Open PDF in new window

Transcription

TOPICAL DIAGNOSIS OF CHIASMAL AND RETROCHIASMAL DISORDERS
and acalculia) in association with a right homonymous hemianopia (289,290). Lesions in the nondominant parietal lobe
may cause impaired constructional ability, dyscalculia, and,
most commonly, inattention or neglect (291,292). Indeed,
left spatial neglect after right hemisphere lesions may accentuate the left hemianopia, hemianesthesia, and hemiplegia
and contribute to poor recovery (293).
LESIONS OF THE OCCIPITAL LOBE AND
VISUAL CORTEX
In many cases the most posterior visual radiation and visual cortex are affected together. Most lesions in the occipital
lobe are vascular or traumatic in origin, with tumor, abscess,
demyelination, and toxic disorders of white matter occurring
less frequently. Cole, a neurologist, has written a first-person
account of his experience as a patient with an occipital
stroke, which includes some of the symptomatology described below (294).
Because of the close anatomic relationship of fibers from
corresponding portions of the two retinas, lesions of the occipital lobe cause defects that are not only overwhelmingly
homonymous but also increasingly congruous the more posteriorly situated the lesion (295–299). Automated static perimetry may inaccurately portray homonymous visual fields
as incongruous, compared to Goldmann or tangent screen
perimetry (300).
Unilateral Lesions of the Posterior Occipital Lobe
Defects seen with these lesions are exclusively homonymous. With lesions of the tip of the occipital lobe (occipital
pole), the field defects are always central homonymous scotomas that are exquisitely congruous (257,296,298,299)
(Fig. 12.33). After a detailed examination of three patients
with homonymous visual field defects produced by well-
539
circumscribed lesions in the occipital lobe, Horton and Hoyt
(298) estimated that the central 10⬚ of the visual field are
represented by at least 50–60% of the posterior striate cortex
and that the central 30⬚ is represented by about 80% of the
cortex (Figs. 12.34 and 12.35). These findings were corroborated by McFadzean et al. (299). Instead of using lesions
for mapping, it is possible to use functional MR imaging to
detect those areas of the cortex activated by visual stimuli,
and thus to produce a retinotopic map noninvasively (301).
Wong and Sharpe used functional MR imaging to map the
retinal representation on the occipital lobe and found that
the central 15% of vision occupied only 37% of the cortex
(302). Probably the most accurate method for determining
cortical magnification (i.e., the degree by which areas of
central retina are processed by magnified areas of visual
cortex) is the use of angioscotoma mapping (303,304).
Adams and Horton (305) used this mapping technique in
squirrel monkeys and found that the central 16⬚ of visual
field is represented by approximately 70% of cortex. Similar
cortical magnification of the central visual field is seen in
the extrastriate areas V2 and V3 (306). The detailed anatomy
of the visual cortex is discussed in Chapter 1.
Lesions located more anteriorly may produce primarily
central field defects that break out into the periphery (Fig.
12.36). Impressive congruity is a feature of such field defects
(257,307,308). The phenomenon of sparing of the macula
is often seen in such cases (see below).
An unusual visual field defect is homonymous hemianopia associated with sparing of the horizontal or vertical
meridian. This results from sparing from injury of the corresponding cortical representation of the meridian, with the
vertical meridian represented at the lips of the calcarine fissure and the horizontal meridian represented at the base of
the fissure (309,310).
Figure 12.33. Exquisitely congruous homonymous hemianopic scotomas in a patient with an infarct of the inferior lip of the
left calcarine cortex. Note the few degrees of macular sparing.
540
CLINICAL NEURO-OPHTHALMOLOGY
Figure 12.34. A 30-year-old woman reported several episodes of flashing colored lights in her right upper quadrant of vision.
A, Merged 30-2 and 60-2 full-threshold visual field tests using a Humphrey Field Analyzer. A homonymous, congruous scotoma
was present in the right upper quadrant. In the inset, the visual fields are mapped at the tangent screen. The scotoma extended
from 6⬚ to 18⬚. B, Parasagittal magnetic resonance image of the left occipital lobe. The lesion (cross-hatched area on the right)
is within the visual cortex (stippled area on the right). The calcarine sulcus (solid arrow) and the parieto-occipital sulcus (open
arrow) are marked by dots. On biopsy, the lesion was a presumed tuberculoma. (From Horton JC, Hoyt WF. The representation
of the visual field in human striate cortex. Arch Ophthalmol 1991;109⬊816–824.)
Both Allen and Carman (311) and Barkan and Boyle (312)
reported a syndrome in which an otherwise healthy patient
develops the sudden onset of an isolated homonymous paracentral scotoma associated with normal visual acuity and
no ophthalmoscopic abnormalities. Clinically, such a lesion
must occur as the result of a vascular event in the posterior
occipital cortex; however, in both cases, the homonymous
scotomas were incongruous. The explanation for this apparent clinical–anatomic discrepancy is unclear.
Unilateral Lesions of the Anterior Occipital Lobe
Because the temporal field in each eye is larger than the
nasal field, the fibers subserving that portion of the peripheral temporal field that has no nasal correlate must be un-
paired throughout the postchiasmal portion of the visual sensory pathway. Damage to these unpaired peripheral fibers
produces a monocular defect in the extreme temporal visual
field. This field defect is crescentic and its widest extent is
in the horizontal meridian, where it extends from 60⬚ out to
approximately 90⬚. Because of its peculiar shape, a defect
in this region has been termed a temporal crescent or halfmoon syndrome (Figs. 12.35C, 12.37, and 12.38). Brouwer
and Zeeman (313) demonstrated that after destruction of the
peripheral nasal retina there was degeneration of a bundle
of fibers localized in the median portion of the optic nerve,
which continued into the optic chiasm to the median portion
of the optic tract and spread ventrally into the lateral geniculate body. Wilbrand (6) confirmed that this bundle contained
541
Figure 12.35. Humphrey visual fields (30-2 program) (A) in a 40-year-old man who suffered a right occipital infarction (B)
involving the superior bank of the calcarine cortex (arrowhead). The infarction spared the posterior striate cortex as well as
the anterior striate cortex at the junction of the parieto-occipital and calcarine fissures (arrow). C, The visual fields showed
sparing of central fixation within 6⬚ as well as sparing of the left monocular temporal crescent. (From McFadzean R, Brosnahan
D, Hadley D, et al. Representation of the visual field in the occipital striate cortex. Br J Ophthalmol 1994;78⬊185–190.)
only crossed fibers. In 1929, Förster (quoted by Walsh [314])
produced the sensation of light in the extreme temporal periphery of one eye by electrically stimulating the most anterior portion of the striate cortex. Polyak (315) confirmed
these findings and postulated that the most anterior portion
of the striate cortex harbored the projected monocular temporal crescent of the contralateral eye. Ask-Upmark (316)
and Kronfeld (317) performed further studies in patients in
whom the temporal crescent was selectively spared or damaged. They agreed that lesions of the posterior striate cortex
tended to spare the temporal crescent, whereas lesions of the
anterior striate cortex could selectively produce a defect in
it or eliminate it entirely. Spence and Fulton (318) extirpated
the entire left occipital lobe in a chimpanzee and later resected part of the right occipital lobe. They subsequently
observed that the animal had vision only in the temporal
periphery of the left eye. Postmortem examination of the
brain disclosed that only the anterior portion of the right
striate cortex was intact, confirming the conclusions reached
by Polyak (315), Ask-Upmark (316), and Kronfeld (317).
According to Bender and Strauss (208), the unpaired nasal
fibers corresponding to the extreme temporal peripheral visual field come together in the most anterior portion of the
striate cortex, with the fibers for the inferior portion of the
542
Figure 12.36. Inferior homonymous hemianopic scotomas that break out to the periphery in a patient with an infarct of the
left superior occipital cortex.
Figure 12.37. Visual field defects in a 22-year-old woman who suffered bilateral occipital lobe infarctions. There is a complete
left homonymous hemianopia and an incomplete right inferior homonymous quadrantanopia. The temporal crescent is absent
from the right visual field.
543
Figure 12.38. Visual field defects in a 56-year-old woman with a right occipital lobe infarction. There is a left homonymous
hemianopia with sparing of the inferior portion of the left temporal crescent.
crescent terminating above the calcarine fissure and the fibers for the superior portion of the crescent terminating
below the fissure.
Bender and Strauss (208) found that in 100 patients with
verified cerebral tumor affecting the posterior optic radiation, there was a unilateral temporal crescentic defect in the
visual field of 10 patients. The recognition of such a defect
has value in the early diagnosis of a lesion of the posterior
optic radiation or anterior visual cortex. Theoretically, a lesion anywhere in the postchiasmal visual pathway, from the
optic tract to the occipital lobe (and particularly the dorsal
LGB), could produce a monocular field defect in the temporal periphery (244); however, in practice, only lesions in
the posterior optic radiation or anterior visual cortex produce
such selective damage.
