Acoustic, mechanical and galvanic stimulation modes elicit ocular vestibular-evoked myogenic potentials

ObjectiveThis study compared the characteristic parameters of ocular vestibular-evoked myogenic potentials (oVEMPs) elicited by the air-conducted sound (ACS) and bone-conducted vibration (BCV) stimulation modes as well as the galvanic vestibular stimulation (GVS) mode.DesignFifteen healthy subjects underwent oVEMP tests using ACS (127 dBSPL), BCV (142 dB force level), and GVS (5 mA) modes. The response rate, latencies of nI and pI, nI–pI interval and amplitude were measured for each mode and compared among modes.ResultsAll 15 healthy subjects (30 ears) had 100% response rates in both BCV- and GVS–oVEMPs, exhibiting a response rate significantly higher than 80% in ACS–oVEMPs. The mean nI latency was the shortest in the GVS mode, followed by BCV and then ACS modes. The variation among the latencies of the three modes was significant. Likewise, the mean nI–pI amplitudes in ACS-, BCV- and GVS modes varied significantly. However, the mean nI–pI interval did not differ significantly among the three modes.ConclusionsAmong the ACS (127 dBSPL), BCV (142 dB force level), and GVS (5 mA) modes, the BCV mode yields a 100% response rate and the largest nI–pI amplitude of oVEMPs.SignificanceThe oVEMPs in ACS and GVS modes may help to differentiate the saccular from the retro-saccular lesions. If ACS–oVEMPs are normal, then oVEMPs in BCV and GVS modes can distinguish between utricular and retro-utricular disorders. Restated, oVEMPs in ACS, BCV, and GVS modes may promote the topographical delineation of the lesion site of the otolithic–ocular reflex pathway.     

The n10 component of the ocular vestibular-evoked myogenic potential (oVEMP) is distinct from the R1 component of the blink reflex

ObjectiveBone-conducted vibration (BCV) in the midline at the hairline (Fz), results in short latency potentials recorded by surface electrodes beneath the eyes – the ocular vestibular-evoked myogenic potential (oVEMP). The early negative component of the oVEMP, n10, is due to vestibular stimulation, however it is similar to the early R1 component of the blink reflex. Here we seek to dissociate n10 from R1.MethodsSurface potentials were recorded from the infraorbital electromyogram of 10 healthy subjects, 6 patients with bilateral vestibular loss, 2 with unilateral vestibular loss, 4 with facial palsy and 3 with facial and vestibular nerve lesions on the same side. BCV was delivered at Fz, the inion, the glabella or the supraorbital ridge using a tendon hammer or a bone-conduction vibrator.ResultsOnset latencies of the n10 evoked by taps at Fz or inion were significantly shorter than the R1 components of blink responses to supraorbital and glabellar stimuli. Upward gaze increased the amplitude of n10 but not R1. The n10 was absent bilaterally in patients with bilateral vestibular loss and beneath the contralesional eye in patients with unilateral vestibular loss, but in both these groups of patients R1 was preserved. In severe facial palsy the R1 component was absent or delayed and attenuated ipsilesionally, but n10 was preserved bilaterally. In subjects with unilateral facial and vestibular nerve lesions (Herpes Zoster of the facial and vestibulocochlear nerves) the dissociation was complete – the ipsilesional R1 was absent or attenuated whereas the ipsilesional n10 was preserved.Conclusionsn10 is distinguished from R1 by its earlier onset, laterality, modulation by gaze position and dissociation in patient groups.SignificanceThe n10 component evoked by BCV at Fz is not the R1 component of the blink reflex.     

Repetitive ocular vestibular evoked myogenic potential stimulation for the diagnosis of myasthenia gravis: Optimization of stimulation parameters

