Vestibular evoked myogenic potentials

By Marcello Cherchi, MD PhD

For patients

Here are answers to some of the most common questions about vestibular evoked myogenic potentials.

What is this test?	The term “vestibular evoked myogenic potentials” (VEMPs) refers to several different tests of inner ear balance function.
What is this test looking for?	This test looks for muscle activity in response to sound.
Is this test experimental or investigational?	This test is approved by the Food and Drug Administration. It is neither experimental nor investigational.
What happens during this test?	During this test the audiologist or technician typically will use an earbud insert earphone in one ear, then the other ear. You will also have surface electrodes placed on various parts of your body (sometimes around the eyes, sometimes on the neck). In some versions of this test you will be lying down and raising your head up. In some versions of this test you may be seated but looking upward. You will hear some loud tones from the earbud insert earphone.
Is this test uncomfortable?	The test is not uncomfortable, though the tones from the earbud insert earphone is fairly loud.
How long does this test take?	The test takes about 30 minutes.
Do I have to prepare for this test?	There are no specific preparations for this test. If you have earwax, this should be removed before the test.
Are there any special instructions for what to do after the test?	There are no special instructions for what to do after this test.

For clinicians

Overview

Vestibular evoked myogenic potentials (VEMPs) are reflex myogenic potentials usually elicited by vibration-induced stimulation of the vestibular end-organ (particularly the saccule and utricle). Responses can be measured from a variety of muscles. The test assesses the pathway from the vestibular end organ, through the vestibular nerve, to the vestibular nuclei, to various targets such as extraocular muscles and skeletal muscles. It is reasonable to check VEMPs in some patients with the symptom of disequilibrium, and some patients with auditory symptoms.

Introduction

The phrase “vestibular evoked myogenic potentials” (VEMPs) refers to the phenomenon of stimulating the peripheral vestibular system (either in the labyrinth, or the vestibular nerve), and measuring a reflex myogenic response using surface electrodes from some part of the body.

There are several ways of stimulating the peripheral vestibular system.

• With physiologic stimuli, meaning actual acceleration. This is the basis for tools such as rotatory chair testing.
• With non-physiologic stimuli, including:
  • Thermal. This is the basis for caloric testing.
  • Galvanic. Properly situated externally applied active electrodes can deliver a direct or alternating current that stimulates the vestibular nerve.
  • With vibration. This is the main methodology for clinically applicable vestibular evoked myogenic potentials. Vibrations can be delivered in several ways:
    • Acoustically — in other words, with an auditory stimulus.
    • With non-acoustic vibration. This is similar to acoustic stimuli, except the vibrations bypass the mechanics of middle ear and are delivered directly to the labyrinth.

History

In 1964, Cody and colleagues (Cody et al. 1964) and Bickford and colleagues (Bickford et al. 1964) reported myogenic potentials induced by sound, and subsequent work showed those myogenic potentials to be of vestibular origin (Colebatch and Halmagyi 1992; Colebatch et al. 1994; Colebatch and Rothwell 1994). In other words, this work demonstrated that acoustic stimuli can induce a vestibular reflex response (Halmagyi et al. 2005a) in normal subjects.

In clinical practice, delivery of acoustic stimuli is the most common method of evoking vestibularly mediated myogenic potentials, largely because of its simplicity. In some cases (typically situations in which there is conductive hearing loss) a bone vibrator or skull taps may be used.

Galvanic vestibular stimulation is mostly restricted to research settings.

Several detailed reviews cover this topic well, such as those by Curthoys and colleagues (Curthoys 2010, 2017; Curthoys and Dlugaiczyk 2019; Curthoys et al. 2019; Curthoys et al. 2014; Grant and Curthoys 2017).

Physiology and neuroanatomy

Given the nature of the myogenic response, there was initially some skepticism that these were triggered through a vestibular pathway, as alternative explanations (such as an acoustically mediated startle response, or an auditory orienting response) seemed plausible. However, several lines of evidence supported that these myogenic responses are, in fact, mediated by the vestibular system.

• Although the stimulus is acoustic, the response is not triggered by cochlear stimulation. It was shown that the myogenic potentials could be elicited when the cochlea was destroyed (Karino et al. 2005; Plontke et al. 2021).
• Sound does stimulate the vestibular end-organelles. It is well-established that the vibrations from sound can stimulate the vestibular receptors of the labyrinth (Colebatch 2001; Halmagyi et al. 2005b), particularly the otolith organs (Curthoys et al. 2018; Curthoys et al. 2011).
• These same myogenic potentials could be elicited via intraoperative stimulation of the vestibular nerve (Basta et al. 2005).

In other words, these data suggested that an acoustic stimulus received by vestibular end-organelles (independently of any activity in the cochlea) initiates a signal mediated by the vestibular nerve that generates a reflex myogenic response identical to what would occur with direct electrical stimulation of the vestibular nerve itself. Taken together, these data strongly support the idea that these myogenic potentials truly are evoked when the vestibular system is stimulated by acoustic stimuli.

A more correct description of this phenomenon would be, “reflex myogenic potentials elicited by vibration-induced stimulation of the vestibular end-organ,” but the simpler and more easily abbreviated term, “vestibular evoked myogenic potentials” (VEMPs) has become entrenched in the literature, so we will use this conventional nomenclature here.

In healthy individuals, vibrations do not significantly stimulate the cupulae of the semicircular canals, but they can stimulate the maculae of the otolith organs. Primary vestibular neurons in the vestibular (Scarpa’s) ganglion receive signals from type 1 vestibular hair cells in the otolith organs, and project to the vestibular nuclei. From there, secondary vestibular neurons project to a variety of targets, including the ocular motor nuclei and the vestibulospinal tracts.