Both upper and lower temporal crescents may be scotomatous in the field of one eye, or only the upper or lower
temporal crescent may be affected. Frequently, such a field
defect is followed later by the development of an homonymous hemianopia (316). Bender and Strauss (208) fairly
often found an unpaired crescentic relative scotoma medial
to the peripheral absolute scotoma, indicating to them that
the optic radiation has a lamellar arrangement.
Just as the temporal crescent may be affected by lesions
destroying the most anterior occipital lobe, it may likewise
be spared when the remainder of the temporal visual field is
affected in an homonymous hemianopia (284,314,319,320)
(Figs. 12.35 and 12.38). Such sparing seems to occur exclusively in patients with occipital lobe lesions that spare the
anterior aspect of the visual cortex (208).
Two important facts should be kept in mind when considering the temporal crescent syndrome. First, monocular peripheral temporal visual field defects are probably caused
most often by retinal lesions, not by intracranial ones. Thus,
the nasal retinal periphery should be carefully examined ophthalmoscopically in such cases. Second, because these defects begin approximately 60⬚ from fixation, central field
testing (i.e., that performed using a tangent screen or most
automated static perimetry programs) gives normal results
and will not detect such defects (321,322) (Fig. 12.39).
Bilateral Effects of Unilateral Occipital Lobe Lesions
Although patients with unilateral lesions of the occipital
lobe typically have visual deficits totally restricted to the
contralateral field, area V1 and adjacent extrastriate areas
often function or fail in concert. Pathologic lesions of the
occipital lobe thus may disrupt not only area V1 but also
adjacent underlying white matter and extrastriate cortex.
Such damage may alter interhemispheric connections along
their presplenial course and even disturb visual cortical–subcortical connections. Nonetheless, there is excellent evidence that there are regions of cortex ipsilateral to the field
defect that process visual information (323,324). Rarely, cortical plasticity in the setting of congenital cerebral lesions
can lead to developmental reorganization of the visual processing areas so that the visual field is represented ipsilaterally (325). There is also indirect evidence for reorganization of projections to extrastriate visual processing areas
after injury to V1 (326).
For example, in a study by Bender and Teuber (327,328),
dark adaptation was delayed in the apparently normal field.
Rizzo and Robin (329) studied the vision of 12 patients with
unilateral lesions of the visual cortex. All patients had homonymous defects in the contralateral visual fields, as expected, and all had visual acuity of 20/30 or better in both
eyes. The first experiment performed by Rizzo and Robin
(329) tested the subjects’ ability to respond to transient sig-
544
Figure 12.39. Migrainous loss of the temporal crescent. A, A 31-year-old woman with a history of migraine without aura
had the sudden onset of a bright light in the temporal field of vision of the right eye, followed by loss of vision in this same
region, and subsequent headache. The field deficit persisted for a few days and then resolved. B, Visual fields performed 5
days later were full in both eyes. Magnetic resonance imaging performed at the time of the initial deficit was completely
normal.
nals presented at unpredictable temporal intervals and spatial
locations among many spatially random and identical distracter elements. The results showed that compared with controls, the lesion group had a significantly reduced sensitivity
to signal and increased response times in both hemifields.
In a second experiment, Rizzo and Robin (329) tested the
useful field of view in two of the patients under conditions
of differing attention demand. Both patients showed bilateral
constriction, consistent with the results of the first experiment. Rizzo and Robin suggested that one possible explanation for the bilateral effects of unilateral occipital lobe lesions is damage to interhemispheric connections along their
presplenial course, affecting the synthesis of visual information from both hemifields. The disturbances are task-dependent and seem to result from a global reduction in visual
attention capacity.
Subsequent experiments by Battelli et al. (329a) showed
ipsilateral effects of right parietal lobe lesions on motion
detection and suggested that some of these effects could be
mediated by specific alterations in attention circuits. Although these disturbances are subtle compared with the homonymous visual field defects that occur in patients with unilateral occipital lobe lesions, they may be responsible for
various unexplained complaints of reduced performance in
tasks with high visual information-processing demands such
as reading and driving.
Macular Sparing
When a portion of the central field of each eye in homonymous hemianopia is preserved as a result of deviation of the
vertical meridian between the functioning and nonfunction-
ing halves of the visual fields, there is said to be ‘‘sparing
of the macula’’ or ‘‘macular sparing.’’ In a majority of such
cases, visual acuity is normal, with the zone of preserved
visual field ranging from 1⬚ or 2⬚ in width to almost 10⬚
(Fig. 12.40). Another type of field in which there is sparing
is the ‘‘overshot’’ field (284). In such a visual field, the
sparing is not central alone but extends along the entire vertical meridian. A complete discussion of this phenomenon
appears in the paper by Safran et al. (330).
Deviation of the vertical meridian accounting for macular
sparing is a much-debated phenomenon. Although it is usually seen in patients with retrogeniculate (usually occipital)
lesions, the absence of sparing does not imply that the lesion
is pregeniculate or noncortical. The etiology of macular spar-
545
ing is controversial. Some authors consider it an artifact of
testing. Others believe it is a real phenomenon related to the
representation of a small portion of each macula in each
occipital lobe. Finally, some authors believe that macular
sparing results from incomplete damage of the visual pathways or occipital lobe. This section addresses each of these
theories in turn.
What has been represented as wide sparing may often be
shown in reality to be much narrower when ocular fixation
is carefully controlled. Hence, imprecisely charted fields
tend to show a wider sparing than actually exists. In many
cases of homonymous hemianopia with sparing, it is possible
to prove a shift in fixation by charting the blind spot in the
single field in which it can be performed. Another method
Figure 12.40. A 28-year-old woman suffered a persistent visual field defect after an unusually severe migraine
attack. A, Full-threshold 60⬚ visual fields using a Humphrey Field Analyzer show the central 15⬚ of the left hemifield is intact. In the inset, the visual fields mapped at the
tangent screen show that the field defect bisects the blind
spot representation of the left eye. B, T1-weighted parasagittal magnetic resonance image through the right occipital
lobe shows an arteriovenous malformation involving the
anterior portion of the right calcarine cortex. The posterior
margin of the lesion is situated 31 mm from the occipital
tip, marking the approximate location of the representation
of the left eye’s blind spot. The calcarine (curved arrow)
and parieto-occipital (straight arrow) sulci are indicated.
(From Horton JC, Hoyt WF. The representation of the
visual field in human striate cortex. Arch Ophthalmol
1991;109⬊816–824.)
546
is performing a tangent screen examination using three or
more identical stimuli slowly brought horizontally from the
defective hemifield toward the vertical meridian. One stimulus moves toward fixation, as the others simultaneously
move inward to points 10–15⬚ above and below fixation.
With genuine macular sparing, the central stimulus is perceived before those above and below it. If there is defective
fixation and shifting of the entire hemifield, however, all
targets are simultaneously perceived.
It is impossible to obtain absolutely stable fixation, and
physiologic movements of the fixing eye so slight that they
cannot be detected by ordinary means probably account for
1–2⬚ of deviation of the vertical meridian about the central
area. Verhoeff (331) emphasized the importance of the optic
fixation reflexes in cases of macular sparing. He suggested
that loss of integration between the seeing field and the blind
field in the conscious visual cortical area in use might result
in eccentric fixation. As a result of fixation slippage, the
original macular field would be split but would appear to
be spared during field testing. Sugishita et al. (332) used
fundus perimetry combined with fundus image analysis in
two subjects with left occipital lesions and ‘‘macular sparing.’’ Two of the four eyes tested showed eccentric fixation,
and miniature eye movements to the blind hemifield were
observed in 4–10% of the trials. Using microperimetry and
a scanning laser ophthalmoscope, Bischoff et al. (333) also
demonstrated rapid fixation shifts in their patients with
‘‘macular sparing’’ of 1–5⬚. These authors concluded that
this typical macular sparing may be a functional adaptation
of the ocular motor system to enhance the central visual field
toward the blind hemifield.
In 1928 Lenz (334) proposed that a small fiber tract subserving the macular region branched from the optic tract
through the corpus callosum to the opposite visual cortex,
providing representation of the macula in both occipital
lobes. The presence of bilateral representation of the macula
was further suggested by Penfield et al. (277), who studied
five patients after radical extirpation of the occipital lobe.
