Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4382_Библиотеки_им_академика_М_И_Перельмана
.pdf
88 Rotational Vestibular Assessment
extensor motorneurons. This tonically active neural input to the extensor muscles is critical when
resisting the persistent forces of gravity (Lysakowski et al., 1998). Without such neural input,
postural control would cripple under the driving
force of gravity and the body would be unable
to support an upright posture. Collectively, the
MVST and LVST provide a sense of postural
“uprightness” that is threatened in the presence
of an acute unilateral labyrinthine lesion. The lack
of tonic symmetry in the presence of an acute
unilateral vestibular lesion, particularly in the
absence of visual cues, creates a leaning posture
and tendency to fall toward the side of lesion due
to a lack of tonic input on the side of the lesion.
This is the underlying principle for an abnormal
Rhomberg or Fakuda Stepping Test. This duality of head and ocular reflex responses becomes
functionally relevant if one is to visualize how
compensatory head and eye movements are critically needed when reading while walking. Not
only is the head “held” stable on the shoulders
with every step, due to the MVST, but concomitant compensatory eye movements are produced
via the VOR in order to maintain visual stabilization, due to the “bouncing” of the head with each
step. A relevant example of pathologic vestibulocollic and vestibulospinal contributions is evident in Mal de Débarquement syndrome, where a
conflict is experienced between spinal and vestibular inputs.
Autonomic Non-Vestibular Projections
Although not completely known or understood, a
variety of other afferent and efferent fibers project
to the vestibular nuclei. The thalamus is known
to have some projections known as the vestibuleophthalamocortical pathways (Arslan, 2001).
Lysakowski and colleagues (1998) hypothesized
that, in the absence of any visual stimulation, thalamic projections may, in fact, provide a conscious
awareness of vertigo, or self-motion.
Finally, a discussion of the peripheral and central
vestibular system would not be complete without
discussing the vascular supply to the vestibular
system (Figure 3–6). The primary blood supply
to the vestibular system and its end organs is
through the labyrinthine artery, also known as the
internal auditory artery. It arises from the anterior
cerebellar artery, the superior cerebellar artery, or
the basilar artery (Lysakowski, et al., 1998). Most
commonly, the labyrinthine artery arises from the
anterior cerebellar artery (Wende, Nakayama,
& Schwerdtfeger, 1975). The labyrinthine artery
courses through the IAC together with the labyrinthine vein (in addition to the VII and VIII cranial nerves). Shortly after exiting the IAC fundus,
the labyrinthine artery branches into two distinct
branches: the anterior vestibular artery, and the
common cochlear artery. Despite the name of the
latter, the vestibular system is supplied by both
branches (Lysakowski et al., 1998). The anterior
vestibular artery supplies the utricle as well as
the superior and horizontal SCC. The common
cochlear artery divides into the proper cochlear
artery and the vestibulocochlear artery. The latter further divides into the posterior vestibular
artery, which is the primary blood supply to the
posterior SCC and the majority of the saccule.
Interestingly, the anatomical organization of the
blood supply to the peripheral vestibular end
organs mimics the innervation pattern of the vestibular nerve and its afferent nerve endings (Lysakowski et al., 1998).
A
Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/
B
FIGURE 3–6. Labyrinthine vascular supply. A. Arterial circulation. B. Venous cir-
culation. From Schuknecht’s Pathology of the Ear (3rd ed.) by S. N. Merchant and
J. B. Nadol, 2010, People’s Medical Publishing House, Shelton, CT. Reprinted with
permission.
89
4
Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/
The Vestibular Ocular Reflex
and Vestibular Nystagmus
THE VESTIBULAR OCULAR
REFLEX (VOR)
The VOR is the best-understood reflex in human
physiology (Schwarz & Tomlinson, 2005). It is an
automatic reflex requiring no cognitive thought
or intent. It is an extremely fast-acting reflex that
occurs hundreds of thousands of times each day.
Often, it is a reflex we take for granted. That is,
until it goes missing or is damaged. In short, the
VOR is the compensatory eye movement that
occurs in the opposite direction to that of head
movement so as to maintain visual fixation on an
intended target during head movement. It is the
reason you can read while walking or running. It
is the reason you can see steady while driving on
a cobblestone road. It is (partly) the reason an athlete can clearly focus on a target while exerting
a high level of active head movement (recall, the
visual system also compliments the VOR for high
frequency head movements where it becomes
less efficient). Ultimately, an effective VOR is possible due to the afferent and efferent projections
between the vestibular periphery, the vestibular
nuclei, the cerebellum, the ocularmotor nuclei,
and the extraocular muscles.
