1 Anatomy Direct laryngoscopy (DL) primarily requires displacement

Anatomy
Direct laryngoscopy (DL) primarily requires displacement of the tongue, epiglottis, and the
hyoid bone (to which the tounge and epiglottis are secured) into the subglossal space. Changes
in axial alignment are useful because they facilitate those displacements.
These illustrations are for review and orientation. Spatial and dynamic relations are better
appreciated by radiology and palpation, which follow.
Fig. A. Cadaver, sagittal section. The preserved structures are more collapsed than in vivo. B.
Larynx, medial view. (Netter).
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Thyroid cartilage,
anterior shield
Hyoid bone,
body
hyoepiglottic
ligament
epiglottis
Vocal cords
(glottis)
Cricoid cartilage
signet plate
Greater horn,
dorsal tip
Fig. Laryngeal skeleton: Xray and co-labeled illustration. The hyoid bone is palpable at the
junction of the neck and chin. The greater horns of the hyoid bone project bilaterally to abut the
posterior pharyngeal wall, forming a rigid strut for the upper larynx. From the hyoid on three
sides a flexible sheet-like ligament, the thyrohyoid membrane, suspends the thyroid cartilage
when the subjext is upright. The cords lie at the mid-thyroid cartilage and mark the rima
glottidis (glottis). The posterior (vocal) cartilages are prominent cephalad projections. The
thyroid cartilage is secured by a posterior articulation to the cricoid cartilage, which restricts
their interaction to flexion and extension. The cricoid cartilage is the only complete ring
structure not only for the larynx, but between the environment and the mainstem bronchi. The
open posterior wall (mouth, pharynx, upper larynx), and soft posterior wall shared by the
trachea and esophagus are important to airway tracking the full range of head movement. (ref)
The leaf-shaped epiglottis is seen on edge passing between the greater horns. The epiglottis is
suspended from the hyoid by the hyoepiglottic ligament and secured by a stem-like ligament to
the anterior wall of the thyroid cartilage. The valecula is the space between the epiglottis and the
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tongue. The valecula is divided by the median fold of the hyoepiglottic ligament, which in DL
functions as a leverage cable to elevate the epiglottis. Via the stylohyoid ligament (SHL) the
hyoid is secured to the styloid process, which is seen projecting from anterior to the mastoid
process, just posterior to the anterior arch of the atlas, toward the hyoid. Direct laryngoscopy
requires the hyoid, tongue and epiglottis be displaced anterior to a line from the upper teeth to
the glottis. Tension in the SHL can prevent forward displacement of the hyoid and therefore
also the tongue and epiglottis.
hyoid
thyroid
cartilage
Styloid process
Fig. Direct laryngoscopy (DL) with a curved blade. DL is primarily forward and caudal displacement
of the tongue and epiglottis, and therefore the hyoid bone, into the subglossal space. Although often
no change in axial alignment is required for DL, in this figure cervical spine flexion and a-o extension
contribute to glottal exposure. C-D indicates lift of the cricoid off the spine. The styloid processes are
visible, one posterior to the ramus, the other crossing the anterior arch of the atlas. (Modified from
Nishikawa.)
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Anatomic basis for head elevation in DL:
Conventionally the objective of axial manipulation is to align the oral, pharyngeal and laryngeal
axes (three axes alignment theory, TAAT). The TAAT has been recognized as deficient (Chou,
Hochman), and reference made to soft tissue displacement and the role of tension in anterior
cervical tissues, but no analysis of axial manipulation in DL has been found satisfactory.
Although maximal head elevation is demonstrably the most effective axial maneuver in difficult
direct laryngoscopy (DL) (Hochman, Jackson, Schmidt, Levitan), the mechanism by which
maximal head elevation improves difficult DL has not been elucidated. We pursued geometric
analysis, as recommended by Chou (Chou ref).
We propose the mechanism by which flexion facilitates DL is based on a strutted cable
suspension system that causes the airway to track the full range of head movements, at the same
time splinting the airway open. (Fig. Strutted cable suspension, and Fig. Strutted cable and
crane suspension of larynx.)
