University of Torino, Via Cherasco 15, 10126 Torino, Italy Full list of author information is available at the end of the article © 2013 Rainero et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Rainero et al. The Journal of Headache and Pain 2013, 14:61 http://www.thejournalofheadacheandpain.com/content/14/1/61 Page 2 of 8 Review In the last two decades, molecular genetic studies provided substantial evidence concerning the potential role of multiple genes in primary headaches. The majority of these studies evaluated genetic factors involved in migraine, while molecular genetics of cluster headache and tension-type headache has been little studied, so far. Vascular genes and primary headaches Migraine and cluster headache have long been considered vascular disorders. Even if the so-called “vascular theory” of migraine has been shown to be inadequate in explaining the complex symptoms of the disorder, both migraineurs and cluster headache patients show abnormalities in both cranial and extracranial vascular reactivity [14,15]. In addition, both disorders are characterized by a significant comorbidity with diseases such as stroke, myocardial infarction, hypertension and Raynaud’s phenomenon [16-18]. Several genes involved in vascular functions, such as endothelin-1 (ETA-1), angiotensin-converting enzyme (ACE), Neurogenic Locus Notch Homolog Protein 4 (NOTCH4) and methylenetetrahydrofolate reductase (MTHFR) genes have been studied in patients with primary headaches and some of these, such as the MTHFR gene, were significantly associated with migraine [19-24]. The MTHFR gene is located on chromosome 1p36.3; it consists of 11 exons and encodes for the methylenetetrahydrofolate reductase, a crucial enzyme involved in purine and thymidylate biosynthesis, methylation of DNA and amino acids, and neurotransmitters synthesis. The MTHFR enzyme catalyzes the reduction of 5, 10methylenetetrahydrofolate to 5-methyltetrahydrofolate, a substrate needed for the conversion of homocysteine to methionine (Figure 1). This pathway is folate-dependent and a lack of dietary folate can produce an increase in homocysteine levels. The clinical consequences of increased homocysteine plasma concentrations include endothelial cells injury and alterations in coagulant properties of blood [25-27]. Furthermore, homocysteine derivatives act as NMDA receptor agonists and they may enhance glutamatergic neurotransmission, thereby increasing the spontaneous trigeminal cells firing and predisposing cortical neurons to hyperexcitability [28,29]. Several genetic variants have been described in the MTHFR gene. Genetic research investigating the role of MTHFR in primary headaches has focused almost exclusively on two common polymorphisms, due to their functional activity. These are a cytosine (C) > thymine (T) change at position 677 in exon 4, that results in a substitution of an alanine into a valine amino acid (Ala222Val) in the catalytic domain, and an adenine (A) > cytosine (C) change occurring at position 1298 in exon 8, that changes a glutamate into an alanine amino acid (Glu429Ala). The C677T genetic variant allele produces a 35% reduction of MTHFR enzyme activity whereas the A1298C variant results in decreased MTHFR activity to a somewhat lesser degree [30]. These two variants have been extensively associated with the pathogenesis of several disorders, such as cardiovascular disease, cerebrovascular disease, and psychiatric disorders [31-35]. A large number of studies also provided evidence of an association between migraine and the C677T polymorphism in the MTHFR gene [22-24]. This association seems significant mainly in patients affected by migraine with aura (MA) while in patients affected by migraine without aura (MO) the results are conflicting. Two recent genetic meta-analysis provided clear evidence of a significant association between MA and the MTHFR gene: the carriage of the T allele was shown to be Folate NADPH + H + + NADP DHF Methionine NADPH + H + SAM DNA RNA proteins phospholipids neurotransmitters methylation NADP THF MTR + MTRR +B12 SAH Homocysteine CBS Adenosine + B6 Cystathionine + DNA synthesis and repair Ser Gly 5,10-methylene-THF MTHFR 5-methyl-THF CTH + B6 Cysteine Glutathione redox cycle Figure 1 Production of homocysteine as part of the amino acid and purine biosynthesis pathway. DHF = dihydrofolate, THF = tetrahydrofolate, MTHFR = methylene-tetrahydrofolate-reductase, TS = thymidylate-synthase, MTR = methionine-synthase, MTRR o MSR = methionine-synthase-reductase. Rainero et al. The Journal of Headache and Pain 2013, 14:61 http://www.thejournalofheadacheandpain.com/content/14/1/61 associated with an approximately two-fold increased risk [36,37]. More recently, in Norfolk Island Population, three MTHFR single nucleotide polymorphisms (SNPs) were associated with migraine. These three SNPs are located in intron 7 (rs6696752), in the 3′ untranslated region (rs4846048), and in exon 11 (rs2274976, non synonymous, producing a substitution of arginine to glutamine, R594Q) and have not previously been reported to show any genetic association with migraine. These findings reinforced the potential role of MTHFR in migraine susceptibility [37]. A recent case–control study examined the association between MTHFR polymorphisms and cluster headache in a group of 147 cases and 599 Caucasians controls. This study found no evidence of association between genotypes of the MTHFR 677C>T polymorphism and cluster headache overall. However, subgroup analyses suggested that carriers of the MTHFR 677 T allele may have an increased risk for chronic cluster headache, suggesting a need for additional studies in order to evaluate a possible role of MHTFR as modifier gene in the disorder [38]. At present, no study examined the potential association between tension-type headache and MTHFR polymorphisms. Hyperomocysteinemia has been reported in patients with migraine [39]. Folic acid, vitamin B6 and vitamin B12 supplementation has been found to be effective in reducing the occurrence of migraine attacks [40]. Therefore, a recent pharmacogenetic study evaluated the effects of different MTHFR and 5-methyltetrahydrofolatehomocysteine methyltransferase reductase (MTRR) genotypes on the occurrence of migraine in a double-blinded placebo-controlled trial of daily vitamin B supplementation. Patients carrying the C allele of the MTHFR C677T variant showed a higher reduction in homocysteine levels, severity of pain and migraine disability, when compared with those with the T allele. MTRR catalyzes the remethylation of homocysteine to methionine, and, similarly, the A allele carriers of the MTRR A66G variants showed a higher degree of reduction in homocysteine levels, severity of pain and percentage of severe migraine disability, when compared with those carrying the GG genotypes [41]. This pivotal study suggests that both MTHFR and MTRR gene variants may influence the response to treatment with vitamin B in migraineurs. Conversely, genetic data concerning the role of vascular genes in tension-type headache are still scarce. A recent meta-analysis investigated the genetic role of the endothelin type A receptor (EDNRA) – one of the two receptors of the potent vasoconstrictor ETA-1 – in migraineurs and in patients with tension-type headache [42]. This meta-analysis included 440 migraineurs, 222 patients with tension-type headaches and 1323 controls from three previous studies reporting Page 3 of 8 conflicting results about EDNRA -231G>A polymorphism. It found a significant difference in the frequency of AA genotype between migraine subjects and healthy controls. However, no differences were found in the distribution of the EDNRA -231G>A SNP between patients with tensiontype headaches and controls. Neuronal genes and primary headaches Several clinical and experimental data support the concept of abnormal cortical excitability as the pivotal physiological disturbance in migraine [43,44]. Mutations in genes that code for ion channels or pumps (CACNA1A, ATP1A2, and SCN1A) have been described in FHM, strongly supporting the hypothesis that migraine may be classified as a “cerebral ionopathy” [45]. CaV 2.1 (CACNA1A) calcium channels are located in the presynaptic terminal of both excitatory and inhibitory neurons, NaV 1.1 (SCN1A) sodium channels are expressed in inhibitory interneurons while Na+/K+ATPase (ATP1A2) is located at the surface of glial cells (astrocytes). Knock-in mouse models carrying such mutations showed an increased susceptibility to cortical spreading depression, the likely underlying mechanism of migraine aura [46,47]. Finally, an interesting comorbidity between migraine and epilepsy has been described, further supporting a role for ion homeostasis genes in migraine pathophysiology [48,49]. However, until few years ago, no ion channel gene involvement has been described in the common form of migraine or in other primary headaches disorders. In 2010, a frameshift mutation in the KCNK18 gene which segregates perfectly with typical MA in a large, multigenerational pedigree was reported [50]. This gene codes for TWIK-related spinal cord potassium channel (TRESK), a member of the two-pore domain (K2P) potassium channel family. Functional characterization of the F139WfsX24 mutation demonstrated that it causes a complete loss of TRESK function and that the mutant subunit suppresses the wild-type channel function through a dominant-negative effect. The KCNK18 gene is located on chromosome 10; it encodes a protein containing 4 transmembrane domains (TMDs), and two pore-forming domains. The extracellular domain located between TMD1 and TMD2 contains a conserved cysteine residue that may form a disulfide bridge to aid channel dimerization, and a conserved N-linked glycosylation site, important for surface expression of the channel [51]. In humans, the family of KP2 channels includes 15 related channels but TRESK is unique in having a large intracellular regulatory domain located between TMD2 and TMD3. TRESK is abundantly expressed in the dorsal root ganglion (DRG), and in other sensory ganglia such as the trigeminal ganglion (TG). TRESK was also found in human autonomic Rainero et al. The Journal of Headache and Pain 2013, 14:61 http://www.thejournalofheadacheandpain.com/content/14/1/61 nervous system ganglia, such as the stellate ganglion and paravertebral sympathetic chain [50,52]. The TRESK is an outwardly rectifying K+ current channel that contributes to the resting potential and is the most important background potassium channel in DRG. TRESK is activated in a complex manner by intracellular calcium signalling. The calcium/calmodulindependent protein phosphatase, calcineurin, activates TRESK function [53,54]. Calcineurin also regulates nuclear factor of activated T cells (NFATs), transcription factors that regulate inducible expression of many cytokines. In addition, recent studies have identified volatile anesthetics as highly potent TRESK agonists, interacting directly with the channel. On the contrary, cyclosporin A and tacrolimus, two potent immunosuppressants that specifically inhibit the calcineurin activation of NFATs, mimic a TRESK loss of function by keeping the channel insensitive to increases in intracellular Ca2+. The physiological functions of TRESK have been mainly investigated in knock-out (KO) mice. The KO mice show no gross anatomical or behavioral phenotype. The gene ablation has minimal effect on the resting membrane potential. However, DRG neurons from TRESK KO mice displayed a lower threshold for activation, reduced action potential duration, and slightly higher amplitudes of after-hyperpolarization, suggesting that DRG neurons from KO mice were more excitable than wild-type DRG neurons [55]. In addition, due to the coupling of TRESK to the histamine H1 receptor, the channel may reduce neuronal excitability in inflammatory conditions when histamine or other inflammatory modulators are released into the surrounding tissue. Taken together, these data suggest an important role of TRESK in both acute and chronic pain conditions [56]. After the identification of KCNK18 gene mutation in a Canadian MA pedigree, a large cohort of unrelated MA patients and healthy controls was screened for gene mutation. Several missense variants (R10G, A34V, C110R, S231P and A233V) were found. These variants either had no apparent functional effect, or they caused a reduction in channel activity. The A34V was identified in a single Australian migraine proband for which family samples were not available, but it was not detected in controls. By contrast, the R10G, C110R, and S231P variants were found in both migraineurs and controls. The authors concluded that the presence of a single nonfunctional variant in the KCNC18 gene is probably not sufficient to determine whether an individual develops migraine [57]. In a recent study, we examined the presence of KCNK18 gene mutations in a large data set of Italian migraine patients (both with MA and MO) and healthy controls. We confirmed the presence of KCNK18 gene mutations in MA and also found gene mutations in MO Page 4 of 8 patients [58]. Some of these gene variants have not previously been described. However, the functional relevance of these mutations still need further investigations. Finally, KCNK18 gene involvement in tension-type headache or cluster headache has not been investigated yet. The TRESK K2P channel is a novel and interesting component of the migraine pathogenesis pathway and represents an excellent opportunity for development of antimigraine therapy, given its highly selective expression pattern in neuronal structures, which is known to be important in disease pathogenesis. Its highly specific expression pattern in TG, DRG and parasympathetic neurons and the presumed role in abating neuronal excitability under inflammatory conditions make it an excellent target for development of new migraine therapeutics [59,60]. Specific agonists could upregulate TRESK activity and may have a potential in both acute and preventive migraine therapy, as well as in other pain disorders. Recently, the transient receptor potential (TRP) channels gained increased interest for their potential involvement in primary headaches [61,62]. There are at least 30 members of the mammalian TRP family, which are coded by several, different genes. TRP channels are distributed in many peripheral tissues as well as central and peripheral nervous system. Several TRP family members, including the TRPV1 (Transient Related Potential Vanilloid Type 1), TRPV2 (Transient Related Potential Vanilloid Type 2), TRPV3 (Transient Related Potential Vanilloid Type 3) and TRPV4 (Transient Related Potential Vanilloid Type 4), TRPM8 (Transient Related Potential Metastatin Type 8) channels, are polymodal sensors expressed in the sensory neurons of dorsal root ganglia (DRG) and trigeminal ganglia (TG) [63]. In particular, TRPV1 receptor is highly co-expressed with calcitoningene related peptide (CGRP) a potent vasodilator with an important role in migraine. TRPV1 is also coexpressed with other pain signalling molecules such as substance P, P2X3 purinergic receptors and other markers of nociceptive C and Aδ fibers [64]. It has been proposed to play a crucial role as mediators of neuropathic pain and have been proposed to play a role in migraineous allodynia and sensitization phenomena [65]. Using a genetic association strategy, in 2012 Carreno et al. found a significant association between SNPs within the TRPV1 and TRPV3 genes and migraine in the Spanish population [66]. Interestingly, TRPV1 and TRPV3 are located in close proximity on the 17p13 chromosomal region and they share a high sequence homology. In the meanwhile, two genome-wide association studies (GWAS) found evidence of a significant association between genetic markers in or near the TRPM8 gene and migraine [67,68]. TRPM8 gene is expressed by a different TRPV1negative neuronal subpopulation in DRG and TG. These Rainero et al. The Journal of Headache and Pain 2013, 14:61 http://www.thejournalofheadacheandpain.com/content/14/1/61 preliminary data significantly support a role for TRP channels in the pathogenesis of primary headaches. TRP channels have been among the most aggressively pursued drug targets over the past few years and several studies suggested these channels as potential therapeutic targets in migraine. Both peripheral and central nerve terminals at the spinal cord can be targeted to induce pain relief by TRPV1 agonists. In particular, the analgesic effects of the TRPV1 antagonist SB-705498 on trigeminovascular sensitization and neurotransmission have been studied in an animal model of neurovascular head pain [69,70]. Recently, a phase II clinical trial using SB-705498 has been conducted for the acute treatment of migraine attacks but results are pending (ClinicalTrials.gov). Neuroendocrine genes and primary headaches A large number of endocrine abnormalities has been described in patients with primary headaches [71,72]. The hypothalamus, with its paramount control of the endocrine system, as well as its widespread connections with both central and autonomic nervous system, exerts a pivotal role in the pathogenesis of both migraine and cluster headache [73]. Therefore, genes that code for proteins involved in endocrine functions are candidate genes for primary headache disorders. Polymorphisms in genes that code for estrogen and progesterone receptors have been intensively studied in migraineurs, with contrasting results [74,75]. In 1998, two research groups independently discovered a new hypothalamic peptidergic system. The former group named the peptides “hypocretins”, because of their hypothalamic location and structural similarity to the incretin family of hormones [76]. The latter group named the peptides “orexins”, due to the appetiteenhancing properties when administered centrally to rats [77]. Subsequent studies revealed complex and interesting neurobiological effects of these peptides, with particular relevance to the pathophysiology of primary headaches [78]. The hypocretins (Hcrt-1 and Hcrt-2), also called orexins, are peptides derived by proteolytic cleavage from the same 130 amino acid precursor peptide (prepro-hypocretin). A single gene located on chromosome 17q21 in humans is responsible for encoding prepro-hypocretin. The human prepro-hypocretin gene consists of 2 exons and 1 intron. The hypocretins bind to 2 G-protein coupled receptors, termed HCRTR1 and HCRTR2. The HCRTR1 gene in humans is located on chromosome1p33 whereas the HCRTR2 gene is located on chromosome 6p11. Both genes consist of 7 exons and 6 introns. Hcrt-1 has equal affinity for both HCRTR1 and HCRTR2, with Hcrt-2 demonstrating a 10-fold higher Page 5 of 8 affinity for HCRTR2 than HCRTR1. Activation of both receptors results in elevated levels of the intracellular Ca2+ concentrations, and this in turn results in the enhancement of the Gq-mediated stimulation of phospholipase C. Hypocretin immunoreactive cell bodies have been observed mainly in the hypothalamus [79]. Hypocretincontaining neurons have widespread projections throughout the CNS with particularly dense excitatory projections to monoaminergic and serotonergic brainstem centers [80]. The hypocretin system influences a wide range of physiological processes in mammals, such as feeding, arousal, rewards, and drug addiction [81,82]. Recently, a number of studies in experimental animals showed that hypocretins are involved in pain modulation within the CNS, and suggested an important role for these peptides in primary headaches [83]. In 2004, our research group investigated the possible involvement of the hypocretin transmission in cluster headache. We selected several DNA polymorphisms of the three genes that constitute the hypocretin system and, using a case–control strategy, we evaluated possible allelic and genotypic differences in a group of 109 CH patients and 211 controls. Genetic analysis revealed that both allelic and genotypic frequencies of the G1246A polymorphism in the HCRTR2 gene were significantly different between CH patients and controls [84]. Subjects homozygous for the G allele, in comparison with the remaining genotypes, were 5-fold more likely to develop the disease. This association was confirmed in a large group of CH patients and controls from Germany [85]. On the contrary, Baumber et al. found no association between CH and the HCRTR2 gene in a cohort of 259 patients of Danish, Swedish, and British origin [86]. To resolve this issue, we performed a genetic metaanalysis of the previous studies (593 cases and 599 controls) and a haplotype analysis: both these studies confirmed the presence of a significant association between the HCRTR2 gene and CH [87]. At present, the possible involvement of the hypocretin system in migraine has been scarcely investigated. Studies in CH patients prompted two independent research groups to evaluate the association of the G1246A polymorphisms in the HCRTR2 gene with migraine. Both studies found no association between this polymorphism and migraine or its clinical subtypes [88,89]. Recently, we performed a genetic case–control study to investigate whether genetic variants in the HCRTR1 gene could modify the occurrence and the clinical features of migraine. Using a case–control strategy we genotyped 384 migraine patients and 259 controls for three SNPs in the HCRTR1 gene. Genotypic and allelic frequencies of the rs2271933 non-synonymous polymorphism were different between migraineurs and controls [90]. The carriage of the A allele was associated with an increased migraine risk. This Rainero et al. The Journal of Headache and Pain 2013, 14:61 http://www.thejournalofheadacheandpain.com/content/14/1/61 study supports the hypothesis that the HCRTR1 gene could represent a genetic susceptibility factor for migraine and suggests that the hypocretin system may have a role also in the pathophysiology of migraine. Unfortunately, no data regarding involvement of HCRTR genes in tension-type headache are currently available. The growing knowledge concerning the role of hypocretins/orexins in different neurological conditions has generated considerable interest in developing smallmolecule hypocretin receptor antagonists as a novel therapeutic strategy. Hypocretin antagonists, especially those that block Hcrtr1 or both Hcrtr1 and Hcrtr2 receptors, have been studied mainly as new drugs for sleep disorders. In experimental animals hypocretin/orexin antagonists (almorexant, suvorexant) clearly promote sleep, and clinical results are encouraging [91-94]. Considering the high frequency of sleep disorders occurring in patients with migraine and CH, these drugs offer a new perspective in the treatment of these disorders. Finally, a pivotal role for these peptides in drug reward and drug seeking has been established and their potential role as anti-relapse medication in drug addiction is currently under investigation in experimental animals [95]. Conclusions The main goal of genetic studies is to unravel molecular pathways underlying primary headache disorders, in order to discover new therapeutic targets. Recent studies have highlighted a potential role for new genes, like MTHFR, KCNK18, TRPV1, TRPV3, and new neurotransmission systems, like the hypocretin system, both in migraine and cluster headache. Additional experimental and clinical studies are needed to better elucidate the involvement of these new genes in primary headaches and to evaluate new therapeutic strategies. Competing interest The authors declare no competing interest regarding this manuscript. Authors’ contribution IR and ER planned the review and wrote the manuscript, with input from other authors. KP reviewed the manuscript. AG and AV performed bibliographic research and draw the pictures. PDM, SG, PS and LP supervised the project. All authors read and approved the final manuscript. Acknowledgements This Review Article will be presented at the XXVII National Congress of the Italian Society for the Study of Headaches – 26–28 September 2013. 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Submit your manuscript to a journal and benefit from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the field 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 REVIEW ARTICLE Open Access Cortical spreading depression as a target for anti-migraine agents Cinzia Costa1,3, Alessandro Tozzi1,3, Innocenzo Rainero2, Letizia Maria Cupini5, Paolo Calabresi1,3, Cenk Ayata4 and Paola Sarchielli1* Abstract Spreading depression (SD) is a slowly propagating wave of neuronal and glial depolarization lasting a few minutes, that can develop within the cerebral cortex or other brain areas after electrical, mechanical or chemical depolarizing stimulations. Cortical SD (CSD) is considered the neurophysiological correlate of migraine aura. It is characterized by massive increases in both extracellular K+ and glutamate, as well as rises in intracellular Na+ and Ca2+. These ionic shifts produce slow direct current (DC) potential shifts that can be recorded extracellularly. Moreover, CSD is associated with changes in cortical parenchymal blood flow. CSD has been shown to be a common therapeutic target for currently prescribed migraine prophylactic drugs. Yet, no effects have been observed for the antiepileptic drugs carbamazepine and oxcarbazepine, consistent with their lack of efficacy on migraine. Some molecules of interest for migraine have been tested for their effect on CSD. Specifically, blocking CSD may play an enabling role for novel benzopyran derivative tonabersat in preventing migraine with aura. Additionally, calcitonin gene-related peptide (CGRP) antagonists have been recently reported to inhibit CSD, suggesting the contribution of CGRP receptor activation to the initiation and maintenance of CSD not only at the classic vascular sites, but also at a central neuronal level. Understanding what may be lying behind this contribution, would add further insights into the mechanisms of actions for “gepants”, which may be pivotal for the effectiveness of these drugs as anti-migraine agents. CSD models are useful tools for testing current and novel prophylactic drugs, providing knowledge on mechanisms of action relevant for migraine. Keywords: Cortical spreading depression; Calcium channels; Sodium channels; Glutamate; Ionotropic glutamate receptors; Calcitonin gene-related peptides; Current prophylactic drugs; Antiepileptics; Tonabersat; Gepants Introduction Spreading depression (SD) is an intense self-propagating wave of depolarization involving neuronal and glial cells in the cerebral cortex, subcortical gray matter or retina, irrespective of functional divisions or arterial territories. This depolarization is followed by a longer lasting wave of inhibition characterised by massive changes in ionic concentrations and slow chemical waves, propagating at a rate of approximately 3–6 mm/min [1]. In both lissencephalic and gyrencephalic cortices, SD can be evoked pharmacologically by the application of K+, * Correspondence: [email protected] 1 Neurologic Clinic, Department of Public Health and Medical and Surgical Specialties, University of Perugia, Ospedale Santa Maria della Misericordia, Sant'Andrea delle Fratte, 06132, Perugia, Italy Full list of author information is available at the end of the article glutamate, and Na+/K+-pump inhibitors or by either electrical or mechanical stimulation. SD can develop over the course of epileptic crises or can be induced by brain tissue injury, as in the case of trauma, hemorrhage or ischemia [2-6]. Several clinical and neuroimaging findings support the concept that cortical SD (CSD) is the pathophysiological correlate of the neurological symptoms in migraine aura [7-9]. Moreover, different experimental models of CSD have been developed which help to better understand the underlying neuronal mechanisms and related vascular changes [10,11]. They also, albeit not consistently, suggest that CSD is able to activate central and peripheral trigemino-vascular nociceptive pathways with a latency matching, generally occurring between aura and headache in migraineurs [12]. © 2013 Costa et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 Familial Hemiplegic Migraine 1 (FHM1) and 2 (FHM2) mutations share the ability to facilitate the induction and propagation of CSD in mouse models, further supporting the role of CSD as a key migraine trigger [13]. CSD is further modulated by endogenous and environmental factors, such as hormones and drugs, and also might be influenced by weather, stress and food [14-16]. Current prophylactic treatments have been investigated for their effects on CSD. Novel drugs can also target CSD and this can account, at least in part, for their mechanisms of action on migraine [17]. Review In this review, we illustrate the main findings concerning basic mechanisms underlying CSD as neurophysiologic substrate of aura and report results on the effects on CSD from available drugs and new therapeutic options. Clinical, neurophysiological and neuroimaging evidence of a link between migraine aura and CSD About a third of migraine patients complain of transient focal aura symptoms beginning from minutes to hours before headache, or occurring during either the headache phase or even in its absence [18]. Usually, migraine aura consists of fully reversible visual, sensory and/or dysphasic symptoms [19]. These aura symptoms are accompanied by fully reversible motor weakness in hemiplegic migraine. This is referred to as familial (i.e., FHM) subtype when the condition is present in at least one first- or second-degree relative, and a sporadic subtype in the absence of family history [20]. Specific genetic subtypes of FHM have been identified. Mutations of three genes all encoding ion-channels or membrane ionic pumps were discovered from 1996 to 2005. They involve a neuronal Ca2+ channel (CACNA1A, FHM1), a glial Na+/K+ pump (ATP1A2, FHM2) and a neuronal Na+ channel (SCN1A, FHM3), respectively [13]. However, these mutations have not been identified in the more common types of migraine with typical aura. These forms are considered polygenic, with an overall heritability nearing 50%, and should be regarded as the result of the interaction of genetic and environmental factors [21]. Descriptions of migraine with aura (MwA) from the year 1870 onwards have reported a slow, gradual progression of aura symptoms. Specifically, in 1941, Lashley, from the meticulous chartering of his own auras, suggested that aura symptoms reflect a cortical process progressing with a speed of 3 mm/min across the primary visual cortex [22]. CSD was first described by Aristides Leão in 1944 [23,24]. Studying experimental