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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4606_Библиотеки_им_академика_М_И_Перельмана
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A. Chern and L. Lustig
nystagmus seen in CNS lesions, where the nystagmus reverses direction every
2min. See Fig.3.11 for an example of positional nystagmus.
Caloric Tests
Caloric vestibular testing is an established technique that is particularly useful in
determining unilateral vestibular hypofunction [3, 23]. Caloric irrigation produces a
convection current of endolymph when the canal is oriented vertically—endolymph
sinks when cooled and rises when warmed. The temperature produced by a cold
stimulus causes the cupula to move away from the utricle, thereby creating a nystagmus that beats toward the contralateral side. A warm stimulus causes the endolymph
to rise, resulting in a nystagmus that beats toward the stimulus. Thus, cool irrigation
causes nystagmus away from the ear, and warm irrigation causes nystagmus toward
the ear. Of note, caloric irrigation is limited by the effectiveness of heat transfer
between the external and inner ears. A small or occluded external ear canal or inadequate irrigation can reduce the intensity of the caloric stimulus to the labyrinth,
resulting in a reduced response.
Equipment
The caloric test uses a caloric stimulator (a water or air irrigator) in addition to the
VNG/ENG recording equipment. There are two types of water stimulators. An
open-loop stimulator delivers water directly into the outer ear canal, while in a
closed-loop stimulator, the water circulates in an expandable rubber medium in
order to preserve temperature. Open-loop systems are thought to be more reliable
and reproducible. However, patients with a tympanic membrane perforation should
undergo caloric testing with an air or closed-loop water stimulus.
Fig. 3.11 Right beating positional nystagmus with vision denied, which is indicative of a peripheral vestibular disorder
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Test Administration andParameters
For caloric tests, the patient is placed supine with the head tilted 30° upward to
bring the horizontal canal into the vertical position, making it more sensitive. The
test can be performed with either a bithermal or a monothermal caloric stimulus.
The bithermal caloric test is stimulated by warm and cold water and is more useful.
Before delivering each caloric stimulus, the system should be calibrated with a saccade of 10°. To optimize the nystagmus response, the patient is asked to perform
mental tasks simultaneously. The recording can be performed with eyes open in
total darkness, with eyes opened and wearing Frenzel glasses, or with eyes closed.
Temperatures 7°C below and above body temperature (30 and 44°C) are used
as cold and warm water stimuli, respectively. A total of 250mL of water is administered to the external auditory canal over a period of 30–40s. During this time, the
patient will experience vertigo for a few minutes. As an alternative to the water
stimulus, two air stimuli that are 24–50°C are used with a ow rate of 8L/min over
60s. Four caloric stimuli are administered at intervals of 5 or more minutes to avoid
superimposition or conicting responses. The recommended order of stimuli is: (1)
right warm, (2) left warm, (3) right cold, and (4) left cold. Nystagmus begins just
prior to the end of the caloric stimulus and peaks at approximately 60s of stimulation, followed by a slow decay over the next minute. At peak nystagmus, patients
are asked to xate their eyes on a central point to assess the xation suppression index.
The method most commonly used to evoke vestibular nystagmus is bithermal
caloric testing. Warm and cool water (or air) irrigations are administered to each ear,
and the maximum velocity of the slow phase of nystagmus from each ear is determined. These data are used in the standard formulas described by Jongkees, discussed below [24]. See Fig.3.12 for an example of normal caloric testing.
Clinical Application
Unilateral weakness (or canal paresis) refers to the relative decrease in the
responses of one side compared with the other. Since this technique directly compares the vestibular function of the right and left sides, caloric testing is highly reliable in detecting unilateral peripheral vestibular loss [22]. In most testing centers, a
canal paresis greater than 22–25% indicates unilateral peripheral vestibular loss
[25]. However, unilateral weakness is not a localizing nding and can be attributed
to lesions from the inner ear to the root entry zone of the vestibulocochlear nerve in
the brainstem, including Meniere’s disease, labyrinthitis, vestibular neuronitis, vestibular schwannomas, and multiple sclerosis. Unilateral weakness can be calculated
as follows: 100 × [(LC+LW) − (RC+RW)/(LC+LW+RC+RW)]=% caloric
response, where RC, RW, LC, and LW indicate peak slow component velocity of
nystagmus from right cool, right warm, left cool, and left warm irrigations, respectively. See Fig.3.13 for an example of unilateral weakness.
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a
b
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A. Chern and L. Lustig
c
Fig. 3.12 Normal bithermal caloric testing indicating normal symmetrical peripheral vestibular function. function. Right (a, b, d) and left (a, c, e) ear caloric testing results with cool and warm irrigation.