The airway skeleton - the mandible, hyoid bone, and thyroid and cricoid cartilages comprise a
tube of rings, all open at the back except the cricoid. Flexibility derives from a primarily
ligamentous suspension from the skull and connection between the rings, and the absence of
bony connection with the somatic skeleton. The ligaments most relevant to the effect of flexion
to facilitate DL are the paired stylohyoid ligaments.
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Fig. Laryngeal support is primarily a strutted cable suspension. The mandible, hyoid bone and thyroid
cartilage are open-ring structures that splint the airway but allow it movement in all directions. The hyoid, a
U-shaped bone palpable at the juncture of the neck and chin, is the primary strut preventing laryngeal
collapse, as essential when pulled backward in cervical extension. The hyoid has no bony connection to the
somatic skeleton and is suspended from the mandible and skull by a complex sling of muscle and ligaments.
The stylohyoid ligaments are the primary suspension for the larynx; they limit movement to a radius their
length, and with cervical extension pull dorsally. The thyroid cartilage is suspended from the hyoid by the
thyrohyoid membrane, which is a flexible sheet of ligament that allows the two cartilages relatively
independent movement. The sling suspension of the hyoid and its dependent larynx causes the airway to
flexibly track movements of the head and neck while fixing the maximal skull-larynx distance.
The only ligamentous hyoid support is the bilateral stylohyoid ligaments (SHL), which extend
from each side of the hyoid up and back to the styloid processes of the skull. The styloid
processes are pen-like projections of about 30 mm in length that originate just in front of the
mastoid processes. Although tension in the sling muscles can initiate hyoid movement in almost
any direction, the cable-like SHL tightly restricts hyoid movement within a radius defined by
the length of the SHL. The remainder of the larynx is suspended from the hyoid by the flexible
sheet-like thryohyoid ligament.
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Fig.: Strutted cable and crane suspension of larynx. Bars indicate styloid process, SHL, greater horns of
the hyoid, thyrohyoid membrane, glottis and anterior tracheal wall. Blue dots approximate centers of
rotation (Penning). When the head moves backward the SHL pulls the hyoid dorsally until the ends of
the U-shaped hyoid (and the posterior walls of the thyroid cartilage) abut the posterior pharyngeal wall.
The SHL cable suspends the larynx and the open-ring structures splint the airway open. Continuing the
engineering analogy, the spine functions as a jointed crane. Cervical spine flexion rotates the mandible
forward and down (regardless of whether the mouth is open). Forward movement of the mandible
increases tension in anterior sling muscles such as the hyomentum, which tends to pull the hyoid forward
and swing away from the posterior wall. The degree of cervical flexion required for the hyoid to swing
free is reduced by the styloid process, which serves as a fixed angle boom extending the suspension point
forward and caudad from its base just anterior to the mastoid process. This forward displacing the center
of SHL rotation by the styloid process allows the elastically-drawn hyoid to be pulled forward as soon as
cervical extension begins. (Hence the advantage even of minimal flexion as commonly practiced.) As the
head moves forward the hyoid moves from dorsal to the thyroid cartilage to forward of it. This
diminished movement of the thyroid cartilage compared to the hyoid is because force forward on the
thyroid cartilage is via elastic tissue (muscle at rest), and because the lower larynx (cricoid cartilage) is
tethered by the cricopharyngeus muscle to C5-6.
It is instructive to palpate this dynamic relationship. By placing a finger on the anterior
prominence of the hyoid bone and thyroid cartilage while moving the head fore and aft,
extension can be felt to move the hyoid posterior to the thyroid cartilage, then flexion to move it
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anterior. Because the tongue and epiglottis are hyoid-based and the glottis is in the thyroid
cartilage, this palpable movement indicates the effect of flexion to facilitate DL. Maximal
flexion may be helpful or even essential for LD in difficult cases.