epilepsy for his PhD thesis, Leão came across a depression of electroencephalographic (EEG) activity moving through the rabbit cortex at a rate of 3–6 mm/min after electrical or mechanical Page 2 of 18 stimulations. The negative wave was sometimes preceded by a small, brief positivity, and always followed by a positive overshoot of 3–5 min [25]. He observed that the threshold of CSD varied among cortical areas also in pigeons and cats, and once triggered, it spread in all directions. During CSD, neither sensory stimulation nor direct cortical stimulation evoked potential waves. Ongoing experimental seizure discharge was also suppressed by CSD, even if sometimes tonic-clonic activity preceded or followed SD [23]. Based on similar propagation of the two processes, Leão hypothesized an association between CSD and seizures [1]. Using microscopy and photography of pial vessels to assess cortical circulation, the researcher was also able to see both arteries dilated “as scarlet as the arteries” and veins, as a consequence of CSD. This latter observation, for the first time, indicated that the cerebral blood flow increase exceeded the increase in oxygen demand. This topic has become a matter of interest for all investigators studying changes in cerebral circulation over the course of CSD [24]. In the early 20th century, aura was considered a vascular process involving an initial vasoconstriction followed by a reactive vasodilation responsible for head pain [8,26]. Later observations by Olesen et al. modified this idea by demonstrating a spreading reduction in cerebral blood flow, called “spreading oligemia”, occurring in patients with MwA [27]. This finding completely redefined the underlying pathogenesis of aura by attributing blood flow changes during aura to changes in neuronal activity [28]. Single photon emission computerized tomography (SPECT) [29] and perfusion-weighted magnetic resonance imaging (MRI) [30] studies have further supported the hypothesis that “spreading oligemia” observed during aura is primarily due to changes in neuronal activity. Additionally, perfusion abnormalities have been suggested to be a response of the autoregulatory mechanisms to underlying neuronal depression. However, in most SPECT and hyper-emia and subsequent spreading hypo-perfusion, patients never experienced symptoms of typical visual auras [7,31]. The first-ever study investigating occipital cortex activation during visual stimulation with functional magnetic resonance (fMRI) by blood oxygenation level (BOLD)-dependent contrast imaging in MwA patients demonstrated that the onset of headache or visual change (only in 2 patients), or both, were preceded by a suppression of the initial activation. This suppression slowly propagated into contiguous occipital cortex at a rate ranging from 3 to 6 mm/min. and was accompanied by baseline contrast intensity increases, indicating that vasodilatation and tissue hyper-oxygenation are associated with the induction of headache [32]. Later, in 2001, Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 Hadjikhani at al. [33], strongly suggested that electrophysiological events consistent with CSD are involved in triggering aura in the human visual cortex. Using highfield fMRI with near-continuous recording during visual aura, the authors identified specific BOLD events in the visual cortex that were strictly linked to the aura percept, in both space (retinotopy) and time. Throughout the progression of each aura, unique BOLD perturbations were found in the corresponding regions of the retinotopic visual cortex. Like the progression of the aura in the visual field, the BOLD perturbations progressed from the paracentral to more peripheral eccentricities, in only the hemisphere corresponding to the aura. The source of the aura-related BOLD changes were localized in the extrastriate visual cortex (area V3A) rather than in the striate cortex (V1). Strikingly, the spread rate of the BOLD perturbations across the flattened cortical gray matter was consistent with previous measures of CSD. As for diffusion changes on MRI, these have been especially observed in cases of prolonged complex migraine aura, suggesting cytotoxic edema in the absence of ischemic lesions [34-37]. To this regard, it is noteworthy the finding of a spreading of cortical edema with reversibly restricted water diffusion from the left occipital to the temporo-parietal cortex in a case of persistent visual migraine aura [38]. In another case of migraine with prolonged aura, hyper-perfusion with vasogenic leakage was detected by diffusion-weighted MRI [39]. A further patient experienced a series of MwA attacks accompanied by slight pleocytosis and gadolinium (GdDTPA) enhancement in proximity of the left middle cerebral artery. In this patient, the migraine attacks and Gd-DTPA enhancement were reversed by prophylactic treatment [40]. Magneto-electroencephalography (MEG) allows the study of direct current (DC) neuromagnetic fields in spontaneous and visually induced migraine patients. Few MEG studies have been conducted with this approach in MwA patients due to technical difficulties. The most relevant study to date has shown multiple cortical areas activated in spontaneous and visually induced MwA patients, unlike activation limited to the primary visual cortex in control subjects. This finding supports the hypothesis that a CSD-like neuro-electric event arises spontaneously during migraine aura or can be visually triggered in widespread regions of the hyper-excitable occipital cortex [41]. MEG has also been used to directly register changes in cortical oscillatory power during aura. Specifically, alpha band desynchronization has been demonstrated with this technique in both the left extra-striate and temporal cortex over the period of reported visual disturbances. These terminated abruptly on the disappearance of scintillations, whereas gamma frequency desynchronization Page 3 of 18 in the left temporal lobe continued for 8 to 10 minutes following the reported end of aura [42]. Neuronal mechanisms of CSD in experimental models When evoked by local extracellular K+ concentrations exceeding a critical threshold, CSD is associated with the disruption of membrane ionic gradients. Both massive K+ (with extracellular concentration increases up to 60 mM) and glutamate effluxes are believed to depolarize adjacent neurons to facilitate spread [43]. Results from studies on ion-selective microelectrodes have shown that an extracellular K+ rise is accompanied by falls in extracellular Na+ and Cl- during CSD, whereas water leaves the extracellular space with significant changes in extracellular pH [44,45]. Specifically, in chick retina, SD induced an initial increase in intracellular pH, which was associated with elevated levels of ADP, P-Creatine, lactate and pyruvate. This was followed by an intermediary acid shift, increases in ATP values and decreases in ADP, a late alkaline rebound, a decrease in P-Creatine levels, and elevations in both ADP and lactate levels. These transient changes in intracellular pH occurring parallel to changes of energy metabolite levels during SD, may be expressions of rapidly modifying metabolic activities of neurons and glial cells. The first alkaline shift was attributed to glial cells, whereas the intermediary acid shift was attributed to neurons. No specific cells were thought to be responsible for the late alkaline shift, which could explain the refractoriness of the neurons in this phase [46]. Accordingly, in rat cerebellum, an initial decrease in [H+] (pH increase) followed by a increase in [H+] (pH decrease) was observed during SD [45]. Further results obtained from a model of CSD showed acidification and a marked depression in the cortical energy status at the wavefront of SD. Afterwards, a residual activation of glycolysis and an accumulation of cGMP persisted for minutes after relatively rapid restorations of high-energy phosphates and pHi [47]. Recovery from this process occurs usually within a few minutes without any tissue damage [43,45]. A causative link between enhanced glutamate release and facilitation of CSD, induced by brief pulses of high K+, has been reported in mouse models of CSD [48-51]. In some of these models, CSD could not be recorded after perfusing cortical slices by Ca2+ free medium or after blocking Ca2+ channels [50,52-55]. Glutamate involvement in CSD is further supported by the finding of CSD blockage by N-methyl- D-aspartate receptor (NMDA-R) antagonists, but not by non-NMDA-R antagonists, in vivo [56-59] or in hippocampal and neocortical slices of rats [5,55]. CSD has also been reported to be blocked by the NMDA-R antagonist 2-amino-5phosphonovaleric acid, in slices of human neocortical Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 tissue [60]. Furthermore, data have demonstrated that NR2B-containing NMDA-R are key mediators of CSD, providing the theoretical basis for the usefulness of memantine and some NR2B-selective antagonists for the treatment of MwA and other CSD-related disorders, such as stroke or brain injury [50,61]. According to the above findings, CSD cannot be induced in brain slices of FHM1 KI mice if either P/Qtype Ca2+ channels or NMDA receptors are blocked. Conversely, blocking N- or R-type Ca2+ channels seems to have only small inhibitory effects on the CSD threshold and velocity of propagation. This suggests that Ca2+ influx through presynaptic P/Q-type Ca2+ channels with consequent release of glutamate from cortical cell synapses and activation of NMDA-R is required for initiation and propagation of the CSD [62]. This is in contrast with results of in vitro and in vivo pharmacological studies where CSD was induced by perfusing cortical slices with a high K+ solution (rather than with brief K+ pulses or electrical stimulation). In these models, NMDA-R antagonists only slightly increased CSD threshold without affecting its velocity. Accordingly, blocking P/Q-type (or the N-type) Ca2+ did not significantly affect the CSD threshold obtained from perfusing cortical slices with progressively increasing K+ concentrations [51,63]. Interestingly, removal of extra-cellular Ca2+ did not block CSD but reduced it to about half the rate of propagation [64]. Different results have been obtained for multiple CSD models induced in vivo by continuous K+ microdialysis or topical application of KCl, where P/Q-type (Cav2.1), or N-type, Ca2+ channel blockers and NMDA-R antagonists led to a strongly reduced frequency, amplitude and duration, but not a complete suppression, of CSD events [50,65,66]. Furthermore, Ca2+ channel blockers have not been reported to affect CSD induced by pinprick in vivo [66]. Therefore, results seem to be strongly influenced by the model used. Currently, electrical stimulation and/ or brief applications of high K+ are considered to be the most appropriate CSD-inducing stimuli, rather than prolonged applications of high K+, for the better understanding of “spontaneous” CSD mechanisms occurring in migraine aura [62]. Specifically, these models best reveal that the excitatory synaptic transmission, involved in CSD initiation and propagation at the pyramidal cortical cells, predominantly depends on presynaptic P/Qtype Ca2+ channels. Earlier studies reported that the Na+ channel blocker TTX was not able to consistently inhibit CSD [67-69]. More recently, Na+ channels have been shown to be involved in the initiation of CSD in hippocampal slices [5]. Their contribution to CSD was confirmed by Tozzi et al. [70] in rat neocortical slices by reducing CSD propagation after applying the voltage sensitive Na+ channel Page 4 of 18 blocker TTX. In another study, Na+ ion channel blockage was also seen to inhibit relative cerebral blood flow (rCBF) changes occurring during CSD induced on both cats and rats. In the same model, voltage-dependent Ca2+ channel blockers had little effect on either the initiation or propagation of CSD spread, as was the case for ATP-activated K+ channel blockers also [71]. It has been demonstrated that the activation of alphaamino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors (AMPA-R) can suppress the actions of NMDA-R in the neocortex [72]. Earlier findings, however, suggested that NMDA-R blockers, but not AMPA-R antagonists, were able to inhibit CSD in rats [70,72,73]. Conversely, a recent study has demonstrated that both 50 μM AMPA, as well as 10 μM of the NMDA-R antagonist 2-amino-5phosphono-pentanoic acid (2AP5), significantly reduce the number of CSD cycles. Additionally, the gammaaminobutyric acid (GABA)-mimetic drug clomethiazole (100 mg/kg i.p.) did not significantly affect the number of CSD cycles [74]. Being so, the suppression of NMDA-R actions in the neocortex by AMPA-R activation, may represent an intrinsic protective mechanism against CSD and could, thus, be a potential therapeutic strategy against CSD-related neurological conditions including migraine aura. In line with the above finding, AMPA-R, as well as GABA(A) and GABA(B)-R agonists, have been shown to inhibit cerebral blood flow changes associated to mechanically-induced CSD in all rats and in a proportion of cats. Furthermore, non-responders showed altered speeds of propagation and times to induction [75]. In contrast, in a recent investigation, reproducible CSD episodes, induced by high extracellular K+ concentrations in rat neocortical slices, were inhibited by antagonists of NMDA-R, but not by AMPA-R [70]. Methodological differences (CSD models, dosages of agonists, outcome measures) could explain discrepancy in the results of the different studies carried out on this topic. Recent autoradiographic findings suggest that selective changes in several receptor-binding sites, in both cortical and subcortical regions, are related to the delayed excitatory phase after CSD. In fact, in neocortical tissues, local increases of ionotropic glutamate receptors NMDA, AMPA, and kainate receptor binding sites have been observed. In addition, receptor binding sites of GABA(A), muscarinic M1 and M2, adrenergic alpha(1) and alpha (2), and serotonergic 5-HT(2) receptors were seen increased in the hippocampus. CSD also up-regulated NMDA, AMPA, kainate, GABA(A), serotonergic 5-HT (2), adrenergic alpha(2) and dopaminergic D1 receptor binding sites in the striatum [76]. Therefore, not only glutamatergic mechanisms, but also changes in monaminergic and cholinergic pathways seem to be involved in CSD. Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 Vascular changes associated to CSD in experimental models CSD has been reported to be associated with changes in the caliber of surface cortical blood vessels. Leão was the first to report arteriole dilatation accompanying electrophysiological changes in CSD of rabbits [24], which was later confirmed in rats and cats [77,78]. A further study using laser Doppler flowmetry, focusing on tissue perfusion rather than arterial diameter, has suggested that CSD is associated with an initial increase in cortical blood flow, which is thought to correspond to arteriolar dilatation [79]. Triggering CSD results in a sustained wave of reduced cortical blood flow after initial vasodilation, as shown by single modality blood flow measurements, including autoradiographic methods [80,81] and laser Doppler flowmetry [82]. Moreover, sustained hypo-perfusion was accompanied by a concurrent reduction in reactivity to vasoactive stimuli [83]. Dual modality methods, such as laser Doppler flowmetry and extracellular electrophysiology, allowed for the concurrent assessment of changes in neuronal firing and cerebral blood flow in CSD but lacked parallel spatial and temporal resolutions [84]. Optical intrinsic signal (OIS) imaging also enables visualization of CSD on the cortical surface with high temporal and spatial resolution [85-87]. The optical correlates of CSD have been evaluated on both a mouse and a rat model by Ayata et al. [88]. Vascular response to CSD propagates with temporal and spatial characteristics, which are distinct from those of the underlying parenchyma, suggesting a distinct mechanism for vascular conduction. Using OIS imaging and electrophysiology to simultaneously examine the vascular and parenchymal changes occurring with CSD in anesthetized mice and rats, Brennan et al. [89] observed vasomotor changes in the cortex which travelled at significantly greater velocities compared to neuronal changes. This observation further reinforces the idea that dissociation between vasomotor and neuronal changes during CSD exists. Specifically, dilatation travelled in a circuitous pattern along individual arterioles, indicating specific vascular conduction as opposed to concentric propagation of the parenchymal signal. This should lead to a complete rethinking of flow-metabolism coupling in the course of CSD. Vascular/metabolic uncoupling with CSD has also been reported by Chang et al. using a combination of OIS imaging, electrophysiology, K+-sensitive electrodes and spectroscopy in mice [90]. The authors identified two distinct phases of altered neurovascular function. In the first phase of the propagating CSD wave, the DC shift was accompanied by marked arterial constriction and desaturation of cortical hemoglobin. After recovery from the initial CSD depression wave, a second phase was identified where a novel DC shift appeared to be Page 5 of 18 accompanied by arterial constriction and a decrease in tissue oxygen supply, lasting at least an hour. Persistent disruption of neurovascular coupling was supported by a loss of consistency between electrophysiological activity and perfusion. Nitric oxide (NO) may play a relevant role in determining changes in cerebro-vascular regulation following CSD. In fact, the NO precursor L-arginine prevented the development of prolonged oligemia after CSD but had no influence on a marked rise of CBF during CSD. Moreover, rats treated with L-arginine recovered their vascular reactivity to hyper-capnia after CSD much faster than controls [91]. The NO donor, 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO) had little effect on CSD but reversed the effects of NO synthase (NOS) inhibition by 1 mM L-NAME, in a concentration-dependent manner, suggesting that the increased formation of endogenous NO associated with CSD is critical for subsequent, rapid recovery of cellular ionic homeostasis. Molecular targets for NO may be either brain cells, through the suppression of mechanisms directly involved in CSD or local blood vessels by means of coupling flow with the increased energy demand associated with CSD. The potent vasoconstrictor endothelin-1 (ET-1) applied on rat neocortices has been demonstrated to induce CSDs through the ET(A) receptor and phospholipase C (PLC) activation. Primary targets of ET-1 mediating CSD seem to be either neurons or vascular smooth muscle cells [92]. This finding provides a bridge between the vascular and the neuronal theories of migraine aura. However, the micro area of selective neuronal necrosis, induced by ET-1 application suggests a role by vasoconstriction/ischemia mechanisms. This observation contrasts with the lack of neuronal damage in several CSD models [93]. Genetic evidence of CSD involvement in migraine Genetic factors are known to enhance susceptibility to CSD, as results from transgenic mice expressing mutations associated with FHM or cerebral autosomal dominant arteriopathy with subcortical infarcts and leuko-encephalopathy (CADASIL) have shown [94-99]. Specifically, P/Q-type Ca2+ channels, located in somatodendritic membranes and presynaptic terminals in the brain, play a pivotal role in inducing potential-evoked neurotransmitter release at CNS synapses [100]. Missense mutations in the gene encoding the pore-forming α1 subunit of voltage-gated P/Q-type Ca2+ channel, responsible for the rare autosomal dominant subtype of MwA FHM1, induce a gain-of-function of human recombinant P/Q-type Ca2+ channels, due to a shift to channel activation at lower voltages [101]. Increased P/ Q-type Ca2+ current density in cortical pyramidal cells has been demonstrated in Knock-in (KI) mice carrying FHM1 mutations [101-103]. Furthermore, FHM1 KI Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 mice have shown a reduced threshold for CSD induction and an increased velocity of CSD propagation [63,104]. These mice represent a powerful tool for exploring presynaptic regulation associated with expression of P/Qtype Ca2+ channels. Mutated P/Q-type Ca2+ channels activate at more hyper-polarizing potentials and lead to a gain-of-function in synaptic transmission. This gain-offunction might be responsible for alterations in the excitatory/inhibitory balance of synaptic transmission, favoring a persistent state of hyper-excitability in cortical neurons which may increase the susceptibility for CSD [101]. In contrast, spontaneous CACNA1a mouse carrying mutations producing partial loss-of-function of the P/Q-type Ca2+ channel, need approximately a 10 fold higher electrical stimulation intensity in order to evoke a CSD compared to wild-type mice [105]. FHM2, the autosomal dominant form of MwA, is caused by mutations of the α2-subunit of the Na+,K+-ATPase, an isoform almost exclusively expressed in astrocytes in the adult brain. In a FHM2 KI mouse model carrying the human W887R mutation in the Atp1a2 orthologous gene, in vivo analysis of CSD in heterozygous F Atp1a2 (+/R887) mutants revealed a decreased induction threshold and an increased velocity of propagation. While several lines of evidence suggest a specific role on the part of glial α2 Na+/K+ pump in active reuptake of glutamate from the synaptic cleft, it is plausible that CSD facilitation in the FHM2 mouse model is sustained by inefficient glutamate clearance by astrocytes, leading to an increase in cortical excitatory neurotransmission [106]. MwA is often the first manifestation of cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), caused by NOTCH3 gene mutations expressed predominantly in vascular smooth muscles. In a recent study, CSD was reported to be enhanced in mice expressing either a vascular Notch 3 CADASIL mutation (R90C) or a Notch 3 knock-out mutation. These findings further support the role of the trigeminal neurovascular unit in the development of migraine aura [107]. Influence of sexual steroids on CSD A relation between migraine and changes in the level of sexual steroids has been well documented and both estrogens and androgens may influence migraine attacks. Accordingly, it has been found that in women with MwA, plasma estrogen concentrations were higher during normal menstrual cycle. Furthermore, it has also been reported that the occurrence of migraine attacks is associated with high circulating estrogen levels as during ovulation, pregnancy and the use of certain oral contraceptives [18-110]. Notably, sex difference in the presentation of attacks has been shown to disappear after Page 6 of 18 oophorectomy and with senescence [111]. Testosterone and its synthetic derivatives have also been demonstrated to improve migraine in both men and women [112-116]. Moreover, males treated with gonadotropins for infertility experienced a marked improvement in their MwA attacks [117]. Conversely, anti-androgen therapy increased MwA frequency in a small cohort of male-to-female transsexuals [118]. Some experimental findings support the excitatory neuronal effect associated with estradiol and the inhibitory effect associated with progesterone. Compared to female hormones, mechanisms of androgenic modulation of excitability are not as well known. Gonadic hormones have been suggested to have a modulating role in CSD susceptibility, which would, at least in part, explain the gender differences in the prevalence of migraine. Accordingly, female FHM1 mutant mice have been shown to be more susceptible to CSD when compared to their male counterparts [119]. On the other hand, testosterone have been reported to suppress CSD via androgen receptor-dependent mechanisms and, accordingly, its inhibitory effect on CSD was prevented by the androgen receptor blocker flutamide. Furthermore, it has been shown that chronic testosterone replacement reversed the effects of orchiectomy on CSD [120]. Astrocytes and gap-junction involvement in CSD Astrocytes, a subset of glial cells, reside next to neurons, establishing together a highly interactive network [121]. Astrocytes play a pivotal role in limiting CSD by acting as a buffer for the ionic and neurochemical changes which initiate and propagate CSD [122]. On the other hand, astrocyte interconnections are believed to contribute to propagating the CSD wave, by way of K+ liberation, allowed for by an opening of remote K+ channels. Moreover, energy failure in astrocytes increases the vulnerability of neurons to CSD [123]. There is increasing evidence suggesting that, while synapses connect neuronal networks, gap-junctions most likely connect astrocyte networks [124]. Clusters of these tightly packed intercellular channels allow for the direct biochemical and electrical communications among astrocytes, contributing to a syncytium-like organization of these cells [125]. Membranes of adjacent astrocytes have connexincontaining hemi-channels which can bridge an intercellular gap to form a gap-junction [126]. This interaction between the two hemi-channels opens them both, allowing for the intercellular passage of ions and small molecules [127]. Approximately 1.0–1.5 nm in diameter, gap-junctions permit the transport of molecules up to about 1 kDa in size. Astrocytes express at least three different connexins at gap-junctions with regional differences in their distributions. Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 Experimental studies have suggested an involvement of gap-junctions in CSD by regulating the milieu around active neurons including extracellular K+, pH and neurotransmitter levels (especially glutamate and GABA), as well as propagating intercellular Ca2+ waves [128]. Nonjunctional connexin hemi-channels may also contribute to the release of adenosine triphosphate (ATP). This extracellular messenger is able to mediate Ca2+ wave propagation directly or via the transfer of a messenger which triggers ATP release from one cell to another [129]. Generation and propagation of CSD may depend on neuronal activation and Ca2+ influx triggered by NMDA-R. Interestingly, NMDA-R antagonists block CSD but, unlike the gap-junction blockers, do not inhibit Ca2+ wave propagation. Astrocytes are known to express several types of glutamate receptors, including NMDA-R. Glutamate release from astrocytes has also have been reported to be mediated via the opening of connexin hemi-channels [127]. For this, gap-junction-mediated propagation of Ca2+ waves may represent the advancing front of CSD, contributing to the triggering of the secondary depolarization of the surrounding neurons, leading to further releases of K+ and glutamate into the extracellular space. Glutamate may then stimulate cytosolic Ca2+ oscillations in astrocytes, providing a feedback loop involved in CSD propagation. If so, gap-junction blockage would represent a viable pharmacological strategy for MwA prevention. Evidence of a gap-junction coupling Ca2+ waves between pia-arachnoid cells and astrocytes has also been reported, suggesting a transfer of Ca2+ signals from cells of the cortical parenchyma into the meningeal trigeminal afferents, all of which might mediate the induction of neurovascular changes responsible for migraine headache [130]. Altered blood–brain barrier (BBB) permeability in CSD CSD alters blood–brain barrier (BBB) permeability by activating matrix metalloproteases (MMPs) [131]. From 3 to 6 hours, MMP-9 levels increase within the cortex ipsilateral to CSD, reaching a maximum at 24 hours and persisting for at least 48 hours. At 3–24 hours, immunoreactive laminin, endothelial barrier antigen, and zona occludens-1 diminish in the ipsilateral cortex, suggesting that CSD altered proteins are critical to the integrity of BBB. Subclinical infarct-like white matter lesions (WMLs) in the brain of some migraine patients, especially those with aura, have been reported to be consistent with CSD-related BBB disruption. Furthermore, increases in plasma levels of matrix MMPs (especially MMP-9 and MMP-2) in migraine patients, in the headache phase, suggest a potential pathogenic role for MMP elevation in both migraine attacks and WMLs [132-135]. Different Page 7 of 18 circulating MMP profiles in MwA and migraine without aura (MwoA) may reflect pathophysiological differences between these conditions. According to Gupta et al. MMPs are responsible for the loosening of the intercellular tight junctions and the expansion of the extracellular matrix of the BBB, consequent to the sudden increase in cerebral blood flow during migraine attacks [136]. In this condition, WML could result from a transient and discrete breakdown of the BBB following sustained cerebral hyper-perfusion rather than hypoperfusion. The relationship between CSD and headache Recent electrophysiological data has provided direct evidence that CSD is a powerful endogenous process which can lead to persistent activation of nociceptors innervating the meninges. Regardless of the method of cortical stimulation, CSD in rat visual cortices induces a twofold increase in meningeal nociceptor firing rates, persisting for 30 min or more. Meningeal nociceptors represent the first-order neurons of the trigeminovascular system, whose activation is involved in the initiation of migraine headache [137]. CSD waves moving slowly across the cortex can promote the releases of K+, arachidonic acid, hydrogen ions, NO and ATP. Critical levels of these substances are thought to cause sensitization and activation of trigeminal neurons in the afferent loop and, in turn, activate second-order neurons in the trigemino-cervical complex. These second-order neurons transmit sensory signals to the brainstem and parasympathetic efferents, the latter projecting from the sphenopalatine ganglion. CSD has been suggested to promote persistent sensitization, thereby provoking the activation of meningeal nociceptors through a mechanism involving local neurogenic inflammation, with contribution of mast cells, macrophages and the release of inflammatory mediators. Local action of such nociceptive mediators increases the responsiveness of meningeal nociceptors. Recent research has provided key experimental data suggesting the role of complex meningeal immuno-vascular interactions leading to an enhancement in meningeal nociceptor responses [137]. CSD also induces increased neuronal activity of central trigeminovascular neurons in the spinal trigeminal nucleus (C1-2) as measured by