The targets in the vestibulospinal tract generate responses that can be clinically detected in many skeletal muscles, including:

• The sternocleidomastoid. These can be detected as cervical vestibular evoked myogenic potentials (cVEMPs), most robustly detected in the sternocleidomastoid ipsilateral to the stimulated ear. This is the most common type of VEMP used in clinical practice.
• The triceps. These can be detected as triceps vestibular evoked myogenic potentials (Cherchi et al. 2009; Cherchi et al. 2015).
• The gastrocnemius (Rudisill and Hain 2008).

The targets in the ocular motor nuclei generate responses that can be clinically detectable as ocular vestibular evoked myogenic potentials (oVEMPs), most robustly detected in the inferior oblique muscle contralateral to the stimulated ear. This is the second most common type of VEMP used in clinical practice.

Like all vestibular responses, vestibular evoked myogenic potentials respond differently at different points along the vestibular tuning spectrum. In healthy individuals, VEMPs usually have the most robust responses at a specific frequency and amplitude (Park et al. 2010), though this can change with healthy aging (Piker et al. 2013). The “tuning curve” of VEMPs may be altered in various disease states such as Ménière’s disease (Jerin et al. 2014; Murofushi et al. 2017a, b; Rauch et al. 2004; Tian et al. 2018; Winters et al. 2012) and perhaps normal pressure hydrocephalus (Inui et al. 2022), but such pattern shifts are not yet used specifically for diagnostic purposes.

Equipment needed

Depending on the clinical scenario, the stimulus generator can be acoustic (through earbud insert earphones) or vibratory (from a bone vibrator). Skull taps (Iwasaki et al. 2007) are less commonly used since it is more difficult to calibrate the stimulus. When feasible it is desirable to use acoustic stimuli because it is easier to stimulate one ear at a time. Using a bone vibrator or skull taps is more likely to deliver a vibratory stimulus to both labyrinths.

The equipment for measuring myogenic responses from surface electrodes is similar to other evoked myogenic potential machines. The software is customized to the timing, amplitude and response-averaging parameters relevant for VEMPs.

What this test assesses

Vestibular evoked myogenic potentials assess the pathways originating in the otolith organs, traveling through the vestibular nerve to the vestibular nuclei, and various destinations from there (which differ depending on the type of VEMP being measured). Lesions anywhere along these pathways can result in an abnormal response.

How to interpret the test results

Abnormal VEMPs include absence of responses, reduced amplitude of responses, delayed latencies of responses, and abnormal thresholds.

The effect of age on VEMP responses

Age has been shown to affect VEMP responses in general.

Age affects have specifically been documented in cVEMP responses.

• Janky and Shepard studied cVEMP responses and reported that advancing age positively correlates with thresholds and negatively correlates with amplitudes, and does not correlate with latencies (Janky and Shepard 2009).
• Su and colleagues studied cVEMP responses and reported that advancing age positively correlates with latency, negatively correlates with amplitude, and negatively correlates with response rates overall (Su et al. 2004).
• Ochi and Ohashi studied cVEMPs and reported that advancing age positively correlates with thresholds and negatively correlates with amplitudes (Ochi and Ohashi 2003).
• Maes and colleagues studied cVEMPs and reported that advancing age positively correlates with thresholds, negatively correlates with amplitudes, and negatively correlates with latencies (Maes et al. 2010).
• Agrawal and colleagues studied cVEMPs and reported that advancing age negatively correlates with amplitudes (Agrawal et al. 2012).
• Welgampola and Colebatch studied cVEMPs and reported that advancing age positively correlates with thresholds and negatively correlates with amplitudes (Welgampola and Colebatch 2001).

Age affects have been specifically documented in oVEMP responses.

• Agrawal and colleagues studied oVEMPs and reported that advancing age negatively correlates with amplitudes (Agrawal et al. 2012).
• Sing and Firdose studied oVEMPs and reported that advancing age positively correlates with latencies, negatively correlates with amplitudes, and negatively correlates with response rates overall (Singh and Firdose 2021).

In summary, there is evidence that advancing age causes an increase in latencies, an increase in thresholds, a decrease in amplitude and a decrease in overall VEMP responses.  Thus, in an older individual, symmetrically absent VEMPs are generally not diagnostically meaningful.

Limitations

Vestibular evoked myogenic potentials that use acoustic stimuli rely on the integrity of the middle ear to transmit the vibrations from sound to the labyrinth. Thus, conductive hearing loss can interfere with VEMP results. For this reason it is desirable to have a recent audiogram when performing acoustically-elicited VEMPs.

Contraindications

Acoustically-elicited VEMPs generally use earbud insert earphones, which is relatively contraindicated in patients with external ear problems such as otitis externa or recent injury.

Pitfalls

Keep in mind that non-vestibular lesions can result in abnormal VEMPs. For example, a patient with hemiparesis may have a weakened sternocleidomastoid (and thus a weak or absent cervical VEMP) even though the peripheral vestibular system is intact. A patient with ocular myasthenia gravis will have weak extraocular muscles (and thus a weak or absent ocular VEMP) even though the peripheral vestibular system is intact.

When is the test indicated

Vestibular evoked myogenic potentials comprise a reasonable tool for testing otolithic vestibular pathways in patients with the symptom of disequilibrium.

Diseases that may be diagnosed by this test

Diseases of the labyrinth and vestibular nerve are sometimes diagnosable if they involve otolithic pathways.

Third window phenomena, such as semicircular canal dehiscence, can manifest with abnormally large-amplitude vestibular evoked myogenic potentials.

Spinal cord lesions that interrupt vestibulospinal tract pathways may also result in abnormal vestibular evoked myogenic potentials (Shirley et al. 2015).

References