Fig. Lifting force in DL can be directly opposed by SHL tension.. The hyoid bone is visible at the
juncture of the chin and the neck. The greater horns of the hyoid (Strut) rest against the posterior
pharyngeal wall. For DL the tongue and epiglottis, both secured to the hyoid, must be lifted above the
line of sight (LOS). In many patients lifting force applied to the hyoid provides glottal exposure without
axial manipulation, i.e., as is desirable in trauma patients. However when the cervical spine is in the
neutral or extended position the lifting force is almost in line with the stylohyoid ligament (SHL). If the
SHL is already tense anterior displacement may not be possible. What then may be interpreted as an
“anterior larynx” is a dorsally secured hyoid. (White arrow indicates a foreign body in the upper
esophageal opening.) (modified from …; permission pending identification of source).
The role of the SHL to hold the hyoid back against the posterior pharyngeal wall is the
mechanism by which extension impairs DL. Specifically, for DL the tongue and epiglottis must
be anterior to the glottal line of sight. The tongue and epiglottis are secured to the hyoid.
Extension causes a ligamentous pull on the hyoid, via the styloid process and SHL, which
secondarily pulls the tongue base and epiglottis dorsal to the thyroid and glottis and the line of
sight. Cervical flexion causes the styloid tip to swing forward, which allows forward
displacement of the hyoid (and tongue and epiglottis) relative to the thyroid cartilage and
glottis. The cricoid remains secured at C5-6.
The airway is a tube of rings. In the neutral or extended position the mid-portion of the tube is
drawn backwards by tension on the highest laryngeal strut, the hyoid, against the spine. The
spine is a jointed crane. Cervical flexion rotates the base of the suspending ligament forward
and caudal, which allows the balance of suspending muscles to move the hyoid closer to and
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forward of the thyroid cartilage (Fig. Tissues anterior to the spine centers of rotation are
approximated by flexion.), from which it can be displaced further by the laryngoscope blade.
The physics dictate that increase in cervical flexion to maximal results in additional slackening
on the SHL cable.
Fig. Tissues anterior to the spine centers of rotation are approximated by flexion. Extension pulls the
styloid and thyroid catilage firmly against the posterior pharyngeal wall (shown). Blue dots
approximate centers of rotation (Penning). Red lines indicates approximate position of the stylohyoid
ligament. Flexion of the cervical spine rotates the mandible around the hyoid so that sling support
tension in the anterior muscles of the hyoid is toward the mandible and forward of the thyroid
cartilage. Since the axes of rotation are behind the styloid process and the hyoid, flexion shortens the
distance between them so also relieves supero-dorsal tension on the stylohyoid ligament. This allows
the hyoid movement forward and provides additional slack for forward displacement of the hyoid by
the tip of the laryngoscope blade. (Modified from Penning)
Supported flexion of the upper thoracic spine
For maximal laryngeal exposure in difficult cases pioneer laryngeal surgeon Chevalier Jackson
emphasized maximal head elevation, and this dictum has been reinforced by Levitan and others
(Jackson 1934, Levitan, Hochman, Schmidt). Maximum head elevation involves flexion of the
upper dorsal spine as well as the cervical spine. Because the lower larynx is secured to the spine
at C5-6 we do not expect flexion of the dorsal spine to directly influence hyoid mobility.
However there appear to be two mechanisms by which flexion below C6 can aid DL. Most
obvious, in many cases contact between the laryngoscope handle and the chest wall impairs
maximal cervical flexion.
Impaired blade insertion due to contact of the laryngoscope handle with the chest may be
caused by morbid obesity, limited a-o extension, small mouth, recessed chin, large breasts, or
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flexing the neck. Mouth opening and blade insertion may be facilitated by use of a shoulder roll
or curved ramp to flex the upper thoracic spine and raise the head and mandible forward relative
to the chest without extending the neck or impairing a-o extension.