Purpose and Properties of the
Vestibular Ocular Reflex
The purpose of the VOR is to stabilize images on
the retina during head movement. This is accomplished by effectively and efficiently generating
extremely quick compensatory eye movements in
the equal and opposite direction to that of head
movement. That is, in order to maintain visual fixation on a traffic sign as your head is moved from
left to right, an eye movement that is equal in velocity and opposite in direction of head movement
(from right to left) must occur. The VOR is capable
of generating effective vestibular-generated eye
movements for head movements up to 350°
second (Leigh & Zee, 2006; Schwarz & Tomlinson,
2005). For velocities higher than this, up to approximately 500° per second, the sensitivity (gain)
of the VOR deteriorates (Schwarz & Tomlinson,
2005). For velocities near or above 500° per second, the visual system attempts to compensate via
the optokinetic system. The VOR must also occur
with extremely short latency so as not to create any
slip or blur of the image on the retina immediately
following the onset of every head movement. The
latency of the VOR is extremely short and measured in humans to be approximately 10 to 12 msec
per
91
92 Rotational Vestibular Assessment
(Schwarz & Tomlinson, 2005). This latency is
quick enough so as not to create any temporal lag
during brainstem integration that would lead to
visual blur.
The direction of the VOR response is always
opposite that of the head turn, and is integrated
by the periphery. Conversely, the velocity of the
response is primarily integrated within the VN
(Gacek, 2005; Leigh & Zee, 2006). The amplitude
and timing of the reflex, (that is the gain and
phase of the VOR), varies greatly with respect
to the velocity of the stimulus. A more equally
“matched” eye velocity response occurs for midto-high frequency stimuli (<350° per second)
(Highstein, 1996; Leigh & Zee, 2006). This suggests
that the VOR is less efficient for low head velocities and more efficient for higher frequencies. This
is indeed the case; however, the inefficiency of
the VOR (in the absence of any supplementing
visual information, or in a vision-denied condition) for low velocity stimuli is “augmented” or
supplemented by an internal “neural drive” of
the system known as velocity storage (discussed
earlier).
The Vestibular Ocular Reflex
and Ewald’s Laws
The calculation of head velocity and direction are
integrated by the vestibular system with respect
to the specific vector and angular/translational
acceleration in space. Each semicircular canal
(SCC) is most sensitive to angular accelerations
in the plane of their respective orientation (Lysakowski, McCrea, & Tomlinson, 1998). That is,
equal and opposite compensatory eye movements
occur in the same plane of the SCCs sharing the
same orientation. Specifically, the h-SCC is most
sensitive to producing compensatory eye movements (VOR) in response to head movements in
the horizontal (yaw) plane, and vertical compensatory eye movements occur as a result of vertical
SCC activation from head movements in the pitch
or roll plane. This property of compensatory eye
movements being produced by activation of SCC
excitation and inhibition in the same plane as the
occurring head movements is known as Ewald’s
first law (Baloh & Honrubia, 2001; Leigh & Zee,
2006). Ewald’s first law specifically states that
maximal stimulation of eye movements always
occurs in the plane of the canal being stimulated
(Leigh & Zee, 2006). Use of this law can be very
useful, not only during physiologic assessment of
the vestibular system, but also when diagnosing
locations of insult within the vestibular system. In
addition, it is also important to be cognizant that,
aside from the VOR produced by the SCCs, the otolith maculae also produce a VOR. Because of the
difference between the two sensory organs’ physiology, the SCC VOR is also sometimes referred to
as the angular VOR (a-VOR), where the maculae
VOR is sometimes referred to as the translational
VOR (t-VOR). We review the physiology of the
VOR for both sensory end organs in this chapter.
Aside from Ewald’s first law, Ernst Julius
Richard Ewald (1855–1921) also proposed two
additional laws of semicircular canal function.
