Sally M. Rosengren

New perspectives on vestibular evoked myogenic potentials.

PURPOSE OF REVIEW Although the vestibular evoked myogenic potential (VEMP) measured from the cervical muscles (cVEMP, cervical VEMP) is well described and has documented clinical utility, its analogue recorded from the extraocular muscles (oVEMP, ocular VEMP) has been described only recently and is currently emerging as an additional test of otolith function. This review will, therefore, summarize recent developments in VEMP research with a focus on the oVEMP. RECENT FINDINGS Recent studies suggest that the oVEMP is produced by otolith afferents in the superior vestibular nerve division, whereas the cVEMP evoked by sound is thought to be an inferior vestibular nerve reflex. Correspondingly, the oVEMP correlates better with caloric and subjective visual vertical tests than sound-cVEMPs. cVEMPs are more complicated than often thought, as shown by the presence of crossed responses and conflicting results of recent vibration studies. Altered inner ear mechanics produced by the vestibular diseases superior semicircular canal dehiscence and Ménières disease lead to changes in the preferred frequency of the oVEMP and cVEMP. SUMMARY The oVEMP provides complementary diagnostic information to the cVEMP and is likely to be a useful addition to the diagnostic test battery in neuro-otology.

A short latency vestibular evoked potential (VsEP) produced by bone-conducted acoustic stimulation

In this paper data are presented from an experiment which provides evidence for the existence of a short latency, acoustically evoked potential of probable vestibular origin. The experiment was conducted in two phases using bone-conducted acoustic stimulation. In the first phase subjects were stimulated with 6-ms, 500-Hz tone bursts in order to obtain the threshold V(T) for vestibular evoked myogenic potentials (VEMP). It was confirmed that the difference between bone-conducted auditory and acoustic vestibular thresholds was slightly over 30 dB. The estimated threshold was then used as a reference value in the second part of the experiment to stimulate subjects over a range of intensities from -6 to +18 dB (re: V(T)). Averaged EEG recordings were made with eight Ag/AgCl electrodes placed on the scalp at Fpz, F3, F4, F7, F8, Cz, T3, and T4 according to the 10-20 system. Below V(T) auditory midlatency responses (MLRs) were observed. Above V(T) two additional potentials appeared: a positivity at about 10 ms (P10) which was maximal at Cz, and a negativity at about 15 ms (N15) which was maximal at Fpz. Extrapolation of the growth functions for the P10 and N15 indicated a threshold close to V(T), consistent with a vestibular origin of these potentials. Given the low threshold of vestibular acoustic sensitivity it is possible that this mode may make a contribution to the detection of and affective responses to loud low frequency sounds. The evoked potentials may also have application as a noninvasive and nontraumatic test of vestibular projections to the cortex.

Vestibular evoked myogenic potentials evoked by brief interaural head acceleration: properties and possible origin

The vestibular system responds to head acceleration by producing compensatory reflexes in the eyes and postural muscles. In this study, we investigated the effect of brief interaural acceleration on the vestibular evoked myogenic potential (VEMP) in 10 normal subjects and 10 patients with bilateral (bVL) or unilateral vestibular loss (uVL). The stimuli were delivered with a handheld minishaker and tendon hammer over the mastoid and produced relatively pure interaural head acceleration with little rotation (mean peak acceleration: 0.14 g at 3.3 ms). VEMPs were recorded from the neck muscles and were characterized in normal subjects by a positive/negative potential ipsilateral to the stimulated side (peak latencies: 15.1 and 22.6 ms) and a positive response contralaterally (20.3 ms), which was sometimes preceded by a negativity (14.5 ms). These peaks were absent in patients with bVL, confirming their vestibular dependence. In the patients with uVL, medial acceleration of the intact ear produced bilateral responses, an initial positivity on the intact side, and a negativity on the affected side, whereas lateral acceleration produced only a late positivity on the intact side. As the acceleration was primarily in the horizontal plane, it is likely to have activated utricular receptors. Consistent with this, we found that VEMPs are very sensitive to the direction of head acceleration and have features consistent with the utriculocollic projections demonstrated in animals.