The second mechanism by which dorsal flexion aids DL is subtler. It was implicit in remarks by
Chevalier Jackson (ref), and suggested by our observation that in some cases when maximal
cervical flexion with minimal dorsal spine flexion provides suboptimal glottal view, the view
can be improved by flexing the upper thorax. We suggest that as the neck is flexed the lower
anterior neck tissues become displaced below the sternal notch, as can be observed in most
subjects by palpation. Less tension in both the anterior muscles and the SHL should allow easier
laryngeal displacement forward but that effect is countered by the lower compliance of the
mediastinum at the sternal notch. The compliance limitation is reduced as the neck structures
move above the sternal notch.
When passive support of the upper trunk has not been placed in advance and difficult intubation
is encountered, the operator seeking the advantage of maximal flexion often must add her own
gross effort to that of assistants called urgently to lift much of the patient’s weight. Although
usually it will prove unnecessary, a shoulder roll or curved ramp construction is occasionally a
vital component of the appropriate starting position for any patient where unpredicted difficult
intubation is a possibility. The ramp must be curved to achieve dorsal flexion; elevation of the
head without changing axial alignment, as by raising the back support of the bed, is not helpful.
Alignment of axes: revised concept:
The alignment of airway axes that must occur for DL is achieved primarily through forward
displacement of the hyoid bone, tongue and epiglottis.
Restricted anterior displacement in the supine position is most likely due to dorsally directed
tension in the SHL as an effect of its role to limit distance of the suspended laryngeal strut from
the skull base.
That distance and SHL tension is diminished by flexion of the lower cervical spine (and less
than full a-o extension), which allow the larynx to swing away from the spine. Flexion of the
upper dorsal spine may also improve glottal view, probably by increase in compliance of the
subglottal space as tissue is lifted from the retrosternal space.
The most prominent effect of axial movement is cervical flexion release of dorsal tension,
which then allows forward swing of the hyoid relative to the thyroid cartilage. Other axial
influences may include thoracic flexion lift of the anterior cervical tissues from the low
compliance thoracic inlet, slight contribution to forward rotation of the axes by thoracic spine
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flexion, and changes in tension in anterior cervical tissues. In addition to forward displacement
of soft tissue, the thyroid cartilage can be rotated forward at its articulation with the cricoid
cartilage, and relaxation of the cricopharyngeus muscle allows the cricoid to be pulled away
from the thoracic spine. The sum effect of these factors can be alignment of airway axes and DL
at any angle from about 20 degrees (in full cervical extension) to past vertical (in maximal head
elevation).
When the head is elevated or sustained by force of the laryngoscope blade tip behind the hyoid,
elevation may be substantially by transfer of lift force through the SHL to the skull, then neck
and even upper thorax. Elevation of the head by an assistant relieves that unnecessary pressure
on laryngeal tissue and may convert the operator’s effort from a gross lifting effort to a delicate
manipulation of the tongue and epiglottis.
Temporomandibular joint
Mouth opening involves a hinge action followed by a gliding forward at the temporomandibular joint (TMJ). The gliding action displaces the lower incisors relative to the upper by
0 to 12 mm (refs). This prognathic movement can be limited by arthrosis or simply tight
ligaments, and can be increased by firm pressure behind the mandibular rami with the jaws
hinged apart. Sustained jaw thrust by an assistant during DL can reduce the operator’s required
lifting force while DL is underway.
Fig. TMJ. The hinge and glide
sequence in the TMJ. (Netter)
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Mouth: operator perspective
The most obvious oral impediments are the teeth and tongue (Fig A). At the back of the tongue
the tonsillar pillars and fauces are visible on either side as they course from the soft palate to the
base of the tongue, in effect creating a tubular inlet to the oropharynx.
Fig. View into mouth: A. The faucial/tonsillar pillars and palatine tonsil between form an
isthmus between mouth and pharynx. Lifting the tongue anterior can tense the palatoglossal
pillar and impair lateral approach. B. Laryngoscopy from a lateral approach. Gray lines: less
distance from teeth to larynx is apparent.