Ewald’s second and third laws grossly state that
vestibular excitation provokes a stronger neural
response than inhibition. This is primarily due to
the fact that inhibition of the resting neural vestibular firing rate is limited to zero, whereas the
excitation of the resting neural firing rate is
essentially “limitless”, with an actually ceiling
likely around 400 to 500 spikes/second. Specifically, Ewald’s second law states that ampullopetal endolymph flow (toward the utricle) in the
horizontal canal causes a greater response than
ampullofugal endolymph flow (away from the
utricle). Moreover, Ewald’s third law pertains
to the vertical canals, and states that ampullofugal flow (away from the utricle) produces a better response than ampullopetal flow (toward the
utricle) when the anterior and posterior canals
are stimulated.
SEMICIRCULAR CANAL
VESTIBULAR OCULAR REFLEX
Each of the semicircular canals forms a three-neuron-reflex pathway from the peripheral sensory
end organ (cristae ampullaris) to a set of distinct
ocular motor muscles. As mentioned in Chapter 1,
Lorente de Nó (1933) first detailed the various
three-neuron arcs that connect each peripheral
vestibular end organ to their respective ocular
muscles (Cohen & Raphan, 2004). The distinctive
4. The Vestibular Ocular Reflex and Vestibular Nystagmus 93
Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/
three-neuron arc for each semicircular canal is
depicted in Figure 4–1. As a general rule, excitatory connections travel along the contralateral
pathways, and inhibitory connections travel along
the ipsilateral pathways for all three semicircular
canal pathways.
Horizontal SCC VOR
The most frequent head movement, both in
everyday life activities as well as during vestibular assessment, occurs in the horizontal plane.
Secondary to this, a detailed discussion will be
A B
D E
C
F
FIGURE 4–1. Vestibular Ocular Reflex (VOR) neural pathways for each semicircular canal. As a general rule,
excitatory connections (A, B, and C) run in the contralateral medial longitudinal fasciculus (MLF), and inhibitory
connections (D, E, and F) in the ipsilateral (MLF). AC (anterior canal); HC (horizontal canal); PC (posterior canal);
ATD (Ascending Tract of Deiters); III (Oculomotor Nucleus); IV (Trochlear Nucleus); VI (Abducens Nucleus); IR
(Inferior Rectus Muscle); MR (Medial Rectus Muscle); LR (Lateral Rectus Muscle); SR (Superior Rectus Muscle);
IO (Inferior Oblique Muscle); SO (Superior Oblique Muscle). From Baloh and Honrubia’s Clinical Neurophysiol-
ogy of the Vestibular System (4th ed.), by R. W. Baloh, V. Honrubia, and K. A. Kerber, 2011, New York, NY: Oxford
University Press. Reprinted with permission.
94 Rotational Vestibular Assessment
given primarily to this reflex, keeping in mind
that the anterior and posterior canals have their
own distinct angular orientation and thus, have
their own distinct three-neuron VOR arc (Leigh
& Zee, 2006).
The horizontal canal VOR (h-VOR) produces
compensatory eye movements in the equal and
opposite directions to that of head movements
within the horizontal (yaw) plane. Again, because
natural head movements in space rarely, if ever,
occur purely within a single plane of orientation, it is important to realize that the resultant
eye movements are likely more a representation
of the VOR produced by all the SCCs (and otoliths). However, during vestibular physiologic
testing, specifically rotational testing, great care
is taken to place the head in such a plane of orientation that the h-SCC is isolated and maximally
stimulated, thus producing compensatory eye
movements that are largely representative of only
the h-VOR. Therefore, the discussion here concentrates on the h-VOR.
Excitatory h-VOR Physiology
The h-VOR is a three-neuron arc that originates
from the crista of the h-SCC and terminates on the
medial and lateral rectus ocular motor muscles
(Leigh & Zee, 2006). For a detailed description
of the excitatory and inhibitory three-neuronarc pathway, the reader is encouraged to review
Leigh and Zee (2006) and Gacek (2005). During
head rotations in the horizontal plane, both the
right and left h-SCC cristae provide an afferent
response. For the h-SCC, the leading ear (right ear
during a rightward rotation and vice versa) displaces the stereocilia toward the kinocilium and
depolarization of the underlying hair cell ensues
(Ewald’s second law) (Baloh & Honrubia, 2001)
(Figure 4–2). Subsequently, an increase in afferent
neural activity is produced on the ipsiversive side,
whereby first-order vestibular neurons saltate the
response through the IAC and terminate in the
MVN and SVN. [For low-frequency stimuli, interneurons work to sustain the afferent responses
Discharge neural firing rate from
the left h-SCC decreases
Left Neural Rate Right Neural Rate
Head turn to right
Resting Neural Rate
Approximately 90 s/s
Discharge neural firing rate from
the right h-SCC increases
Utricle Utricle
EXCITAT ION INHIBITION
FIGURE 4–2. Image depicting left and right cupulae deflection and underlying stereocillia
deflection of the horizontal semicircular canals in response to a right head turn. Green arrows
depict the direction of endolymph flow. Source: Adapted from Barin, K., & Durrant, J. (2000).