single-unit recording. It therefore represents a "nociceptive stimulus" capable of activating both peripheral and central trigemino-vascular neurons underlying the headache phase of MwA [137]. Recent evidence suggests that central trigeminal neurons are activated by CSD. Specifically, an increase in the spontaneous discharge rate, following the induction of CSD by cortical injection of KCl was not reversed through the injection of lignocaine into the trigeminal ganglion 20 min after CSD induction. Lignocaine Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 injection prior to the initiation of CSD also failed to prevent the subsequent development of CSD-induced increases in discharge rates [138]. In these experiments, lignocaine at a dosage of 10 μg (capable of interrupting stimulus-induced responses to either electrical stimulation of the dura mater or mechanical stimulation of the craniofacial skin) reduced basal the discharge rate of second-order trigeminovascular neurons. This increased traffic in the second-order neurons induced by CSD, however, was not influenced by the blockage of conduction in first-order neurons which was due to lignocaine injection into trigeminal ganglion after CSD induction by cortical pinprick. A time point of 20 min post-lignocaine injection was chosen because responses to evoked stimulation reached a minimum at this time. It has been suggested that CSD may produce a rapid sensitization at first sensory neurons which could become “locked-in” and, therefore, would not be influenced by a later reduction in sensory traffic, like that induced by the injection of lignocaine into the trigeminal ganglion [139]. An increase in discharge rate produced by CSD has also been observed when lignocaine is injected into trigeminal ganglion, prior to the induction of CSD. This is further evidence that CSD does not act solely by increasing continuous traffic in primary trigemino-vascular fibers through a peripheral action alone, but rather exerts its effect through a mechanism intrinsic to the CNS. Accordingly, pain in MwA may not always be the result of peripheral sensory stimulation, but may arise via a central mechanism [140]. The principal opposition to this hypothesis is based upon the belief that mediators released as a consequence of CSD induction cannot be sustained in the perivascular space to induce persistent trigeminal sensitization and the subsequent hours-lasting headache because of the glia limitans barrier (astrocyte foot processes associated with the parenchymal basal lamina surrounding the brain and spinal cord, regulating the movement of small molecules and cells into the brain parenchyma) and the continuous cerebrospinal fluid (CSF) flow [141]. Additionally, the delay of 20–30 min between aura and headache suggests that a time lag is required for the transduction of algesic signals beyond glia limitans via inflammatory mediators. In support to this rebuttal, Karatas et al. demonstrated that intense depolarization and NMDA receptor overactivation due to CSD, opens neuronal Pannexin1 (Panx1) mega-channels [142]. Panx1 activation induces a downstream inflammasome formation involving caspase-1 activation and the sustained release of pro-inflammatory mediators from glia limitans such as high-mobility group box 1 (HMGB1) and IL-1β, both of which take part in the initiation of the inflammatory response [143-146]. A subsequent NF-kB translocation was Page 8 of 18 observed inside the cortex, involving astrocytes, forming or abutting glia limitans, followed by the activations of both cyclooxygenase (COX)2 and inducible NOS (iNOS). The inhibition of Panx1 channels or HMGB1 resulted in a reversal of this effect. A CSD-induced neuronal megachannel opening may therefore promote sustained stimulus required for both sensitization and activation of meningeal trigeminal afferents through the maintenance of inflammatory responses which may be involved in the subsequent headache pain. The effects of anti-migraine drugs on CSD Symptomatic drugs There is no evidence that acute anti-migraine drugs affect CSD due to the fact that they are not able to block or reduce aura symptoms. In one of the few studies carried out on this, sumatriptan was reported to decrease the amplitude of NO release but was seen to enhance extracellular superoxide concentrations in both lissencephalic and gyrencephalic cortices during CSD [147]. In another study, the same drug failed to inhibit CSD and CSD-related events [148]. Preventive treatment Currently, numerous drugs are available for the prophylactic treatment of migraine, including: tricyclic antidepressants, beta-blockers, calcium channel blockers and antiepileptics. Many of these have been demonstrated to be effective for both MwA and MwoA, suggesting that multiple targets are involved not only in cortical areas but also in sub-cortical structures [17,149]. Results from studies investigating the effects of currently available or novel drugs in animal models for CSD are reported in Additional file 1: Table S1. Current prophylactic drugs Kaube and Goadsby were the first to investigate the effectiveness of anti-migraine agents in the prevention of CSD by measuring cortical blood flow with laser Doppler flowmetry and cortical single unit activity in alphachloralose-anaesthetised cats [150]. None of the tested drugs, including dihydroergotamine (DHE), acetylsalicylic acid, lignocaine, metoprolol, clonazepam and valproate at a single dose prior to CSD induction, were able to inhibit CSD, reduce the rate of propagation or change the amplitude of the cortical blood flow increase. CSD, on the other hand, was blocked by both the NMDA-R blocker MK-801 and halothane. More recently, diverse prophylactic drugs, which are efficacious for the prophylactic treatment of MwA and MwoA, have been shown to experimentally suppress SD susceptibility. In particular, chronic daily administration of topiramate (TPM), valproate, propranolol, amitriptyline, and methysergide Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 dose-dependently were seen to inhibit CSD frequency by 40 to 80% and increase the cathodal stimulation threshold. Longer treatment durations produced stronger CSD suppression. However, the acute administration of these drugs was ineffective [151]. In a further study, peroral administration of TPM, once-daily for 6 weeks, inhibited KCl-induced CSD frequency and propagation. This effect emerged for high plasma levels of the drug, while low levels were ineffective [152]. According to these results, prophylactic drugs should be administered daily for at least 1 month to up 3–6 months, in order to be effective on CSD. This prolonged treatment is necessary to reduce attack frequency and severity in migraineurs. This is due to the fact that pharmacokinetic factors, which determine the gradual achievement of therapeutic tissue levels, as well as the mechanisms involved in gene expression and ultrastructural changes, require such a time period to be effective. In contrast, Akerman and Goadsby demonstrated that mechanically-induced CSD could be prevented by a single dose of topiramate 30 min after administration [153]. Similarly, Hoffmann observed a suppression of CSD susceptibility within 1 hour after a single intravenous dose of gabapentin [154]. No data are available on the effect of high doses of chronic oral gabapentin treatment. Lamotrigine, a potent Na+ channel blocker and glutamate receptor antagonist, has been demonstrated to affect MwA in open-label studies but not MwoA in controlled trials vs placebo or vs an active drug [155-159]. The drug has also been tested for its effect on CSD. Chronic treatment with this drug appeared to exert a marked suppressive effect on CSD, which is in line with its selective action on the migraine aura [160]. Specifically, lamotrigine was seen to suppress CSD by 37% and 60% at proximal and distal electrodes, respectively. Conversely, valproate had no effect on distal CSD, but reduced and slowed propagation velocity by 32% at the proximal electrodes. In the same study, riboflavin had no significant effect. Furthermore, frontal Fos expression was decreased by lamotrigine and valproate, but not by riboflavin. Single dose lamotrigine has never been tested for its specific effects on CSD. Conflicting results have been obtained regarding the effects of the Ca2+ channel blocker flunarizine on CSD events [161-166]. These discrepancies are likely due to the different experimental designs utilized, as well as technical limitations characteristic of earlier studies. Moreover, this drug has never been tested on currently used CSD models. For many years it has been suggested that magnesium (Mg2+) deficiency could be involved in migraine pathogenic mechanisms by increasing neuronal excitability via glutamate receptors. To this regard, several studies have reported low values of Mg2+ in serum and blood cells Page 9 of 18 over the interictal periods, as well as reduced Mg2+ ionized levels in more than 50% of migraine patients during attacks [167-170]. Low brain Mg2+ levels have also been detected in migraineurs by brain phosphorus spectroscopy [171]. Furthermore, in MwA patients, magnesium sulphate administration was seen to significantly relieve pain and alleviate migraine-associated symptoms [172]. Accordingly, systemic administration of Mg2+ has been shown to reduce CSD frequency induced by topical KCl in rat neocortices [173]. The predictive values of CSD models, specifically for the efficacy of drugs in migraine prophylaxis, can be further reinforced by negative findings from the testing of molecules demonstrated to be ineffective in migraine. This was the case for oxcarbazepine which failed to suppress CSD susceptibility either acutely after a single dose or after chronic treatment for 5 weeks [174] and carbamazepine tested in vitro on a CSD rat model [70]. Dpropranolol, which is anecdotally ineffective in migraine, also did not suppress CSD, indicating an enantiomer specificity for its efficacy [151]. Novel therapeutic options The novel benzopyran compound tonabersat is a unique molecule, which has been demonstrated to exert activity as a gap-junction inhibitor in animal studies. It has been suggested to be useful in both acute treatment and prophylaxis of migraine [175]. However, conflicting results have been obtained in two-dose ranging, placebocontrolled trials concerning its ability to relieve attacks [176] and also in the two randomized clinical trials aimed at investigating its effectiveness as a preventive drug [177,178]. This lack of efficacy was attributed to the slow absorption of tonabersat, suggesting that a daily administration of higher dosages should be tested for migraine prophylaxis. Interestingly, in another randomised, double-blind, placebo-controlled crossover trial, 40 mg tonabersat, administrated once daily, had a significant preventive effect on MwA but not MwoA, compared to placebo [179]. These findings concur with results of preclinical studies where the drug was reported to markedly reduce CSD and CSD-associated events, compared to sumatriptan [148]. Additionally, a study using repetitive diffusion weighted MR imaging (DWI) detected in vivo CSD modulation for tonabersat and, in part, also for sumatriptan [180]. Another mechanism of action of tonabersat includes its ability to inhibit gap-junction communication between neurons and satellite glial cells in the trigeminal ganglion [181]. This mechanism, together with its suppressive effect on CSD and its good pharmacokinetic profile, render the drug a potential candidate for preventive treatment of MwA. A recent open-labeled pilot study on the Na+/H+ exchanger amiloride showed that it was clinically effective Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 by reducing aura and headache symptoms in 4 of 7 patients with intractable MwA. Preclinical findings from the same study suggested that the drug blocks CSD and inhibits trigeminal activation in in vivo migraine models, via the acid-sensing ion channel (ASIC)1 mechanism [182]. This mechanism could be a novel therapeutic target for MwA. ASIC3 are expressed in most trigeminal neurons where they mediate proton stimulation of calcitonin gene-related peptide (CGRP) secretion [183]. The stimulatory effects of protons (pH 5.5) on CGRP secretion, due to ASIC3 activation, appear not to be limited to peripheral trigeminal neurons, but may also involve dural trigeminal afferents. Activation of dural trigeminal afferents by acidic pH has been shown to be mediated by ASICs channels (most likely ASIC3) but not TRPV1 [184]. Amiloride, due its nonselective inhibitory effect, might influence the activation of these extracellular proton key sensors in trigeminal neurons, therefore reducing CGRP release at both peripheral and central levels. CGRP is the main mediator of trigeminal pain signals. This neuropeptide acts at several steps in the cascade, from the trigeminal nerve to the CNS. It is released from trigeminal ganglion neurons, both peripherally at the dura and centrally in the spinal trigeminal nucleus and other sites within the CNS. Specifically, both CGRP and CGRP receptors have been observed in structures implicated in the pathogenesis of migraine including cortex and meninges [185]. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release on spinal neurons and increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent CGRP mechanisms. CGRP also regulates glia activity within the spinal cord and this activity contributes to central sensitization [186]. Whal et al. investigated for the effect of the CGRP competitive inhibitor CGRP-(8–37) (10−7 M) and NO inhibitor NOLAG (10−4 M) on dilation of pial arteries accompanying transient negative DC shift during KCl-induced CSD in cats. The authors reported a 75% inhibition of the CSD-induced dilatation during simultaneous application of both compounds, indicating that the initial dilatation during CSD is mediated, at least in part, by releases of CGRP and NO [187]. The latter two may act as mediators of the coupling between neuronal activity and cerebral blood flow during migraine aura. In a further study, topical administration of 12.8 μM CGRP(8–37) reduced CSD-induced pial vasodilation in urethaneanesthetized rabbits. In the same experiment, the removal of the receptor antagonist from the brain surface restored CSD-induced dilation, further supporting a pivotal role for CGRP in determining transient arteriolar dilation in the first phases of CSD [188]. The above results argue in favor of a local release of CGRP in the Page 10 of 18 meninges, where this neuropeptide may contribute to sensitizing local sensory neurons. However, a recent investigation on cultured cortical astrocytes has shown that CGRP possesses a modest proinflammatory action, as does satellite glia [189]. CSD does not seem to significantly influence CGRP outflow in jugular blood, but local increases of CGRP in the cortex during CSD cannot be excluded [190]. Furthermore, peripheral injection of CGRP in MwA patients has triggered a typical aura in 28% of patients, and migraine-like attacks without aura in the remaining. If this induction of aura can be confirmed, it will indicate that this neuropeptide has an upstream role in CSD [191]. Recent OIS imaging findings support the release of endogenous CGRP during CSD in rat neocortical slices in a calcium-dependent manner [70]. Additionally, three different CGRP receptor antagonists have shown dose-dependent inhibitory effects on CSD events, suggesting the critical role of CGRP in CSD. If so, antagonists targeting central CGRP receptors could be useful as anti-migraine agents [70]. Presently, gepants, a CGRP antagonist class of molecules, might offer a new non-vasoconstrictive approach in the acute treatment of migraine. Four chemically unrelated CGRP receptor (CGRP-R) antagonists (olcegepant, telcagepant, MK-3207 and BI 44370 TA) have displayed efficacy in the treatment of migraine [192]. They have fewer adverse effects, and act for a longer period than triptans [193]. Their development has been slowed by a liver toxicity when used as preventives. New CGRP-R antagonists, such as BMS-927711 and BI 44370 TA, are currently under study. The latter has shown a dosedependent effectiveness as an acute anti-migraine drug in a recent trial [194]. It remains to be established if these molecules are effective in antagonizing MwA. Other molecules of interest Kynurenic acid, a derivative of tryptophan metabolism, is an endogenous NMDA-R antagonist whose cerebral concentrations can be increased by the systemic administration of its precursor L-kynurenine. L-Kynurenine administration suppresses CSD in adult Sprague–Dawley rats, most likely by increasing kynurenic acid levels in the cortex. Females are more sensitive to the suppressive effects of L-kynurenine, further highlighting the role of sex hormones in migraine [195]. Ketamine, is a drug primarily used for the induction and maintenance of general anesthesia (usually in combination with a sedative) and also for sedation in intensive care, analgesia (particularly in emergency medicine), and the treatment of bronchospasm. Like other drugs of the same class, such as tiletamine and phencyclidine, it is also used as a recreational drug. Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 From a pharmacologic point of view, ketamine is an NMDA-R antagonist which, at high fully anesthetic level doses, binds to μ-opioid receptors type 2, without agonist activity and to sigma receptors in rats [196,197]. The drug also interacts with muscarinic receptors, monoaminergic receptors in descending pain pathways and voltage-gated Ca2+ channels. In earlier experiments, ketamine was shown to block CSD in rats [56,198]. As for MK-801, tolerance to ketamine was observed after repeated injections. That is, there was a gradual decline in their CSD blocking effects, which might have been due to some conformational changes at binding site(s) in NMDA-R [199]. Ketamine has been proposed as a putative treatment option for severe and prolonged aura. The first study on ketamine was carried out on 11 patients with severe, disabling auras resulting from FHM. In five of these, the drug reproducibly reduced the severity and duration of Page 11 of 18 the neurologic deficits, whereas in the remaining 6 patients no benefits were observed [200]. A recent double-blind, randomized, parallel-group, controlled study including patients with prolonged aura, reported that intranasal 25 mg ketamine, but not intranasal 2 mg midazolam, reduced aura severity but not duration [201]. However, ketamine has several side effects, including hallucinations, elevated blood pressure, dissociative anesthesia which strictly limit its use in clinical practice. Early anecdotal evidence indicated that propofol could have been effective in terminating refractory migraine. This rapidly acting water-insoluble non-barbiturate anesthetic agent has been recently reported to be effective on CSD. Specifically, intraperitoneal propofol hemisuccinate (PHS), a water-soluble prodrug of propofol, administered 15 min prior to KCl–induced CSD on the cortex of mice, decreased the number of CSD deflections at doses of 120 and 200 mg/kg without any effect on CSD Figure 1 Mechanisms and structures involved in the pathogenesis of migraine with aura. CSD underlies aura symptoms. Initiation and propagation of CSD are determined by massive increases in extracellular potassium ion concentration and excitatory glutamate. CSD biochemical changes may trigger the activations of meningeal trigeminal endings and trigemino-vascular system, causing the headache phase. The latter can occur through matrix metalloproteases activation that increases vascular permeability and also through the release of nociceptive molecules from mastcells, including proinflammatory cytokines. The pain phase is due to peripheral and central sensitization of the trigeminal system, as well as to the release of CGRP, both peripherally and centrally. CGRP is considered a key mediator in migraine and, together with NO, is the main molecule responsible for vasodilation consequential to neurogenic inflammation. CGRP is also released from cortical slices during CSD and this calcium-dependent release can mediate the dilatation of cortical arterioles. Periacqueduttal gray matter (PAG), locus coeruleus (LC), nucleus of raphe magnum (NRM) are brainstem structures implicated in the processing of trigeminal pain. Functional and structural PAG abnormalities occurring in migraineurs, contribute to the hyper-excitability of trigeminal nociceptive pathways. Functional alteration of noradrenergic nuclei of the LC are believed to be involved in cortical vasomotor instability. Thus, dysfunction in brainstem pain-inhibiting circuitry may explain many facets of the headache phases, even in MwA. CSD participates in this dysregulation by antagonizing the suppressive effect exerted by NRM on the responses of trigeminal neurons. In this scenario, amitriptyline may influence CSD by preserving 5-HT and perhaps NA neurotransmission in the cortex and/or inhibiting high-voltage-activated (HVA) Ca2+ channels and Ca2+ currents. Propranolol, on the other hand, may reduce neuronal excitability and susceptibility to CSD via a beta-adrenergic blockage. Original brain template was designed by Patrick J. Linch, medical illustrator and C. Carl Jeffe, MD, cardiologist. Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 amplitude [202]. In contrast, Kudo et al. failed to demonstrate any inhibitory effect of propofol on the frequency of KCl-induced CSD in rats [203]. This result may have been due to the fact that a water-insoluble formulation of propofol widely administered as a anesthetic in clinical setting was used. In above mentioned study by Dhir et al. a water-soluble, non-commercially available prodrug PHS, was tested [202]. Considering these contrasting findings, it should be investigated whether metabolites produced during PHS activation, rather than propofol per se, can mediate an inhibitory effect on CSD. Given the varying effects of anesthetics on CSD, future studies will have to follow uniform protocols using only anaesthetics having a minimal impact on CSD in order to guarantee reliability and comparability of the results [204]. Page 12 of 18 TRPV1 receptors play an important role in modulating trigeminal sensory processing and for this have been proposed as potential targets for migraine treatment. The TRPV1 receptor antagonist, A-993610, has been tested in a model of mechanically-induced CSD but it failed to show any effects. This lack of effect should be confirmed for other TRPV1 receptor antagonists in future research [205]. Finally, cannabis has been empirically used for centuries for both symptomatic and prophylactic treatment of different types of headaches including migraine. Recent findings have demonstrated a dose-dependent suppression of CSD amplitude, duration and propagation velocity after Delta9-tetrahydrocannabinol (THC) in rat neocortical slices, which had been antagonized by cannabinoid CB1 agonist, WIN 55,212-2 mesylate but not by Figure 2 Main potential targets of currently utilized preventive drugs and those under-investigation for migraine. The mechanisms mediating CSD inhibition by several migraine preventive drugs are not completely understood. However, it is believed that this inhibitory action is exerted by influencing ion channels and neurotransmission. In particular, topiramate and valproate are believed to exert their antagonizing effects on CSD by blocking Na+ channels, inhibiting glutamatergic transmission and increasing GABaergic transmission. Topiramate, but not valproate, can also block L-type Ca2+ channels. Lamotrigine has a marked suppressive effect on CSD, which may be due to its selective action on Na+ channels. Furthermore, gabapentin is able to modulate the activities of P/Q- and, to a lesser extent, N-type high voltage-activated channels located presynaptically at the cortical level and controls the neurotransmitter release, in particular that of glutamate. This effect and the modulation of GABA-mediated transmission might explain CSD inhibition by gabapentin. Magnesium exerts an inhibitory effect on CSD by interfering with NMDA receptor function and reducing glutamatergic transmission as well as regulating glutamate uptake by astrocytes and modulating NO system in the cortex. The blockage of L-type Ca2+ channels may also account for the inhibitory effects of flunarizine on CSD (not shown). The novel benzopyran compound tonabersat seems to inhibit CSD in experimental models and has shown some efficacy in the prophylactic treatment of MwA. It is not known if tonabersat acts on gap-junctions in CNS, as it has already been demonstrated for trigeminal ganglion. CSD inhibition can also be achieved by CGRP-R blockage due to CGRP antagonists, such as olcegepant. This class of drugs might exert its inhibitory effects at both cortical neuronal and cerebrovascular levels. Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 cannabinoid CB2 agonist, JWH-133 [206]. These finding suggests that cannabinoids might have inherent therapeutic effects for MwA but their known side effects and risk of dependence must be properly weighed before cannabinoid being considered for treatment. Conclusions A number of mechanisms have been shown to have a role in fostering CSD wave initiation and propagation including: ion diffusion, membrane ionic currents, osmotic effects, spatial buffering, neurotransmitter substances, gap junctions, metabolic pumps, and synaptic connections (Figure 1). In spite of this knowledge, CSD remains an enigma necessitating further theoretical investigations. Experimental findings to date suggest that chronic daily administration of certain migraine prophylactic drugs (topiramate, valproate, propranolol, amitryptiline, and methysergide) dose-dependently suppress CSD. At a molecular level, targets of the inhibitory effects of antiepileptic drugs tested exert their inhibitory effects on CSD targeting Ca2+ and Na2+ channels, as well as glutamatergic and/or GABAergic transmissions based on their mechanisms of action. Additionally, the antiepileptic drug lamotrigine has a proven suppressive effect on CSD, which could explain its selective action on migraine aura. Preservation of 5-HT, and maybe even NA neurotrasmission in the cortex, could in some way be responsible for the effects of amitryptiline on CSD, while beta-adrenergic blockage by propranolol might facilitate a reduction in cortical neuronal excitability and thus in turn reduce susceptibility to CSD (Figure 2). Mechanisms of action for some novel molecules are currently under investigation. To date, it has been found that CGRP-R antagonists exert a dose-dependent inhibitory effect on CSD. For this reason, it can be hypothesized that CGRP plays a determining role in CSD and its modulation may be effective for the preventive treatment of MwA (Figure 2). Additionally, tonabersat, a novel benzopyran compound, has been reported to exert an inhibitory action on CSD and on neurogenic inflammation in animal models of migraine. These inhibitory actions might be an effect of the blockage of neuronalglial cell gap-junctions, as it has only been demonstrated to be for the trigeminal ganglion. The most significant anti-migraine action on the part of tonabersat seems to derive from its inhibitory action on CSD. In fact, clinical trials on tonabersat have shown its preventive effect on MwA attacks but not on MwoA attacks. Based on recent clinical findings, intranasal ketamine, has been shown to be effective in CSD models, and therefore it has been administered for cases of migraine with prolonged aura, but its use is limited due to its Page 13 of 18 relevant side effects. Noteworthy, the effectiveness of ketamine adds to the existing evidence that glutamatergic transmission plays a role in human aura. The proven capacities of other molecules (i.e. amiloride) in blocking CSD suggests that they might also have a role in preventing MwA. Further investigations on molecules with evidence of targeting CSD not only would lead to a better understanding of the underlying mechanisms for CSD, but also supply meaningful insight into their potential roles in MwA, as well as other diseases, such as epilepsy, ischemic stroke, intracranial hemorrhage, and trauma, where CSD is thought to be a pathogenic mechanism. Additional file Additional file 1: Table S1. A summary of the most relevant studies on CSD experimental models regarding the effects of currently used drugs and drugs under investigation for migraine prophylaxis. Abbreviations 5-HT: 5-hydroxytryptamine, serotonin; AMPA: Alpha-amino-3-hydroxy-5methyl-4-isoxazole propionate; AMPA-R: Alpha-amino-3-hydroxy-5-methyl-4isoxazole propionate receptor; ATP: Adenosine-5'-triphosphate; ATP1A2: ATPase, Na+/K+ transporting, alpha 2 (+) polypeptide; BBB: Blood–brain barrier; BOLD: Blood oxygenation level dependent; Ca2 + : Calcium; CACNA: Ca2+ channel alpha 1A; CADASIL: Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy; CBFLDF: Cortical blood flow laser Doppler flowmetry; CGRP: Calcitonin gene-related peptide; COX: Cyclooxygenase; CSD: Cortical spreading depression; CSF: Cerebrospinal fluid; DC: Direct current; DEA: (N,N-diethylamino)-diazenolate-2-oxide; DHE: Dihydroergotamine; EEG: Electroencephalographic; ET-1: Endothelin-1; FHM: Familial Hemyplegic migraine; fMRI: Functional magnetic resonance imaging; GABA: Gamma-aminobutyric acid; GABA-R: Gamma-aminobutyric acid receptor; Gd-DTPA: Gadolinium-diethylenetriaminepenta-acetic acid; HMGB1: High-mobility group box 1; IL-1β: Interleukin-1 β; iNOS: Inducible NO synthase; K+: Potassium; KCl: Potassium chloride; KI: Knockin; L-NAME: L-NG-Nitroarginine Methyl Ester; MEG: Magnetoelectroencephalography; Mg2 + : Magnesium; MMP(s): Metalloprotease(s); MR: Magnetic resonance; MwA: Migraine with aura; MwoA: Migraine without aura; Na+: Sodium; NA: Noradrenalin; NMDA-R: N-methyl- D-aspartate receptor; NO: Nitric oxide; NOLAG: NO-nitro-L-arginine; NOS: NO synthase; OIS: Optical intrinsic signal; Panx1: Pannexin1; PET: Positron Emission Tomography; PHS: Propofol hemisuccinate; PLC: Phospholipase C; SCN1A: Sodium channel, voltage-gated , type I, alpha subunit; SD: Spreading depression; SPECT: Single-photon emission computed tomography; THC: Tetrahydrocannabinol; TPM: Topiramate; TRPV1: Transient receptor potential cation channel subfamily V member 1; TTX: Tetrodotoxin; WMLs: White matter lesions. Competing interests The authors declare that they have no competing interests. Authors’ contribution All authors equally contributed to the conception and drafting of the manuscript. PC, CA, and PS critically revised the manuscript for important intellectual content. PS, CC, and AT also contributed in the preparation of figures and table. All authors read and approved the final manuscript. Acknowledgements We thank Mr. Thomas Kilcline for editing the English. This Review Article will be presented at the XXVII National Congress of the Italian Society for the Study of Headaches – 26–28 September 2013. Costa et al. The Journal of Headache and Pain 2013, 14:62 http://www.thejournalofheadacheandpain.com/content/14/1/62 Author details 1 Neurologic Clinic, Department of Public Health and Medical and Surgical Specialties, University of Perugia, Ospedale Santa Maria della Misericordia, Sant'Andrea delle Fratte, 06132, Perugia, Italy. 