Anatomy and Mobility of the tongue
The mobility of the tongue and its floor are fundamental to soft tissue displacement. The floor
of the mouth is predominantly formed by the mylohyoid muscle, which slopes from attachments
around the mandible to form a midline raphe anteriorly and to the hyoid bone posterior. This
arrangement is comparable to the levator ani that forms the floor of the pelvis (ref Last). Above
the mylohyoid the geniohyoid muscle adds to the floor of the mouth and contracts the tongue
base.
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A.
C.
B.
Fig. Floor of mouth. A. From below. The mylohyoid forms a midline raphe anterior to the hyoid and
A.
attaches to the anterior hyoid (body). B. From above. The laterally-directed mylohyoid is supplemented
by the antero-posterior geniohyoid. C. Coronal section. The hyoglossus muscle secures the tongue to the
hyoid. A paraglossal channel is apparent between the tongue root and the mandible. x
Fig. Muscles of the tongue. Laterally the tongue is secured to the hyoid by a vertical sheet of muscle,
the hyoglossus muscle. The bulk of the tongue is the intrinsic genioglossus muscle, which arises from
the mandible anteriorly and extends fibers to the hyoid bone posterior; the hyomental distance is a measure
of the length of the tongue base.
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Often adhesion of the laryngoscope blade to the tongue distorts the tongue back against the epiglottis
and interposed between the blade and the hyoid. (Fig. Peardrop).
Fig. Peardrop distortion. Rather than sliding into the
valecula the blade has distorted the tongue caudally,
impairing further anterior displacement. The line is from
the incisors to the anterior airway point T. (Horton).
Bimanual laryngoscopy should minimize this problem.
The posterior third of the dorsum of the tongue looks backward, and contains numerous
submucous adenoid collections and lymph follicles called the lingual tonsil. Hypertrophy
of the lingual tonsil has been described as a common and important cause of unpredicted
difficult intubation where the epiglottis cannot be lifted from a dorsal approach over the
tongue (Ovassapian 2002) (Fig. Hypertrophic lingual tonsils).
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Hypertrophic lingual tonsils impair displacement and are prone to bleed with minimal trauma.
Bimanual examination enables delicate manipulation. (Fig. Bimanual laryngoscopy.) Pressure
on the hyoid should minimize displacing the tongue while gently exploring with the blade. A
lateral straight blade approach should be considered if there is suspicion of hypertrophied
lingual tonsils.
Tongue, epiglottis and the laryngeal inlet: dynamic anatomy.
Styloglossus muscle in anterior tonsilar pillar
Hyoepiglottic ligament, median fold
Valecula
Epiglottis
Pharyngoepiglottic fold
Piriform recess
Fig. Section through pharynx, superior view of larynx. The valecula, between the epiglottis and
the tongue, is divided by the median fold of the hyoepiglottic ligament.
The epiglottis is a slightly curled leaf-like cartilage with a stem secured anteriorly to the thyroid
cartilage by a band ligament just above the vocal ligaments (thyroepiglottic ligament). The
epiglottis is suspended from the hyoid bone by the hyoepiglottic ligament (HEL), which is
comprised of a midline and two lateral folds. The lateral folds extend to the pharyngeal wall,
forming the pharyngoepiglottic fold, which separates the valecula from the piriform fossa posterior.
A midline fold divides the valecula into two oval recesses. When covered with mucous membrane
the midline fold is called the glosso-epiglottic membrane. This median hyoepiglottic fold forms a
sensitive lever to elevate the epiglottis.
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The laryngeal inlet (aditus)
The vestibule of the larynx is a deeper and more tubular structure than is apparent from the
operator’s perspective. Entry is via an oval introitus that is slanted away from the operator.
vestibule
Vocal ligament/glottis
A.
hyoepiglottic
ligament.