Applied physiology of the vestibular system. In Canalis, R. F. & Lambert, P. R. (Eds.).
4. The Vestibular Ocular Reflex and Vestibular Nystagmus 95
Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/
through the process of velocity storage, a process
facilitated by the cerebellum and the commissural
pathways] (Curthoys & Halmagyi, 1996). From
here, a group of interneurons within the ipsiversive MVN cross the midline and terminate in the
contralateral abducens nucleus (VI) in order to
innervate the contralateral lateral rectus motor
units. Excitation of these motor units creates a
muscle contraction of the contralateral eye in the
opposite direction, with a peak eye velocity that is
(ideally) equal to the velocity of head movement.
A second group of interneurons within the SVN
sends the excitation response ipsilaterally through
the ascending tract of Dieters (ATD) to terminate
on the ipsilateral medial rectus muscles motor
units via the ipsilateral ocular motor nucleus (III).
Excitation of these motor units causes a conjugate
contraction of the ipsilateral medial rectus muscle
in the opposite direction, also with a peak eye
velocity that is (ideally) equal to head velocity.
A smaller, third group of neurons also cross the
midline from the contralateral abducens nucleus,
through the MLF and eventually innervate the
ipsilateral medial rectus motor units via the ipsilateral ocular motor nucleus (joining the ascending fibers from the ipsilateral ATD). The excitation
neural pathway for the horizontal semicircular
canals is summarized in Figure 4–3.
FIGURE 4–3. Vestibular Ocular Reflex (VOR) neural pathways for the
horizontal semicircular canal. Medial longitudinal fasciculus (MLF); ATD
(Ascending Tract of Deiters); IR (Inferior Rectus Muscle); MR (Medial
Rectus Muscle); LR (Lateral Rectus Muscle); SR (Superior Rectus Muscle); IO (Inferior Oblique Muscle); SG (Superior Nerve Branch Ganglia).
From Baloh and Honrubia’s Clinical Neurophysiology of the Vestibular
System (4th ed.), by R. W. Baloh, V. Honrubia, and K. A. Kerber, 2011.
New York, NY: Oxford University Press. Reprinted with permission.
96 Rotational Vestibular Assessment
Inhibitory h-VOR Physiology
In light of the complimentary and antagonistic
ocular muscle arrangement that maintains eye
position in the center of the viscous environment
within the ocular orbit, an inhibitory response
must be provided to the antagonistic ocular muscles to facilitate the VOR muscle contraction in the
opposite direction (Leigh & Zee, 2006). The contraversive (trailing ear) h-SCC periphery generates this inhibitory response (Barber & Stockwell,
1980). Bending of the stereocilia in the trailing
ear away from the kinocilium hyperpolarizes the
underlying hair cell and produces an inhibitory
afferent response to eventually terminate in the
contraversive MVN and SVN. Through similar
pathways, as previously described for the excitatory response, second and third-order neurons
eventually terminate on the contralateral medial
rectus muscle and the ipsilateral lateral rectus
muscle. This pathway provides an inhibitory
(relaxed) muscle response, which facilitates the
opposing (excitatory) contraction of the eyes in
the opposite direction (Figure 4–4).
FIGURE 4–4. Vestibular Ocular Reflex (VOR) neural pathways for the
horizontal semicircular canal. Excitatory pathways depicted in red and
inhibitory pathways depicted in blue for a head turn toward the right. MR
(Medial Rectus); LR (Lateral Rectus); OM Nerve (Ocular Motor Nerve);
OM Nuc (Ocular Motor Nucleus); MLF (Medial Longitudinal Fasciculus);
A Nerve (Abducens Nerve); A Nuc (Abducens Nucleus); ATD (Ascending
Tract of Dieters); MVN (Medial Vestibular Nucleus); VN (Vestibular Nerve);
hSCC (Horizontal Semicircular Canal). From Electronystagmography and
Videonystagmography: ENG/VNG by D. L. McCaslin, 2012, San Diego,
CA: Plural Publishing. Reprinted with permission.