2Neurology II, Department of Neuroscience, University of Torino, Ospedale Molinette, Via Cherasco 15, 10126, Turin, Italy. 3Fondazione Santa Lucia I.R.C.C.S., Via del Fosso di Fiorano, 00143, Rome, Italy. 4Neurovascular Research Lab., Department of Radiology, Stroke Service and Neuroscience Intensive Unit Department of Neurology Massachusetts Hospital, Harvard Medical School, 02115, Boston, MA, USA. 5 Neurologic Clinic, Ospedale S. Eugenio, Piazzale Umanesimo, 00144, Rome, Italy. Received: 5 June 2013 Accepted: 8 July 2013 Published: 23 July 2013 References 1. Somjen GG (2005) Aristides Leão's discovery of cortical spreading depression. J Neurophysiol 94:2–4 2. 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Kazemi H, Rahgozar M, Speckmann EJ, Gorji A (2012) Effect of cannabinoid receptor activation on spreading depression. Iran J Basic Med Sci 15:926–936 doi:10.1186/1129-2377-14-62 Cite this article as: Costa et al.: Cortical spreading depression as a target for anti-migraine agents. The Journal of Headache and Pain 2013 14:62. Submit your manuscript to a journal and benefit from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the field 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Lionetto et al. The Journal of Headache and Pain 2013, 14:55 http://www.thejournalofheadacheandpain.com/content/14/1/55 REVIEW ARTICLE Open Access The omics in migraine Luana Lionetto1*, Giovanna Gentile2, Elisa Bellei3, Matilde Capi1, Donata Sabato1, Francesco Marsibilio4, Maurizio Simmaco1,2, Luigi Alberto Pini5 and Paolo Martelletti4,6 Abstract The term omics consist of three main areas of molecular biology, such as genomics, proteomics and metabolomics. The omics synergism recognise migraine as an ideal study model, due to its multifactorial nature. In this review, the plainly research data featuring in this complex network are reported and analyzed, as single or multiple factor in pathophysiology of migraine. The future of migraine biomolecular research shall be focused on networking among these different and hierarchical disciplines. We have to look for its Ariadne’s tread, in order to see the whole painting of migraine molecular biology. Keywords: Genomics; Proteomics; Metabolomics; Migraine Review The omics synergism Personalized Medicine aims to evaluate the individual profile of the subject and according to it to proceed to a specific therapeutic strategy. This strategy is made possible by the shift from a hierarchical to a holistic vision of the biological system. In the first case the control of the biological system is generated from the genome up to the lower hierarchical levels represented by proteomics and metabolomics; in the second case, the interactions among these disciplines are evaluated in a synergic way (Figure 1). This new approach is certainly the most appropriate for the understanding of the physiopathology of migraine that, as a multifactorial disease, can only be assessed through the integrated study of the omics sciences. Nowadays, it is evident that migraine patients react differently to given drugs, but the exact mechanism has not yet been clarified. In this way, genomic and proteomic research aimed to the identification of molecular pathways involved in drug action is of pivotal importance, in order to focus attention of pharmacogenomics studies on the right targets [1]. Genomics In an effort to identify genetic variants involved in migraine risk and influencing the appropriate pharmacological * Correspondence: [email protected] 1 Sant’Andrea Hospital, Advanced Molecular Diagnostics Unit, Via di Grottarossa 1035 – 1039, Rome 00189, Italy Full list of author information is available at the end of the article treatments, many genomic studies have been performed in the last years. Due to neurological origin of migraine, some researchers have studied receptors involved in mediation of neuronal activities. Chen et al. [2] characterized one polymorphism in GABRG2 gene encoding the GABAA receptor gamma-2-subunit (rs211037) on a migraine case– control population of 546 subjects. No significant correlation was found. Carreno et al. [3] studied the transient receptor potential (TRP) superfamily of non-selective cationic channels accountable of multimodal sensory and pain perception, central and peripheral sensitization, and regulation of calcium homeostasis, relevant steps of migraine physiopathology. They carried out a migraine-control genetic association study genotyping 149 SNPs covering 14 TRP genes. Consistent results were obtained for TRPV3 rs7217270 in the Migraine with aura group and TRPV1 rs222741 in the overall migraine group, suggesting the involvement of the vanilloid TRPV subfamily of receptors to the genetic susceptibility to migraine [4]. Another gene analyzed is the calcium-activated potassium ion channel gene (KCNN3) involved in neural excitability and in migraine susceptibility. Cox et al. [5] performed a gene-wide SNP genotyping in a high-risk genetic isolate from Norfolk Island, characterized by high percentage of migraineurs. A total of 85 SNPs spanning the KCNN3 gene were genotyped in a sub-sample of 285 related individuals. Only four intronic SNPs displayed gene-wide significance: rs4845663, rs7532286, rs6426929 and rs1218551, with the minor allele in each case conferring protection against migraine risk. Using the same population, Cox et al. [6] carried out © 2013 Lionetto et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Lionetto et al. The Journal of Headache and Pain 2013, 14:55 http://www.thejournalofheadacheandpain.com/content/14/1/55 Page 2 of 6 AMBP, CYTC, IGKC, ITH4, UROM, ZAZG ADARB2, EDNRA, GABRG2, GNB3, GRM7, HTR7, KCNN3, MTHFR, OPRM1, TRPV, BDNF, CRP, Lactate, NAA, PCT, sCD40L THE “OMICS” SYNERGISM IN MIGRAINE Figure 1 Framework of the multiple interactions taking place in the migraine omics scenario. another pedigree-based genome-wide association (GWA) study found a significant statistical association with SNP rs4807347 in ZNF555 gene, coding for a zing finger protein. This result has been confirmed in unrelated cohort with more than 500 patients affected by migraine. They also found 4 SNPs in neurotransmitter-related genes (ADARB2, GRM7 and HTR7 genes) suggesting an alteration in the serotoninergic pathway. Maher et al., characterized the entire X chromosome in an association study on the Norfolk population and provides evidence for the SNP rs102834 in the UTR of the HEPH gene, which is involved in iron homeostasis [7,8]. Ligthart et al. [9] performed a metaanalysis of GWA studies for migraine in six populationbased European cohorts consisting of 2446 cases and 8534 controls. A total of 32 SNPs showed marginal evidence for association to migraine and the best result was obtained for SNP rs9908234 in the nerve growth factor receptor NGFR- gene but those results were not replicated in other cohorts. Besides, they found a modest gene-based significant association between migraine and the rs1835740 near the metadherin gene, but further replication studies did not validated this association [10,11]. The opioid system plays an important role in various biological functions including analgesia, drug response and pain reduction. Menon et al. [12] studied the influence of polymorphism in gene coding for the μ-opioid receptor highlighting the association between the OPRM1 A118G SNP and head pain severity in a clinical cohort of 153 female migraineurs with aura. In particular, G118 allele carriers were more likely to be high pain sufferers compared to homozygous carriers of the A118 allele. Migraine is a complex condition that may in part be related by endothelial and cerebrovascular disruption [13]. Some studies showed that supplementation of B vitamins lowered homocysteine levels and reduced the occurrence of migraine in women [14]. Besides, polymorphisms in genes coding for key player enzymes in the folate metabolic pathway have been investigated in order to define a relation with the pathology and its treatment. The C677T variant in the methylenetetrahydrofolate reductase (MTHFR) has been associated with increased risk of migraine with aura [15,16]. The C allele carrier is also related to higher reduction in homocysteine levels, severity of pain in migraine and percentage of high migraine disability in patient Lionetto et al. The Journal of Headache and Pain 2013, 14:55 http://www.thejournalofheadacheandpain.com/content/14/1/55 supplemented with B vitamins [14]. The same approach has been adopted for the polymorphism A66G in methionine synthase reductase gene (MTRR): the A allele carriers showed a better response to B vitamin administration [14]. Roecklein et al. [17] performed a haplotype analysis of migraine risk and MTHFR, MTRR and methionine synthase (MTR), including subjects with non-migraine headache (N = 367), migraine without aura (N = 85), migraine with aura (N = 167), and no headache (N = 1347). Haplotype analysis suggested an association between MTRR haplotypes and reduced risk of migraine with aura. Miao J et al. [18] performed a meta-analysis to determine if polymorphisms in genes linked to the vascular system could be related to migraine. They focused their attention on the receptor for endothelin-1, (EDNRA) known as a potent vasoconstrictor investigating the EDNRA -231G > A SNP. Three studies were included in their meta-analysis for a total of 440 migraineurs, 222 subjects with tension-type headaches (TTHs) and 1323 controls. They found statistical difference between migraineurs and controls with AA genotype vs. AG + GG, suggesting a potential association of -231G > A SNP and migraine. These data suggest that individual SNPs will provide only a small piece of a much larger puzzle composed by a variety of (un) know clinical (phenotypic) information. Gentile et al. [19] genotyped panel of SNPs involved in triptans pharmacokinetics and pharmacody-namics in a chronic migraine (CM) population. In particular they studied the 30 bp VNTR in MAO A (monoamine oxidase A) promoter, CYP 1A2 *1C and *1 F, CYP3A4 *1B and C825T SNP in the gene coding the G protein b3 subunit (GNB3). A significant association with CM was found for the long allele of monoamine oxidase A 30 bp VNTR and CYP1A2*1 F variant. The same authors performed an analysis of the association between genotypic and allelic frequencies of the analyzed SNPs and the grade of response to triptan administration: a significant correlation for MAOA uVNTR polymorphism was found. Further stratification of patients in abuser and non-abuser groups revealed a significant association with triptan overuse and, within the abusers, with drug response to the CYP1A2*1 F variant [20]. Proteomics With the completion of the human genome project in 2002, it has become clear that organism complexity is generated more by a complex proteome than by a complex genome. Actually, the term “clinical proteomics” defines proteomic technologies employed to examine clinical samples [21]. The available genomic data have now been translated to proteomics, making the discovery of biomarkers increasingly feasible. With its high-throughput and Page 3 of 6 unbiased approach to the analysis of variation in protein expression patterns (actual phenotypic expression of genetic variation), proteomics promises to be the most suitable platform for biomarker discovery. This hopefulness is based on the increasing advances and ability of proteomic technologies to identify thousands of proteins and peptides in complex biological tissues and biofluids, such as plasma, serum and urine [22]. In the past decade, many novel proteomic technologies were emerged and applied to several biological systems for the understanding of cellular activities, disease development and physiological responses to therapeutic interventions and environmental perturbations [23]. One of such emerging field is the application of proteomic technologies to toxicological research, which has given rise to a new area called toxicoproteomics [24]. Drug induced toxicity represents a significant problem in healthcare delivery, therefore the early detection of organ toxicity may provide great benefits to patients, preventing further onset of adverse events and complications of disease management. Incidences of toxicity in liver, heart, brain, kidney and other organs have been reported with the use of different therapeutic drugs. In particular, various epidemiologic investigations proved that different drug types could cause nephrotoxicity, especially in chronic patients. In this regard, Bellei and co-authors reported two proteomic studies in which they analysed the urinary proteome of medication-overuse headache (MOH) patients, in comparison with healthy non-abuser individuals as control, with the purpose to identify possible differences in excreted proteins induced by the excessive consumption of antimigraine drugs, that could lead to nephrotoxicity [25,26]. Comparing the proteomic profiles of patients and controls, they found a significantly different protein expression at various MW levels, especially in the NSAIDs group, in which six proteins over-secreted from kidney were strongly correlated with various forms of kidney disorders: uromodulin (UROM), alpha-1-microglobulin (AMBP), zinc-alpha-2-glycoprotein (ZAZG), cystatin C (CYTC), Ig-kappa-chain (IGKC), and inter-alpha-trypsin heavy chain H4 (ITIH4). These preliminary results have allowed to define the urinary protein pattern of MOH patients, that was found to be correlated to the abused drug. While adverse effects from long-term triptans use are unknown, the overuse of analgesics may cause wellknown unwanted events, including liver dysfunction, gastrointestinal bleeding, addiction and renal insufficiency [27]. Moreover, a recent review concerning the epidemiology of drug-induced disorders has demonstrated that medication overuse could lead to nephrotoxicity and potential renal damage [28]. Currently, the most pertinent application in nephrotoxicology is the proteomic analysis of urine. In recent years, it has proposed a broad range of urinary enzymes and proteins as possible early biomarkers of drug-induced nephrotoxicity [29-31]. Lionetto et al. The Journal of Headache and Pain 2013, 14:55 http://www.thejournalofheadacheandpain.com/content/14/1/55 Metabolomics While the genomics and proteomics suggest a possible mode of operation of the system, metabolomics gives the actual representation of the system. In agreement with the multifactorial nature of the disease, the studies conducted to date relate to different classes of analytes. In fact, several different mechanisms, such as neuro-inflammatory, neurological and cerebrovascular, have so far been suggested in migraine pathophysiology [32]. Whereas the presence of inflammatory processes during migraine attacks, serum levels of different proinflammatory molecules have been investigated. Several studies have found elevated levels of C-Reactive Protein (CRP), a marker of chronic low-grade inflammation, in patients with migraine [33,34]. Also the procalcitonin levels were investigated as indicators of inflammation in migraine patients with and without aura. Turan et al. found significantly high levels of PCT in patients with migraine during the attack compared to the interictal period (0.0485 vs 0.0298 ng/ml) [35]. An interesting fact was found by Guldiken et al. about the increase of soluble CD40 ligand (sCD40L). In this study, sCD40L amount was higher in the subgroup of migraineurs with aura than in those without aura and in rare attacks rather than in the frequent ones. In both comparisons the difference in the levels of sCD40L was not significant but these data may be related to the association of migraine with cardiovascular diseases [36]. Also the correlation between migraine and molecules of pain was investigated. Several observation suggested a relationship between low serum levels of vitamin D and higher incidence of chronic pain [37,38]. However, in a large cross-sectional study, Kjaergaard et al. have found that low level of serum vitamin D were associated with nonmigraine type of headache [39]. Further studies should be conducted to clarify the potential association of vitamin D and migraine pain. Brain-derived neurotrophic factor (BDNF) is a neurotrophin associated with pain modulation and central sensitization [40]. Fisher et al. have demonstrated that in migraineurs, BDNF was significantly elevated during migraine attacks (31.24 ± 9.31 ng/ml) compared with headache-free periods (24.50 ± 9.17 ng/ml), tension-type headache (20.97 ± 2.49 ng/ml) and healthy controls (21.20 ± 5.64 ng/ml) [41]. This study, showing a correlation between migraine and BDNF, supports the hypothesis that BDNF has a role in the pathophysiology of migraine. The results are in agreement with a pilot study of Tanure et al. [42]. However, NMR or mass spectrometry techniques are able to provide, with an acceptable probability, the description of the current biochemical state of an organism. The development of proton magnetic resonance spectroscopy (1H-MRS) has been used to assess noninvasively the metabolic status of human brain [43]. Several studies have employed 1H-MRS achieving numerous results for Page 4 of 6 metabolites including N-acetylaspartate (NAA), as a marker of neuronal functioning [44], choline (Cho), as a marker for membrane turnover [45], total creatine (tCr) and lactate, for energy metabolism [46], and myo-inositol (a glial marker) [47]. With the exception of NAA, the results obtained for these molecules are heterogeneous and sometimes contradictory. Indeed the levels of NAA are also increased in the serum [48]. In our opinion, to date, it has not been identified molecules that can be used as a marker for migraine. Undoubtedly, spectroscopy Nuclear Magnetic Resonance and Mass Spectrometry represent the most powerful and most widely used techniques for the simultaneous analysis of different molecules, allowing together with functional genomics and proteomics to open new roads knowledge in the pathophysiology of migraine. We can suppose that also the progression of the disease may be evaluated by a therapeutic metabolite monitoring approach [49]. Conclusions A conclusive scene of omics network in migraine pathophysiology is still far to be sharpened. We know a lot of genomics, but less of proteomics and metabolomics of migraine. However, there are numerous scientific evidences assigning an increasingly dominant role to external factors, environmental or behavioural, that interact with the pathophysiology of migraine. Although genomics represents the hierarchical basis of migraine mechanism, proteomics and metabolomics mirror its real image. We have to continue to study the connections among omics in order to unsnarl the Ariadne’s tread of migraine. Abbreviations SNP: Single nucleotide polymorphism; CM: Chronic migraine; MOH: Medication-overuse headache; MW: Molecular weight; NSAIDs: Nonsteroidal anti-inflammatory drugs; PCT: Procalcitonin; NMR: Nuclear magnetic resonance. Competing interests All authors declared no competing interest related to the content of this manuscript. Authors’ contributions LL, GG, EB contributed equally in the preparation of the manuscript. LAP and PM conceived the study, provided overall and oversaw the implementation of the study. All the authors approved the final draft of the manuscript. No external assistance in selection, writing or reviewing data has been utilized. Author details 1 Sant’Andrea Hospital, Advanced Molecular Diagnostics Unit, Via di Grottarossa 1035 – 1039, Rome 00189, Italy. 2NESMOS Department, Sapienza University of Rome, Rome, Italy. 3Department of Diagnostic Medicine, Clinic and Public Health, University of Modena and Reggio Emilia, Modena, Italy. 4 Regional Referral Headache Centre, Sant’Andrea Hospital, Rome, Italy. 5 Inter-Department Headache and Drug Abuse Centre, University of Modena and Reggio Emilia, Modena, Italy. 6Department of Clinical and Molecular Medicine, Sapienza University of Rome, Rome, Italy. Received: 6 June 2013 Accepted: 22 June 2013 Published: 1 July 2013 Lionetto et al. The Journal of Headache and Pain 2013, 14:55 http://www.thejournalofheadacheandpain.com/content/14/1/55 References 1. Simmaco M, Borro M, Missori S, Martelletti P (2009) Pharmacogenomics in migraine: catching biomarkers for a predictable disease control. Expert Rev Neurother 9(9):1267–1269 2. 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Lionetto L, Capi M, Vignaroli G, Negro A, Martelletti P (2012) Deciphering the task of N-acetyl aspartate in migraine. Expert Rev Neurother 12 (9):1057–1059 doi:10.1186/1129-2377-14-55 Cite this article as: Lionetto et al.: The omics in migraine. The Journal of Headache and Pain 2013 14:55. Submit your manuscript to a journal and benefit from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the field 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 REVIEW ARTICLE Open Access Headache and comorbidity in children and adolescents Benedetta Bellini1, Marco Arruda2, Alessandra Cescut1, Cosetta Saulle1, Antonello Persico3, Marco Carotenuto4, Michela Gatta5, Renata Nacinovich6, Fausta Paola Piazza7, Cristiano Termine8, Elisabetta Tozzi9, Franco Lucchese10 and Vincenzo Guidetti1* Abstract Headache is one of the most common neurological symptom reported in childhood and adolescence, leading to high levels of school absences and being associated with several comorbid conditions, particularly in neurological, psychiatric and cardiovascular systems. Neurological and psychiatric disorders, that are associated with migraine, are mainly depression, anxiety disorders, epilepsy and sleep disorders, ADHD and Tourette syndrome. It also has been shown an association with atopic disease and cardiovascular disease, especially ischemic stroke and patent foramen ovale (PFO). Keywords: Headache; Comorbidity; Children; Adolescents Review Introduction Headache is one of the most common somatic complaints in children and adolescents [1]. The prevalence is estimated to be 10–20% in the school-age population, with progressive increase with age, up to values about 27–32% at the age of 13–14 years (considering monthly crisis), 87–94% (considering the presence of headache at least once a year). Until puberty, it hasn’t been seen gender differences (with a slight male predominance), at a later stage it has been noted an increase among females with a ratio of 2.5:1, except that lasts into adulthood [2,3]. Prevalence of migraine in the pediatric population ranges from 3,3% to 21,4% and it increases from childhood to adolescence [4]. Children and adolescents with headache, and in particular migraine, have worse outcomes, compared to those without migraine, as far as quality of life and school attendance [5] are concerned and they are more likely to have other somatic symptoms (e.g. abdominal pain) [6], anxiety and mood disorders, such as depression [7,8]. Due to its negative impact, on the World Health Organization’s ranking of causes of disability, headache disorders are * Correspondence: [email protected] 1 Department of Pediatrics and Child Neuropsychiatry, Sapienza University of Rome, Via Dei Sabelli 108, Rome, Italy Full list of author information is available at the end of the article brought into the 10 most disabling conditions for the two genders, and into the five most disabling for women [9]. In fact, primary headaches, and in particular migraine, is associated with several comorbid conditions. Comorbidity is the presence of coexisting additional condition in a patient with particular disease index, or the association of non-random two disorders [10]. In children and adolescents, headache and migraine are commonly associated with various diseases, such as psychiatric and neurological comorbidity, in particular depression and anxiety, epilepsy, sleep disorders, ADHD. It also has been shown an association with atopy, cardiovascular disease, especially ischemic stroke and PFO [11-14]. Headache and psychiatric disorders: anxiety and depression Since past decades, numerous population- and hospitalbased studies have revealed a relationship between migraine or headache and psychopathology in children [15-17]. Depression is more prevalent in headache patients than in the headache-free population [18]. Recently, Pavone et al. (2012) [19] studied the frequency of some comorbidities in primary headaches in childhood. They enrolled two hundred and eighty children (175 males and 105 females), aged 4 to 14 years, affected by primary headaches. In direct interviews, parents and children gave information about the association of their © 2013 Bellini et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 headaches with different conditions. The Authors found a significant association of primary headache with anxiety and depression. Migraine is probably the best studied pain disorder in the context of comorbidity with anxiety and/or depression [20]. In a psychiatric setting Masi and collegues [21], in an exploratory study, examine the prevalence of somatic symptoms in a sample of 162 Italian children and adolescents from emotional and/or behavioral disorders. The sample was divided according to gender (96 males, 66 females), age (70 children younger and 92 adolescents older than 12 years), and psychiatric diagnosis (Anxiety, Depression, Depression/Anxiety, Other). The Authors observed that headache was the most frequent somatic symptom in children and adolescents referred for anxiety, depression and behavioral disorders, with a prevalence of females. Cahill and Cannon [22] defined migraine as a subtype of headache of particular interest for psychiatrists, as they found a linkage between migraine, psychiatric disorders (mainly anxiety and depression), personality traits and stress. The nature of the relationship between migraine and anxiety is still not clear and we do not know if that relationship is specific to migraine or related to attack frequency [23], even if some evidence confirms that linkage [24]. It is well known that the risk of migraine is higher in patients with comorbid anxiety and/or depression [25] and that anxiety predicts the persistence of migraine and tension-type headache [26]. While only phobic disorder seems to be a predictor of the onset of migraine [27], anxiety, more than depression, predicts long-term migraine persistence, headache-related disability and reduces perceptions of efficacy with acute treatment [26,28]. Phobic disorder is also associated with more frequent and longer migraine attacks, particularly among males [29]. The increased risk of anxiety disorder in children and adolescents with migraine, compared to patients without migraine, is noticed in many studies. Arruda and Bigal [5], in their population-based study, confirmed the higher prevalence of anxious symptoms in children and adolescents with migraine. In a meta-analysis of 10 studies published between 1996–2011 (406 patients, mean age 11,6 ± 2,3 ys) Ballottin and collegues [23] found that migraine children show more psychological symptoms, detected by using Child Behavior Checklist (CBCL), than healthy controls. They also emphasized the need for studies to compare migraine children with children affected by other chronic pain in order to understand whether the psychopathological profile is migraine- related or chronic pain-related. Some studies suggest that psychiatric disorders might not specifically relate to migraine, but to chronic illness in general: comparing migraine and chronic non-headache Page 2 of 11 pain samples, Cunningham [30] found no difference in anxiety and depression levels between the two groups with chronic pain, with respect to pain-free controls. Another study [6] comparing headache patients and patients with recurrent abdominal pain did not find differences by the psychological point of view (internalizing vs. externalizing disorders). One of the hypotheses for the comorbidity is that common genetic and/or environmental risk factors may underlie both migraine and psychiatric disorders [27]. Gonda and collegues [31] found that high levels of anxiety and migraine were associated with specific gene polymorphism, supporting the hypothesis of a shared genetic linkage between these two conditions. Instead other studies show no correlation between migraine, anxiety and depression, as Kowal and Pritchard [32] that studied twenty-three volunteer subjects, compared with 23 (matched) control subjects on self and parental ratings of anxiety, depression, shyness-sensitivity, sleeping difficulties, perfectionism, psychosomatic problems (unrelated to headache), other behavioural disturbances, major life stress events and parental expectations (i.e. achievement orientation). Results indicated that the headache children showed significantly higher shynesssensitivity, psychosomatic problems and behavioural disturbances and significantly lower parental expectations than the control group children. No other differences were found. While none of the variables were predictive of the frequency or intensity of head pain, measures of anxiety, perfectionism, and life stress events contributed significantly to the prediction of the severity of head pain. Also the study by Laurell et al. [33] show conflicting data. The Authors interviewed 130 schoolchildren and their parents and showed a predominance of comorbidity with other pains rather than psychological and social problems. In addition to migraine, Chronic daily headache (CDH), defined as 15 or more headaches per month, is associated with increased functional disability and impaired quality of life [34]. Functional disability in children with recurrent headache has also been shown to be a risk factor for psychiatric conditions such as depression [35]. While research in the area of adult headache has made great strides, little is known about the prevalence of psychiatric comorbidity in children with chronic headache conditions. Some researchers have suggested that children with headaches are at increased risk for psychological