In two instances, the posterior portion of the lobe was excised. In both, there was complete homonymous hemianopia
with splitting of the macula. In two other cases, a large portion of the lobe was removed, and it was thought that the
entire calcarine cortex was included. Nevertheless, following surgery, the patient had an homonymous hemianopia
with macular sparing and retention of normal visual acuity.
The fifth and most interesting case studied by Penfield et
al. (277) was one in which there was complete destruction
of the calcarine cortex of one side. This pathologic process
resulted in homonymous hemianopia with sparing. When
the diseased occipital lobe was extirpated, the line of resection was so far forward that it was adjacent to the splenium
of the corpus callosum. Subsequent to the resection there
were further field changes in that the vertical meridian failed
to deviate about the point of fixation. Thus, ‘‘sparing of the
macula’’ was converted into ‘‘splitting of the macula.’’ Ohki
et al. (335) described a case that was the reverse of Penfield’s
fifth case. The patient was a 62-year-old man who experienced a left parieto-occipital hemorrhage. When first examined, the patient had a right homonymous hemianopia with
macular splitting. As the hemorrhage resolved, the patient
developed macular sparing, initially of 5⬚ and then of 10⬚.
From a study of reported cases of occipital lobectomy and
five cases studied personally, Dubois-Poulsen et al. (336)
also supported the concept of macular representation in both
occipital lobes as an explanation for macular sparing. These
researchers endorsed a theory that had first been proposed
by Morax (337), who suggested that in the foveal region of
the retina there is a small vertical band of ganglion cells,
some of which have crossed and others uncrossed fibers.
Gramberg-Danielsen (338) supported this thesis in a thorough review of the problem of macular sparing. He emphasized the inexact nature of the vertical border between the
two hemifields and called attention to a functional vertical
overlap of the two sides of the visual field in the form of a
narrow band that is widest near the fovea. At the retinal
level, ganglion cells and their dendritic processes overlap
and intertwine without any morphologic suggestion of the
functional vertical separation of the two halves of the retina,
corresponding to the temporal and nasal halves of the visual
field. Halstead et al. (339) carefully studied the visual fields
of two patients who underwent complete occipital lobectomy
and found ‘‘splitting’’ in one case and ‘‘sparing’’ in the
other. From a study of occipital lobectomy cases, Huber
(340) (who subsequently declared macular sparing to be an
artifact [333,341]) also confirmed the phenomenon of macular sparing, favoring the theory of bilateral foveal representation probably explained at the level of the retina.
Cowey (342) found occasional anomalous receptive fields
for cells in the squirrel monkey’s cortical fovea. All of his
animals had previously had an occipital lobectomy. Recording from a microelectrode in a single cortical unit in the
foveal area of the monkey’ intact occipital lobe, he was able
to evoke responses from the parafoveal area of the ‘‘blind’’
visual field. He recognized the similarity of his experimental
finding to the clinical observation of macular sparing following occipital lobectomy in man.
It is clear from histochemical studies that, to some extent,
a small portion of each macula probably has representation
in each occipital lobe. As noted in Chapter 1, there is a small
area of nasotemporal overlap on either side of the vertical
meridian in which some axons from ganglion cells temporal
to the fovea cross within the chiasm, whereas some axons
from ganglion cells nasal to the fovea remain uncrossed
(210,211,213,214,343). In the monkey, the nasotemporal
overlap is smallest (about 1⬚ of visual angle) near the fovea
and increases at least twofold in more peripheral retina
(213,214). Scanning laser ophthalmoscopy has demonstrated this zone of nasotemporal overlap in humans that
extends into the blind hemifield, with a slightly concave
shape (344).
Despite the presence of a nasotemporal overlap that exists
in nonhuman primates and presumably in humans as well,
the mere presence of an overlap does not necessarily have
major visual consequences. Sugishita et al. (332) concluded
that dual macular representation, if it exists in humans, must
be less than 0.4⬚ wide, and Brysbaert (345) also questioned
the clinical significance of bilateral macular representation.
Of more importance, however, is that a ganglion cell located
on one side of the fovea does not necessarily receive input
from a photoreceptor located on the same side. The much
larger width of the cone pedicle compared with the foveal
cone soma creates a packing problem that is only partly
resolved by lateral displacement of cone pedicles and via
the nerve fiber layer of Henle. This lateral shift could theoretically connect photoreceptors on one side of the vertical
meridian to seemingly distant ganglion cells on the opposite
side (266). Such an anatomic relationship would have no
visual consequences, because it is the position of the photoreceptor and not the ganglion cell body that determines the
location of visual input to the brain. The anatomic inexactness of the vertical demarcation can be observed clinically
by visual field testing (289).
Nonetheless, the most likely cause of true macular sparing
in clinical practice is incomplete damage to the visual pathways, particularly the occipital lobe. Wilbrand (347) suggested that macular sparing was the result of residual intact
areas of the visual pathways. McAuley and Russell (308)
noted an association between macular sparing and retained
portions of the most posterior portion of the occipital cortex
(Fig. 12.41). These observations are supported by the work
of Horton and Hoyt (298), who demonstrated the extremely
high cortical magnification of the macular representation
(Fig. 12.40). The unique dual blood supply of the occipital
cortex from the posterior cerebral and middle cerebral arteries (see Chapters 1 and 39) provides a mechanism for this
partial damage. In patients with homonymous hemianopia
and substantial macular sparing, not only is the location of
the lesion almost always the occipital lobe, but also the pathogenesis is likely posterior cerebral artery infarction. In these
cases, the most common cause for macular sparing is reten-
547
tion of some functioning posterior occipital cortex. In other
cases, macular sparing may reflect fixation artifact or bilateral representation. Together, the consequences of these theories can explain the great variability in macular sparing
seen in retrochiasmal disease.
Bilateral Occipital Lobe Lesions
Bilateral lesions of the occipital lobes may occur simultaneously or consecutively. Because such lesions are neurologically asymptomatic except with respect to the visual system, patients with unilateral lesions that have caused a
homonymous field defect may not be aware of the defect
until their attention is called to it (e.g., during a routine ocular
examination or after a motor vehicle accident) or until they
experience a similar event on the opposite side.
Förster first described a patient with a well-documented,
partial, bilateral homonymous hemianopia in 1890 (348).
The patient initially sustained a right homonymous hemianopia followed by a left homonymous hemianopia with sparing of central vision. Numerous case reports subsequently
documented this condition and its many causes (349–358).
Bilateral homonymous hemianopia may appear simultaneously. In a majority of these cases, there is initially complete visual loss; however, this total blindness is usually
transient, lasting from minutes to days (359,360). Nepple et
al. (356) reviewed the findings in 15 patients with bilateral
homonymous hemianopia. All patients had similarly shaped
visual field defects on corresponding sides of the vertical
midline for each eye, symmetric (and usually normal) visual
acuity, normal pupils and fundi, and normal ocular motility
unless there was a coexisting brain stem lesion. The majority
Figure 12.41. A 52-year-old man with coronary artery disease and thrombocytopenia purpura noticed trouble seeing to the
left side with his left eye. A, Examination was normal except for visual fields, which showed an incongruous left homonymous
quadrantic defect, denser in the left eye. (Figure continues.)
548
Figure 12.41. Continued. B and C, Magnetic resonance images (B, T2-weighted; C, T1-weighted after gadolinium) showed
a curvilinear enhancing lesion in the medial, anterior, right occipital lobe consistent with infarction (arrows).
of patients had bilateral posterior cerebral artery insufficiency caused by arteriosclerosis (40%), uncal herniation
from subdural hematoma (20%), or migraine (13%). Most
of these patients had a history suggestive of simultaneous
bilateral occurrence. Brain tumor did not occur in this series;
conversely, in 849 patients with visual field defects and brain
tumors reviewed by Schaublin (361), there were no patients
with bilateral homonymous hemianopias.
Bilateral homonymous hemianopia may also occur from
consecutive events, invariably vascular (Fig. 12.42). This is
a much more common phenomenon than is bilateral simultaneous homonymous hemianopia. In such cases, the patient
Figure 12.42. A, Bilateral homonymous hemianopic defects secondary to bilateral posterior cerebral artery infarction. Note
the dense right homonymous hemianopia combined with a left inferior homonymous quadrantanopia with sparing of the left
temporal crescent. (Figure continues.)
549
Figure 12.42. Continued. B–D, T2-weighted magnetic resonance
images demonstrated bilateral posterior cerebral artery infarctions,
worse on the left than the right, and extending less inferiorly and less
anteriorly on the right.
experiences an acute homonymous hemianopia with retention of normal vision with or without sparing of the macula.
At a later time, varying from weeks to years, the patient
develops a sudden homonymous hemianopia on the opposite
side, again with or without macular sparing. After this second event, the patient is either blind or retains only a small
central field around the point of fixation, similar to that seen
in simultaneous cases.