Stochastic galvanic vestibular stimulation produces a small reduction in sway in Parkinson's disease.

We investigated the effects of bicathodal stochastic galvanic vestibular stimulation (GVS) on body sway in normal subjects and in Parkinsons Disease (PD) patients. Twenty normal subjects and five PD patients were stimulated with four stimulus intensities between 0 and 0.5 mA and sway was measured in two stance conditions (on a compliant surface with either eyes open (EOCS) or closed (ECCS)). Subjects stood facing forward with their feet together on a force platform. Centre of pressure (CoP) displacement over 26 seconds was measured in the anteroposterior (AP) and mediolateral (ML) planes. GVS had no significant overall effect on sway in the normal subjects. In the patients a small (4.5%) significant decrease in sway was seen in the ECCS condition with low intensity (0.1 mA) stimulation (P=0.02). Similar changes were seen in the normal subjects. This work indicates that low intensities of stochastic GVS can reduce sway levels in PD patients for certain stance conditions.

The relative effectiveness of different stimulus waveforms in evoking VEMPs: significance of stimulus energy and frequency.

We compared the effectiveness of a series of different sound stimulus waveforms in evoking VEMPs in normal volunteers. The waveforms were clicks (0.1-0.8 ms), biphasic clicks (0.8 ms) and sine waves (1250 Hz, 0.8 ms and 500 Hz, 2 ms) with different peak intensity and duration but similar root mean square area. VEMP amplitudes varied widely (corrected values 0.35 to 1.06), but when the amplitudes were plotted against the physical energy content and A-weighted intensity (L(Aeq): a measure of acoustic energy) of the waveforms, the relationship was found to be highly linear. However, when the stimuli were matched for their A-weighted energy, a 500 Hz 2 ms sine wave was the most effective waveform, suggesting that frequency tuning in the vestibular system is also an important factor. VEMP amplitude is thus determined by three stimulus-related factors: physical energy, transmission through the middle ear and vestibular frequency tuning. Use of a 500 Hz stimulus will maximise the prevalence and amplitude of the VEMP for a given sound exposure level.

Clinical utility of ocular vestibular-evoked myogenic potentials (oVEMPs)

Over the last years, vestibular-evoked myogenic potentials (VEMPs) have been established as clinical tests of otolith function. Complementary to the cervical VEMPs, which assess mainly saccular function, ocular VEMPs (oVEMPs) test predominantly utricular otolith function. oVEMPs are elicited either with air-conducted (AC) sound or bone-conducted (BC) skull vibration and are recorded from beneath the eyes during up-gaze. They assess the vestibulo-ocular reflex and are a crossed excitatory response originating from the inferior oblique eye muscle. Enlarged oVEMPs have proven to be sensitive for screening of superior canal dehiscence, while absent oVEMPs indicate a loss of superior vestibular nerve otolith function, often seen in vestibular neuritis (VN) or vestibular Schwannoma.

Bilateral vestibulopathy: Diagnostic criteria Consensus document of the Classification Committee of the Barany Society