Cricothyroid articulation
(behind cricoid shoulder )
thyrohyoid membrane
B.
aryepiglottic fold
Fig. Larynx, lateral view. A. Sagittal view. Hyoepiglottic ligament is loose areolar tissue that
pressed edge-on serves to leverage the epiglottis. Cricothyroid articulation allows thyroid cartilage
to flex forward on cricoid. B. Addition of membranes (aryepiglottic fold) shows depth of vestibule.
Papillae of dorsal tongue
Epiglottis
Tubercle (cushion) of
epiglottis
Valecula
Vocal cords
Vestibular folds (false
cords)
Aryepiglottic fold
Glottis
posterior (vocal)
cartilages
Interarytenoid notch
Piriform recess
Fig. Laryngeal vestibule, operator view. A. The depth of the vestibule is not apparent from the
operator’s perspective. Prominent projection of the posterior cartilages can be discerned. They and
the inter-arytenoid notch are important landmarks when only the posterior portion of the larynx is
visualized. Descending into the laryngeal inlet, one first encounters the vestibular folds, or “false
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vocal cords.” A red horizontal structure below the cords is the superior edge of the cricoid cartilage.
The dark space above the cricoid is the site of the cricothyroid membrane; it is a depression in the
anterior tracheal wall that readily catches the tip of an anteriorly directed endotracheal tube. (Sobata)
B. Enhanced operator view. Elevation of the epiglottis stretches the lateral aryepiglottic folds, which
are elastic membranes that slope backwards from the epiglottis to the arytenoid cartilages. The edgeon appearance of the aryepiglottic folds makes their height above the glottis unapparent to the
operator, which explains their common diversion of the tip of the endotracheal tube into the piriform
recess and the esophagus. (Martini)
The “epi flip”: direct and indirect observation of leveraging function of median fold of the
hyoepiglottic ligament.
Removal of the skull just above the TMJ of a supine cadaver exposed the oropharynx and allowed
direct independent manipulation of the tongue and the membrane-covered hyoepiglottic ligament
(HEL). Gently lifting the tongue opened the median fold of the HEL to its unstressed length; additional
lift caused the epiglottis to rise with the tongue, with the HEL appearing as a cable between the two (as
in Fig. Section through pharynx, superior view of larynx.). With the tongue elevated by one blade,
pressure by a second blade tip against the center of the leading edge of the median fold caused a brisk
elevation of the epiglottis simulating cable leverage. Simulation of clinical probing, i.e., using the same
blade to lift the tongue at the same time as it served to probe for the optimal means to lift the epiglottis,
changed the dynamics only slightly. Movement of the blade tip against the HEL gave approximately
the same sensitive elevation of the epiglottis but the optimal blade tip placement and direction that
provided maximal elevation of the epiglottis was slightly altered by the altered angle of the blade and
by subtle changes in how the tongue rested on the blade. Most important for clinical purposes,
continued small forward and lifting motions while repositioning the blade tip allowed rapid empiric
location of the optimal point and direction. (link to video)
Right: From the right side
of the mouth, looking
down the lumen of a
straight blade at the
leading edge of the
epiglottis, which obscures
the larynx except one of
the posterior cartilages.
Fig. Less than full elevation of the epiglottis causes the laryngeal aperture to slant away from the operator
and obscure the glottis. The optimal maneuver to elevate the epiglottis usually is not apparent by DL.
Left: Above the prominent tip of the epiglottis the mucosal pattern typical of dorsal tongue indicates
blade tip placement too shallow relative to the hyoepiglottic ligament. This useful view is not available
with curved blade except via fiberoptic viewing. Right: Often the direct view does not indicate whether
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placement is too deep or too shallow. Then trial bimanual manipulation of blade and externally on the
larynx effectively defines optimal positioning.
Line drawing and corresponding xrays convey a range of relations between the blade, epiglottis,
hyoid and glottis.