The syndrome of bilateral nonsimultaneous homonymous
hemianopia from infarction of both occipital lobes is most
frequently observed in elderly atherosclerotic individuals
and may be associated with hypertensive crisis. In some
instances, there are premonitory visual disturbances such as
flashing lights or visual hallucinations. Because the vascular
supply to association areas of the visual cortex may be interrupted, there may also be visual agnosia and other disturbances of higher cortical function.
Bilateral vascular lesions of the occipital lobes produce
550
bilateral homonymous lesions that are characteristic and
vary only in their extent. They may be complete or scotomatous, and they may or may not be accompanied by macular
sparing. Bender and Furlow (362,363), for instance, described ‘‘central scotomas’’ produced by bilateral lesions
of the occipital lobes. Such defects are, in reality, bilateral
homonymous scotomas that may be detected by careful field
testing along the vertical midline (364) (Fig. 12.43A and B).
Occasionally, such scotomas have enough central sparing to
produce ‘‘ring’’ scotomas. One such patient had complete
blindness transiently following a tractor accident. When vi-
sion returned, the patient noted that although he could see
clearly in the exact center of his field, the paracentral area
was blurred. Visual field testing demonstrated bilateral, congruous, homonymous scotomas with macular sparing (Fig.
12.43C and D).
Bilateral occipital lobe disease, whether from infarction,
tumor, or trauma, may result in various degrees of bilateral
homonymous hemianopia (Fig. 12.44). There may be complete (cortical or cerebral) blindness. The hemianopia may
be complete on one side and incomplete (and congruous) on
the other, or there may be a hemianopia on one side and a
Figure 12.43. A and B, Bilateral homonymous hemianopic scotomas in a patient with bilateral nonsimultaneous occipital
lobe strokes. C and D, Note vertical step that differentiates these defects from a true central scotoma with macular sparing in
a patient who suffered trauma to the occipital region. The tractor on which he was riding overturned, pinning him underneath
for several minutes. Initially, he was completely blind, but vision returned within several minutes. He subsequently realized
that he had a ‘‘ring’’ of blurred vision around fixation. Note that the homonymous scotomas respect the vertical midline.
Figure 12.44. A 40-year-old man suffered occipital head
trauma with a depressed skull fracture and visual complaints. Visual acuity was 20/20 in both eyes. A, Visual
fields revealed exquisitely congruous bilateral homonymous scotomas, larger on the left than the right. A computed tomography scan (B) revealed a right occipital pole
lesion, but magnetic resonance images (C and D) confirmed bilateral occipital pole involvement.
551
552
Figure 12.45. After strenuous exercise, a 33-year-old man suffered intermittent confusion, gait disturbances, and right facial
weakness, followed by permanent left hemiparesis and difficulties with vision. A, Visual fields revealed a right homonymous
hemianopia with macular sparing and a left superior homonymous quadrantanopia. B, Magnetic resonance imaging revealed
the corresponding infarctions in the occipital lobes bilaterally. Note the involvement of the inferior occipital lobe on the right
and the entire calcarine cortex on the left, with sparing of the most posterior pole. The patient also had cerebellar and brain
stem infarctions, all secondary to artery-to-artery emboli from a vertebral artery dissection.
quadrantanopia on the other (307,365) (Figs. 12.42 and
12.45). In some instances, there may be a bilateral homonymous hemianopia with bilateral macular sparing of a different degree on each side. The remaining visual field thus
appears to be severely constricted, and such patients may be
thought to have bilateral optic nerve or retinal disease or
may even be thought to have nonorganic visual field loss. As
with bilateral homonymous hemianopic scotomas, however,
careful testing along the vertical midline will establish the
bilateral nature of the field defect and its correct origin (Fig.
12.46).
Crossed quadrant hemianopia results when patients develop bilateral quadrantic defects, either simultaneously or
more commonly consecutively, that affect the superior oc-
cipital lobe above the calcarine fissure on one side and the
inferior occipital lobe below the fissure on the other side.
Such defects are sometimes called checkerboard fields and
are infrequent (Fig. 12.47). Felix (366) described a case and
reviewed the literature up to 1926, which included only two
other cases. In the case reported by Felix and in one other
case, this field defect followed a simultaneous bilateral homonymous hemianopia, although a case described by Groenouw (quoted by Felix [366]) occurred after two separate
vascular events occurring 10 months apart.
Although bilateral altitudinal field defects are usually
caused by bilateral anterior optic nerve disease (e.g., glaucoma or nonarteritic anterior ischemic optic neuropathy),
they can also be caused by bilateral occipital lobe disease
553
Figure 12.46. Bilateral homonymous hemianopias with macular sparing in a patient with severe cerebrovascular occlusive
disease. The patient initially presented with 20/20 vision in both eyes but with ‘‘tubular’’ fields. The macular sparing respects
the vertical midline.
(e.g., from trauma, infarction, hemorrhage, and rarely tumors). Such lesions were studied extensively by Holmes and
Lister (375) and were subsequently described in more detail
by Holmes (367), who observed that bullet wounds interrupting both occipital lobes were usually fatal if the lower portions were damaged because death occurred from intracranial bleeding as a result of laceration of the dural sinuses in
the region of the torcular herophili. When the upper portions
of the visual cortex or posterior radiations were damaged,
however, the patient often survived, and the resultant field
defects in such cases were altitudinal, with loss of the entire
inferior field of both eyes.
Money and Nelson (1943) described field defects similar
to those reported by Holmes (367), and Wortham et al. (368)
reported bilateral inferior hemianopia occurring during
or immediately following thoracoplasty. The defect was
Figure 12.47. Crossed quadrant (checkerboard) hemianopia. These field defects occurred suddenly in a 70-year-old woman
with basilar artery disease. Note the quadrantic defects in the right upper field and the left lower field with the narrow congruous
isthmus near fixation. Also note the sparing of the left temporal crescent and the incongruity of the field defects along the
upper vertical meridian and the right horizontal meridian. (Courtesy of University of California Hospital, San Francisco.)
554
thought to have resulted from anoxia, and slight improvement occurred over the next several months’ observation.
Newman et al. (369) described two cases of bilateral altitudinal hemianopia, one inferior and one superior, secondary to
occipital infarctions that were demonstrated by CT scanning,
and a similar case was confirmed both on neuroimaging and
pathology by Vanroose et al. (370). Other cases secondary
to occipital infarction or hemorrhage occur, occasionally in
the setting of cardiac surgery or hypoxia (308,353,368,
371–376).
Based on the preceding, it can be understood why superior
altitudinal defects from occipital lobe trauma are less common than inferior altitudinal defects. An example of the former, occurring in a patient where a bullet passed from right
to left through the inferior portions of both occipital lobes
but missing the dural sinuses, is depicted in Figure 12.48.
On the other hand, both superior (Fig. 12.49) and inferior
bilateral altitudinal defects can occur with vascular disease
affecting the occipital lobes. When the lesion is far posterior,
the field defect remains altitudinal but is central and scotomatous (Fig. 12.50). Spalding (296) confirmed the impressive congruity of these small field defects.
Cortical (Cerebral) Blindness
Cortical blindness and cerebral blindness are considered
in the same section because in many instances it is impossible to differentiate between them on clinical grounds. The
term ‘‘cortical blindness’’ indicates loss of vision in both
eyes from damage to the striate cortex. ‘‘Cerebral blindness’’ is a more general term indicating blindness from damage to any portion of both visual pathways posterior to the
lateral geniculate bodies. Thus, cortical blindness is a subset
of cerebral blindness. Patients with cerebral blindness may
have other neurologic deficits, including hemiplegia, sensory disorders, aphasia, and disorientation. Because of the
similarity in the visual system symptomatology between
these two conditions, for the sake of simplicity the term
‘‘cortical blindness’’ will be used in this section.
The essential features of cortical blindness as outlined by
Marquis (377) are (a) complete loss of all visual sensation,
including all appreciation of light and dark; (b) loss of reflex
lid closure to bright illumination and to threatening gestures;
(c) retention of the reflex constriction of the pupils to illumination and to convergence movements (the near-response);
(d) integrity of the normal structure of the retinas as verified
with the ophthalmoscope; (e) retention of full extraocular
movements of the eyes, unless there is also damage to ocular
motor structures. Many neuro-ophthalmologists use the term
‘‘cortical blindness’’ not only where the visual acuity is light
perception or no light perception but also with any level of
visual acuity, as long as the visual acuity is equal in the two
eyes (assuming the anterior visual pathways are normal).