This paper describes the diagnostic criteria for bilateral vestibulopathy (BVP) by the Classification Committee of the Bárány Society. The diagnosis of BVP is based on the patient history, bedside examination and laboratory evaluation. Bilateral vestibulopathy is a chronic vestibular syndrome which is characterized by unsteadiness when walking or standing, which worsen in darkness and/or on uneven ground, or during head motion. Additionally, patients may describe head or body movement-induced blurred vision or oscillopsia. There are typically no symptoms while sitting or lying down under static conditions.The diagnosis of BVP requires bilaterally significantly impaired or absent function of the vestibulo-ocular reflex (VOR). This can be diagnosed for the high frequency range of the angular VOR by the head impulse test (HIT), the video-HIT (vHIT) and the scleral coil technique and for the low frequency range by caloric testing. The moderate range can be examined by the sinusoidal or step profile rotational chair test.For the diagnosis of BVP, the horizontal angular VOR gain on both sides should be <0.6 (angular velocity 150-300°/s) and/or the sum of the maximal peak velocities of the slow phase caloric-induced nystagmus for stimulation with warm and cold water on each side <6°/s and/or the horizontal angular VOR gain <0.1 upon sinusoidal stimulation on a rotatory chair (0.1 Hz, Vmax = 50°/sec) and/or a phase lead >68 degrees (time constant of <5 seconds). For the diagnosis of probable BVP the above mentioned symptoms and a bilaterally pathological bedside HIT are required.Complementary tests that may be used but are currently not included in the definition are: a) dynamic visual acuity (a decrease of ≥0.2 logMAR is considered pathological); b) Romberg (indicating a sensory deficit of the vestibular or somatosensory system and therefore not specific); and c) abnormal cervical and ocular vestibular-evoked myogenic potentials for otolith function.At present the scientific basis for further subdivisions into subtypes of BVP is not sufficient to put forward reliable or clinically meaningful definitions. Depending on the affected anatomical structure and frequency range, different subtypes may be better identified in the future: impaired canal function in the low- or high-frequency VOR range only and/or impaired otolith function only; the latter is evidently very rare.Bilateral vestibulopathy is a clinical syndrome and, if known, the etiology (e.g., due to ototoxicity, bilateral Menières disease, bilateral vestibular schwannoma) should be added to the diagnosis. Synonyms include bilateral vestibular failure, deficiency, areflexia, hypofunction and loss.

Effects of muscle contraction on cervical vestibular evoked myogenic potentials in normal subjects

OBJECTIVE Cervical vestibular evoked myogenic potentials (cVEMPs) are vestibular-dependent muscle reflexes recorded from the sternocleidomastoid (SCM) muscles in humans. cVEMP amplitude is modulated by stimulus intensity and SCM muscle contraction strength, but the effect of muscle contraction is less well-documented. The effects of intensity and contraction were therefore compared in 25 normal subjects over a wide range of contractions. METHODS cVEMPs were recorded at different contraction levels while holding stimulus intensity constant and at different intensities while holding SCM contraction constant. RESULTS The effect of muscle contraction on cVEMP amplitude was linear for most of the range of muscle contractions in the majority of subjects (mean R(2)=0.93), although there were some nonlinearities when the contraction was either very weak or very strong. Very weak contractions were associated with absent responses, incomplete morphology and prolonged p13 latencies. Normalization of amplitudes, by dividing the p13-n23 amplitude by the muscle contraction estimate, reduced the effect of muscle contraction, but tended to underestimate the amplitude with weak contractions. CONCLUSIONS Minimum contraction levels are required for accurate interpretation of cVEMPs. SIGNIFICANCE These data highlight the importance of measuring SCM contraction strength when recording cVEMPs.

Vestibular-evoked myogenic potentials

The vestibular-evoked myogenic potential (VEMP) is a short-latency potential evoked through activation of vestibular receptors using sound or vibration. It is generated by modulated electromyographic signals either from the sternocleidomastoid muscle for the cervical VEMP (cVEMP) or the inferior oblique muscle for the ocular VEMP (oVEMP). These reflexes appear to originate from the otolith organs and thus complement existing methods of vestibular assessment, which are mainly based upon canal function. This review considers the basis, methodology, and current applications of the cVEMP and oVEMP in the assessment and diagnosis of vestibular disorders, both peripheral and central.

Safe levels of acoustic stimulation: comment on '"effects of acoustic stimuli used for vestibular evoked myogenic potential studies on the cochlear function '".