Directional preponderance (or unidirectional preponderance) is a phenomenon
dened by Jongkees where the intensity of bithermal caloric-induced nystagmus is
greater in one direction compared to the other. In other words, the slow phase velocity of the nystagmus beating toward one side is signicantly greater than the mean
peak slow phase velocity of the nystagmus beating toward the opposite side.
Directional preponderance values can be calculated as follows: 100 × [(LC+RW)
− (RC+LW)/(LC+LW+RC+RW)]=% directional preponderance.
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Fig. 3.13 Caloric testing demonstrating a unilateral left-sided peripheral caloric weakness; 41%
weakness in the left ear (a). Right ear results with cool (b) and warm (d) irrigation, left ear results
with warm (c) and cool (e) irrigation
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A. Chern and L. Lustig
A value of greater than 26–30% is generally used to suggest the presence of a
directional preponderance [25]. A directional preponderance is often associated
with spontaneous nystagmus because spontaneous nystagmus enhances the nystagmus beating in its direction while eliminating the nystagmus beating in the opposite
direction. Since directional preponderance may be observed in lesions from the
inner ear to the cortex, this is a poor localizing nding. The directional preponderance is toward the lesion site for the labyrinth and eight nerve lesions, while it is
toward the uninvolved site for brainstem and cortex lesions. Some suggest that a
directional preponderance without spontaneous nystagmus is suggestive of a CNS
disorder; however, this is controversial [26]. A directional preponderance and unilateral weakness observed together are suggestive of an acute unilateral peripheral
lesion. See Fig.3.14 for an example of unilateral peripheral weakness with normal
directional preponderance, suggesting complete compensation of the contralateral ear.
Patients may also have bilateral weakness, where the total response to a warm
stimulus is <11°/s and the total response to a cold stimulus is <6°/s [25]. These
patients present with oscillopsia and may have experienced vestibulotoxic antibiotic
therapy or bilateral Meniere’s disease. However, bilateral weakness may also be
found in patients with vestibular nuclei lesions, Lyme disease, pseudotumor cerebri,
neurodegenerative diseases of the brainstem and cerebellum, and Cogan syndrome.
Fig. 3.14 Bithermal caloric testing demonstrating a right-sided unilateral peripheral vestibular
weakness. Of note, directional preponderance is within normal limits, which suggests complete
compensation of the right ear
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Hyperactive caloric responses can also be observed, with responses varying
between 40°/s and 80°/s [25]. These are associated with cerebellar lesions or atrophy from the removal of the cerebellar inhibitory effect on vestibular nuclei. See
Fig.3.15 for an example.
Failure of xation suppression (FFS) may be observed with caloric testing.
Recall that the patient is asked to xate on a central point during peak caloric
response, during which vestibular nystagmus is normally suppressed by visual xation. The xation index expresses this quantitatively, calculated by the difference
between the slow-phase velocity in the dark and in the light divided by the slowphase velocity in the dark. Visual suppression of the caloric response is typically
>50%. Less than 50% indicates impaired xation suppression and “failure” [25].
Fixation suppression is mediated by the occulus of the cerebellum; thus, cerebellar
lesions affecting the occulus cause impaired xation suppression.
Fig. 3.15 Bilateral hyperactive caloric response; 112°/s right beating nystagmus; 83°/s left beating nystagmus (a). Only warm air (b) evaluated (patient intolerance); cerebellar dysfunction cannot be ruled out
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Inversion of caloric nystagmus may be seen in individuals with a tympanic
membrane perforation. When warm air is used as a caloric stimulus, there is a cooling effect from moisture evaporation from the middle ear mucosa.
Premature caloric reversal may be seen in patients with brainstem lesions or
Friedreich ataxia. Typically, a normal caloric response begins to decay at 90s of
stimulation and disappears after 200s, with nystagmus beating toward the contralateral side. In premature caloric reversal, nystagmus reversal occurs earlier, at
140–150s [25]. Preexisting spontaneous nystagmus should not be considered a premature caloric response.
Ice water caloric irrigations are an even stronger stimulus, sometimes used as a
supplement to bithermal calorics when the responses are poor or in assessing comatose patients. In a comatose patient, an intact vestibular response causes slow, tonic
movement of the eye toward the side irrigated with ice water. In coma, there are no
corrective fast phases of nystagmus, so only the slow component of nystagmus
is seen.
Disadvantages of caloric testing include wide variability in caloric vestibular
responses due to individual differences in external auditory canal size in the size and
efciency of thermal energy transfer across the middle ear (dependent on temporal
bone pneumatization) [27]. Caloric irrigation only evaluates vestibular responses at
one frequency. Since the external ear is closest to the horizontal SCC, most responses
originate only from the horizontal SCC.
A. Chern and L. Lustig
Rotary Chair Testing
The rotary chair test, otherwise known as rotational testing, is used to evaluate the
horizontal VOR reex pathway between the horizontal SCC and eye muscles. The
patient is positioned such that only the horizontal SCC is stimulated. This test has
three functions: (1) to conrm bilateral impairment of the horizontal SCCs, (2) to
determine the presence of a central vestibular dysfunction, and (3) to quantify any
progression of a known vestibulopathy.
