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LESSON 160

VEMP testing

Year 2
Spring semester
Vestibular Assessment I
Vestibular assessment
Duration: 12:02

This lesson covers the vestibular evoked myogenic potential tests that assess the otolith organs: the cervical VEMP (saccule, inferior vestibular nerve, sternocleidomastoid muscle) and the ocular VEMP (utricle, superior vestibular nerve, inferior oblique muscle). We explain the recording setup, waveforms, asymmetry ratio, thresholds and the effect of conductive hearing loss.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn the vestibular evoked myogenic potential tests that assess the otolith organs: cVEMP and oVEMP.
For both tests, we will cover the pathway, the recording, the asymmetry ratio and what they show in the clinic.
The caloric test and the video head impulse test assess the semicircular canals; they do not examine the otolith organs.
The utricle and saccule are the two otolith organs that sense linear acceleration and gravity.
The vestibular evoked myogenic potential, or VEMP, gives information about the function of these organs.
High-intensity sound or bone vibration stimulates the otolith organs; the saccule in particular is quite sensitive to sound.
The reflex response to this stimulus is recorded with surface electrodes from muscles in the neck or around the eye.
So a VEMP is not a hearing response but the recording of a muscle reflex that starts in the otolith organs.
Two types of VEMP are used in the clinic.
The cervical VEMP, or cVEMP, is recorded from a neck muscle and mainly reflects the saccule and the inferior vestibular nerve.
The cVEMP response is a brief relaxation of a contracting muscle, that is, an inhibitory response.
The ocular VEMP, or oVEMP, is recorded below the eye and mainly reflects the utricle and the superior vestibular nerve.
The oVEMP, by contrast, is an excitatory response arising from a brief contraction of an eye muscle.
Together, the two tests help assess the two divisions of the vestibular nerve separately.
The most common stimulus for cVEMP is a high-intensity tone burst of about five hundred hertz, or a click.
The stimulus activates the saccule on the same side.
The signal travels along the inferior vestibular nerve to the vestibular nuclei in the brainstem.
From there it descends to the neck muscle through the medial vestibulospinal tract and the accessory nerve, the eleventh cranial nerve.
The response is recorded from the musculus sternocleidomastoideus, that is, the sternocleidomastoid muscle on the same side.
So a stimulus to the right ear is recorded from the right neck muscle, and a stimulus to the left ear from the left neck muscle.
The active electrode is placed on the upper or middle part of the sternocleidomastoid muscle.
The reference electrode usually goes on the upper end of the sternum, and the ground electrode on the forehead.
Because the response appears only in a contracted muscle, the patient must keep the muscle tense during the test.
The patient sits and turns the head to the opposite side, or lies supine and lifts and turns the head off the pillow.
Because amplitude changes with the level of contraction, muscle activity is monitored on screen and amplitude is corrected if needed.
Each ear is stimulated separately, and the responses from the two sides are compared.
The cVEMP response is a biphasic wave that appears shortly after the stimulus.
The first peak is a positive peak at about thirteen milliseconds, so it is called P13.
It is followed by a negative peak at about twenty three milliseconds, called N23.
The difference between the two peaks is the response amplitude, measured in microvolts.
Latencies are quite stable; clinical interpretation relies mostly on amplitude, the difference between sides, and threshold.
A prolonged latency, however, can be seen in some central disorders and is noted separately.
Three measures are used to interpret the cVEMP.
The asymmetry ratio is the amplitude difference between the ears divided by the sum of the amplitudes, expressed as a percentage.
The upper limit of the asymmetry ratio varies by laboratory; it is usually reported between about thirty and forty five percent.
Threshold is the lowest stimulus intensity giving a repeatable response; in a healthy ear, the threshold is usually high.
In third window disorders such as superior canal dehiscence, the threshold drops markedly and amplitude increases.
Latency is compared between the two sides; prolonged latency may bring central pathways to mind.
Now let us look at a purely example cVEMP result.
In this example, P13 latency is within normal limits in both ears.
Amplitude is one hundred fifty microvolts in the right ear and fifty in the left.
The difference is one hundred and the sum is two hundred, so the asymmetry ratio is fifty percent.
This value is well above the laboratory upper limit and shows a reduced response on the left side.