adjustment problems, including symptoms of anxiety and depression [36,37]. A single published study of a large sample of school-children in Taiwan that did utilize standardized interviews, written by Wang SJ et al. [15], indicated that nearly half (47%) of the sample of 122 children (out of more than 7000 children) who reported chronic headaches had one or more psychiatric disorders, primarily mood or anxiety disorders. Two years later the same Authors identified a higher frequency of suicidal ideation in younger adolescents with Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 migraine with aura or high headache frequency. These associations were independent of depressive symptoms [38]. Parisi P. [39] stressed that antidepressant and antiepileptic usage in adolescents was potentially associated with an increasing suicide risk and that these two medications are frequently used in adolescents with migraine. Moreover, Wang SJ et al. [38] did not exclude the diagnosis of early onset juvenile bipolar disorders (JBD). Although the onset of JBD before the age of 10 is rare and the first manifestation occurs most frequently between the ages of 13 and 15, the diagnosis of JBD is more difficult in children and adolescent populations vs the adult population due to varying symptoms. For example, in children and adolescents, dysphoria is more likely than a euphoric or depressive mood. Asymptomatic intervals rarely exist, yet rapid cycling prevails. In addition, it has been shown that antidepressants in JBD-affected children can have severe adverse effects, particularly the amplification of suicidal ideation. Parisi P., in this respect, indeed, stressed that the possibilities of manic switching and occurrence of suicidal ideation have to be closely monitored when clinicians prescribe antidepressants for treatment of either migraine or depression in adolescents. Slater et al. (2012) [34] assessed comorbid psychiatric diagnoses in youth with CDH and examined relationships between psychiatric status and CDH symptom severity, as well as headache-related disability. Results showed that 29.6% of CDH patients met criteria for at least one current psychiatric diagnosis. Of those, anxiety disorders were the most common (16.6% of the sample). Mood disorders, on the other hand, were less prevalent (9.5%). The most common anxiety diagnoses were specific phobia (14 of 169), generalized anxiety disorder (10 of 169) and obsessive compulsive disorder (eight of 169). Of the 16 participants with a depressive disorder diagnosis, eight had major depressive disorder, four had a diagnosis of dysthymia, and four met criteria for other mood disorders. Moreover, 34.9% met criteria for at least one lifetime psychiatric diagnosis. No significant relationship between psychiatric status and headache frequency, duration, or severity was found. However, children with at least one lifetime psychiatric diagnosis had greater functional disability and poorer quality of life than those without a psychiatric diagnosis. It is, furthermore, important to consider the impact of headache on family life and dynamics. Children with migraine seem to be characterized by a higher prevalence of familial headache recurrence and parents’ psychiatric disorders than children with other headache subtypes [40]. Only in the case of migraine, higher familial headache recurrence correlates with higher psychiatric comorbidity in children. The association between migraine and anxiety leads us to think of the need for an integrated, medical and Page 3 of 11 psychological, approach to the taking care of these young patients and their families. Headache and sleep disorders The existence of an intimate relationship between sleep and headache has been recognized for over a century, although the nature of this association is still enigmatic. It is known as sleep deprivation, or, on the contrary, a prolonged sleep, can favor the onset of headache, in particular migraine attack [41]. On the other hand, in many cases, and especially in children, sleep, spontaneous or induced by hypnotics, constitutes the decisive factor for resolution of a migraine attack [42]. Also melatonin seems to effectively reduce the number, intensity and duration of headache attacks per month in the children but the mechanism remains unclear, even though there is much evidence to support the analgesic and anti-inflammatory effects of melatonin [43]. Headaches are known to occur during sleep, after sleep, and in relationship with various sleep stages. Nocturnal migraine attacks can be a result of disrupted sleep, and primary headaches may also emerge during nocturnal sleep, causing sleep disruption [44]. About the variety of phenomena that can disrupt the sleep macrostructure and can impact its restorative function, the periodic limb movements disorder (PLMd) can be considered as the most powerful. No studies are known about the role of PLMd in the pathophysiology of migraine in children. Esposito et al. [45] assess the prevalence of PLMd and migraine and their relationship with disability and pain intensity in a pediatric sample, referred for migraine without aura by pediatricians. This study indicates the potential value of the determination of the PLMd signs, and the importance of the nocturnal polysomnography evaluation in children affected by migraine, particularly when the clinical and pharmacological management tend to fail in the attacks control. Children who suffer from headache have usually a high rate of sleep difficulties, such as insufficient sleep, cosleeping with parents, difficulties falling asleep, anxiety related to sleep, restless sleep, night waking, nightmares, and fatigue during the day [46]. Furthermore, an higher prevalence of parasomnias in children, particularly of sleepwalking, bedwetting and pavor, has been documented in migraine patients than in controls [47,48]. The prevalence of sleepwalking in migraineurs seems to swing between 30% and 55% [49]. Different studies propose a model of interaction between headache and sleep [50]. Table 1 shows a model combining clinical data and experimental evidence. To date, there are no epidemiological studies to investigate systematically specific comorbidity between headache and the spectrum of sleep disorders. The lack of such Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 Table 1 Models of relations between sleep and headache 1 Sleep as trigger factor for headache (excessive, reduced or disrupted, increased deep sleep) 2 Sleep as relieving factor for headache 3 Sleep disturbance as cause of headache (es. sleep apnea) 4 Headache as cause of sleep disturbance (es. attacks occurring during sleep) 5 Sleep disorders in headache patients (parasomnias, sleepwalking) 6 Sleep related headache: a) Temporal relationship (during or after sleep) b) Sleep stage relationship: 1. REM sleep (migraine, cluster headache, chronic paroxysmal hemicrania) 2. Slow-wave sleep (migraine) 7 Headache/sleep association: a) Intrinsic origin (modulation through the same neurotransmitters) b) Extrinsic origin (i.e. fibromyalgia syndrome) c) Reinforcement (bad sleep hygiene) studies is compounded by the difficulties of classification of the two disease entities. Among the epidemiological studies carried out so far, who have analyzed the relationship between sleep and headache, there is a European study, performed by 18,980 telephone interviews. This study showed the presence of chronic headache in the morning in 7.6% of subjects [51]. These, also, than non-sufferers, had more frequently sleep disorders including insomnia, circadian rhythm disorders, snoring, sleep-related breathing disorders, frightening dreams and other dyssomnia. Headache and epilepsy Migraine and epilepsy are the commoner brain diseases and comorbidity of these conditions is well known. This comorbidity is most frequent in childhood and adolescence. The International Classification of Headache Disorders (ICHD-2) committee recognizes three nosographic entities concerning the relationship between epilepsy and headache: migralepsy, hemicrania epileptica, post-ictal headache [52]. Recent scientific evidences on the ictal epileptic headache have demonstrated that the ‘migralepsy’ concept is exceptional or even it does not exist [53]. On the other hand, migralepsy is neither included in the currently used International League Against Epilepsy (ILAE) seizure classification nor in the recent recommendations of the ILAE Commission on Classification and terminology. In particular, migralepsy, which in the recent ICHD-II is defined as a seizure developing during or within 1 h of a migraine aura, is extremely rare. The concept of migralepsy, Page 4 of 11 according to the current definition, is too narrow and inadequate and it should be revised keeping in the mind that headache or visual symptoms can be the epileptic “aura” of a seizure [54]. Parisi et al. [52] suggest to add, to the forthcoming ICHD-3 classification, the term “ictal epileptic headache” (IEH) which defines a condition diagnosed when a headache attack is the only clinical feature of epileptiform discharges [52-56]. The classification criteria for “ictal epileptic headache” (IEH), was based on twelve well-documented cases that have been published in the literature. The “migraine-epilepsy” sequence, defined, as said, “migralepsy”, may often merely be a seizure starting with an ictal headache, followed by a sensory-motor partial or generalized seizure, which fits into the codified “Hemicrania Epileptica” [53,57]. To date, headache and epilepsy classifications have ignored each other. In the International League Against Epilepsy (ILAE) classification, headache is considered exclusively as a possible semiological ictal phenomenon among the “non-motor” features. In particular, headache is described as a “cephalic”sensation and is not considered as the sole ictal expression of an epileptic seizure. Moreover, headache is not classified as a “pain” (among the “somatosensory” features) or “autonomic” sensation, whereas signs of involvement of the autonomic nervous system, including cardiovascular, gastrointestinal, “vasomotor” and thermoregulatory functions, are classified as “autonomic” features. Now, although still considered a controversial issue, we must consider that headache pain could originate in the terminal nervous fibers (“vasomotor”) in cerebral blood vessels; consequently, headache should be classified as an “autonomic” sensation in the ILAE Glossary and Terminology. Headache could thus be interpreted as the sole expression of an epileptic seizure and classified as an autonomic seizure [53]. In according to these criteria, Parisi et al. [58] propose the term “ictal epileptic headache” for cases in which headache is the sole ictal manifestation, whereas the term “ictal headache” should be applied when the headache, whether brief or long-lasting, is part of a more complex seizure including other sequential or overlapping (sensory-motor, psychiatric or non-autonomic) ictal manifestations. This new classification proposal (headache as an isolated ictal autonomic manifestation in IEH) has very different prognostic implications because the outcome in people with long-lasting autonomic status epilepticus is very different (i.e., benign) from that of people with additional ictal motor-sensitive semiology. In addition, headache as an autonomic phenomenon is crucial when attempting to understand why headache may be the sole ictal epileptic manifestation: the reasons have been thoroughly explained in Panayiotopoulos syndrome, whereas the Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 threshold required to trigger an ictal autonomic phenomenon is believed to be lower than that required to trigger sensitive sensorial or motor ictal semiology [53]. The criteria of IEH have been proposed by Belcastro et al. [59] to identify the case of headache (as sole ictal manifestation) of epileptic origin in order to promptly obtain an EEG recording and confirm the diagnosis. Table 2, taken from the paper of Parisi et al. [52], shows the proposed criteria for ictal epileptic headache (Diagnostic criteria A–D must all be fulfilled to make a diagnosis of ‘IEH’). This clinical picture is extremely rare and has only been documented in about 10–12 cases and its epileptic nature is documented with ictal EEG. For this reason, it is difficult to obtain firm conclusions about the frequency of IEH based on epidemiological studies. Using these criteria, we will be able to clarify if IEH represents an underestimated phenomenon or not [59]. Regarding the third entity concerning the relationship between epilepsy and headache, the post-ictal headache, a multicentric italian study from 2006 at 2009 on 142 children, shows that post-ictal headaches were most frequent (62%). Pre-ictal headaches were less common (30%). Inter-ictal headaches were described in 57.6%. Clear migrainous features were present in 93% of preictal and 81.4% of post-ictal headaches. Inter-ictal headaches meet criteria for migraines in 87%. The association between partial epilepsy and migraine without aura is most common and reported in 82% of our patients with peri ictal headache and in 76.5% of patients with postictal headache [60]. The term “hemicrania epileptica” should be maintained in the ICHD-II, introduced into the ILAE, and be used to classify all cases in which an “ictal epileptic headache” “coexists” and is associated synchronously or sequentially with other ictal sensorymotor events [55]. As regards the possible causes of comorbidity, the first hypothesis provides a causal relationship of migraine and epilepsy, which seems, however, unlikely considering that some epileptic syndromes such as benign partial epilepsies Table 2 Proposed criteria for ictal epileptic headache (IEH) A. Headache lasting seconds, minutes, hours or days; B. Headache that is ipsilateral or contralateral to lateralized ictal epileptiform EEG discharges (if EEG discharges are lateralized); C. Evidence of epileptiform (localized, lateralized or generalized) discharges on scalp EEG concomitantly with headache; different types of EEG anomalies may be observed (generalized spike-andwave or polyspike-and-wave, focal or generalized rhythmic activity or focal subcontinuous spikes or theta activity that may be intermingled with sharp waves) with or without photoparoxysmal response (PPRs) D. Headache resolves immediately after i.v. antiepileptic medication Page 5 of 11 are observed more frequently [61]. If the association of the two disorders were purely random, the expected prevalence of epilepsy was 1% in migraineurs and the prevalence of migraine was 12% in epileptics, while the literature reports prevalence data significantly higher than expected on basis of random association. The risk for unprovoked seizures was increased in children with migraine with aura and not in patients with migraine without aura [62]. Several epidemiological studies indicate an association of migraine and epilepsy with an increased prevalence of migraine in patients with epilepsy and vice versa. In particular, the prevalence of epilepsy in patients with migraine varies from 1 to 17%, with an average of 5.9%, but this percentage greatly exceeds that of the general population that is approximately 0.5–1%. The overall prevalence of migraine in children with epilepsy varies from 8 to 15%, with values also increased in children with central-temporal EEG spikes (63%) and epilepsy with absences (33%) [63,64]. The risk of migraine is more than twice as high in subjects with epilepsy both in probands than in relatives, compared to people without epilepsy [65]. As a second hypothesis has been suggested a causal unidirectional relationship, for instance in case of migraine can cause cerebral ischemia or cerebral damage, and consequently epilepsy, or in the case of "migralepsy", where migraine aura can trigger a seizure. More often a seizure triggers an attack of headache post-critical, often with migraine characteristics, in this case it has been hypothesized that epilepsy can trigger migraines through activation of the trigeminal-vascular system or through mechanismsencephalic trunk [66]. However, the unidirectional hypothesis has been contradicted in a study for verified the relationship between migraine and epilepsy in 395 adult seizure patients, conducted by Marks et al. [67], since in the majority of patients with migraine and epilepsy (66/79, 84%) attacks were completely independent. A third hypothesis requires that common environmental risk factors, such as head injury, can cause both migraine and epilepsy. In fact, it has been found an increased risk of migraine in people with epilepsy caused by head trauma, and that in each subgroup of epilepsy, defined on the basis of seizure type, age at onset, etiology, and family history [68]. On the other hand, the presence of shared environmental factors do not explain the increased risk of migraine in patients with idiopathic epilepsy and several studies have documented the association between migraine and rolandic epilepsy and idiopathic occipital epilepsy [69,70]. Headache may occur before, during or after an epileptic seizure, as well as vomiting. In idiopathic occipital epilepsy, crises are, in fact, often characterized by Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 vomiting associated with visual symptoms, focal seizures and headache. The existence of a possible constitutional common ground between migraine and epilepsy was initially proposed on the basis of significantly greater familiarity of migraine in epileptics (28%) and for epilepsy in migraineurs (2–3%) [70]. The genetic hypothesis (fourth hypothesis) was tested by Ottman et al. [65], which have suggested a higher incidence of migraine in families with genetic forms of migraine than those with non-genetic forms and that the relatives of patients with migraine and epilepsy had an increased incidence of epilepsy compared to the relatives of patients with only epilepsy. However, this hypothesis was not confirmed by other studies [68]. Subsequent work reported data in favor of possible genetic factors common to the two conditions. In fact, in some families with idiopathic temporal lobe epilepsy was found a higher prevalence of migraine [71,72]. An extended family with several individuals with occipital and temporal lobe epilepsy was also featured, which segregated with an autosomal dominant mode of transmission; epileptic patients had migraines with aura are independent of seizures [73]. Most obvious is the association between migraine with aura (MWA) and epilepsy. In fact, in a study of 134 children and adolescents with headache, there was a high prevalence of MWA (30.4%) than other types of primary headache in children with seizures [74]. Another study of population-based case–control study documented that the risk of seizures was increased in children with MWA and not in those with migraine without aura (MOA) [75]. In addition, in a study of adult patients, the frequency of MWA was significantly higher in patients with epilepsy in comorbidity (41%) compared to patients only with migraine (25.8%) [76]. Finally, considering the comorbidities as a result of an alteration in brain excitability, Leninger et al. [76] investigated whether the clinical features associated with diffuse cortical depression (the so-called “Spreading Depression,” CSD) were more severe in patients with comorbidities. Despite the frequency of epileptic seizures and syndromes did not differ between patients with epilepsy alone, compared to subjects with comorbidy, migraine with aura, worsening pain with physical activity, phonophobia and photophobia were significantly more frequent in patients with comorbidities compared with patients with epilepsy or migraine alone. These differences are in favor of the hypothesis that the link between migraine and epilepsy is based on the CSD as an expression of neuronal hyperexcitability. The altered neuronal excitability may cause an increased sensitivity to the CSD resulting in an increased activation of the trigeminal nociceptive fibers and consequently in more severe migraine attacks [76]. Page 6 of 11 Therefore it is likely that the altered neuronal excitability threshold, involved in migraine and epilepsy, and due to altered levels of neurotransmitters, is attributable to genetic factors, in particular the disorders of membrane ion channels, the so-called channelopathies. The epilepsy and migraine, in fact, share common pathogenetic mechanisms partially related to the dysfunction of ion channels, it is assumed, therefore, that channelopathies may be the link between epilepsy and migraine, particularly when these disturbances are in comorbidy. However, when headache and epilepsy overlap as a result of the crossing of the cascade of events at the cortical level, in both of the events (CSD and epileptic focus), their onset and propagation are triggered when these events reach a certain threshold, which is lower for CSD than for seizure. These two phenomena may be triggered by more than one pathway converging (at cortical level) upon the same destination: depolarization and hypersynchronization [53]. Finally, further studies are warranted to better delineate the complex link between epilepsy and migraine. Headache and general medical conditions Compared to comorbidity between headache and general medical conditions, an interesting epidemiological study, led by Lateef et al. [77] on the child population, highlights the correlation between headache and other general medical conditions, including asthma, hay fever and frequent ear infections. The 41.6% of children with headache had at least one of these conditions, and in general, the group examined had a probability of 3.2 times higher to present two of the above conditions and a probability of 13.6 times greater to submit all three. The increased comorbidity between headache and general medical conditions was found from 4 to 11 years. Other conditions most frequently observed in children with frequent or severe headaches are: ADHD, especially as regards hyperactive/impulsive behavior [78], learning disabilities, stuttering, anemia, obesity, bowel disease. Regarding the girls, it was found that most of those who had frequent headaches had their first menstrual cycle before the age of 12 years [77]. It was also found a higher comorbidity of headache, in particular migraine, with atopic disorders (asthma, rhinitis or eczema), studied in a sample of children presenting with such disorders. The prevalence of migraine was significantly higher in children with atopic disorders than those without. In particular, the greater association was detected with rhinitis [12]. Recent researches suggest that obesity was significantly correlated with migraine frequency and disability in children, as well as in adult population studies. Translational and basic science research shows multiple areas of overlap Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 between migraine pathophysiology and the central and peripheral pathways regulating feeding. Specifically, neurotransmittors such as serotonin, peptides such as orexin, and adipocytokines such as adiponectin and leptin have been suggested to have roles in both feeding and migraine. A relationship between migraine and body mass index exists, and therefore, interventions to modify body mass index may provide a useful treatment model for investigating whether modest weight loss reduces headache frequency and severity in obese migraineurs [79]. The effect of obesity and weight change on headache outcomes may have important implications for clinical care. Recently, Verrotti et al. [80] investigated the real impact of a weight loss treatment on headache in a sample of obese adolescents. In all, 135 migraineurs, aged 14– 18 years, with body mass index (BMI) greater than or equal 97th percentile, participating in a 12-month-long program, were studied before and after treatment. The program included dietary education, specific physical training, and behavioral treatment. Decreases in weight, BMI, waist circumference, headache frequency and intensity, use of acute medications, and disability were observed at the end of the first 6month period and were maintained through the second 6 months. Both lower baseline BMI and excess change in BMI were significantly associated with better migraine outcomes 12 months after the intervention program. So, initial body weight and amount of weight loss may be useful for clinicians to predict migraine outcomes [80]. Headache and cerebro and cardio-vascular diseases Although migraine is an accepted cause of cerebral infarction in adults, this association is not recognized in children. The mean annual incidence of stroke in children is about 2.5 per 100,000 [78]. The causes of cerebral infarction in children may include: heart disease, vascular disease, blood disorders, primary hypercoagulable states or congenital metabolic disorders, but 50% of strokes are considered idiopathic [81]. In the adult population is generally accepted that cerebral infarction may occur during a migraine attack [82]. In young adults, ischemic strokes could be the result of migraine in a percentage ranging from 10% to 27% [83]. In contrast, in children, the diagnosis of stroke caused by migraine is still questioned, in fact, until now, only a few cases have been reported in subjects under the age of 16 years [84,85]. In most patients, the ischemic stroke occurred in a middle cerebral artery territory [85] but may be involved also areas of the brain sprayed from basic. In particular, there appears to be a complex relationship in a bidirectional association between migraine and stroke, including migraine as a risk factor for cerebral ischemia, migraine caused by cerebral ischemia, migraine as a cause of stroke, the presence of a common cause for Page 7 of 11 migraine and cerebral ischemia or migraine associated with subclinical vascular injury of the brain. Some studies of young adults seem to confirm this association [86,87]. A history of migraine with aura seems to be more common among victims of ischemic stroke than among controls and an acute attack of migraine may precede, accompany or follow a thromboembolic transient ischemic attack or a stroke, this seems to occur more often among migraineurs compared patients without migraine [88,89]. Adults suffering from migraine with aura are at increased risk of cardiovascular disease and stroke [90], but it is necessary to consider that in adults, the analysis of this association is complicated by a frequent presence of additional risk factors such as smoking, hypertension and diabetes mellitus. In children, these and other potential confounding factors are much less common. There are relationships arising from small clinical samples of pediatric age who demonstrate the association of migraine with dyslipidemia [91], hyperhomocysteinemia and genetic variants related to homocysteine which appear to be risk factors for the development of stroke in children [92]. Furthermore, in a national representative sample of children, the severe or recurrent headache was associated with higher levels of adiposity measured by the body mass index (BMI) [77]. In general, below the age of 55, migraine with aura is a risk factor for ischemic strokes. However, it’s important to point out that part of the latter, can be linked to the presence of a patent foramen ovale (PFO). The PFO is the result of incomplete fusion of the septum “primum” and “secundum”, which normally occurs shortly after birth, when the left atrial pressure exceeds that of the right atrium. Epidemiological studies have shown a clear comorbidity between migraine with aura and PFO. In fact, the available data suggest that PFO is more common in women with migraine with aura (present in about 50% of cases) and that migraine with aura is more common in patients with PFO [93-95]. The mechanism underlying the possible relationship between migraine and PFO is not yet very clear: is there a causal relationship with migraine attacks, or have common genetic factors? The pathophysiological mechanism is considered a passage of microemboli and vasoactive chemicals through the PFO, which would circumvent the filtering pulmonary triggering migraine symptoms. The widespread cortical depression, which is the mechanism behind the migraine aura, could be favored by the presence of a PFO. Among the various hypotheses, it seems interesting to Pierangeli et al., who claim that a particular genetic predisposition could lead to a co-development of atrial septal abnormalities and migraine [96]. In fact, if the aura has occurred due to a malfunction of the Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 cerebral perfusion, the symptoms should occur with a sudden onset and not gradual. It’s likely that the association of migraine and PFO is random, given the frequency of both disorders. Recently, Steenblik et al. [97] sought to examine the familial risk of isolated interatrial shunt, caused by either atrial septal defect or patent foramen ovale, and explore associated comorbidities of stroke, transient ischemic attack (TIA), and migraine using a population database. They found that there is a strong familial inheritance pattern for isolated interatrial shunt, with significantly higher risk of interatrial shunt among affected patients’ siblings, first-, and second-degree relatives. Relatives of affected individuals also had a higher risk of TIA, a trend toward an increased risk for stroke, but no increased risk of migraine headache. The relevance of genetic factors with respect to the preparation and transmission of PFO and migraine with aura is still under discussion [98-100]. Page 8 of 11 A recent study of Ghosh et al. (2012) [104] analyzed the frequency of occurrence of headaches in children and adolescents with TS to address their possible inclusion as a comorbidity. Using a prospective questionnaire, administered directly, the author interviewed a total sample size of 109 patients with TS ≤21 years of age. The questionnaires were then analyzed according to the International Headache Society’s diagnostic criteria. The author found that headaches were present in 55% of patients, with two most common headache types being migraine headaches and tension-type headaches. The rate of migraine headache within the TS group was found to be 4 times greater than that of general pediatric population, as reported in literature. In addition, the rate of tensiontype headache was found to be more than 5 times greater than that of general pediatric population. Overall, the high rates of migraine and tension-type headache within this population support the proposition that