Fig. Supine at rest x-ray. The epiglottis is projecting between the greater horns of the hyoid which rest
against the posterior pharyngeal wall. Suspension of the epiglottis via the median fold of the HEL can
be appreciated in the open curved tissue immediately below the body of the hyoid. The edges of the
delicate aryepiglottic folds curve back from the tip of the epiglottis to form a “Robin Hood” hat, and
end on the arytenoids, which rest on the shoulders of the cricoid. The vertical black-white line
interface dropping from the anterior thyroid cartilage, (the back of the ‘hat’), is the vocal cords,
therefore the glottis. With the head resting on the table, in mild extension, the hyoid is drawn dorsal to
the thyroid cartilage. Arrow: Foreign body marks the piriform sinus. Overlapping styloid processes
cross the posterior edge of the anterior arch of the atlas, pointing toward the hyoid body.
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throhyoid
membrane
Flip
E
Fig. DL x-ray (enhanced) and drawing. The blade tip is directly under the hyoid body, inferred
pressing against the hyoepiglottic ligament; the tongue is fully displaced above the line of sight,
which is through the mid-glottis (line). E indicates displacement forward of the thyroid and
cricoid cartilages, which demonstrates the potential, were glottal view not adequate, for external
pressure to move the thyroid cartilage, therefore glottis, into view. In the x-ray both styloid
processes are visible, one pointing into the ramus, the other crossing the anterior arch of the
atlas.
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Sighting and controlling the epiglottis is fundamental to success in difficult cases. Two categories are
back-flexing of the epiglottis and indeterminate view of the epiglottis.
Back-flexed epiglottis: When the epiglottis is not sought carefully during initial advance of the blade,
frequently it is flexed backwards. It can be flexed back on itself, rising out of sight and revealing the
glottis. This experience of success encourages the novice to insert the blade to an estimate of the right
depth or deeper and lift as a first try, and use caution only when that fails.
The back-flexed epiglottis may remain in view, distorted into shapes suggestive of laryngeal landmarks
that invite inappropriate probing. Or the back-flexed epiglottis may cover the laryngeal opening and the
stretched open esophageal orifice can resemble a satisfying view of the larynx.
Fig. Epiglottis backflexed over glottis (left), and into glottis (center). Right: Esophagus exposed.
Mucosal irregularities help simulate entry to larynx.
Indeterminate view. It is common to sight the epiglottis but not obtain a brisk and complete
elevation. Four indeterminate views that are problematic and correctible include: peardrop
distortion, discussed above; shallow placement; deep fixed placement; and ‘can’t turn corner’.
Fig. Indeterminate view. A straight blade inserted via the
right corner of the mouth reveals the brightly-lit tip of an
epiglottis obscuring all but one posterior cartilage. Strong
blade lift of the blade yielded little improvement. There
are several blade tip-anatomy positions in which the
epiglottis can be seen but lifts poorly that merit discussion
because clinically they can be distinguished and corrected.
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Fig. Indeterminate blade position, shallow placement. Tongue villi above the epiglottis (fiberoptic view on right)
informs shallow placement, but usually this is not visible on DL. Blade advancement might locate the blade on
the “sweet spot, but instead might convert to other problem views (see below). Bimanual laryngoscopy facilitates
identification of the problem and usually will prevent its occurrence.
Fig. Indeterminate blade position, deep blade tip with fixed epiglottis. When the blade is advanced keeping the
epiglottis in sight but without a pulsing blade or external pressure the blade tip can contact the HEL too low
(dorsal) and immobilize the epiglottis. Absent a visible blade tip indicator, contact with the epiglottis may not be
appreciated. The immobilized epiglottis can appear in a normal position but lift sluggishly, as with other
indeterminate positions.
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Fig. Indeterminate position: Can’t turn corner. Possible causes include: contact between flange and upper teeth
prevent adequate forward rotation of the blade, or the blade may be shorter than ideal for that patient, or
anatomic factors (e.g., short thyromental distance) may contribute, or elasticity of the dorsal tongue may be
diminished, as by lingual tonsil hypertrophy. In most cases it is likely that the epiglottis can be elevated by
pressing on the larynx to move the ‘sweet spot’ back onto the blade (Levitan, Benumof), especially if the head is
maximally elevated. Manipulation of lingual tonsil hypertrophy may cause profuse bleeding; bimanual
laryngoscopy allows more complex and delicate manipulation than probing only with the blade tip.