Hypoxia or ischemia of the occipital lobes is the most
common etiologic factor producing cortical blindness. Such
damage is necessarily bilateral. Most commonly, an infarction in the posterior cerebral artery territory is initially unrecognized, but this previously silent hemianopia contributes
to complete cortical blindness when a contralateral lesion
occurs (378,379). The most common mechanism for the infarction is cerebral embolism from either the heart or the
more proximal vessels of the vertebrobasilar system
(380,381) (Fig. 12.51). Prolonged hypotension can cause
cortical blindness from bilateral watershed infarctions at the
parieto-occipital junction. Courville (382), among others,
recognized that hypoxia causes laminar cortical necrosis and
that the visual, premotor, and parietal areas are particularly
apt to be damaged by this process. Courville (382) expressed
the opinion that these areas seemingly had a higher oxygen
requirement. Lindenberg (383) emphasized that the affected
areas represent the border zones of cerebral circulation.
Cortical blindness is observed under many circumstances
other than infarction, including trauma, neoplasm, malignant
hypertension, toxemia of pregnancy, diseases of white matter (e.g., Schilder’s disease, adrenoleukodystrophy, Pelizaeus-Merzbacher disease, metachromatic leukodystrophy,
progressive multifocal leukoencephalopathy), CreutzfeldtJakob disease, mitochondrial encephalopathy lactic acidosis
and stroke-like episodes (MELAS), cerebral angiography,
ventriculography, blood transfusions, uremia, acute intermittent porphyria, syphilis, infectious and neoplastic meningitis,
bacterial endocarditis, subacute sclerosing panencephalitis,
hepatic encephalopathy, sudden elevation or reduction in
intracranial pressure, cardiac arrest, hypoglycemia, correction of hyponatremia, epilepsy, electroshock, and after exposure to carbon monoxide, nitrous oxide, ethanol, licorice,
methamphetamine, mercury, lead, cis-platinum, cyclosporin
A, tacrolimus (FK506), methotrexate, vincristine, vindesine,
and interferon (379,384–418). The mechanism of injury underlying the cortical blindness caused by these events or
substances is not always known, but vascular insufficiency
plays a role in many of them.
In certain situations, cortical blindness is transient. This
is particularly true in patients who experience transient vascular insufficiency in the vertebrobasilar system, in hypertensive syndromes following restoration of normal blood
pressure, after removal of many of the toxic agents listed
above, and after trauma (417,419–421). Children are more
likely to experience recovery from cortical blindness than
adults, regardless of the underlying cause. Barnet et al. (419)
studied six children who developed transient cortical blindness after respiratory or cardiac arrest, head trauma, or meningitis. In all cases, visual acuity returned to some extent
in 1–10 weeks. Greenblatt (422) emphasized three clinical
patterns of transient cortical blindness following head
trauma:
1. Juvenile (up to age 8 years). These patients experience
blindness of short duration (hours), accompanied by somnolence, irritability, and vomiting, with an excellent prognosis for full recovery.
2. Adolescent (late childhood through teenage years). These
patients have blindness that does not develop immediately after the trauma but occurs several minutes later.
The blindness lasts minutes to hours, is usually unaccompanied by other neurologic or systemic deficits, and also
tends to resolve completely.
3. Adult. These patients have immediate blindness, a
Figure 12.48. A 24-year-old man suffered a gunshot wound to the left temporal region with exit of the bullet via the right
occipital pole. Visual acuity was 20/20 in both eyes, but there was cerebral dyschromatopsia and prosopagnosia. A, Visual
fields showed a right superior homonymous quadrantanopia (probably resulting from damage to the left inferior radiations at
the temporal–occipital lobe junction) and a left superior homonymous quadrantic scotoma (probably from damage to the right
inferior occipital pole). B–E, Magnetic resonance images showed the hematoma created by the bullet track.
555
556
Figure 12.49. A 71-year-old man had acute onset of complete visual loss, followed by clearing inferiorly. A, Examination
was normal except for visual fields that showed bilateral superior altitudinal visual field defects (bilateral superior homonymous
quadrantanopias). B–F, T2-weighted magnetic resonance images (B–D, axial views; E and F, sagittal views to the right and
left of midline, respectively) demonstrated bilateral posterior cerebral artery infarctions involving primarily the inferior occipital
lobes. (Figure continues.)
557
Figure 12.49. Continued.
protracted course with other neurologic defects, and a
variable visual outcome.
Postictal cortical blindness is not uncommon (395,
423,424), and ictal cortical blindness as a manifestation of
occipital seizures may occur (409,425–429). Ashby and
Stephenson (430) analyzed 11 cases of blindness after seizures and concluded that there is a form of amaurosis that
occurs in young children and infants that is postictal and is
caused by depression of the visual centers. The seizures that
accompany such blindness are usually violent and may be
associated with aphasia and paresis of hemiplegic distribution. The hemiplegia may be permanent, although the visual
loss is generally transient. Joseph and Louis (429) reviewed
the literature concerning seizure-induced bilateral blindness
and divided affected patients into three groups. Fifteen patients, all under 12 years of age, had a long succession of
seizures or status epilepticus. They developed blindness that
was permanent or that lasted many months. Ten patients,
seven of whom were between the ages of 10 and 20 years,
had bilateral occipital seizures with blindness corresponding
to atypical spike and wave activity. The blindness lasted less
than 30 minutes in all cases. Finally, 18 patients had a focal
seizure originating in or adjacent to an occipital lobe, with
ictal spread to involve both occipital lobes. In these patients,
blindness occurred either ictally or postictally. Because focal
lesions may be responsible for ictal amaurosis, MR imaging
558
Figure 12.50. Occipital injury (shell fragment). Bilateral inferior hemianopic quadrantic scotomas, according to Holmes.
(From Traquair HM. An Introduction to Clinical Perimetry, 3rd edition. St Louis, CV Mosby, 1940.)
should be performed in all cases (409). Complete recovery
from ictal or postictal blindness can occur even though the
blindness lasts for several days.
Visual-Evoked Responses in Cortical Blindness
Figure 12.51. Bilateral simultaneous posterior cerebral artery occlusion
secondary to embolic infarction from a cardiac arrhythmia. The patient was
initially cortically blind with no light perception vision. Over the course
of a few weeks he had 20/30 vision in each eye because of 5⬚ of macular
sparing.
The relationship between cortical blindness and the visual-evoked response (VER) is unclear. One of the earliest
human studies on this subject was performed by Kooi and
Sharbrough (431). Their patient had suffered head trauma
and was initially blind with no evoked responses to light
stimuli. Subsequently, both visual acuity and VERs underwent gradual and parallel improvement. This promising correlation was corroborated by other investigators (432–434).
However, Spehlmann et al. (435) reported persistence of the
VER to light in a blind patient who had extensive bilateral
destruction of cortical visual areas and their association
areas. Similarly, Celesia et al. (436) and Hess et al. (437)
demonstrated normal VERs in both complete and incomplete
cortical blindness. Aldrich et al. (379) found abnormal pattern and flash VERs in 15 of 19 patients with cortical blindness, but there was no correlation with the severity of visual
loss or with visual outcome. Indeed, all four patients with
normal VERs during blindness had poor visual outcomes,
and three patients with good outcomes had no initial responses to pattern-reversal VERs. These investigators found
that electroencephalography was a more helpful diagnostic
indicator: the presence of a posterior dominant alpha rhythm
responsive to eye opening appears to be incompatible with
complete or incomplete cortical blindness. Hence, it would
appear that at least in adults, VERs are not useful in establishing either the diagnosis or prognosis in patients with cortical blindness (379,438).
The usefulness of VERs in children with cortical visual
dysfunction is controversial. Barnet et al. (419) reported two
children who showed well-formed VERs shortly after the
onset of their acute disease, even though they were completely blind. Bodis-Wollner et al. (439) described normal
VERs to both flash and alternating grating patterns in a blind
child in whom CT scanning showed destruction of the cerebral cortex corresponding to areas 18 and 19 bilaterally with
preservation of primary visual cortex (area 17). Frank and
Torres (440) recorded VERs to brief light flashes from occipital regions in a group of 30 cortically blind children aged
4 months to 15 years. These investigators compared their
results with those obtained from 31 children of similar age
range who had similar central nervous system diseases but
no evidence of blindness. Only one patient with cortical
blindness showed no response, and the presence of abnormal
responses was not incompatible with normal vision. Mellor
and Fielder (441) reported that poor or even absent VERs
in infancy did not always indicate a poor prognosis, and Hoyt
(442) showed a similar lack of correlation. Other authors
also noted poor correlation of VERs and visual function in
children with bilateral cerebral abnormalities, especially
those with associated neurologic deficits (443–445), and it
is not uncommon to see patients free of clinical neurologic
or visual deficits who have incidentally discovered abnormal
VERs.
