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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4612_Библиотеки_им_академика_М_И_Перельмана
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Audiology Review: Preparing for the Praxis and Comprehensive Examinations
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Monothermal irrigations should only be used if:
●
Right ear and left ear responses are both >11°/second
●
There are no additional abnormalities noted in oculomotor testing
●
CP is <10% to 15%
●
If these conditions are not met, bithermal irrigations are indicated.
Caloric testing is the most common VOR task completed in everyday clinical practice. There are
some additional considerations to remember when completing this protocol.
n
Alerting tasks
Any vestibular test completed without fixation requires alerting tasks. This keeps the
patient alert and allows for consistent nystagmus generation. Tasks vary but should involve
interaction between the patient and examiner. This may include having the patient recall
things from memory (e.g., city/state names) and should be moderately challenging to the
patient. Using topics of patient interest is also helpful.
If the patient is not sufficiently alert, the VOR response will likely be reduced and may not
appropriately represent inner ear performance.
n
Order effect
ANSI (2009) and BSA (2010) standards recommend beginning with warm irrigations.
ANSI standard (ANSI, 2009) recommends beginning with the right ear, while the BSA
standard (BSA, 2010) does not specify.
n
Waiting time between irrigations
3 to 5 minutes is recommended between irrigations to ensure that any residual temperature
effect will not influence the next caloric irrigation (ANSI, 2009).
n
Caloric testing is generally not recommended for children <6 years of age due to poor
tolerance of the procedure.
Rotational Chair Assessment
Rotational chair assessment provides another measure of horizontal SCC and superior vestibular nerve
function. There are several subtests, but most laboratories complete sinusoidal harmonic acceleration
(SHA) testing. For this task, the patient is secured in the examination chair and fit with infrared
goggles like those used in VNG testing. SHA testing must be done with vision denied. The patient is
oscillated across a range of frequencies (0.01–0.64 Hz) at a set maximum chair velocity (typically 50°
or 60°/second). Nystagmus is calculated and compared to normative values for the following measures
(Figure 7–9).
n
Gain: ratio of recorded nystagmus velocity to the set maximum chair velocity
Normative values vary depending on the stimulus frequency.
Gain is measured to identify the presence of bilateral peripheral or central hypofunction or
an acute unilateral lesion.
Gain for bilateral hypofunction remains reduced over time; acute lesions will demonstrate
increased gain as compensation occurs.
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FIGURE 7–9. Comparison of sinusoidal harmonic acceleration (SHA) responses across frequencies for
normal and various abnormal presentations.
n
Phase: timing relationship between chair movement and eye movement
Phase describes the central processing of horizontal SCC information.
Normative values depend on the stimulus frequency.
Abnormal phase can occur due to peripheral or central vestibular system dysfunction.
Abnormal low-frequency phase lead is the most common abnormality and suggests history
of peripheral or central vestibulopathy.
Phase abnormalities do not change with compensation.
n
Symmetry: comparison of right-beating versus left-beating nystagmus
Comparable to directional preponderance as described in caloric testing.
Asymmetry generally occurs with present spontaneous nystagmus.
Besides SHA testing, there are several other subtests that may be completed, depending on patient
presentation.
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
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334
n
VOR fixation/VOR suppression testing is completed to determine the patient’s ability to
suppress the VOR response. A fixation point is provided to the patient and repeated SHA
oscillations are completed, commonly at 0.08 or 0.16 Hz. The nystagmus velocity during
the fixation condition is compared to the vision-denied condition to determine the fixation
index (%). VOR fixation in this paradigm should be between 75% and 90% (Barber &
Stockwell, 1980).
Abnormal fixation index suggests cerebellar involvement. Confirm with other measures of
cerebellar performance (e.g., smooth pursuit).
n
Visually enhanced VOR (VVOR) testing evaluates the interaction between the visual and
vestibular systems. It is key to identifying the site of lesion for bilateral hypofunction, that is, is
the bilateral loss related to central or peripheral system dysfunction? In this subtest, the patient
is oscillated as described in SHA testing; however, the OKN stimulus is projected onto the
walls of the enclosure. The nystagmus recorded during this trial is a summation of VOR and
smooth pursuit systems and should result in gain between 0.7 and 1.1. Patients with bilateral
hypofunction associated with peripheral vestibulopathy (e.g., ototoxicity) will demonstrate
increased gain and generally fall within normal limits. Those with bilateral hypofunction
associated with central vestibulopathy (e.g., cerebellar degeneration) will not. These patients
should also demonstrate other central signs such as poor smooth pursuit and reduced fixation
suppression ability.
n
Trapezoidal acceleration/step testing may be completed with either a low- or high-velocity
paradigm. Low-velocity testing (60°/second) is used to measure the rate of nystagmus decay
in response to abrupt accelerations. High-velocity testing (240°/second) is used to support the
diagnosis of unilateral peripheral vestibulopathy.