In the most challenging cases the tongue mass cannot be displaced sufficient even to see the tip of the epiglottis.
However, this occurred in only two of 1500 OR cases when maximal head elevation and bimanual laryngoscopy
was applied (Schmidt).
Anatomy of bimanual laryngoscopy
Bimanual laryngoscopy converts laryngoscopy from probing an unstable complex tissue
with a bar to multidirectional manipulation of stabilized tissues.
External pressure anywhere on the laryngeal skeleton can improve or compromise
glottal view through unpredictable changes in soft tissue relationships. However, specific
manipulations are more likely to be useful in particular stages of DL.
As the blade is advanced over the tongue adhesion between tongue and blade can cause the
tongue to be distorted dorsally, which can prevent sighting the epiglottis tip, or entry of the
blade tip into the valecula, and/or can cause the tongue to press the epiglottis dorsally. Bimanual
laryngoscopy can prevent or break adhesion by stabilizing the tongue.
Therefore as the blade is advanced the optimal site for external pressure may be on the
hyoid, to stabilize the tongue and prevent dorsal distortion. As the blade tip approaches pressure
on the hyoid can prevent its forward migration, or press the valecula back onto the tip of the
blade (ref Levitan). Small oscillations of external pressure on the hyoid may facilitate
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identification of the ‘sweet spot’ more easily than similar manipulations of the blade tip
internally.
Pressure on the thyroid cartilage will not cause movement dorsally until the hyoid has
been lifted anterior by the laryngoscope with the thyroid following passively, as seen in Fig.
(Nishikawa). However cephalad pressure on the thyroid cartilage may aid glottal exposure by
subtending the hyoid.
Pressure on the cricoid cartilage is more likely to compromise than improve glottal view.
However sustained cricoid pressure during DL is indicated if inadvertent gastric insufflation
may have increased gastric pressure above opening pressure of the lower esophageal sphincter,
in which case the esophagus may be an open conduit for flow of gastric content, even after
chemical paralysis.
Anatomy of cricoid pressure
Recommendations to apply pressure on the cricoid cartilage to prevent gastric insufflation (GI)
and regurgitation are anatomically rational based on the normal position of the esophagus
between the spine and the broad dorsal signet plate of the cricoid ring. Studies have consistently
shown cricoid pressure prevents GI, but protection from regurgitation and aspiration is much
less reliable.
Protective effect of natural barriers to GI is collectively measured as the opening pressure of the
upper esophageal sphincter. We presume the barrier is more than the muscular tone of the
cricipharyngeus muscle sphincter because the latter varies from rigid fixation that resists
forceful lift (Jackson) to wide open with minimal lifting force (Fig.___). In code situations
adhesion of the anterior and posterior surfaces of the piriform recess might provide important
protection from GI, because in fresh cadavers moderate to high mask pressure is required to
initiate GI, but once the upper esophageal sphincter has been breached, GI occurs at much lower
mask pressure. Especially important in codes, jaw thrust can cause GI to become voluminous
even at low pressure.
Failure of cricoid pressure to protect from regurgitation is at least partly because the
esophagus can slip to the side of the cricoid signet, so not be compressed (ref). Cricoid pressure
during active emesis can rupture the esophagus (ref). Of equal importance to the prophylactic
value of cricoid pressure is awareness of its propensity to cause airway obstruction and interfere
with laryngoscopy.
Fig. Cricoid pressure causing laryngeal
obstruction. The airway can be obstructed even
when pressure is only on the cricoid. Diagnosis
may require changing or removing cricoid
pressure.
Cricoid pressure may distort view on the larynx,
as here or worse. Concern for imminent
regurgitation, e.g., possible gastric insufflation,
variceal bleeding, may justify continued cricoid
pressure during laryngoscopy until the
endotracheal tube is as close as possible.
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