headaches are a comorbidity of TS. Headache and tourette syndrome Tourette syndrome (TS) is recognized as one of the most common childhood movement disorders, characterized by motor and phonic tics often associated with neurobehavioral comorbidities, such as obsessivecompulsive disorder. Neurotransmitter dysregulation, particularly involving the serotonin system, has been implicated in the pathogenesis of TS, obsessive-compulsive disorder, and migraine headache. The frequency of migraine headache in a clinic sample of TS subjects was nearly 4-fold more than the frequency of migraines reported in general population. In particular, of 100 patients with TS, 25 (25.0%) satisfied the diagnostic criteria for migraine headache, significantly greater than the estimated 10% to 13% in general adult population and the estimated 2% to 10% in general pediatric population [101]. The first study that has examined the comorbidity between Tourette syndrome and headache was conducted by Barabas et al. (1984) [102]. The authors studied the incidence of migraine among children with Tourette’s Syndrome (TS). Among 60 children with TS (mean age of 11.9 yrs), migraine was prevalent in 26.6%. This figure is substantially greater than that reported for general population of school-aged children (4.0–7.4%) or for 2 control groups consisting of 72 children with seizure disorders and 62 children with learning disabilities. The prevalence rates for these two control groups were 11.3% and 8.0%, respectively. Subsequently, in 1986, Lacey D.J. [103] have shown a correlation between Tourette’s syndrome and several other disorders in children, including: thought and behavioral disorders, sleep disturbances, headaches, and school difficulties-including attention deficit disorder. Headache and ADHD Primary headache syndromes (eg, migraine and tensiontype headache [TTH]) and attention-deficit/hyperactivity disorder (ADHD) are prevalent in childhood and may cause impairment in social and academic functioning. In particular, Migraine and ADHD are highly prevalent, affecting between 5 to 10% of the pediatric population [4,105]. Coincidentally, the burden caused by both neuropsychiatric disorders reaches a common range of negative outcomes impairing quality of life [106,107], school achievement [108,109], social [7,110], and family functioning [111,112]. Thus, studying the association of both conditions is of utmost clinical importance. According to a systematic review of clinical studies on psychological functioning and psychiatric comorbidity of migraine in children, there is no evidence that ADHD is more frequently diagnosed in this group compared with no headache controls [7]. In a cross-sectional epidemiological study specifically designed to examine this association, we have found that migraine are not comorbid to ADHD overall, but are comorbid to hyperactive-impulsive behavior [88]. In this study ADHD was assessed according to the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) criteria by the validated Brazilian version of the Multimodal Treatment Study of Children with ADHD – Swanson, Nolan, and Pelham IV (MTA-SNAP-IV) scale [113] fulfilled by parents and teacher. Mental health status was assessed with the validated Brazilian version of the Child Behavior Checklist (CBCL) [114]. The prevalence of ADHD was not significantly different comparing children with migraine to controls (no headache). For inattention symptoms, Bellini et al. The Journal of Headache and Pain 2013, 14:79 http://www.thejournalofheadacheandpain.com/content/14/1/79 no significant differences were found. The prevalence of hyperactivity-impulsivity symptoms was 8.1% in children without headache, 23.7% in children with migraine (relative risk [RR] = 2.6; 95% confidence interval [CI] = 1.6– 4.2), and 18.4% in children with probable migraine (RR = 2.1; 95% CI = 1.4–3.2). According to the multivariate analyses, ADHD or inattention symptoms were not predicted by headache subtypes or headache frequency. On the other hand, hyperactivity-impulsivity symptoms were significantly associated with any headache (p < 0.01), tension-type headache (TTH) (p < 0.01), or migraine (p < 0.001) [88]. An association between childhood migraine and inattention symptoms have been reported by some populational [115,116] and clinical [6] studies. However, the findings must be understood in the context of some methodological limitations. The behavior rating scales adopted by these studies add symptoms of inattention and hyperactivity/impulsivity in the same domain preventing the distinction between them. Among the 11 questions comprising the attention domain in the CBCL, only three capture symptoms of inattention (“Can’t concentrate, can’t pay attention for long”, “Daydreams or gets lost in his/her thoughts”, and “Stares blankly“). The remaining questions focus on hyperactivity, impulsivity, executive dysfunctions and lack of coordination (“Acts too young for his/her age”, “Can´t sit still, restless, or hyperactive”, “Confused or seems to be in a fog”, “Impulsive or acts without thinking”, “Nervous, high strung, or tense”, “Nervous movements or twitching”, “Poor school work”, and “Poorly coordinated or clumsy”) [117]. Likewise, of the five questions that encompass the hyperactivity scale of the Strengths and Difficulties Questionnaire (SDQ), two are destined to identify inattention symptoms and three to hyperactivity/impulsivity [118]. Adopting the MTA-SNAPIV scale we could separate both dimensions of ADHD symptoms in our study [88]. In accordance to our findings, neuropsychological studies with clinical samples have found no attentional impairment in children with migraine compared to controls, in spite of a rather impulsive response profile [119-121]. Given the possible comorbidity between migraine and hyperactivity-impulsivity symptoms, providers and educators should be aware of the association. Conclusions Primary Headaches In Childhood and Adolescence are often associated with,and deeply influnced by,many comorbid situations. In this review are analyzed the most relevant of them. It is foundamental to take care of any kind of comorbidity to establish the most effective treatment strategy. Competing interest The authors declare that they have no competing interest. Page 9 of 11 Authors’ contributions All authors contributed to the writing of each paragraph of the manuscript. All authors read and approved the final manuscript. Author details 1 Department of Pediatrics and Child Neuropsychiatry, Sapienza University of Rome, Via Dei Sabelli 108, Rome, Italy. 2Riberao Preto, Sao Paulo, Brasil. 3 University “Campus Biomedico”, Rome, Italy. 4II University of Naples, Naples, Italy. 5Padua University, Padua, Italy. 6San Gerardo Hospital University of Milano-Bicocca, Monza, Italy. 7Mondino Institute, Pavia University, Pavia, Italy. 8 Insubria University, Varese, Italy. 9L’Aquila University, L’Aquila, Italy. 10 Department of Dynamic and Clinical Psychology, Sapienza University of Rome, Rome, Italy. Received: 5 August 2013 Accepted: 17 September 2013 Published: 24 September 2013 References 1. Perquin CW, Hazebroek-Kampschreur AA, Hunfeld JA, Bohnen AM, van Suijlekom-Smith LW, Passchier J, van der Wouden JC (2000) Pain in children and adolescents: a common experience. Pain 87:51–58 2. Guidetti V (2005) Fondamenti di neuropsichiatria dell’infanzia e dell’adolescenza. Il Mulino, Bologna 3. 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Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 REVIEW ARTICLE Open Access Habituation and sensitization in primary headaches Gianluca Coppola1*, Cherubino Di Lorenzo2, Jean Schoenen3 and Francesco Pierelli4 Abstract The phenomena of habituation and sensitization are considered most useful for studying the neuronal substrates of information processing in the CNS. Both were studied in primary headaches, that are functional disorders of the brain characterized by an abnormal responsivity to any kind of incoming innocuous or painful stimuli and it’s cycling pattern over time (interictal, pre-ictal, ictal). The present review summarizes available data on stimulus responsivity in primary headaches obtained with clinical neurophysiology. In migraine, the majority of electrophysiological studies between attacks have shown that, for a number of different sensory modalities, the brain is characterised by a lack of habituation of evoked responses to repeated stimuli. This abnormal processing of the incoming information reaches its maximum a few days before the beginning of an attack, and normalizes during the attack, at a time when sensitization may also manifest itself. An abnormal rhythmic activity between thalamus and cortex, namely thalamocortical dysrhythmia, may be the pathophysiological mechanism subtending abnormal information processing in migraine. In tension-type headache (TTH), only few signs of deficient habituation were observed only in subgroups of patients. By contrast, using grand-average responses indirect evidence for sensitization has been found in chronic TTH with increased nociceptive specific reflexes and evoked potentials. Generalized increased sensitivity to pain (lower thresholds and increased pain rating) and a dysfunction in supraspinal descending pain control systems may contribute to the development and/or maintenance of central sensitization in chronic TTH. Cluster headache patients are chrarcterized during the bout and on the headache side by a pronounced lack of habituation of the brainstem blink reflex and a general sensitization of pain processing. A better insight into the nature of these ictal/interictal electrophysiological dysfunctions in primary headaches paves the way for novel therapeutic targets and may allow a better understanding of the mode of action of available therapies. Keywords: Migraine; Tension-type headache; Cluster headache; Trigeminal autonomic cephalalgias; Sensitization; Habituation; Evoked potentials; Reflex; Pain mechanisms; Thalamocortical dysrhythmia Review Introduction Among the general population, headaches are highly prevalent and receive growing attention not only because they affect people’s quality of life, but also because they have a significant economic impact. Idiopathic or primary headache syndromes are disorders in which there is a temporary or permanent dysfunction of the central nervous system, often genetically determined, without apparent organic lesion. They include migraine, tension * Correspondence: [email protected] 1 Department of Neurophysiology of Vision and Neurophthalmology, G.B. Bietti Foundation IRCCS, Via Livenza 3, 00198, Rome, Italy Full list of author information is available at the end of the article headache, and the trigeminal autonomic cephalalgias among which "cluster headache". Progress in headache research has benefited from the International Classification of Headache Disorders (ICHD), and its revisions [1,2], because they have provided operational diagnostic criteria allowing for a better comparison of clinical data between headache centres. In the recent publication of the Global Burden of Disease survey 2010, tension-type headache and migraine are the second and third most prevalent disorders in the world, and migraine is recognized as the seventh highest cause of disability in the world [3]. The large and still growing scientific knowledge on their pathophysiological mechanisms has contributed the recognition © 2013 Coppola et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 of headaches as neurological conditions worth of interest. In fact, although scientists have not completely disentangled the complicated puzzle of primary headache pathophysiology, great advances were made during the last 2 decades with the help of the new research armamentarium. Clinical neurophysiology methods, in particular, have allowed in vivo measurements of the headache patients’ electrocortical responses to various sensory stimuli. They are atraumatic, non-invasive and complementary to modern neuroimaging techniques, and thus suitable to study functional disorders as primary headaches. In their episodic forms, primary headaches are defined as paroxysms (the attacks) separated by remissions of variable lengths. Several studies focused therefore for ictal versus interictal electrophysiological abnormalities, in order to understand the predisposition and the recurrent character of attacks. In this respect, among the various primary headaches, migraine is doubtless the beststudied headache type. Migraineurs are characterized interictally by a generally increased sensitivity to visual (sensitivity to light), auditory (to sound), or somatic stimuli (allodynia) not only during the attack, but also outside of the attack. Researchers, trying to explain this phenomenon, observed that interictally migraineurs with and without aura show a time dependent amplitude increase of scalp-evoked potentials to repeated stereotyped stimuli with respect to normal subjects. This phenomenon was called “deficient habituation” and was only seen during the painfree period for almost all sensory modalities. However, this is not a static phenomenon, but, as shown in several studies, habituation changes with the proximity to an attack, during the attack and when episodic migraine evolves to chronic migraine, a complication of migraine where sensitization, the opposite of habituation, makes its appearance, changing fundamentally the response pattern. The present article, after providing an overview of the general concept of habituation and its opposite “sensitization”, will review the studies on habituation/ sensitization in migraine and other primary headaches, performed with different clinical neurophysiology methods and emphasize in particular the more recent data. General concept Habituation is defined as “a response decrement as a result of repeated stimulation” [4] and is a common feature of responses to any kind of sensory stimulation. It is an ubiquitous phenomenon observed in different experimental settings and in neuronal circuits of a wide range of complexity, from the withdrawal reflex of the gill and siphon in Aplysia to the autonomic and behavioral Page 2 of 13 component of the whole-of-body reflex called the “orienting response” in humans [5-7]. Habituation is a multifactorial event of which the accompanying synaptic plastic mechanisms are still not totally elucidated. Several theories, or at least hypotheses, have been proposed over the years to explain this phenomenon [8]. In the 70's, Groves and Thompson proposed the “dual-process” theory, stating that two separate and opposing processes, depression (habituation) and facilitation (sensitization), compete to determine the final behavioural outcome after a sequence of repetitive stimuli [6]. According to the dual-process theory, sensitization is the side of the pendulum that, when present, prevails at the beginning of the stimulus session and accounts for the initial transitory increase in response amplitude, whereas habituation occurs later during the course of the recording session and accounts for the delayed response decrement [6]. At the synaptic level, the stimulus–response pathway interacts with an external “state” system represented by various “tonic” non-specific and motivational circuits, including the ascending reticular activating system and related structures. In humans, these structures comprise the monoaminergic nuclei in the brainstem, that are critically involved in the central processing of arousal, control of the signal-to-noise ratio generated by sensory stimuli at cortical and thalamic levels, and endogenous antinociception [9]. To avoid semantic misunderstanding, it should be noted that a response dishabituation does not refer to lack of habituation, but to a response sensitization or “heterosynaptic facilitation” as termed by Kandel and associates in their works on Aplysia [10]. In fact, dishabituation is an actual recovery of the habituated response caused by the interference of an unexpected stimulus markedly different from the habituating ones. Sensitization is an elementary form of behavioral plasticity, perhaps equal in importance to habituation and apparently generated by somewhat different neuronal mechanisms [11]. Habituation depends on a series of parametric properties or characteristics [5], which were revised and refined during a workshop in Vancouver in 2007 [12]. These characteristics dealt with both short- as well as longterm habituation and its opposite dishabituation. The phenomenon of habituation is considered useful for studying the neuronal substrates of behavior, the mechanisms of learning processes, or information processing in the CNS both in health and in disease. It must be noted that, during the last three decades, sensitization has assumed a wider meaning than habituation. Sensitization is not only considered a general behavioral response of augmentation to innocuous sensory stimuli, but it has also acquired a particular significance Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 in the field of pain as an augmentation of sensory signalling in the central nervous system as a consequence of noxious stimulation (for a review see Woolf, 2011 [13]). The latter may be a peripheral injury activating smallfiber afferents that produce an increase in excitability, i.e. sensitize, nociceptive spinal cord neurons. This increase in excitability is responsible for the plastic changes in those neural structures belonging to the socalled “pain matrix”. It results in decreased nociceptive thresholds and increased responsiveness to noxious and innocuous peripheral stimuli, as well as expansion of the receptive fields of central nociceptors [14]. Overall, this transient or persistent state of higher reactivity is called “central sensitization”. Studies in the headaches field disclosed changes in pain sensitivity that were interpreted as reflecting central sensitization. A wellrecognized clinical expression of central sensitization is cutaneous allodynia, which is prevalent during episodic migraine attacks [15,16] and in chronic migraine [17-19]. There is no common agreement yet about what causes and where starts the cascade of events that lead to central sensitization in migraine or in other primary headaches. However, some evidences point towards sequential sensitization of first-order or second-order trigeminovascular nociceptors via overt (aura) or possibly silent (without aura) cortical spreading depression waves or, more likely, via an indirect activation of pain modulatory structures in the brainstem (raphe magnus, locus coeruleus and other aminergic nuclei) and the forebrain (periaqueductal gray, rostroventral medulla) [20,21]. In human research, time-locked cortical potentials (EP) evoked by a sensory stimulation and reflex responses (RR) after electric stimulation of a peripheral nerve have been frequently used to study the habituation and sensitization phenomena. A common way of analysing these responses consists of either analysing single trials or averaging in blocks single epochs of EP or RR per subject, identifying the latencies and amplitudes or areaunder-the-curves of the considered components, and using these parameters as a dependent variables in the statistical analysis. Data are usually expressed either in absolute or log-transformed values, and between single trials or blocks habituation is calculated either with linear regression (slope of the linear regression line) or with block ratios (change in amplitude expressed in percentage). Habituation in migraine Neurophysiological data suggest that lack of habituation during stimulus repetition despite an initial normal or slightly lower response amplitude is a functional, probably genetically determined, property of the brain in migraineurs between attacks. Page 3 of 13 The very first study showing that habituation is decreased in patients affected by migraine without aura between attacks was carried out with contingent negative variation (CNV), a slow negative cortical response related to higher mental functions [22-26]. The CNV abnormality was more evident for the early than for the late component [24,27-33], and was assessed by visual [34,35] or auditory [36-38] oddball paradigms. The lack of habituation was confirmed several times by analyzing visual evoked potentials (VEP) in response to checkerboard pattern [39-49], and also by using magneto electroencephalography [50-52]. The abnormal visual information processing in migraine seems to be characterized by an initial response of normal or slightly lower amplitude followed by an amplitude increase, that, as stated above, corresponds neither to response sensitization nor to dishabituation, but to a lack of habituation [40-50]. The same phenomenon was also found with somatosensory [53-55] and auditory [56,57] evoked cortical potentials in migraine between attacks. Sand et al. (2008) recorded brainstem auditory evoked potentials (BAEPs) in migraine interictally and found lack of habituation in the wave IV-V [58], a datum that was recently confirmed also in a few migraineurs experiencing vertigo, especially when symptomatic [59]. Moreover, Sand et al. also observed a positive relationship between BAEP amplitudes and blood serotonin level in healthy controls, but not in migraine, a result that was interpreted as an evidence in support of the notion that a dysregulation of the serotonin system is linked to migraine pathogenesis [58]. Besides the innocuous evoked potentials, researchers have verified if the same habituation deficit might exist after noxious stimulation. The blink reflex (BR) obtained after supraorbital stimulation with a so-called “nociception-specific” electrode, a way to more specifically explore trigeminal nucleus caudalis activation, discloses an interictal habituation deficit during short [60,61] as well as long time courses [44,62]. Amplitude and habituation of another brainstem reflex, the clickevoked vestibulocollic reflex, was equally reduced in migraineurs between attacks compared with healthy subjects [63,64]. Brief radiant heat pulses, produced by CO2 laser stimulation, activate selectively Aδ and C fibres and generate an evoked potential that can be recorded from the temples (early component, N1) and the vertex (late components, N2-P2) of the skull. In episodic migraine the late component of laser evoked potentials (LEPs), mainly generated in the insular and anterior cingulate cortices, elicited by either cephalic (usually supraorbital) or extracephalic (usually hand dorsum) stimulation does not habituate during stimulus repetitions over short [65] or long durations [66-68]. Deficient habituation was also observed for the early N1 LEP component, mainly Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 generated in the secondary somatosensory cortex [66,69]. In two studies performed in episodic migraine between attacks using contact-heat evoked potentials (CHEPS) and source localization with standardized LORETA (sLORETA) mapping it was shown that the lack of habituation to the noxious heat stimuli is probably related to the inability of the orbitofrontal cortex (OFC) to filter out correctly the pain information [70,71]. Since evidence from animal and human studies suggests a role for serotonin in the OFC-mediated descending inhibition of pain [72], the authors argue that a possible alteration of brain 5-HT neurotransmission may be responsible for this abnormal pain information processing in migraine, where decreased serotonergic disposition was previously reported [73-75], Genetic load seems to play an important role in the mechanisms that produce physiologically altered habituation. In fact, the habituation deficit in migraineurs has a family character. This abnormality is present interictally not only in adults, but also in children in whom it is significantly correlated with that of their parents [28,76]. Moreover, asymptomatic subjects defined to be "at risk" for developing migraine, i.e. healthy but with first-degree relatives with migraine, present the same habituation deficit in evoked potentials and nociceptive blink reflex as established migraineurs, so that deficient habituation can be regarded as neurophysiological marker of pre-symptomatic migraine [29,62]. Since migraine is a recurrent paroxysmal disease characterized by attacks (ictal period) and variable pain-free periods (interictal), it was of interest to perform sequential recordings during the days preceding the attacks, immediately before or during the attack. During the days preceding the attack (pre-ictally) VEP and SSEP amplitudes increase and habituate normally [50,54,58,77,78], whereas CNV and P300 habituation is minimal and amplitude peaks [30,79,80], suggesting that, depending on sensory modality, the habituation deficit worsens interictally, reaches its maximum a few days before the attack and then normalizes during the attack. Finally, it must be mentioned that among more than fifty positive studies, the habituation deficit in migraine during the pain-free phase was not confirmed in some studies [77,81-87]. It is not easy to explain why some research groups did not retrieve any habituation deficit in migrainous patients between attacks. For some Authors lack of blindness for diagnosis during the recording sessions may be an explanation [87], but the same research group did not find lack of habituation even with no blindness [77]. Another one could be the use of different patients’ selection criteria, such as recruitment of university students or medical staff instead of patients who spontaneously visited a Headache Clinic, the latter experiencing more day life discomfort from their migraine. Page 4 of 13 Whatever the explanation, we must take into account that habituation deficit is not constant in migrainous patients. In fact, habituation degree may change not only interictally vs. pre-ictally vs. ictally, but also within the pain-free period with the distance since the last or next attack [88]. Moreover, specific genetics influence [76,89] and clinical fluctuations, such as spontaneous clinical worsening or improving of attacks frequency [90,91], may vary the baseline level of thalamocortical activation [92] and then the degree of habituation in migraine [55]. Mechanisms of the habituation deficit in migraine As mentioned above, the neural mechanisms underlying habituation remain poorly understood, and this uncertainty helps to explain why the abnormal habituation pattern in migraine still lacks a definitive consensual interpretation [93-95]. Neuromodulatory techniques, like repetitive transcranial magnetic stimulations (rTMS) and transcranial direct current stimulations (tDCS), were used to shed more light on the interictal abnormal information processing in migraine. In migraineurs activating high frequency rTMS over the visual or somatosensory cortices was able to increase for some minutes the amplitude of the first VEP and somatosensory evoked potentials (SSEP) block and to normalize habituation over successive blocks, whereas inhibiting low frequency rTMS had negligible effects on both. By contrast, in healthy volunteers, inhibiting rTMS reduced first VEP and SSEP amplitude block as well as habituation, while the activating protocol had no effect [42,55]. A longer lasting effect (several weeks) on VEP was induced with 5 consecutive daily sessions of inhibiting rTMS over the visual cortex in healthy subjects, while the effect of activating rTMS in migraineurs lasted only hours or a few days [45]. Recently, Viganò et al. (2013) applied another activating neuromodulation method, anodal tDCS over the visual area in migraineurs, and reported that, similar to 10Hz rTMS, the the 1st VEP block increased in amplitude and habituation normalized [96]. In the second phase of their study, the same authors performed a preventive pilot trial with 2 sessions of anodal tDCS over the visual cortex per week for 8 weeks in 15 migraine patients and found a clear beneficial effect on several clinical endpoints up to an average of 4.8 weeks after the tDCS treatment period [96]. Overall, the neuromodulatory studies indicate that only procedures that enhance cortical excitability are able to normalize the abnormal interictal information processing in migraine. Further information on the pathophysiology of the interictal dysfunction in migraine was obtained from the more sophisticated studies of the high-frequency oscillations (HFOs) embedded in common somatosensory and visual evoked potentials. Early somatosensory HFOs, Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 reflecting spike activity in thalamo-cortical cholinergic drives, were decreased interictally in migraine and normalized during the attack while late HFOs, reflecting primary cortical activation, were normal [97] or decreased [98]. Moreover, the reduction early HFOs was associated with a worsening in the clinical course of migraine [90]. In a recent study in migraineurs activating rTMS over the sensorimotor cortex was able to increase the interictal low thalamo-cortical drive. This was not the case in healthy volunteers probably because their thalamo-cortical activity was already maximal before the rTMS [97]. This finding supports the hypothesis that deficient habituation in migraine is due to a reduced thalamic control of the activity in sensory cortices i.e. a low pre-activation level. Further evidence for an abnormal thalamic control in the migraine brain intericatlly comes from the analysis of visual HFOs (gamma-band oscillations, GBO) [46]. We demonstrated a significant habituation deficit of the late GBO components in migraineurs relative to healthy subjects, which we interpreted as indicative of a dysfunction in cortical oscillatory networks that could in turn be due to an abnormal thalamic pacemaker rhythmic activity, namely “thalamo-cortical dysrhythmia” [46]. The latter may reconcile the long-lasting controversy between excessive excitation and deficient inhibition in migraine, since a deficient thalamo-cortical drive, i.e. a low level of cortical preactivation, results in dysfunction of both inhibition and excitation. Lower inhibition and preactivation may thus co-exist, since the latter can promote the former via reduction of lateral inhibition [93]. Refined VEP techniques have shown that it is possible to enhance the relative contributions that arise from short- and long- range lateral inhibition between neurons through differential temporal modulation of adjacent regions of radial windmill-dartboard (W-D) or partial-windmill (P-W) visual patterns [99-101]. This may represent a further tool for investigating migraine pathophysiology. According to our recent study, the degree of short-range lateral