Nevertheless, some investigators argue that the VER is
indeed helpful in establishing both diagnosis and prognosis
in children with cortical visual dysfunction. In a study of
six children with cortical blindness after either bacterial
meningitis or head trauma, Duchowny et al. (446) found that
changes in short-latency VER components correlate with
visual ability, whereas changes in longer-latency VER components correlate with the level of psychomotor function.
Kupersmith and Nelson (447) reported that the presence of
a VER wave, whether normal or abnormal, indicated the
future development of vision. Lambert et al. (448) and Bencivenga et al. (449) also reported some correlation of the
VER response to cerebral visual function. McCulloch and
Taylor (450) argued that flash VERs are an excellent indicator of visual recovery for those children who have acuteonset cortical blindness and no preexisting neurologic disorder. In this subset of patients, many with presumed hypoxia,
those with abnormal VERs had permanent visual impairment
or blindness, and all but one patient with absent VERs remained blind (451,452). The variable results from previous
studies may reflect the many different underlying etiologies
of cortical blindness in children and the many other associated neurologic factors that might contribute to poor recovery.
Optokinetic Nystagmus in Cortical Blindness
Optokinetic nystagmus (OKN), the slow component of
which is generally considered a conscious pursuit movement, is often used to distinguish patients with true blindness
from those with nonorganic blindness. Most investigators
agree that OKN is absent in patients with total cortical blindness (385,453–455). In fact, Brindley et al. (455) repeatedly
559
examined a patient with total cortical blindness over a 3.5year period and could never elicit OKN.
Ter Braak (456) separated OKN into two types: passive
and active. According to this investigator, passive OKN is
produced by the movement of the majority of contrasts in
the visual field and may still be present after ablation of the
visual cortex, whereas active OKN depends on the movement of specific objects in the visual field and thus requires
‘‘vision’’ for its production. Using these definitions, Ter
Braak et al. (457) examined a 71-year-old man with longstanding cortical blindness and found passive OKN in one
direction. Subsequent necropsy revealed almost total destruction of both striate cortices and degeneration of both
LGBs.
Despite these observations, the presence of OKN in an
individual complaining of complete blindness or perception
of light only is almost always evidence of nonorganic visual
loss (see also Chapter 21).
Cortical Blindness with Denial of Blindness
(Anton’s Syndrome)
Patients with cortical blindness are often unaware of their
deficit. This denial of blindness, a type of anosognosia, is
called Anton’s syndrome (458). The syndrome also occurs
in patients with blindness from causes other than occipital
lobe disease, including cataracts, retinopathies, or optic atrophy (459). The explanation for Anton’s syndrome is unclear.
Lessell (459) reviewed the various theories and concluded
that each explains some of the cases. Geschwind (quoted by
Lessell [459]) suggested that patients with denial commonly
have an alteration in emotional reactivity. Such patients have
emotions that are described as coarse and shallow and may
predispose them to deny their blindness. Lessell also stated
that ‘‘psychiatric’’ denial may occur as an accentuation of
a common response to illness. In patients with Korsakoff’s
syndrome, denial of blindness may represent a memory disorder. Such patients cannot remember from minute to minute
that they truly are blind. Finally, it is probable that in many
patients with cortical blindness, there are lesions in various
areas of the brain responsible for recognition and interpretation of visual images. In such patients, denial of blindness
is caused not by the lesion in the primary visual pathway
but by a lesion in another region of the brain.
Other Visual Features of Occipital Lobe Damage
Bender and Teuber (327) made noteworthy observations
based on their study of 140 individuals who suffered penetrating head wounds in World War II. It was their belief that
when macular function is spared, there is either an incomplete homonymous hemianopia or development of a pseudofovea. They also noted that when an incomplete homonymous field defect is produced by an occipital lobe lesion,
there are often disturbances in the area of the homonymous
visual fields that seem spared when ‘‘ordinary’’ methods of
perimetry are performed. For instance, critical flicker fusion
frequency falls in field areas adjacent to the scotoma and to
a lesser degree in field areas far distant from the scotoma.
560
Dark adaptation is delayed in the apparently normal field.
Luminous objects can be discerned within the scotomatous
field when patients are placed in total darkness. Finally,
when simple figures are presented to the patient so that part
of the figure is in the area of scotoma and part in the seeing
field, the entire figure is seen (completion phenomenon)
(460). There are several different explanations for these observations. First, the methods of perimetry used in the 1940s
were clearly less sensitive than automated static perimetry.
It thus is likely that many of the patients studied by Bender
and Teuber (327) actually had larger field defects than were
detected by what was then ‘‘routine’’ perimetry. Second, it
is likely that the observation of luminous objects in blind
areas of the field represented artifacts of testing, since it is
clear that some ‘‘luminous’’ objects have a larger area of
stimulation of the retina than might normally be assumed.
In addition, minor movements of the eyes during testing that
might not be detected in total darkness with the equipment
available to Bender and Teuber (327) may have moved the
luminous target into the seeing field adjacent to the blind
field.
Some patients with homonymous hemianopia, especially
those with a vascular occlusion in the occipital lobe, report
phosphenes in the blind visual field, particularly early in the
course of their disorder (461–464) (see also the section on
visual hallucinations in Chapter 13). Among 96 patients with
homonymous hemianopia or quadrantanopia, most of vascular origin, Kolmel (463) noted that 14 had experienced colored patterns in their blind hemifield, typically red, green,
blue, and yellow. Many of the visual field defects in these
patients resolved substantially, suggesting that the phosphenes may be viewed as a prognostically favorable symptom. Of 32 patients with ischemic infarction of the retrochiasmal pathways studied prospectively by Vaphiades et al.
(464), positive spontaneous visual phenomena in the blind
hemifield were present in 13 patients (41%). Classifying
these positive visual phenomena as photopsias, phosphenes,
palinopsia, or visual hallucinations had no value in localizing
the site of the lesion. However, there was a significant difference in the severity of associated neurologic deficits between
hemianopic patients with and without these positive visual
phenomena; specifically, larger lesions destroying anteriorly
located visual association areas precluded the development
of these positive phenomena. The authors proposed that visual association areas bordering damaged primary cortex
were the source of these visual symptoms when they were
released from normal inhibitory inputs from primary visual
cortex.
An unusual visual phenomenon associated with unilateral
or bilateral occipital lesions is cerebral diplopia or polyopia,
in which two or more images are perceived monocularly
(465,466). Cerebral polyopia may be due to reorganization
of one or more of the multiple representations of the visual
field in the visual cortex (466). Unlike monocular polyopia
from irregularities of the ocular refractive media, cerebral
polyopia does not improve with pinhole. This entity is discussed further in Chapter 13.
Lesions of the Striate Cortex Without Defects in the
Visual Field
It is generally assumed that any lesion of the striate cortex
produces a defect in the field of vision, but there are cases
on record that suggest that this assumption may be incorrect.
Teuber (467) stated that he had records on an extraordinary
case of a 1-year-old child who sustained an accidental gunshot wound through the right parietal lobe. The bullet remained in the right calcarine area, where it could still be
shown radiologically 30 years later. Teuber et al. could not
demonstrate any evidence of a field defect, even though they
used refined tests of flicker fusion and light thresholds.
Weiskrantz (468) stressed that it is conceivable that an injury
of the striate cortex must produce a critical minimum amount
of damage before it produces any demonstrable scotoma.
Evidence supporting this has been contributed by N. R.
Miller, who searched in vain for tiny scotomas produced
by stereotactically implanted depth electrodes (0.5 mm in
diameter) that were inserted directly into and through the
macular area of the occipital pole and the occipital portion
of the optic radiations. It is unlikely that standard white-onwhite automated perimetry can identify visual field defects
associated with these small injuries, although it is possible
that static color field testing could identify such defects.
Similarly, stereotactic pallidotomy, a procedure used to treat
patients with Parkinson’s disease, requires precise positioning before the electrode is activated. The positioning includes placement of the electrode in the ipsilateral optic tract,
the cells of which are then identified by recording from the
electrode (468a). The electrode is then pulled back out of
the optic tract into the globus pallidus, where a lesion is then
made. It is difficult to identify visual field defects in the
majority of patients in whom stereotactic pallidotomy is performed, regardless of whether the testing is performed using
kinetic or static automated perimetry.