In both protocols, the patient is abruptly accelerated to the maximum velocity (either 60°
or 240°/second) and maintained at this velocity until the observed nystagmus is 37% of its
peak value (Stockwell & Bojrab, 1997). The time it takes to reach this point is called the
time constant or decay time. Typical responses for 60°/second stimuli should be between 10
and 30 seconds (Goulson et al., 2016).
●
Time constants <10 seconds are associated with abnormal performance of the peripheral
vestibular system, vestibular nerve, and/or vestibular nuclei.
●
Time constants >30 seconds suggest abnormal central vestibular system processing.
●
Time decay is typically only evaluated for low-velocity trials (60°/second).
Gain is also evaluated as the peak slow-phase velocity response divided by the target
velocity. Gain is typically only calculated for high-velocity trials (240°/second). At this
velocity, the lagging ear is inhibited to the point that there is minimal contribution to the
VOR, allowing for the comparison of right versus left VOR performance.
Video Head Impulse Test (vHIT)
VHIT provides another method of evaluating the horizontal SCC and superior vestibular nerve. Most
systems also evaluate RALP and LARP planes, allowing for a clinically accessible method of evaluating
these pathways. Importantly, vHIT provides a measure of high-frequency performance not captured
by other VOR measures. It is not equivalent to other VOR measures but is complementary.
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Video goggles are typically used for this test, and most evaluations can be completed in a few
minutes. Some systems use remote cameras, especially when testing young children. The patient
focuses on a target while the examiner moves the head abruptly to stimulate the appropriate SCC
plane. Trials should be done at >150°/second for consistent results. The infrared camera records the eye
movements as the patient attempts to maintain visual fixation. VHIT systems document the following
(Figure 7–10):
n
Gain: ratio of eye velocity to head velocity
Appropriate gain >0.7
Gain may improve over time with central compensation
n
Corrective saccades occur in patients with reduced peripheral system performance to
reposition the eyes on the target.
These eye movements are part of the central compensation mechanism and adapt for
a deficient VOR. At the initial insult, these saccades occur randomly, but over time
become more time locked as compensation occurs. There are two types of corrective
saccades.
●
Overt saccades: eye movements that occur after the head stops moving; often seen on
bedside head thrust testing (see Table 7–3).
●
Covert saccades: eye movements that occur during the head movement; cannot be seen
on bedside head thrust testing.
FIGURE 7–10. A. Typical video head impulse response (vHIT). Note that the head movement
and VOR are overlaid for easy visualization of gain abnormalities. Head movement is noted in
light gray; VOR performance is in dark gray. B. Abnormal vHIT is characterized by reduced gain
and the presence of covert and/or overt saccades.
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AUDIOLOGY NUGGET
There are numerous options for evaluating horizontal SCC performance
(Table7–8). Consider which test protocols are most appropriate for your patient
to obtain the information needed for appropriate management.
TABLE 7–8. Comparison of Common Vestibulo-Ocular Reflex (VOR) Tests
CALORIC SHA vHIT
Objective? Yes Yes Yes
Independent SCC
assessment?
Frequency (Hz) Low Low-mid High
Tasking effect? Yes Yes No
Cost Less expensive Expensive Less expensive
Tolerated? Yes Yes Yes
Minimum age >6 years >6 months >3 months
Note. SHA: sinusoidal harmonic acceleration; vHIT: video head impulse test.
Yes No Ye s
Vestibular Evoked Myogenic Potentials (VEMPs)
Until VEMPs, there was no clinically available method to evaluate the otolith organs. These tests have
expanded the understanding of vestibular system function. VEMPs use surface electrodes to record the
underlying electromyogram (EMG) of the target muscle.
Cervical VEMPs (cVEMPs)
The cVEMP reflex pathway includes the saccule, inferior branch of CN VIII, CN XI, and sternocleidomastoid (SCM) muscle. The recorded response is inhibitory, meaning that the SCM must be
contracted to record the response. Most clinicians use air-conducted stimuli, but bone conduction and
bone-tapping devices may also be used (Table 7–9).
n
After electrodes and earphones are placed, the patient is instructed to contract the SCM
during the stimulus. This is commonly done by asking the patient to raise the head from a
reclined position and turn to the contralateral side.
n
Current evoked potential systems provide EMG monitoring to allow the patient to maintain
a consistent level of muscle contraction during the test. This is important — cVEMP response
amplitude will increase with increased muscle contraction (McCaslin et al., 2014).