inhibition in the visual cortex during W-D visual stimulation is more pronounced in migraine patients than in healthy volunteers at the beginning of the stimulus session (1st block). Over successive blocks of recordings, however, it decreases in migraineurs, but remains unchanged in healthy controls. During the migraine attack, short-range lateral inhibition is on the contrary much reduced, but it increases during stimulus repetition. There was no significant between group difference in the P-W amplitude, reflecting longrange lateral inhibition, and its attenuation [88]. These results favour a migraine cycle-dependent imbalance between excitation and inhibition in the visual cortex that results in a heightened cortical response to repeated stimuli, i.e. a lack of habituation. We hypothesized that an interictal hypoactivity of monaminergic pathways Page 5 of 13 may cause a functional disconnection of the thalamus in migraine leading to an abnormal intracortical shortrange lateral inhibition, which could contribute to the habituation deficit observed during stimulus repetition. That in migraine the thalamus abnormally controls the cortex via thalamorcortical loops is further underscored by the recent study of the paired associative stimulation (PAS) paradigm, a protocol that uses in humans a design principle very similar to those producing long-term depression (LTD) or potentiation (LTP) in animal studies [102-104]. In migraine, depressing PAS paradoxically increased motor evoked potential amplitudes instead of decreasing them, and enhancing PAS induced only a slight non significant response potentiation [105]. This suggests that impaired long-term associative synaptic plasticity mechanisms characterize migraine without aura patients between attacks. Because we observed, at least in a subgroup of subjects, that the PAS-induced plastic changes were inversely related with thalamocortical activation, as assessed by early somatosensory HFOs, we suggested that the malfunction in PAS-induced effects in migraine might reflect low cortical preactivation, which prevents short-term and longer-term changes in cortical synaptic effectiveness [105]. Sensitization in migraine The majority of the studies on the dynamic behavior of peripheral reflexes and evoked cortical responses in migraine have focused on habituation instead of sensitization, probably because it is particularly difficult to assess sensitization by calculating “sliding” averages using successive blocks of few responses. However, if sensitization, defined as facilitation occurring at the beginning of the stimulus presentation, is impaired in migraine (i.e. enhanced in comparison to habituation), it should be detectable as a higher first block absolute amplitude value of the considered response. Indirect signs of sensitization were observed during migraine attacks and in chronic migraine patients with or without medication overuse. It was reported several times that within the 12–24 hours preceding the attack the habituation pattern of reflex and non-noxious evoked potentials normalize. This has been shown with CNV [26,79,80], VEP [50,78,88], visual P300 latency [35], and nociceptive blink reflexes [60]. Pain-related responses may behave in a partially different way. During migraine attacks, the area-under-thecurve of the nociceptive blink reflex R2 component is temporary increased on the affected side in comparison with the non-affected side was observed [106]. Similar results were obtained using another noxious stimulation, the radiant laser CO2: amplitude of the N2–P2 complex at the vertex was increased on the Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 affected side compared to the non-affected side [107-109]. Interestingly, in episodic migraine LEPs did not habituate not only interictally, but also during the attacks, underscoring the different cerebral processing of noxious versus innocuous stimuli. These data could represent the electrophysiological counterpart of central sensitization of cerebral structures belonging to the so-called “pain matrix” that seems to be related to the mechanism of the migraine attack and of the chronification of migraine. As abovementioned, from a physiological point of view, sensitization refers to the plastic changes in neural structures belonging to the “pain matrix” that result in decreased nociceptive thresholds and increased responsiveness to noxious and innocuous peripheral stimuli, and expansion of the receptive fields of CNS nociceptive neurons [14]. As a matter of fact, reduced pain thresholds have been found clinically with quantitative sensory testing immediately before [110] and during [16] a migraine attack, but sometimes even earlier in the interictal period in some [111,112] but not all the studies [110,113-115]. During attacks of migraine, reduced cutaneous pain thresholds on both symptomatic and non-symptomatic sides are accompanied by significantly increased N2-P2 complex of LEP [107] and changes in its dipolar source localization [108]. These abnormalities worsen with the increase in attack frequency [107,108]. In a group of chronic migraine (CM) patients, a trend for an increase in LEP N2P2 amplitudes [116] and a reorganization of the cortical areas devoted to pain processing [117], was also detected. Moreover, in CM due to medication overuse LEP N2-P2 amplitude still showed reduced habituation after both hand and face stimulation, similarly to the response behavior found with LEPs during interictal and ictal periods [118]. Interestingly, withdrawal from the acute medication overuse normalized the habituation curve [118]. Recently, somatosensory evoked potentials (SSEPs) proved to be ideal for disclosing sensitization (reflected by an increased response amplitude to low numbers of stimuli) and habituation (reflected by a decrease in response amplitude after high numbers of stimuli) in both episodic and chronic forms of migraine. While SSEPs confirmed a lower initial amplitude and late abnormal habituation in migraineurs studied interictally [53,54], a clear-cut sensitization, as reflected by a significant increase in SSEP 1st N20-P25 block amplitude, was found during an attack, followed by a normal habituation [54]. In medication overuse headache (MOH) patients, we managed to record SSEPs in a pain-free state or during mild headaches and found that N20-P25 SSEP amplitude was initially (1st block) greater in MOH patients than in the subgroup of episodic migraineurs studied interictally and healthy controls [54]. The increased SSEP amplitude Page 6 of 13 in MOH was proportional to the duration of headache chronification. We interpreted these results as reflecting reinforcement and perpetuation of central sensitization due to the medication overuse and increased headache frequency [54]. We further noted that the presence of such sensory sensitization depends on the class of drugs overused, since initial SSEP amplitudes were smaller in triptan overusers than in NSAIDs or combined overusers. The abnormalities in cortical responses to somatosensory stimulation seem to be strongly influenced by genetic factors. MOH patients carrying the D/D polymorphic variant of the angiotensin converting enzyme (ACE), that plays a role in neural plasticity and dependence behaviour, showed less SSEP habituation, in proportion with the duration of the overuse headache, and increased sensitization depending on the overused drug compared to I carriers [89]. Sensitization in migraine patients who evolved to chronic daily headache due to medication overuse was also demonstrated with pain-related evoked potentials (PREPs). Ayzenberg and co-workers recorded PREPs after electrical stimulations of cephalic (forehead) and extracephalic (hand dorsum) sites with a nociceptionspecific electrode. They observed a significant increase in PREP amplitudes both after cephalic and extracephalic stimulations in all patients with MOH irrespective if they overused NDAIDs or triptans. Withdrawal from the acute medication overuse normalized the PREP amplitude [119]. Sensitization phenomena might also manifest themselves at the spinal level. Perrotta and coworkers explored the spinal cord pain processing by studying threshold, area and temporal summation threshold (TST) of the lower limb nociceptive withdrawal reflex in a group of 31 MOH patients before and after acute drug withdrawal. A significantly lower reflex threshold, higher amplitude and lower TST was found in MOH patients before detoxification in comparison with episodic migraine and controls [120]. All these neurophysiological abnormalities tended to improve after a detoxification program [120], which was coupled with an increased activity of the endocannabinoid system [121]. Habituation in tension-type headache There are only a few reports about habituation in tension-type headache (TTH). No habituation deficit was observed with visual evoked or event-related potentials in episodic [41] or chronic TTH patients [34,41]. Episodic TTH sufferers had normal habituation of P300 latency, while P300 amplitude also showed some degree of habituation, although not of statistical significance [122]. Patients affected by chronic TTH showed a normal reducing behavior (habituation) in scalp potentials evoked by CO2 laser stimulation (LEPs) of the hand and facial Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 skin [66]. Mismatch negativity, which is believed to reflect the automatic central processing of a novel stimulus, and P300 habituation were significantly lower in migraine and TTH children than in healthy subjects in one study, where P300 habituation also positively correlated with behavioral symptomatology [123]. In TTH, habituation was also investigated using sympathetic skin responses (SSR), a tool used to evaluate autonomic dysfunction. Ozkul and Ay explored SSR changes with the same stimulus at a constant intensity by four blocks of 20 responses and found that in both episodic migraine without aura and TTH patients there was a lack of habituation compared to normal controls [124]. The electrophysiological similarities between the episodic forms of migraine and TTH support the hypothesis that some patients with TTH might be at the mild end of the migraine spectrum. Since lack of habituation was not always observed in TTH, it seems to be relevant only for a subgroup of patients. Sensitization in tension-type headache In TTH and in cluster headache, like in migraine, the vast majority of neurophysiological studies have only indirectly assessed sensitization, through the measure of the grand-average area under the curve or amplitude of the given test. The few studies in which the dynamic behavior of responses was analyzed using successive blocks of a few averagings were unable to find clear evidence for sensitization, i.e. for increased amplitude of the first block. This was the case in episodic TTH for VEPs [41], visual P300 [122], LEPs [123] and sympathetic skin responses [124], in chronic TTH for visual P300 [34], and LEPs [66]. By contrast, some indirect evidence for sensitization was found in TTH, chiefly in its chronic form, with nociceptive specific reflexes and evoked potentials. Normal amplitude, area, latency [125-128] and slower recovery cycle [127] of the blink reflex R2 component was found in chronic TTH. Two separate groups found reduced latencies of the trigemonocervical reflex in patients with chronic TTH [129-131]. Using a nociceptionspecific electrode lower values of the normalized root mean square and area under the curve of the blink with control subjects [132]. More convincing evidence for central sensitization in CTTH has come from studies of pain sensitivity in pericranial or lower limb tissues. Sandrini et al. (2006) studying the nociceptive lower limb flexion RIII reflex found significantly lower subjective pain thresholds and RIII reflex threshold in chronic TTH than in controls [115]. These findings were associated with a paradoxical facilitation of the RIII reflex response during the cold pressor test, which indicates deficient descending Page 7 of 13 inhibition, an abnormality also found by others [133]. Previous studies have found normal pressure pain thresholds (PPTs) in episodic TTH [134,135]. In chronic TTH instead, PPTs were decreased [15,136,137] especially on the anterior part of the temporalis muscle [15,135-139] and in the upper part of the trapezius muscle [140]. Cathcart et al. (2010) investigated temporal summation, defined as the increase in pain perception to repeated noxious stimulation (indirect measure of sensitization), by an algometer and heterotopic noxious conditioning stimulation (HNCS) in chronic TTH vs. controls. Pain from repeated algometer pressures increased more in the CTTH sufferers compared with healthy controls, both at finger and shoulder, and was less inhibited by conditioned HNCS [141]. Lower pain thresholds in muscle and skin of the cephalic region but not of the extracephalic region with higher rating to suprathreshold single and repetitive (2 Hz) electrical stimulation were reported in patients with chronic TTH than in healthy controls [142]. In a LEP study, the heat pain threshold was similar in chronic TTH patients and controls at the level of both the hand and pericranial skin. The total tenderness scores (TTS) at pericranial sites were higher in TTH patients than in controls. The amplitude of the N2a–P2 LEP complex elicited by stimulation of the pericranial zone was greater in TTH patients than in controls and this was significantly associated with the TTS score [143]. Habituation in cluster headache and other trigeminal autonomic cephalalgias (TACs) During the last decades, great advance in the understanding of the cluster headache pathophysiology was made with the modern techniques of functional neuroimaging [144]. Electrophysiological methods contributed to the study of cognitive and nociceptive processes. Normal cognitive habituation was found in two visual event-related potential studies in cluster headache either during the bout or outside, and in chronic paroxysmal hemicrania [34,145]. Formisano et al. were the first to found abnormal habituation of the blink reflex in a small number of CH patients during the attack, but comparison with control subjects was lacking [146]. Habituation of both the R2 and the R3 blink reflex components are impaired in CH patients on the affected side compared to healthy controls [147]. The lack of habituation in CH patients was even more pronounced than that found in episodic migraine [147]. We recently replicated these results by using the nociception-specific concentric stimulating electrode: R2 reflex area and habituation were reduced on the affected CH side (data published in abstract form [148]). Conversely, Holle et al. (2012) failed to detect Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 altered habituation of the nBR R2 in episodic and chronic CH within or outside a bout. In the latter study, however, the majority of CH patients were taking one or several prophylactic medications at the time of recordings, which may biased the results [149]. Sensitization in cluster headache and other trigeminal autonomic cephalalgias (TACs) Classical blink reflex studies did not disclose any sign of sensitization in CH [150,151]. In 10 episodic cluster headache patients within a bout, Lozza et al. (1997) found a significantly faster R2 blink reflex recovery curve on the symptomatic side after paired supraorbital stimuli, probably reflecting an indirect sign of sensitization within the spinal trigeminal nucleus. Furthermore, in the same study the R2 recovery curve was faster on both affected and unaffected sides in CH patients when the supraorbital stimulus was preconditioned by a peripheral stimulation of the index finger. Since naloxone injection transiently reverted this bilateral R2 sensitization, the authors postulated that the faster R2 recovery reflects hypoactivity of reticular nuclei, due to reduced descending opiatergic inhibition [152], a mechanism that was recently supported by functional neuroimaging studies [153,154]. The threshold of the corneal reflex was reduced on the affected side in a mixed group of episodic (during the bout) and chronic CH patients, and normalized in the remission phase [155]. Others were not able to confirm such lateralized abnormalities. Researchers found that both in- and out-side the bout patients had lower thresholds for pressure pain [156], electric pain and RIII reflex [157] on the affected than on the unaffected side both in episodic (in and outside of a bout) and chronic CH [158]. These signs of sensitization within the nociceptive system were coupled with a phase shift of the normal circadian rhythmic variations in RIII threshold in episodic bouts of CH when compared with the remission period, and with absence of circadian rhythmicity in chronic CH patients [158]. Perrotta et al. (2013) recently studied the functional activity of the descending diffuse noxious inhibitory controls (DNIC) (or conditioned pain modulation system) elicited by a cold pressor test (CPT) in a group of episodic CH patients during active and remission phases. Compared to healthy subjects, the RIII reflex threshold and TST were lower and the R2 area higher during, but not outside of a bout. CH patients during the bout had a significant reduction of TST compared both to controls and to CH patients outside of a bout. Only during the bout but not outside, the CPT had no effect on TST and reflex area [159]. The authors concluded that CH patients have a dysfunction of the supraspinal control of pain that depends on the clinical activity of the disease Page 8 of 13 and leads to facilitation of pain processing predisposing to the CH attacks. Further evidence for lateralized abnormalities came from the study of Procacci et al. (1989) who found cutaneous and deep hyperalgesia to both mechanical and electrical stimuli with earlier appearance of pain after an ischaemic test in the upper limbs on the affected side of the body in episodic CH patients [160]. By contrast, with quantitative sensory testing, perception of warmth, cold and pressure pain was reduced on the cluster side as compared with the contralateral asymptomatic side in a pooled group of episodic and chronic CH patients [161,162]; warm detection thresholds and thermal sensory limen on the affected side correlated negatively with elapsed time since last attack [162]. We are aware of only one study on sensitization in other trigeminal autonomic cephalalgias. In 12 patients with chronic paroxysmal hemicrania and 12 with hemicrania continua, pain pressure threshold, subjective pain perception after sural nerve stimulation as well as RIII reflex threshold were reduced mostly on the affected side, compared to healthy subjects [163]. Moreover, although there were no abnormalities in the blink reflex, corneal reflex thresholds were significantly reduced on both sides only in chronic paroxysmal hemicrania patients. Discussion Neurophysiological studies have disclosed various abnormalities of spinal, brainstem and cortical responsivity to external innocuous or noxious stimuli in primary headaches. These abnormalities can be summarized as follows: Abnormalities of the habituation/sensitization mechanisms were discovered in migraine. In episodic migraine, most published EP studies show two characteristic changes: a lack of habituation on recordings performed between attacks and sensitization during the attack, especially with somatosensory stimuli. The habituation deficit normalizes during attacks, whereas sensitizations vanishes between attacks, but in the immediate preictal phase both sensitization and deficient habituation may variably co-exist in response to non-noxious and pain stimuli. In patients who developed MOH the cortical response pattern could be locked in a pre-ictal state associating both initial sensitization and late deficient habituation, which contrasts with episodic migraine where these cortical states alternate (Figure 1). Recent works suggest that an abnormal rhythmic activity between thalamus and cortex, namely thalamocortical dysrhythmia, may be the pathophysiological mechanism subtending abnormal information processing in migraine. Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 Page 9 of 13 Figure 1 Schematic representation of the changes in habituation and sensitization in an healthy subject and over the migraine cycle (interictal, ictal, and chronic migraine due to medication overuse [MOH]). In tension-type headache, available studies are limited. Some, though only in subgroups of patients, found some evidence of deficient habituation, chiefly with cognitive potentials (mismatch negativity and P300) and sympathetic skin responses. By contrast, using grand-average responses indirect evidence for sensitization has been disclosed in chronic TTH with nociceptive specific reflexes and evoked potentials. These studies provide evidence for generalized increased sensitivity to pain (lower thresholds and increased pain ratings) and a dysfunction in supraspinal conditioned pain modulation in CTTH, which may contribute to the development and/or maintenance of central sensitization in this disorder. The deficient habituation of the blink reflex found in episodic CH patients during the bout suggests that interictal migraine and cluster headache probably share some pathophysiological mechanisms. However, the more pronounced habituation deficit found in the CH with respect to the migraine group suggests that additional dysfunctional neurobiological factors are at work in CH patients. Only during the bout but not outside, a sensitization of pain processing was observed. Several possible non-mutually exclusive causes could be responsible for this: i) dysfunctioning descending aminergic, especially dopaminergic, control [164,165], ii) malfunctioning hypothalamo-trigeminal control [166], and iii) altered descending opiatergic pain control system [153,154]. Given that CH patients had no habituation deficit of event-related potentials [34,145], it is likely that the pathogenic factors involved in CH produce functional changes at the level of the trigeminal system but not at cortical level. Altogether, these data indicate that in cluster headache lateralised abnormalities may occur throughout the body, probably because of sensitization in the central nervous system and activation of nociceptive reflexes. Conclusions In migraine research, progress will largely depend on a better understanding of the mechanisms underlying the habituation deficit, its variations with the migraine cycle and its relation with changes in thalamo-cortical rhythms and brain stem-(thalamo-)cortical aminergic pathways. Future studies will have to determine whether there is an interaction between abnormal sensory processing and metabolic abnormalities, for instance decreased brain ATP content. It will also be of importance to gather more data on the geno- phenotype correlations in migraine and in the other primary headaches. Such genotype/phenotype correlations could help to tailor treatment to the individual patient depending on his genetic profile [89]. In cluster headache, future electrophysiological works should try to understand the role of the descending monoamine and opioid systems in the mechanism of sensitization and lateralization of pain. Moreover, they may help to unravel the mechanisms that periodically ignite the cluster period and the culprits for the transformation of episodic into chronic CH. Finally, better characterizing headache patients from a neurophysiological point of view will allow to optimize the protocols of the minimally invasive techniques and non-invasive neurostimulation methods, and to improve their therapeutic efficacy [96,167,168]. Abbreviations BR: Blink reflex; CH: Cluster headache; CM: Chronic migraine; CNV: Contingent negative variation; CPT: Cold pressor test; CTTH: Chronic tension-type headache; EP: Evoked potential; GBOs: Gamma-band oscillations; HFOs: High-frequency oscillations; HNCS: Heterotopic noxious conditioning stimulation; LEP: Laser evoked potential; MOH: Medication overuse headache; OFC: Orbitofrontal cortex; PAS: Paired associative stimulation; PREPs: pain-related evoked potentials; PPTs: Pressure pain thresholds; P-W: Partial-windmill; RR: Reflex response; rTMS: repetitive Transcranial Magnetic Stimulations; SSEP: Somatosensory evoked potential; SSR: Sympathetic skin responses; tDCS: transcranial Direct Current Coppola et al. The Journal of Headache and Pain 2013, 14:65 http://www.thejournalofheadacheandpain.com/content/14/1/65 Stimulations; TST: Temporal summation threshold; TTH: Tension-type headache; TTS: Total tenderness score; W-D: Windmill-dartboard. Competing interests The authors declare that they have no competing interests. Authors’ contributions GC made substantial contributions to review the literature as well as in drafting the manuscript. CDL, JS, and FP were implied in drafting the manuscript. All authors read and approved the final manuscript. Author details 1 Department of Neurophysiology of Vision and Neurophthalmology, G.B. Bietti Foundation IRCCS, Via Livenza 3, 00198, Rome, Italy. 2Don Carlo Gnocchi Onlus Foundation, Milan, Italy. 3Headache Research Unit, University Department of Neurology & GIGA-Neurosciences, Liège University, Liège, Belgium. 4IRCCS Neuromed, Pozzilli (IS), Italy. Received: 7 June 2013 Accepted: 21 July 2013 Published: 30 July 2013 References 1. Headache Classification Subcommittee of the International Headache Society (2004) The International Classification of Headache Disorders: 2nd edition. Cephalalgia 1:9–160 2. Olesen J, Bousser MG, Diener HC, Dodick D, First M, Goadsby PJ, et al. 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Submit your manuscript to a journal and benefit from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the field 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 RESEARCH ARTICLE Open Access A six year retrospective review of occipital nerve stimulation practice - controversies and challenges of an emerging technique for treating refractory headache syndromes Stefano Palmisani1*, Adnan Al-Kaisy1, Roberto Arcioni2, Tom Smith1, Andrea Negro3, Giorgio Lambru1, Vijay Bandikatla1, Eleanor Carson1 and Paolo Martelletti3 Abstract Background: A retrospective review of patients treated with Occipital Nerve Stimulation (ONS) at two large tertiary referral centres has been audited in order to optimise future treatment pathways. Methods: Patient’s medical records were retrospectively reviewed, and each patient was contacted by a trained headache expert to confirm clinical diagnosis and system efficacy. Results were compared to reported outcomes in current literature on ONS for primary headaches. Results: Twenty-five patients underwent a trial of ONS between January 2007 and December 2012, and 23 patients went on to have permanent implantation of ONS. All 23 patients reached one-year follow/up, and 14 of them (61%) exceeded two years of follow-up. Seventeen of the 23 had refractory chronic migraine (rCM), and 3 refractory occipital neuralgia (ON). 