Weiskrantz and Cowey (469) contributed neurophysiologic evidence to support the aforementioned observations
and conclusions. They showed that quantitative lesions in
the macular cortex of the monkey produce a smaller drop
in visual acuity than would be predicted from equivalent
lesions of the macula. They suggested that the discrepancy
between retinal and cortical effects is probably best explained in terms of lateral interaction of neuronal elements at
the retinal level, the geniculate level, or both. An alternative
explanation is that the visual cortex has a higher relative
magnification factor for the foveal representation than is accounted for by the concentration of retinal ganglion cells in
the macula (305).
Lesions of the Extrastrate Cortex with Defects in the
Visual Field
As noted in Chapter 1, the striate cortex (V1, or Brodmann
area 17) is the primary visual cortex and the principal recipient of output from the LGB. Surrounding the striate cortex
within the occipital lobe are two visual association areas,
Brodmann areas 18 and 19, also called the extrastriate visual
cortex. Studies from primate electrophysiologic and pathway
tracing methods indicate that areas 18 and 19 together repre-
sent at least five distinct cortical areas devoted to visual
processing (278,470–472). These visual areas are designated
V2, V3, V3a, V4, and V5 in the monkey, and corresponding
regions are present in the human occipital cortex (472).
Functional MR imaging has recently been able to demonstrate multiple visual areas in the human, each with their
own retinotopic mapping (473,474). Regions V2 and V3
are the major recipient areas for projections from both the
magnocellular and parvocellular systems in V1, the primary
striate cortex. However, V1 also directly projects to areas
V4 and V5, bypassing V2 (475). The sizes of V1 and V2
are highly correlated, but not V1 and V3 (306).
Feinsod et al. (476) suggested that a lesion in the extrastriate cortex alone might cause defects in the visual field. Others reported superior visual field defects from damage to the
561
‘‘nonvisual’’ lingual and fusiform gyri (373,477). Polyak
(478) reported a case in which paracentral quadrantic scotomas were caused by an infarction primarily located in a region corresponding to the ventral portion of areas V2 and
V3. Horton and Hoyt (278) reported two patients with homonymous quadrantic defects caused by tumors, one in the
cuneus of the occipital lobe and the other in the upper peristriate cortex (Figs. 12.52 and 12.53). These investigators
proposed not only that a lesion of V2/V3 may be sufficient
to create a visual field defect, but also that such lesions are
the principal cause of quadrantic defects that strictly respect
the horizontal meridian. This hypothesis has been supported
by functional MR imaging studies demonstrating selective
loss of activation of extrastriate cortex areas in a patient with
an upper right homonymous quadrantic defect (479).
Figure 12.52. A 39-year-old woman had experienced multicolored visual hallucinations in her left lower quadrant since
childhood. A, Magnetic resonance imaging revealed a lesion within the cuneus of the right occipital lobe that on en bloc
resection was found to be a grade I astrocytoma. B, Postoperative magnetic resonance image revealed the area of resection.
C, Postoperative visual fields demonstrated a left inferior homonymous quadrantanopia with precise respect of the horizontal
and vertical meridians. The patient could detect gross hand motion within the quadrantic defect. (From Horton JC, Hoyt WF.
Quadrantic visual field defects. A hallmark of lesions in extrastriate [V2/V3] cortex. Brain 1991;114⬊1703–1718.)
562
Figure 12.53. A 40-year-old man experienced increasing episodes of flashing
lights in the left lower quadrant of vision and was found to have a large tumor
of the right parieto-occipital cortex. A, On magnetic resonance imaging the
calcarine sulcus (between arrows) and the occipital pole are preserved. B, After
resection of the mass, he had a left lower quadrantanopia precisely bordering
the horizontal meridian with sparing of the central 10⬚. (From Horton JC, Hoyt
WF. Quadrantic visual field defects. A hallmark of lesions in extrastriate [V2/
V3] cortex. Brain 1991;114⬊1703–1718.)
Cortical Visual Loss with Normal Neuroimaging
William F. Hoyt has suggested that the essence of neuroophthalmology is diagnosis in the presence of normal neuroimaging. A not uncommon example occurs when there is
unilateral or bilateral homonymous hemianopia in the presence of normal MR imaging. Brazis et al. (480) reviewed
several causes of this phenomenon, including the Heidenhain
variant of Creutzfeldt-Jakob disease (481,482), variant Alzheimer dementia (483,484), nonketotic hypoglycemia, and
ischemia without infarction. Many of these disorders can
be detected with positron emission tomography scanning or
functional MR imaging, which is sensitive to the effects of
neural activity on blood oxygenation levels (485). Another
etiology for visual loss without abnormalities on conventional MR imaging is occipital lobe seizures, which can disrupt visual perception not only during the ictus but also postictally (486) and even permanently (487).
Dissociation of Visual Perception
The chapter that follows (Chapter 13) discusses the topographic diagnosis of disorders of visual processing more dis-
tal to the primary visual cortex. Because many of these disorders result from lesions in the extrastriate cortex of the
occipital lobes, it is worth mentioning a few of the more
prominent syndromes in this chapter. As suggested by studies in monkeys, the human visual cortex is highly specialized
with respect to specific functions (472). For instance, there
are distinct areas of the cortex that subserve color vision.
These can be identified either by studying patients with
known lesions or by functional MR imaging mapping of
activation of cortex after appropriate stimulation. One area
in the lingual and fusiform gyri of the prestriate cortex corresponds to area V4 in the monkey and is responsible for color
(472,488,489). Other areas adjacent to V3 can be identified
by functional MR imaging as containing retinotopic maps
for color (473). Lesions in any of these regions may spare
the visual field but produce acquired dyschromatopsia in the
contralateral hemifield (477,490,491). The disturbance of
color vision in such cases may be permanent, especially if
there are bilateral lesions. Conversely, transient achromatopsia may reflect temporary ischemia in this region, a phenomenon that may occur in migrainous aura (492).
The area of functional specialization for visual motion is
localized to the temporoparieto-occipital junction, a region
corresponding to area V5 in the monkey (472,493,494). Lesions in this region, especially when bilateral, may cause a
selective deficit in the perception of visual movement without a visual field defect and without impairment of nonvisual
movement perception (i.e., movement perceived by acoustic
or tactile stimulation) (495,496). A similar loss of motion
detection, but in a single direction at a time, can be reproduced by electrical stimulation of the posterior temporal lobe
(497).
In some patients, damage to an occipital lobe can cause
a complete homonymous hemianopia to nonmoving objects,
with retention of the ability to detect moving objects within
the blind hemifield: static–kinetic dissociation, also called
the Riddoch phenomenon. In 1917 Riddoch (498) noted that
some wounded soldiers with complete homonymous hemianopia developed or retained the ability to perceive small
moving objects in the affected hemifield. Riddoch divided
his cases into three groups: those who perceived motion only
on the affected side; those who perceived both moving and
stationary objects but to a different degree; and those with
no dissociation between moving and stationary objects.
Static–kinetic dissociation may have prognostic significance
since it usually means that some degree of recovery can be
expected. Such a phenomenon has been observed not only
after occipital trauma but also after removal of tumor, cortical blindness from cardiac arrest, and occipital apoplexy in
patients with occipital arteriovenous malformations (see also
(499)). Preservation of the human correlate of area V5 may
be required for intact visual motion processing, perhaps via
extrageniculostriate pathways (500) (see below).
Static–kinetic dissociation is not a purely pathologic phenomenon, nor is it limited to the striate cortex. In fact, all
normal individuals perceive moving objects better than static
objects of the same size, shape, and luminance. If objects
are sufficiently small or dull, they will be perceived only
during kinetic perimetry and not during static perimetry. Safran and Glaser (501) demonstrated variable degrees of physiologic dissociation of kinetic and static stimuli in 7 of 15
normal subjects. Interestingly, the dissociation occurred for
achromatic target perception but not for chromatic perception of red. Safran and Glaser attributed this physiologic
dissociation to a successive lateral summation of images, as
previously noted by Greve (502). Spatial summation is more
pronounced for rods than for cones, and this may explain
the lack of dissociation of static and kinetic targets when hue
recognition is required. Selective excitation of movementdependent channels appears to be required for perception
of movement (503). In addition, pathologic static–kinetic
dissociation occurs in patients with nonoccipital lobe lesions. Zappia et al. (504) reported two patients, one with a
lesion in the optic tract and one with a lesion of the optic
chiasm. Both patients showed a dissociation between perception of static and kinetic objects in their affected hemifields.
In addition, Safran and Glaser (501) studied 11 patients with
compression of the anterior visual pathways and found some
degree of static–kinetic dissociation in the defective field of
all of them.