To the Editor: Krause et al. (1) have reported the effects on cochlear function of acoustic stimuli used in eliciting cervical vestibular evoked myogenic potentials (cVEMPs). They reported some subjective complaints of muffling of hearing, no significant change in pure-tone audiometry, but some reduction in distortion product otoacoustic emissions in the high-frequency range whenmeasured 5 minutes after the cVEMP. Although the abnormal features settled the following day and were not considered to represent clinically relevant temporary hearing loss, the authors felt that patients need to be informed about the risk of adverse effects on hearing. The safety of diagnostic tests is an important consideration, although it is also recognized that some investigations do entail some risk to patients (e.g., ionizing radiation with conventional radiology). Any test that is to be used in large numbers of patients or in healthy volunteers should be as safe as possible. The cVEMP has become a popular additional investigation of inner ear function and can provide clinically relevant information in diseases such as superior canal dehiscence (SCD), otosclerosis, Méneière’s disease, vestibular neuritis, and other conditions (2). The intensities of sound required to evoke the cVEMP are high (3), and correctly calibrated stimuli and audiometric equipment are essential. The technique has been widely used and very few side effects have been reported to date, although caution is suggested with patients with tinnitus. Although technically tone bursts are impulse noise, they are not typical of naturally occurring impulse noise and are probably better regarded simply as an interrupted sinusoidal waveform. Hearing damage is generally thought to relate to the peak intensity of sound pressure (blast-type injuries) or the total sound energy delivered to the ear (4). Industrial legislation in many countries reflects these concerns by specifying both the maximum sound pressure level (SPL) and the total sound energy exposure measured during a specified period (e.g., 8 h), with more intense sound requiring a reduction in the duration of exposure. For example, the European Union (2003) and U.K. (2005) guidelines for occupational exposure specify an upper limit of 200 Pa (140 dB peak SPL, C weighted) and an exposure equivalent to 87 dB LAeq,8h. Although these guidelines are not intended to cover medical contexts, it is prudent to consider the recommended levels and the reason behind them when selecting VEMP stimuli. Commercial VEMP systems usually allow users to choose between clicks and tone bursts and generally specify stimulus intensity in decibel SPL or normal hearing level (nHL). However, these intensities do not reflect the total sound energy in a stimulus because the duration is not taken into account. Longer stimuli contain proportionately greater sound energy. The options for stimulus duration and shape are often limited in commercial systems, and sine waves sometimes require a minimum number of rise/fall or plateau cycles. Tone bursts of up to about 6 ms in duration are therefore quite common in the VEMP literature. LAeq is a measure of equivalent ‘‘A’’-weighted sound intensity over the measurement period, the usual reference being 1 second and thus gives a measure of energy delivered to the ear over the period specified. A-weighting is commonly used, possibly because this is similar to the attenuation of the middle ear (5). Total energy exposure is the product of sound intensity and time exposure and is also measured in decibels, where the reference energy (0 dB) is a pressure of 20 KPa applied for 1 second. An 87-dB LAeq stimulus given over 8 hours represents 132 dB of energy delivered to the ear, compared to this reference. Rosengren et al. (6) investigated the effects of different acoustic waveforms and energies on the VEMP and concluded that waveform energy was an important determinant of cVEMP amplitude. They calculated that, to stay within the required ‘‘upper exposure action values’’ (of 85 dB LAeq,8h, slightly less than the absolute upper limit of 87 dB), a 105-dB LAeq,1s stimulus could be presented at a rate of 5/s for a total of almost 5 minutes to each ear. For stimuli given at 5/s, a 105-dB LAeq,1s stimulus consisting of 0.1-ms duration click would be expected to have an intensity of 138 dB peak SPL; and for a 2-ms 500 Hz tone burst, an intensity of 131 dB peak SPL (see Appendix). Krause et al. (1) used a 500-Hz, 133-dB SPL stimulus to evoke cVEMPs, but the duration (10 ms) was longer than commonly used. The sound energy delivered by this stimulus (200 repetitions of a 500-Hz, 10-ms tone burst at 133 dB SPL at 5/s, assuming their intensity is measured as root mean square [RMS]) is equivalent to 133 dB. If all 200 stimuli are delivered, the energy slightly exceeds the maximum LAeq exposure specified by the EU limits (by 1 dB). The results of Krause et al. show that stimulating near the recommended limit in their sample of young subjects with normal hearing did not have any serious effects. But at this intensity/duration, there is little scope for safely increasing the number of stimuli given, which is needed when the reflex waveform is small or unclear, as often occurs in older subjects. Longer stimuli increase the amount of sound energy delivered to the ear but do not necessarily yield larger or clearer reflexes. Welgampola and Colebatch (7) showed Otology & Neurotology 35:932Y935 2014, Otology & Neurotology, Inc.