If muscle tension is not similar on both sides, such a difference can falsely look like an asymmetry.
For oVEMP, bone-conducted vibration, such as a brief tap or vibration on the forehead, or high-intensity sound can be used.
This response mainly reflects utricular function, especially when it is evoked with bone vibration.
The signal reaches the vestibular nuclei through the superior vestibular nerve.
From there it crosses to the opposite side along the vestibulo-ocular reflex pathway and reaches the eye muscle nucleus.
The response is recorded from the inferior oblique muscle below the opposite eye.
So when the right ear is stimulated, the response is picked up by the electrode below the left eye.
The active electrode is placed just below the eye, and the reference electrode a little below it.
The patient looks upward during the test, which brings the inferior oblique muscle closer to the electrode below the eye.
The gaze angle varies by protocol; often the highest comfortable point is chosen and kept the same on both sides.
The oVEMP response is much smaller than the cVEMP; its amplitude is usually only a few microvolts.
Bone-conducted vibration often produces larger oVEMP responses than air-conducted sound.
The response is also interpreted with the asymmetry ratio, and the upper limit is set by the laboratory.
The oVEMP response is also biphasic, but its first peak is negative.
The negative peak at about ten milliseconds is called N10, and the positive peak that follows is called P15.
Amplitude is the difference between N10 and P15, measured in microvolts.
Because the response is small, it is very important that the patient does not blink and keeps gaze steady during recording.
With age, both cVEMP and oVEMP amplitudes decrease; in older adults, bilaterally absent responses may not be meaningful on their own.
An air-conducted sound stimulus reaches the otolith organs through the middle ear.
So even a small conductive hearing loss can reduce or completely abolish an air-conducted VEMP response.
In this case, an absent response should not be interpreted as a vestibular loss.
In patients with conductive hearing loss, a bone-conducted vibration stimulus is preferred.
In otosclerosis the air-conducted VEMP is usually absent, while in third window disorders it grows; this helps differential diagnosis.
So the audiogram and tympanometry results are always checked before VEMP testing.
In superior vestibular neuritis, the cVEMP is usually preserved, while the oVEMP is reduced or absent.
In the rarer inferior involvement, the reverse is true: the cVEMP is affected and the oVEMP may be preserved.
In superior canal dehiscence, the cVEMP threshold drops and the oVEMP amplitude grows.
In conductive hearing loss, both responses may be absent with an air-conducted stimulus; this is not a vestibular finding.
In Meniere's disease, the findings vary with the stage; reduced or normal responses may be seen.
These patterns are examples; interpretation is made together with the history, hearing tests and other vestibular tests.
The sound level used for VEMP is very high, so the total number and duration of stimuli are kept no higher than needed.
Before the test, the patient is asked about any history of sound sensitivity or tinnitus.
Because children's ear canals are small, the same setting produces a higher sound pressure; the stimulus level is chosen accordingly.
For a reliable comparison between sides, similar muscle tension on both sides is ensured.
Stimulus type, level and normal limits are set according to the device and the laboratory.
A clinical tip: before interpreting a cVEMP asymmetry, compare the muscle tension on both sides.
If the patient turns the head more weakly to one side, the response on that side will look smaller.
Each recording is repeated at least twice, and the waveforms are checked for overlap.
An irregular wave that cannot be repeated is not accepted as a response.
The cVEMP reflects the saccule and inferior vestibular nerve and is recorded from the sternocleidomastoid muscle on the same side.
The oVEMP reflects the utricle and superior vestibular nerve and is recorded below the opposite eye.
Interpretation uses the asymmetry ratio, threshold and latency; normal limits are laboratory specific.
Conductive hearing loss can abolish an air-conducted VEMP; in that case, a bone-conducted stimulus is preferred.
Test yourself: which organ and nerve division does the cervical VEMP, or cVEMP, mainly reflect?
The correct answer is B: the saccule and the inferior vestibular nerve.
The lateral canal, on the other hand, is assessed with the caloric test and the video head impulse test.
In the next lesson we will learn why the caloric test and the video head impulse test can give different results.
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Prerequisites

LESSON 159 Video head impulse test (vHIT)

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