The Riddoch phenomenon of static–kinetic dissociation
563
is one form of a general category of visual phenomena designated as ‘‘blindsight’’ (468,505–510). In a classic experiment, Mohler and Wurtz (511) removed the occipital lobe
in trained monkeys. The monkeys appeared to be blind in
the appropriate hemifield for several weeks but then began
to make appropriate saccades into the previously blind hemifield. Mohler and Wurtz then extirpated the superior colliculi
in the monkeys, who then became permanently blind in the
hemifield. In another set of monkeys, Mohler and Wurtz
first removed the superior colliculi. The monkeys showed
some mild and transient difficulty with saccadic eye movements but no evidence of visual loss. The monkeys were
then subjected to unilateral occipital lobe excision, following
which they became permanently blind in the contralateral
hemifield of both eyes. These experiments indicated that
nonhuman primates have a subcortical vision sensory system
that likely involves connections between the retina and the
superior colliculus in addition to the retinogeniculo-occipital
pathway for normal visual function. A similar subcortical
pathway may exist in humans. Indeed, some patients with
extensive damage to the occipital lobes appear to retain a
rudimentary form of vision involving the perception of visual stimuli other than just moving objects. Most patients are
not consciously aware of this ability to look, point, detect,
and discriminate without truly ‘‘seeing.’’ In many cases
blindsight is likely a result of islands of preserved area 17,
as demonstrated by positron emission tomography, single
photon-emission computed tomography, and image stabilization perimetry (512,513). Celesia et al. (512) argued that
the sparing of portions of primary visual cortex, combined
with probable scatter of light into the intact visual field during testing (514), accounts for blindsight in humans. However, other investigators contend that blindsight is a genuine
phenomenon that reflects nonstriate visual pathways (see
Cowey and Stoerig [508] for review), which can occur in
the absence of functioning area 17 (515). In humans, examples of pathways invoked as an explanation for blindsight are
a retinocollicular projection that reaches extrastriate cortical
visual areas via the pulvinar nucleus (516) and a direct
geniculo-extrastriate cortical projection (508). Interestingly,
there appears to be an increase in the sensitivity of blindsight
in a given individual over time, suggesting that training of
visual function may have rehabilitative implications.
Another way in which visual perceptions may be dissociated is in the syndrome of unilateral inattention or neglect.
Patients with this defect may appear to have normal visual
function if tested in routine fashion, since they can correctly
perceive objects in each hemifield with either eye. However,
when two test objects are presented in the right and left
hemifields of each eye simultaneously, the patient perceives
only the test object in the hemifield ipsilateral to the lesion.
This phenomenon, called visual extinction, can occur following damage to the parietal or occipitoparietal cortex as
well as to the frontal lobe and thalamus. It can be demonstrated not only in patients with partial hemianopic defects
on the same side but also in patients with previously recovered hemianopias or no documented visual field defects.
Unilateral neglect can result from lesions in several different
regions of the brain. This finding reflects the complex inte-
564
grated network that exists for the modulation of directed
attention within extrapersonal space (291). Functional MR
imaging can demonstrate activation of visual cortex without
activation of more distal cortical processing areas, demonstrating an anatomic basis for extinction (517).
lated to direct downward pressure on the cerebellum by the
tumor.
Nonvisual Symptoms and Signs of Occipital Lobe
Disease
Spontaneous recovery of homonymous visual field defects occurs in no more than 20% of patients within the first
several months after brain injury (523–525). Thus, despite a
certain amount of plasticity even in the adult cerebral cortex
(526,527), patients with visual field defects have a consistently poor rehabilitation outcome (528–530). The exact anatomic location of the lesion causing the homonymous hemianopia does not appear to affect the functional outcome
(531); however, the greater the number of associated neurologic deficits, the more difficult the rehabilitation and the
poorer the functional performance (532). Contributing further to the poor functional improvement is the advanced age
of most of these patients, a factor associated even in normal
individuals with progressive cognitive, sensory, and motor
deficits (533). Finally, the presence of neglect, especially in
patients with nondominant hemisphere lesions, also interferes with the rehabilitation process.
Several techniques may assist in the treatment and rehabilitation of patients with homonymous hemianopia (534). A
mirror attachment can be used to project the mirror image
of the blind field into the seeing half-field (535). The mirror
is attached to the nasal side of the spectacle frame behind
the lens. Burns et al. (536) found that three of six patients
learned to make some use of the device. For many patients,
however, such mirrors do not prove satisfactory because the
patient has to turn the head toward the mirror, because the
mirror produces a scotoma that blocks a portion of the seeing
field, and because spatial disorientation is common. Images
seen in the mirror are reversed and projected to the opposite
side of the midline (537), causing confusion. Mirrors on
prongs fixed to a frame are available, but they are usually
discarded by the patient because of the cosmetic appearance
of such devices. Mintz (538) devised a small adjustable mirror that is mounted on a clip placed over the spectacle frames
on the nasal side. Waiss and Cohen (539) suggested the use
of a temporal mirror coating on the back surface of spectacles.
Smith (540) advocated the use of Fresnel press-on prisms
in patients with a homonymous hemianopia. The prism is
placed on the outside half of the lens ipsilateral to the hemianopia with the base oriented toward that side (i.e., for a
patient with a left homonymous hemianopia, a 30-diopter
prism was placed base-out on the left half of the left lens).
Perlin and Dziadul (537) advocated placing 20 prism-diopter
Fresnel prisms in front of both eyes, allowing for a 5⬚ movement of the eyes before they encounter the prism edge (see
also [541]). Prism power and placement are then modified,
depending on patient adaptation. The goal is to increase the
patient’s scanning skills over time. Rossi et al. (542) randomly assigned 39 patients with stroke and homonymous
hemianopia or unilateral visual neglect to treatment with
15 prism-diopter plastic press-on Fresnel prisms or to no
treatment. Although the prism-treated group later performed
As might be expected, vascular lesions of the occipital
lobe are usually asymptomatic, except with regard to the
visual system. Many patients with occipital infarctions experience acute pain of the head, brow, or eye ipsilateral to the
lesion. This presumably reflects the dual trigeminal innervation of the posterior dural structures and the periorbital region (518). Patients with homonymous hemianopia may also
complain of disturbances of equilibrium, particularly a feeling that their body is swaying toward the side of the hemianopia. Rondot et al. (519) called this phenomenon ‘‘visual
ataxia’’ and proposed that the postural defect reflects unopposed tonic input of vision from the intact hemifield rather
than any true vestibular impairment or neglect.
If a homonymous hemianopia is unrecognized by the patient, the first signs of its presence may be the functional
consequence of hemifield blindness. Most clinicians have
never seen the ‘‘door sign’’ (520), in which a patient with
homonymous hemianopia acquires a linear wound in the
forehead ipsilateral to the field defect from inadvertently
striking the head on a door frame. However, it is not unusual
to see patients with homonymous hemianopia that was discovered after a motor vehicle accident in which little or no
injury was sustained to account for the field defect or the
infarct in the occipital lobe (subsequently detected by neuroimaging) that clearly predated the accident.
If the posterior cerebral artery is occluded proximally,
patients with an infarct in the occipital lobe may also have
hemiplegia from damage to the posterior internal capsule
or cerebral peduncle, language dysfunction from damage to
posterior parietal structures, or symptoms reflecting damage
to the ipsilateral thalamus (380,521). In addition, because
the condition is ordinarily seen in patients with severe atherosclerosis, there may be other evidence of vertebrobasilar
insufficiency, including ocular motor abnormalities referable to the rostral brain stem. Patients who develop a bilateral
homonymous hemianopia in this setting may be more severely impaired than patients with blindness from bilateral
retinal or optic nerve disease and may be difficult to rehabilitate (see below).
Tumors of the occipital lobe may cause nonvisual manifestations by virtue of their mass effect. Parkinson and Craig
(522) examined the signs and symptoms of 50 patients with
verified tumors limited to the occipital lobe. Headache was
the most common symptom, occurring in 45 patients (90%).
Other symptoms and signs included nausea and vomiting
(46%), ataxia (30%), hallucinations that were usually unformed (24%), seizures (18%), and mental changes (16%).
Most of the signs and symptoms noted by Parkinson and
Craig are nonlocalizing and related to increased intracranial
pressure. Ataxia and other cerebellar signs are usually re-
TREATMENT AND REHABILITATION FOR
HOMONYMOUS HEMIANOPIA

Open PDF in new window

Transcription

Similar documents

Dynamic Range

Class 8 (3/10) review presentation

What Is the evaluatIOn fOr a hOMOnyMOus heMIanOpIa?

2010 - Stanford Math Brain

Colpocephaly

Somatosensory Cortex

Neuroanatomy - UCSD Cognitive Science

LECTURE 5Crystalline Lens pathology

Prevention of Occipital Pressure Ulcers In Neonates