CHAPTER 7 Vestibular Assessment and Differential Diagnosis
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TABLE 7–9. Common Recording Parameters for cVEMPs and oVEMPs
cVEMP oVEMP
337
Electrode montage Noninverting: upper half
of SCM
Inverting:
junction
Ground:
Electrode impedance <10 kOhms <10 kOhms; interelectrode
Channels 1 channel 1 or 2 channels
Amplification (gain) × 5,000 × 30,000–50,000
Artifact rejection Disabled Enabled
Filter 10–1000 Hz 1–1000 Hz
Window/epoch 100 ms (20 ms prestimulus) 100 ms (20 ms prestimulus)
Number of sweeps ~100 100–200
Number of averages 2 2
Stimulus 500 Hz tonebursts
Rarefaction
120 dB pSPL
5 Hz repetition rate
Blackman gating
2-ms rise/fall, 0-ms plateau
sternoclavicular
forehead
Noninverting: infraorbital
midline
Inverting:
Ground: forehead
differences ≤2 kOhms
500 Hz tonebursts
Rarefaction
120 dB pSPL
5 Hz repetition rate
Blackman gating
2-ms rise/fall, 0-ms plateau
inner canthus
n
To reduce the risk of response asymmetry due to asymmetric muscle contraction, systems can
apply EMG limiting (i.e., only accepting sweeps within a set EMG range) and/or response
normalization. Response normalization adjusts the response amplitude by the amount of
prestimulus EMG activity (Figure 7–11).
The recorded cVEMP response is evaluated in terms of peak latency, response amplitude, and
amplitude asymmetry ratio. Absolute cVEMP amplitude demonstrates considerable variability and is
therefore not as clinically meaningful. Table 7–10 provides basic interpretation for cVEMP responses.
n
Peak latency
p1: 13 ms; abnormal >21 ms
n1: 23 ms; abnormal >26 ms
Significantly prolonged latencies are associated with retrolabyrinthine involvement.
Note: SCM muscle fatigue may also lead to prolonged n1 latencies.
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FIGURE 7–11. Typical cervical vestibular evoked myogenic potential (cVEMP) response. This normalized
response demonstrates typical cVEMP presentation. For the right ear (left side), p1 and n1 occur at 14.00
and 20.67 ms, respectively, and p1-n1 normalized amplitude is 1.164. For the left ear (right side), p1 and
n1 occur at 13.67 and 21.00 ms, respectively, and p1-n1 normalized amplitude is 1.833. Amplitude asymmetry is 22%.
TABLE 7–10. Basic VEMP Interpretation
FINDING INTERPRETATION
Absent responses bilaterally • Consider technical error, especially in younger
patients
Bilateral peripheral or central otolith reflex
•
pathway involvement
•
Common in vestibular migraine, advancing age
Absent response unilaterally;
asymmetrical response amplitude
• Unilateral peripheral or central otolith reflex
pathway involvement
• Common in peripheral lesions, vestibular migraine
Significantly enhanced amplitude •
Consider third window disorder
Significantly reduced threshold • Consider third window disorder
Present response at 4000 Hz • Consider third window disorder
Prolonged latencies • Central otolith reflex pathway involvement (e.g.,
multiple sclerosis)
• Consider SCM fatigue for cVEMP
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n
Response threshold: lowest intensity with repeatable cVEMP responses
Normal thresholds >75 dB nHL
Thresholds <75 dB nHL suggest third window disorder (Zuniga et al., 2013).
n
Amplitude asymmetry: comparison of right and left ear responses
339
Amplitude asymmetry (%) =
right ear amplitude − left ear amplitude
right ear amplitude + left ear amplitude
Normal: 20% to 45% (McCaslin et al., 2013; Welgampola & Colebatch, 2005)
Q & A
Question: What are the differences between cervical versus ocular VEMPs?
Answer:
Cervical VEMPs measure an inhibitory pathway describing the saccule,
inferior vestibular nerve, and descending vestibulo-spinal pathway to the SCM.
This pathway is a vestibulo-collic reflex. Ocular VEMPs measure an excitatory
pathway describing the utricle, superior vestibular nerve, and ascending vestibuloocular reflex. These two pathways are different and cannot be used interchangeably.
Ocular VEMPs (oVEMPs)
The oVEMP reflex pathway includes the utricle, superior branch of CN VIII, CN III, and the contralateral inferior oblique muscle. The oVEMP is an excitatory contralateral reflex, meaning that right
ear stimulation is recorded from the left inferior oblique. This reflex pathway describes a translational
VOR — a specific otolith-ocular reflex pathway that encodes linear acceleration.