4. The Vestibular Ocular Reflex and Vestibular Nystagmus 97
Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/
Synergistic h-VOR Afferent
Tract Advantage
Complementary and synergistic excitation and
inhibition pathways are critical for effective operation of the h-VOR (as well as for any of the SCC or
maculae VOR pathways). These separate neural
pathways fundamentally provide the capability
of applying varying amounts of contraction and
“displacement” of each eye, independent from
one another (Wong, 2008; Schwarz & Tomlinson,
2005). This becomes important as it permits fixation of targets at varying distances (vergence), or
at various angles within the periphery in coordination with the VOR, by independently regulating muscle contractions from each eye (Schwarz
& Tomlinson, 2005). This concept is critical when
considering pathologic processes in view of the
fact that documenting abnormal movement of one,
or both eyes, can be different from one another.
In addition, this reflex should occur within 10 to
12
ms of head movement. Any increase in the
latency or amplitude (gain) of either, or both eyes,
can be significant and diagnostically useful, as
with internuclear ophthalmoplegia (INO).
Anterior SCC VOR
Excitation of the anterior semicircular canal
produces an excitatory and compensatory eye
response that is in the same roll (side-to-side)
plane as the anterior canal (Ewald’s first law).
This response is governed by the specific muscles
innervated by the three-neuron arc of the anterior VOR (a-VOR) (see Figure 4–1). Although the
general aspects of the a-SCC three-neuron arc are
similar in myeloarchitecture to the h-VOR, the
exact neural centers, pathways, and ocular muscle innervations are slightly different (Schwarz
& Tomlinson, 2005). The specific eye movements
of the a-VOR can be described as a synchronized
(conjugate) upward deviation of both eyes concomitant with a counter-rolling of the eyes, such
that the upper poles of both eyes move toward the
contralateral side (i.e., the ipsilateral eye intorts
and the contralateral eye extorts) (Schwarz &
Tomlinson, 2005). Of note, it is important to recall
that the stereocilia and kinocilia arrangement on
the a-SCC is opposite that of the h-SCC. That is,
the kinocilium is located on the canalicular side of
the cupula. This orientation is significant because
it allows the leading ear to displace the stereocilia
toward the kinocilium, whereby depolarization
of the underlying hair cell ensues (Ewald’s third
law). With respect to the anterior canals, the lead-
ing ear would apply to the right ear during a forward pitching of the head over the right shoulder
(i.e., the RALP plane as described in Chapter 2),
and a forward pitching of the head over the left
shoulder for the left ear (LARP plane).
Posterior SCC VOR
Excitation of the posterior canal produces an
excitatory and compensatory eye response that is
in the same pitch (front-to-back) plane (Ewald’s
first law). This response is governed by the specific muscles innervated by the three-neuron arc
of the posterior VOR (p-VOR) (see Figure 4–1).
Like the a-VOR, the general aspects of the p-VOR
three-neuron arc are similar in myeloarchitecture
to the h-VOR, with the exact neural centers, pathways, and ocular muscles innervations also being
slightly different (Schwarz & Tomlinson, 2005).
The specific eye movements of the p-VOR are very
similar to that of the anterior semicircular canal,
and can be described in much the same fashion.
With the p-VOR pathway, synchronized (conjugate) downward deviation occurs in both eyes concomitant to counter-rolling, such that the upper
poles of both eyes move toward the contralateral
side (i.e., the ipsilateral eye intorts and the contralateral eye extorts) (Schwarz & Tomlinson, 2005).
While both the a-VOR and p-VOR share the same
torsional vestibular slow-phase component of the
nystagmus (always toward the contralateral, or
inhibitory, ear), the primary distinction between
the anterior and the posterior semicircular canal
VOR lies the vertical component of eye movement, as the anterior canal vestibular slow-phase
deviation is upward and the posterior canal vestibular slow-phase deviation is downward. This
distinction actually becomes critical when considering the differential diagnosis between anterior
versus posterior canal benign paroxysmal positional vertigo (BPPV).
As with the a-SCC, it is important to recall that
the stereocilia and kinocilium arrangement on the
p-SCC is opposite that of the h-SCC. That is, the
Соседние файлы в папке Библиотека им академика М.И. Перельмана