11 of the 19 rCM patients had been referred with an incorrect headache diagnosis. Nine of the rCM patients (53%) reported 50% or more reduction in headache pain intensity and or frequency at long term follow-up (11–77 months). All 3 ON patients reported more than 50% reduction in pain intensity and/or frequency at 28–31 months. Ten (43%) subjects underwent surgical revision after an average of 11 ± 7 months from permanent implantation - in 90% of cases due to lead problems. Seven patients attended a specifically designed, multi-disciplinary, two-week pre-implant programme and showed improved scores across all measured psychological and functional parameters independent of response to subsequent ONS. Conclusions: Our retrospective review: 1) confirms the long-term ONS success rate in refractory chronic headaches, consistent with previously published studies; 2) suggests that some headaches types may respond better to ONS than others (ON vs CM); 3) calls into question the role of trial stimulation in ONS; 4) confirms the high rate of complications related to the equipment not originally designed for ONS; 5) emphasises the need for specialist multidisciplinary care in these patients. Keywords: Headache; Chronic migraine; Occipital neuralgia; Neuromodulation; Occipital nerve stimulation * Correspondence: [email protected] 1 Pain Management & Neuromodulation Centre, Guy’s & St Thomas NHS Trust, London, UK Full list of author information is available at the end of the article © 2013 Palmisani et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 Background Chronic Daily Headache (CDH) is an umbrella term for headache disorders with a high rate of reoccurrence (15 or more days per month for 3 consecutive months). CDH represents a major worldwide health problem as affects 3–5% of adults [1-3] who experience substantial disability. Chronic migraine (CM), the most prevalent form of CDH, is defined as headache occurring more than 15 days/ month for at least 3 consecutive months, with headache having the clinical features of migraine without aura for at least 8 days per month [4]. Recently published results from the American Migraine Prevalence and Prevention Study (AMPP) found the prevalence of CM in the United States is approximately 1% [5]. The World Health Organization recognizes migraine as a major public health problem, ranking it at 7th place among all worldwide diseases leading to disability [6]. Compared to episodic migraine, CM is associated with higher disability, inferior quality of life and greater health resource utilization [7]. Despite substantial advances in migraine therapy [8], some individuals with chronic migraine are either resistant or intolerant to guideline-based treatments [9]. This subset of patients requires the development of further treatments and in recent years peripheral neuromodulation, in the form of occipital nerve stimulation (ONS), has emerged as an option for this subset of patients [8,10]. Several published small retrospective studies reported promising safety and efficacy data for ONS in primary headaches. Open label studies in trigeminal autonomic cephalalgias have shown significant, long-term benefit in 67% of refractory chronic cluster headache patients [10] and in 89% of refractory SUNCT and SUNA (short-lasting neuralgiform headache attacks with conjunctival injection and tearing/autonomic symptoms) patients [11]. Encouraging results in refractory chronic migraine patients led to three, commercially funded, multi-centre randomized trials [12,13]. The benefits shown in these trials were less dramatic than hoped for, however the studies have been criticised for methodological weaknesses, unmitigated placebo effect, and a high rate of surgical complications, which may have obscured the full beneficial effect of ONS. Limited data on relevant endpoints was available at the time of studies’ design and poor endpoint choice may have masked the true efficacy of ONS [13]. Thus, the literature leaves many questions unanswered about the role for ONS in chronic daily headache. Our institutions are large, tertiary neuromodulation centers with a special interest in headaches. We agreed to pool resources and retrospectively audit our own data on ONS for CDH to help guide us on future clinical Page 2 of 10 indications for ONS, identify areas for improved clinical practice, technical practice and data collection. This paper reports the results of our audit and relates these to the literature. We discuss the importance of specialists within a multidisciplinary treatment team, question the use of temporary trials to select ONSresponders, and look at surgical strategies to limit hardware-related complications. Methods Two large tertiary neuromodulation centers (Guy’s & St Thomas NHS Trust, London, United Kingdom and Sapienza University at Sant’Andrea Hospital, Rome, Italy) retrospectively audited outcomes of patients receiving ONS from the previous 6 years. The audit results were analyzed with reference to available literature on ONS for CDH. Ethics committee approval was not required for this audit. Audit process All patients receiving a trial of ONS in the last 6 years at both institutions were included in the audit. Patient demographics, headache phenotype and technical details of the surgical procedure(s) were collected from patient medical records. Telephone reviews (up to three per patient) were performed by one headache specialist for each site (GL and PM) to confirm data accuracy, system efficacy and, when needed, to re-code patients’ diagnosis according to the ICHD-II classification [14]. ONS indication At both sites, the indication for ONS was refractory chronic headaches. Patients had failed to significantly improve after adequate trials of four classes of preventive medicines and three classes of acute drugs with established efficacy [15]. ONS candidates were advised not to proceed with surgery when psychological evaluation identified conditions which could be aggravated by the treatment or cause confusion in interpreting clinical results (including, but not limited to, intractable epilepsy, active major depression, psychosis, somatoform disorder, severe personality disorder). Surgical procedure The ONS surgical procedure was performed by four different operators, with equipment and surgical technique (particularly lead insertion and anchoring) varying between operators and over time (Figure 1). All patients underwent a trial of therapy. One or two percutaneous lead(s) were inserted under sedation in the subcutaneous tissue above the peripheral branches of the occipital nerves at approximately C1 level, and left in place for 7 – 10 days to evaluate the efficacy and Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 Page 3 of 10 Figure 1 Example of 3 different approaches for ONS. From left to right, 1) single lead monolateral ONS; 2) dual lead, bilateral ONS; 3) single lead bilateral ONS. tolerability of the treatment before being removed. If the trial was “successful”, i.e. the patient reported at least 50% decrease in headache intensity and/or frequency associated with a decrease headache medication use, a permanent implant was then performed under general anaesthesia. Leads were implanted as in the trial, but this time they were anchored to fascia, tunnelled, and connected to an IPG sited in a subcutaneous abdominal pocket. The practice of leaving stress relief loops in each of the subcutaneous incisions was implemented in some of the subjects implanted after the technique was widely published as part of large multi-centre study [16]. Outcome The patients were treated by different physicians in different centres across a 6-year timeframe and a variety of outcomes were measured for both trial and full implant efficacy. To homogenously evaluate ONS outcomes and be consistent in neuromodulation trial evaluation, we decided a patient implanted with a permanent ONS system would be considered a “success” if a sustained decrease of at least 50% in headache intensity and/or frequency was reported by the patient during the telephone review. Those patients with whom we did not make telephone contact were excluded from the outcome analysis regardless of the information reported in their medical notes. Cognitive Behavioural Therapy (CBT) based implant preparation In one of the two centres involved in the study (GSTT), some patients were required to attend a pre-implant programme (PIP) before proceeding to the trial stage. The PIP involves groups of up to 11 patients engaging in 7–9 days activity spread over two weeks. Physicians, psychologists, physiotherapists, nurses and occupational therapists provide a variety of broadly CBT based interventions, which explicitly seek to reduce emotional distress [17] and improve social and physical functioning [18]. This is done by addressing an individual’s interpretation, evaluations and beliefs about their health condition [19]. Several outcome measures are routinely collected during the course of the PIP, many of those reflecting the IMMPACT recommendations [20] and measuring painrelated disability as a primary outcome variable. Among those: 1) The Pain Disability Index (PDI), which measures the extent to which chronic pain interferes with daily activities [21]; 2) the Beck Depression Index (BDI), which measures the severity of self-reported depressive symptoms [22]; 3) the Pain Self Efficacy Questionnaire (PSEQ), which evaluates how confident patients feel about carry out a variety of tasks despite their pain [23]; 4) the Pain Catastrophizing Scale (PCS), which measures the extent of catastrophising thoughts and feelings associated with pain [24]; 5) the Tampa Scale of Kinesiophobia (TSK), which is a measure of pain-related fear of movement or re-injury [25]. Patients at GSTT who did not do a PIP attended a “Technology Day” instead. This examines patient expectations of treatment with a psychologist, includes information and question and answer sessions given by a physiotherapist and nurse on the stimulator itself and briefly educates on chronic pain and ways of managing this more effectively. Formal psychological data is not gathered, and CBT-based interventions are not provided. Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 Statistics Descriptive statistics has been used to interpret data as appropriate, and data were presented mean ± standard deviation if not stated otherwise. Wilcoxon signed-rank non-parametric test has been used to compare psychological variables in the small subgroup of patients who attended the PIP. Significance level was set at α = 0.05. Results Twenty-five patients underwent a trial of ONS between January 2007 and December 2012 (Male/Female: 7/18; Average age: 49 ± 14 years) (Table 1). Only three patients did not report enough relief during the period of percutaneous stimulation to consider the trial a successful (success rate = 88%), but one patient still requested and received a permanent system. Therefore, 23 patients who Page 4 of 10 received a permanent ONS system were included in the following analysis (Table 2). All patients reached one-year follow-up, and 14 of them (61%) exceeded two years of follow-up (Average 36 ± 23 months, median 28 months). Ten (43%) subjects underwent at least one surgical revision after an average of 11 ± 7 months from permanent implantation, and 90% of the revision surgeries were needed because of problems with leads. Battery replacements were not considered as surgical revisions, unless battery depletion was caused by high lead impedances. Nine patients required at least one surgical revision to replace the stimulating lead because of displacement (3), high impedances (2), local infection/skin erosion (2) or painful paresthesia (2). Eight subjects (35%) had their system removed after an average implant time of 30 ± 21 months (range 2 – 61 months), either for inefficacy (4/23), infection (1/23) or both (2/23). One Table 1 Diagnoses, laterality and site of the pain of the sample of headache patients trialled with Occipital nerve stimulation Diagnosis Pain distribution Sex Definitive Preliminary Trigger Bilateral Length Area of origin Radiation 1 F CM ON No Y 2 ys Occipital Vertex 2* F CM CM No Y 16 ys Neck/Occipital Occipital 3 F CM ON Yes (S) Y 6 ys Occipital Forehead 4 F CM CDH Yes (S) Y N/A Ear Ear/Face 5 F CM Migraine No N 15 ys Eye Eye 6* M CM ON No N 17 ys Eye Forehead 7 F CM ON Yes (T) Y 1 ys Occipital Vertex 8 F IIH ON No Y 2 ys Occipital Holocranic 9* M CM ON No Y 8 ys Neck Occipital 10 M ON ON No Y 10 ys Occipital Holocranic 11 F ON ON No Y 4 ys Neck/Occipital Vertex 12 F CH CH No N 5 ys Occipital Eye 13 M CM ON No N N/A N/A N/A 14 F ON ON No Y 3 ys Occipital Shoulders 15 F CM ON No Y 15 ys Neck Temple 16 M CM Migraine No N 3 ys Occipital Hemicranium 17 M CM ON No Y 15 ys Neck/Occipital Forehead 18 F CM ON No Y 15 ys Neck/Occipital Eye 19 F CM ON Yes (T) Y 19 ys Occipital Forehead 20 M Cerv.H. ON Yes (T) Y 10 ys Occipital Forehead 21 F CM ON Yes (T) N 3 ys Neck Vertex/Eye 22 F CM CM N/A N N/A Temple Temple 23 F CM CM N/A N N/A Temple Forehead 24 F CM CM N/A N N/A Eye Hemicranium 25 F CM CM N/A Y N/A Forehead Holocranic CM chronic migraine, IIH idiopathic intracranial hypertension, ON occipital neuralgia, CH Cluster Headache, CDH chronic daily headache, Cerv.H Cervicogenic Headache, N/A data not available, T Post-traumatic, onset of the headache within a week following a head/neck injury, S Post-Surgical, onset of the headache within a week from a scheduled or un-scheduled surgery; * Pts considered to have failed the ONS trial. Diagnosis Side shift Origin of pain Implant success Lead(s) Paraesthesia Last coverage Fw/up Revision surgery Time to revision Removal Time to removal 1 CM Y Occipital No 1 Quadripolar Good 77 – – – – 2 CM Y Neck/ Occipital No 1 Quadripolar Excellent – – – Painful paraesthesia - inefficacy 10 3 CM Y Occipital Yes (100%) 2 Quadripolar Good 71 Battery site hyperalgesia 2 – – 4 CM Y Ear Yes (90%) 2 Octopolar Moderate – – – Granuloma and skin erosion 29 5 CM N Eye Yes (100%) 2 Octopolar Moderate 42 – – – – 6 CM Y Occipital Yes (90%) 2 Octopolar Excellent 18 Infection lead (×2) 11 – – 7 IIH Y Occipital Yes (100%) 2 Octopolar Excellent 21 – – – – 8 ON Y Occipital Yes (100%) 1 Quadripolar Excellent 28 – – – – 9 ON Y Neck/ Occipital Yes (70%) N/A Excellent 31 Tilted IPG N/A – – 10 CH N Occipital Yes (50%) N/A N/A 28 Lead replacement (High Imp.) 24 – – 11 CM N N/A No (<50%) 2 Octopolar Poor – – – Inefficacy 54 12 ON Y Occipital Yes (50%) N/A Good 28 – – – – 13 CM Y Neck Yes (100%) N/A Excellent 48 Skin erosion (×3) N/A – – 14 CM N Occipital No N/A N/A – – – Inefficacy and implant site infection 2 15 CM Y Neck/ Occipital No 2 Octopolar Excellent – 1st: painful paraesthesia; 2nd: SO lead added 12 Inefficacy 35 16 CM Y Neck/ Occipital No 1 Octopolar Excellent 79 – – – – 17 CM Y Occipital No 1 Octopolar Excellent – Lead migration 8 Inefficacy 20 18 Cerv.H. Y Occipital No 2 Octopolar Good – Several granulomas, lead breakage N/A Inefficacy and implant site infection 61 19 CM N Neck Yes (50%) 2 Octopolar Moderate 31 – – – – 20 CM N Temple Yes (70%) 2 Quadripolar Poor 13 – – – – 21 CM N Temple Yes (50%) 2 Quadripolar Poor 11 Lead and IPG replaced (High Imp.) SO lead added 7 – – 22 CM N Eye Yes (50%) 2 Quadripolar Poor 12 – – – – 23 CM Y Forehead No 2 Quadripolar Poor – Lead migration – Pt request despite effective 12 Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 Table 2 Paraesthesia coverage, types of implants, outcome, complications and removal rate of patients implanted with occipital nerve stimulation FI full implant, IPG implanted pulse generator, N/A data not available, SO supraorbital, Paresthesia Coverage % of original painful area covered by paresthesia. Last follow/up (Fw/up), Time to Revision and Time to removal all expressed in months. Page 5 of 10 Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 patient requested the removal of the system for psychological reasons despite receiving significant benefit from it. All 25 patients were reviewed by the headache specialists during the telephone interview. Nineteen patients (76%) were diagnosed with refractory chronic migraine (rCM), 3 (12%) with refractory occipital neuralgia, 1 (4%) with refractory chronic cluster headache and 2 (8%) with other forms of chronic headache (see Table 1). Interestingly, only 7 of the rCM patients were referred with this diagnosis for ONS, while 11/19 were wrongly labelled as occipital neuralgia and 1/19 as chronic headache refractory to medical treatment. Seventeen patients with a diagnosis of rCM received a permanent ONS system, and all but three had a successful trial before the implant (84% success rate). Two of the subjects with an unsuccessful trial did not proceed to the full implant. One patient, against medical recommendation, decided to undergo full implantation despite limited benefit from the trial and reported a mild benefit (< 50% relief ) after 5 years of follow-up. Nine subjects (53%) reported significant pain relief (> 50% relief in attacks’ intensity and/or frequency) after an average follow-up of 40 ± 27 months (range 11–77 months). In 5/17 (4 of which with sustained pain relief ), migraine attacks originated in the trigeminal nerve distribution, while 11/17 patients had their original pain in the occipital area and of those, 5 reported significant relief over time. It should be noted that in most patients headache pain radiates in both territories as the migraine attacks progress. Seven of the eight patients who had their system removed were classified as rCM. Five were removed for inefficacy (despite a successful initial percutaneous trial), and one for acquired infection not responding to antibiotic therapy. Three subjects were classified as refractory occipital neuralgia, with a history of tenderness over the occipital area and temporary pain relief following at least one occipital nerve block with local anaesthetic and/or steroids. All had a successful trial of stimulation and all of them (100%) report significant relief (well over 50% reduction in severity and frequency) after 28, 28 and 31 months of follow/up from the permanent insertion of the ONS system, respectively. Seven patients (6 rCM and 1 ON) attended a specifically designed, multi-disciplinary, two-week pre-implant programme (PIP). Attending the programme was associated with improved scores across all measured psychological and functional parameters. Statistically significant improvement occurred in the BDI scores, with a mean decrease of 7.4 (95% CI: 2.3 – 12.5), and in the TSK scores, with an average decrease of 8 points (95% CI: Page 6 of 10 2.2 – 13.8). The analysed population was very small and differences observed between responders and nonresponders did not reach statistical significance. However, long-term responders seemed to have higher values of PCS scores before the PIP than non-responders, and were able to decrease their BDI values during the course more than those who failed ONS treatment. Discussion ONS is a promising treatment for some refractory primary headaches, but its role needs further definition. We have presented 6 years ONS experience in two European neuromodulation centres closely working as twin teams with tertiary headache centres. Our data is consistent with published studies that suggest ONS has a place in the management of patients with refractory chronic migraine and with refractory occipital neuralgia but that much work needs to be done to refine patient selection and optimise the treatment. Our analysis has highlighted important specific areas to focus on in the future clinical and research use of ONS. The concept of a multidisciplinary approach to refractory headaches In order to face the clinical challenge of refractory chronic headaches there is a need of at least three different specialists to be involved in the selection of refractory headaches patients as potential candidate for ONS: a referring headache specialist, a pain physician with expertise in neuromodulation and a psychologist with expertise in chronic pain. The presence of a headache specialist with expertise in ICDH-II diagnostic classes must be considered mandatory in future. Many patients included in our analyzed cohort were reclassified when reviewed by a trained headache specialist: only 25% of the patients were correctly labelled as CM at the time of the referral, and only 19% of the subjects originally labelled as occipital neuralgia fulfilled the ICDH-II criteria for this diagnosis. Our results are in line with previous ONS retrospective analysis, where patients reviewed by an headache specialist were often re-coded [26]. As evidenced by the difference between long-term efficacy (53% CM vs 100% ON) and system removal rates (7 patients with CM vs 0 with ON) in our series, correct diagnosis is essential for scientific and economic evaluation of ONS. Inappropriate use of the words “refractory” and “intractable” might also led healthcare professionals to improperly label patients as “refractory” even if they have not been on an appropriate trial of acute treatments or have never been tried on a preventive medication at an adequate doses for a reasonable period of time [9,15]. Efficacy of onabotulinumtoxinA as a preventive treatment of chronic migraine has been shown in the PREEMPT studies [27] Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 and it should now be added to the list of preventive therapies to be tried before labeling a migraine patient as refractory and offering them invasive treatments. 15 of the patients included in our series had their system implanted before the PREEMPT publication (and therefore did not receive onabotulinumtoxinA treatment), it is possible that some of them might have responded to onabotulinumtoxinA treatment without the need of an ONS implant. Patients with chronic migraine experience the same complex spectrum of biopsychosocial problems seen in other chronic pain conditions [28,29]. Anxiety, depression, sleep interference, employment interference, relationship interference and decreased physical and social activity are important factors in overall morbidity and should be assessed and addressed. The GSTT subgroup in our data who participated in a pre-implant pain management approach (PIP) experienced improvement across all measured psychosocial domains leading to improved quality of life and health outcomes. However, in our series more patients had a successful implant who did not have a PIP (7/9 vs 3/7), suggesting that while the PIP is efficacious itself, in its current form it may not provide the best preparation of patients for an implant. The current PIP focuses on encouraging patients to manage their pain and maximise activity and does not focus on patient selection for ONS. Careful assessment of psychosocial domains should lead to improved ONS patient selection and outcomes this is widely observed recognised in other neuromodulation areas [19,30,31]. Pain duration, psychological distress, pain catastrophising, psychiatric conditions including personality disorders, history of abuse, and significant cognitive deficits are associated with poor outcomes from pain treatments in general [32]. Depression has been identified as the single most important factor predictive of efficacious Spinal Cord Stimulation [33], and other factors including somatization, anxiety, poor coping also predict poor response [34]. Reports on ONS to date have focussed on technical details and patient outcomes have centred on pain scores as a measure of patient benefit [10] and there is an absence of literature looking at patients’ psychosocial and physical status and examining outcomes with quality of life measures. Stimulation trial as a reliable predictor for long-term success A successful temporary trial of stimulation has been considered the best predictor of long-term outcome [35] in different groups of chronic pain patients who are candidates for neuromodulation. However, a positive trial does not guarantee long term success. The two largest, multicenter, prospective trials of spinal cord stimulation Page 7 of 10 for the treatment of chronic pain after spine surgery required a positive trial as key inclusion criteria for patients enrollment [36,37]. Despite an high initial trial to implant ratio (83% in both studies), successful outcome at one year dropped dramatically (55% - 47%) [37,38]. There is no available literature on the ability of a percutaneous trial to predict long-term benefit of ONS implant [39]. Subgroup analysis of data coming from one large RCT of ONS in CM showed that patients who positively responded during a percutaneous trial before the permanent implant reported a decrease in headache days per month significantly greater than those who failed the trial [16]. However, only short term data was published so we do not know if the successful trial predicted long-term benefit. Moreover, we do not know if a longer period of stimulation in those who failed the trial might have resulted in benefit in the longer term. In our series of patients, despite an initial trial success rate of 88%, 7/23 systems were removed due to inefficacy, and only nine subjects (53%) with a diagnosis of chronic migraine reported significant pain relief (>50% relief in attacks’ intensity and/or frequency) after an average follow-up of 40 months. A retrospective review of ONS in heterogeneous headache patient population has been recently published reporting similar data in terms of trial success rate (89%), system efficacy (56%), and long-term benefit in CM patients (42% at an average of 34 months) [40]. Rarely ONS-induced improvements are evident within days, as the neuromodulatory processes involved are believed to occur slowly in different areas of the whole nociceptive system [10]. The reported benefit of a short (7 – 10 days) percutaneous trial might represent a placebo effect in a cohort of subjects who usually have unrealistic expectations on the surgery, after having failed most of the available treatments. The view of the International Headache Society Clinical Trials Subcommittee is that the subjective nature of migraine features and a high placebo effect invalidate open and singleblind trials of any prophylactic intervention and that the number of migraine attacks and number of migraine days should be collected prospectively for an interval of time long enough to be compared with a prospective baseline of at least 1 month [41]. A one or two weeks percutaneous ONS trial will not satisfy this standard. Furthermore, when a one-month, semi-permanent, tunnelled trial was employed to test ONS system efficacy before implantation, the long-term outcome in CM patients was still only 47%, despite an accurate evaluation of trial outcomes through specific pain questionnaires [26]. Therefore, the use of a trial test of ONS is now highly questionable. Its ability to select long-term responders appears poor and with >80% of patients going on to full implantation anyway, a trial poses additional risk and Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 inconvenience for patients and an economic burden to the health care system. Long-term treatment efficacy Neuromodulation is an invasive and expensive treatment, and should be reserved for specific subset of chronic pain patients following evidence-based guidelines [42]. 350 patients have been enrolled in three large, industry sponsored, randomized control trials in the efforts to evaluate safety and efficacy of ONS to treat rCM [12,13,16]. Two found no significant support for an adequate therapeutic effect (responders defined as 50% reduction in headache days per month), and the other found only a moderate benefit (responders defined as 30% improvement in pain) in 39% of the treated subjects. Different study designs, with controversial end-point choices, do not allow a direct comparison of the trials’ results. Furthermore, no conclusions on long-term treatment efficacy can be drawn as only 3 months follow/up data have been reported to date. In our patients the average time of system removals for inefficacy is around 23 months (range 2 – 54). An analysis of the available ONS literature reported long-term implant response rate is high (88% to 100%) when peripheral stimulation is performed to elicit paresthesia in the whole painful area, compared to a low response rate (40%) in those studies reporting nonconcordant paresthesia [43]. Some authors hypothesized that the combined stimulation of areas innervated by both the occipital nerves (ON) and supraorbital nerves (SON) might benefit those patients who perceived pain in a hemicephalic or global extent [43,44]. Interestingly, in our series we found no differences among the patients who reported Excellent/Good paresthesia in terms of long-term positive outcome (64% vs 66%), and 4 out of 5 patients with migraine origin in the trigeminal area had good long-term outcome. Moreover two patients had a supraorbital lead added later on in the attempt of increase paresthesia coverage and system efficacy, but only one of them reported significant benefit. As adding supraorbital leads increases surgical times and complexity, a carefully designed trial is warranted to establish the long-term benefit of this new approach. Hardware-related complications Currently available ONS technology, originally designed for epidural use, is associated with troublesome complications when used subcutaneously for ONS. Skin erosion, lead breakage, lead migration, and pain around the battery site can occur. These are not only direct adverse events for the patient, but also impact on ONS efficacy, and dramatically increase health care expenditure as further surgical procedures and new equipment are Page 8 of 10 often required. In our series, almost 43% of the patients required at least one surgical revision to treat such problems. In 90% of cases leads or the intermediate connections were the culprit. Similar numbers have been reported in another recent retrospective review of ONS in heterogeneous headache patient population, with 58% of patients needing a surgical revision [40]. In the larger RCTs, where only 3 months data have been disclosed, surgical revision rates were already between 19% [13] and 37% [12]. Lead migration and lead breakage, major causes of ONS-related surgical revision, are related to repeated lead and extension traction events due to the high mobility of the implanted area. Over the years, some authors have described techniques to minimize these complications. Bennett suggested securing each lead ipsilaterally to the lateral pocket fascia using 2 suture sleeves separated by a strain relief loop, and anchoring each sleeve to the fascia with 3 sutures and intraluminal medical adhesive [16]. Franzini et al. recommend securing the distal end of the lead to the lateral portion of the superficial cervical fascia (with two additional skin incisions) to prevent lead migration and report no displacement at 1 year follow-up in 17 patients [45]. Additional strain relief loops are recommended at the upper thoracic level (T2- T4), at the implantable pulse generator (IPG), and at any other incisions [16]. Finally, IPG implantation sites other than the traditional gluteal region may have the advantage of less pathway length change during patient movement. Thus, infraclavicular and low abdomen IPG sites may result in less lead migration/rupture [46]. This literature reveals that specialist expertise by the neuromodulator is important factor in outcome. Limitations Our audit has several weaknesses. Its design is flawed by the well-known limitations of retrospective case-series studies [47]. Lead/anchor technology and our surgical technique have evolved so some of the problems we have highlighted are already being addressed. Different measures were collected over the years, and our choice of using patients’ subjective report of headache’s intensity/ frequency reduction to define long-term success is not highly robust. Any prospective trial should now endorse the outcome measures defined by Task Force of the International Headache Society Clinical Trials Subcommittee [41]. Finally, we couldn’t collect enough information to report and comment on medication-overuse headache. Conclusions Our audited series of 25 patients treated with ONS in two tertiary neuromodulation centers is consistent with literature suggesting that ONS is a therapeutic option for patients with refractory chronic migraine (9 of 17 Palmisani et al. The Journal of Headache and Pain 2013, 14:67 http://www.thejournalofheadacheandpain.com/content/14/1/67 patients reporting >50% reduction in headache frequency and or intensity at long-term follow up), and refractory occipital neuralgia (all patients reporting >50% reduction in pain frequency and or intensity at long-term follow up). There is a need to refine patient selection for ONS and ensure optimal medical, psychological and surgical management at all stages - a multidisciplinary team comprising of headache, psychology, and neuromodulation specialists is essential for this. Such teams should be used in future randomized controlled trials with long-term follow-up to further determine the place for ONS in refractory chronic headache management and improve patient outcomes. Page 9 of 10 9. 10. 11. 12. 13. 14. 15. Competing interests SP has received travel reimbursement from Medtronic and Nevro Corp. AA has received travel sponsorship and speaker fees from Medtronic and Nevro Corp, and he is the principal investigator in separate studies sponsored by Medtronic and Nevro Corp. RA has received travel reimbursement from Medtronic and Nevro Corp. TS has received travel sponsorship and speaker fees from Medtronic and Nevro Corp. PM has received travel grants from Nevro Corp and St Jude Medical. AN, EC, VB and GL do not declare any competing interest. 16. 17. 18. 19. Authors’ contributions SP designed the study, supervised the data collection, performed data analysis and drafted the initial manuscript. AA, RA, TS and SP performed the surgical procedures. AN, GL and PM reviewed patient’s notes and diagnosis. VB and EC collect data and took part in data analysis. All authors revised the manuscript. All authors read and approved the final manuscript. 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