Rotational testing can be performed using active (volitional) or passive, low frequency or high frequency, and head-only or whole-body (en bloc) rotations. There
are two primary advantages to rotational testing over caloric testing. Rotational testing does not depend on the effectiveness of thermal energy transfer across the middle ear and temporal bone. Moreover, rotational testing allows the precise application
of multiple frequencies of rotational stimuli, while caloric testing is equivalent to a
single, very low-frequency (0.003Hz) vestibular stimulus [28].
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Equipment
A rotatory chair test typically consists of a stimulus device (a chair whose rotational
speed is controlled by a computer), a response recording (made of electrodes to
record horizontal eye movements) and its analysis, a light-proof video camera, and
a two-way communication system.
Types ofStimuli
The main two types of rotary chair testing include (1) sinusoidal harmonic acceleration and (2) velocity step tests. In sinusoidal harmonic acceleration, a series of
rotary stimuli is employed at the octaves of frequencies from 0.01 to 1.28Hz, to the
right and to the left. Chair velocity is set at 50–60°/s. The rotational stimulus at a
given frequency is employed for multiple cycles. The velocity step test applies a
series of velocities. First, an acceleration impulse of 100°/s2 is initiated until a xed,
desirable rotational stimulus of 60–180°/s is achieved. Once the velocity has been
reached and applied for 45–60s, the chair is decelerated to 0°/s with the same magnitude of acceleration. The test is repeated in the opposite direction. Following each
stimulus protocol, the computer detects slow-component eye velocity while omitting the fast component of the induced nystagmus (i.e., the response to the rotational
stimulus). During these tests, eye position, eye velocity, and chair velocity (i.e.,
head velocity) are assessed.
Test Administration andParameters
The patient sits in the chair with the seatbelt fastened and his or her head secured in
the head support. Eye movements are recorded with an infrared camera or electrodes on the skin at the lateral canthi. Testing is performed in complete darkness
with eyes opened. During testing, the patient is kept mentally alert with arithmetic
tasks. A two-way communication system is used to provide instructions and arithmetic tasks to the patient.
There are three important parameters to be evaluated in a sinusoidal harmonic
acceleration test: phase, gain, and symmetry data. Phase describes the timing relationship between head velocity and slow-component eye velocity (reexive eye
response). The difference between the two is dened as the phase angle and
expressed as a measure of degrees. When the eye velocity is greater than the head
velocity, this is known as phase lead. The opposite is referred to as the phase lag.
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To maintain the position of objects in the retina, eye velocity needs to be equal to
head velocity, thus providing eye movement equal to head movement in the opposite direction. Under these circumstances, the phase angle would be 180°. Another
measure of the angle is the time constant of the response, which is inversely correlated to it. Gain is the ratio of the amplitude of the slow-component eye velocity to
the head velocity. This represents the response capability of the vestibular system at
tested frequencies. Values outside of the normal range are considered abnormal as
long as the system is calibrated and the patient is alert. Symmetry data demonstrates the side toward which asymmetry exists and whether slow-component eye
velocities are equal on both sides.
The main parameter in the velocity step test is the time constant—the time
needed for slow component eye velocity to decrease to 37% of its initial value. The
second parameter for the velocity step test is the gain (the same as its equivalent in
the sinusoidal harmonic acceleration test).
A. Chern and L. Lustig
Clinical Application
The test is most useful in determining the amount of central compensation and
residual vestibular function in patients with bilateral vestibular loss. As a result, one
of the main disadvantages of rotational testing is that it affects both ears simultaneously, making it less helpful in detecting unilateral lesions. In a sinusoidal harmonic
acceleration test, an increased phase angle may imply a peripheral system insult (or,
less commonly, vestibular nucleus involvement), while a decreased phase angle
may suggest a cerebellar lesion. High gain may be observed in a cerebellar lesion,
while low gain is associated with a bilateral peripheral insult. Asymmetry may suggest the involvement of a central or decompensated peripheral vestibular system.
With an intact CNS, this suggests a paretic lesion where either asymmetry or an
irritative lesion of the contralateral side is likely. This is analogous to directional
preponderance (from caloric testing). With the velocity step test, an acute peripheral
insult results in a low gain and a decreased time constant of the response to the
rotational stimulus toward the side of the lesion. See Figs.3.16 and 3.17 for examples of normal and abnormal rotary chair results.
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Fig. 3.16 Normal rotary chair results. Multifrequency phase (a), gain (b), and symmetry (c) are
within normal limits. Rotary chair testing assesses a range of frequencies from 0.01 Hz to 0.64
Hz (d-g)
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