Once electrodes are placed, the patient is instructed to gaze upward by approximately 30°. The
change in gaze position leads to increased oVEMP amplitude due to (1) reduced distance from the
muscle to the recording electrode and/or (2) increased underlying EMG activity (Rosengren et al.,
2013). The recorded oVEMP is evaluated in terms of peak latency, response threshold, response
amplitude/n1-p1 amplitude, and amplitude asymmetry ratio (Figure 7–12). Table 7–10 provides basic
interpretation for oVEMP responses.
n
Peak latency
n1: 12 ms; abnormal >15 ms
p1: 17 ms; abnormal >21 ms
Significantly prolonged latencies are rare; generally associated with central pathology.
n
Threshold: lowest intensity with repeatable oVEMP responses
Normal thresholds >85 dB nHL
Thresholds <85 dB nHL suggest third window disorder (Zuniga et al., 2013)
n
Response amplitude/n1-p1 amplitude
There is less variability in raw oVEMP amplitude compared to raw cVEMP amplitude.
Electrode montage significantly impacts response amplitude.
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FIGURE 7–12. Typical ocular vestibular evoked myogenic potential (oVEMP) response. This response demonstrates typical oVEMP presentation. For the right ear (left side), n1 and p1 occur at 11.00 and 16.67 ms,
respectively, and n1-p1 amplitude is 3.017 µV. For the left ear (right side), n1 and p1 occur at 12.00 and
17.67 ms, respectively, and n1-p1 amplitude is 2.281 µV. Amplitude asymmetry is 14%. There is no repeatable response noted at 90 dB nHL.
Using the electrode montage provided in Table 7–10 should provide mean (standard
deviation) amplitude of 5.67 ± 3.42
n
Amplitude asymmetry: comparison of right ear versus left ear responses
µV (Sandhu et al., 2013).
Amplitude asymmetry (%) =
right ear amplitude − left ear amplitude
right ear amplitude + left ear amplitude
Normal: <40% (Piker et al., 2011; Zaleski et al., 2015)
Electrocochleography (ECochG)
ECochG is an evoked potential describing the cochlear microphonic, summating potential (SP), and
whole nerve action potential (AP) generated by CN VIII. See Chapter 5 for recording parameters and
specific considerations for ECochG. Remember that the SP/AP ratio is the reported outcome measure,
with ratios >0.4 considered abnormal depending on the electrodes used (Kileny & McCaslin, 2021).
n
Some clinics use ECochG as part of the vestibular test battery, especially in cases of suspected
Ménière disease or third window disorder, because this test may be sensitive to altered
hydrodynamic forces within the inner ear. In these cases, the SP/AP ratio may be abnormally
elevated (Aso et al., 1991; Kileny & McCaslin, 2021).
n
Remember that electrode placement is extremely important for reliable ECochG
recordings — transtympanic electrode placement will produce better results than those
obtained from tympanic membrane electrodes.
Functional Assessment
Identifying the functional impact of an underlying vestibular disorder allows for appropriate understanding of the impact on the patient’s daily activities as well as provides guidance for rehabilitation
recommendations.
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Balance Testing
Poor balance is often the primary concern for a patient coming in for vestibular evaluation. There are
numerous options for balance testing. Some may be done at bedside as described in Table 7–3. Other
options are computerized and use additional technology. The most common computerized balance test
is called computerized dynamic posturography (CDP). CDP is typically available in larger vestibular
laboratories and physical therapy practices. This system allows for evaluation of the patient’s ability to
use visual, vestibular, and somatosensory information in maintaining appropriate balance. There are
three main subtests:
n
Sensory organization test (SOT) evaluates six conditions to identify which system(s)
contributes to reduced balance (Figure 7–13). The conditions are:
Eyes open/firm surface
1.
2. Eyes closed/firm surface
3. Eyes open/firm surface/sway-referenced visual surround
4. Eyes open/sway-referenced surface
5. Eyes closed/sway-referenced surface
6. Eyes open/sway-referenced surface/sway-referenced visual surround
Balance performance is measured by calculating the center of gravity sway angle over the
course of each 20-second trial.
An equilibrium score is provided for each trial, defined as a percentage that compares the
patient’s peak amplitude of anterior-posterior sway with the theoretical anterior-posterior
limits of stability. Scores closer to 100% suggest very little sway and those closer to 0%
suggest significant sway and that the patient is reaching/exceeding the limits of stability.
FIGURE 7–13. The SOT protocol showing the six sensory test conditions. Source: From Handbook of Balance Function Testing. Jacobson
et al., 1997.
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