ObjectiveTo determine the most effective stimulation parameters for the diagnosis of ocular myasthenia gravis (MG) using repetitive ocular vestibular evoked myogenic potentials (oVEMP) for quantification of the extraocular muscle response decrement.MethodsRepetitive bone-conducted oVEMPs were elicited in 18 MG patients and 20 healthy subjects. We compared four different stimulus repetition rates (20 Hz, 30 Hz, 40 Hz, 50 Hz) and 100 Hz continuous stimulation, as well as recordings from the inferior oblique muscles and the lateral rectus muscles to determine the most sensitive and specific oVEMP parameters for decrement detection.ResultsRepetitive stimulation at all tested repetition rates with recordings from inferior oblique muscles allowed for effective differentiation between MG patients and healthy subjects. Among all repetition rates, 30 Hz showed a trend towards superiority, with a sensitivity of 71% and a specificity of 94% (area under the curve (AUC) 0.88) when using the smaller decrement of the two eyes and −10% as cutoff. Considering the larger decrement for analysis (−9% as cutoff), sensitivity increased to 82%, but specificity decreased to 78% (AUC 0.81).ConclusionsOur study demonstrates, that repetitive oVEMP stimulation elicits a robust decrement in the inferior oblique muscles of MG patients at repetition rates between 20 Hz and 50 Hz, with a probable optimum at 30 Hz.SignificanceRepetitive inferior oblique oVEMP stimulation with optimal stimulus parameters facilitates early and accurate diagnosis of ocular MG.     

Cervical and ocular vestibular-evoked myogenic potentials in vestibular neuritis: comparison between air- and bone-conducted stimulation

To clarify the changes of cervical (cVEMP) and ocular (oVEMP) vestibular evoked myogenic potentials induced by air-conducted sound (ACS) and bone-conducted vibration (BCV) in patients with vestibular neuritis (VN), patients with VN (n = 30) and normal controls (n = 45) underwent recording of cVEMP and oVEMP in response to ACS (1,000 Hz, 5 ms, tone bursts) and BCV (500 Hz, short tone burst). Patients with VN showed a high proportion of oVEMP abnormalities in response to both ACS (80.0 %) and BCV at the forehead (Fz, 73.3 %) or the mastoid (76.7 %). In contrast, cVEMPs were mostly normal with both ACS and BCV in the patients. The dissociations in the abnormalities of cVEMP and oVEMP induced by ACS and BCV at the mastoids and at the forehead in patients with VN suggest that oVEMP reflects functions of the superior vestibular nerve and most likely the utricular function. The results of our study suggest that oVEMP induced by either ACS or BCV appears to depend on integrity of the superior vestibular nerve, possibly due to the utricular afferents travelling in it. In contrast, cVEMP elicited by either ACS or BCV may reflect function of the saccular afferents running in the inferior vestibular nerve.     

Ocular vestibular evoked myogenic potentials to bone conducted vibration of the midline forehead at Fz in healthy subjects.

OBJECTIVE: To provide the empirical basis for using ocular vestibular evoked myogenic potentials (oVEMPS) in response to Fz bone conducted vibration (BCV) stimulation to indicate vestibular function in human subjects. To show the generality of the response by testing a large number of unselected healthy subjects across a wide age range and the repeatability of the response within subjects. To provide evidence that the response depends on otolithic function. METHODS: The early negative component (n10) of the oVEMP to brief BCV of the forehead, in the midline at the hairline (Fz) is recorded by surface EMG electrodes just beneath the eyes. We used a Bruel and Kjaer 4810 Mini-Shaker or a light tap with a tendon hammer to provide adequate BCV stimuli to test a large number (67) of unselected healthy people to quantify the individual differences in n10 magnitude, latency and symmetry to Fz BCV. A Radioear B-71 bone oscillator at Fz is not adequate to elicit a reliable n10 response. RESULTS: The n10 oVEMP response showed substantial differences in amplitude between subjects, but is repeatable within subjects. n10 is of equal magnitude in both eyes with an average asymmetry around 11%. The average n10 amplitude for Mini Tone Burst BCV is 8.47microV+/-4.02 (sd), the average latency is 10.35ms+/-0.63 (sd). The amplitude of n10 decreases and its latency increases with age. CONCLUSIONS: oVEMPs are a new reliable, repeatable test to indicate vestibular and probably otolithic function. SIGNIFICANCE: This study shows the optimum conditions for recording oVEMPs and provides baseline values for individual differences and asymmetry. oVEMPs can be measured in senior subjects without difficulty.     

A critical review of the neurophysiological evidence underlying clinical vestibular testing using sound,vibration and galvanic stimuli

In addition to activating cochlear receptors, air conducted sound (ACS) and bone conducted vibration (BCV) activate vestibular otolithic receptors, as shown by neurophysiological evidence from animal studies – evidence which is the foundation for using ACS and BCV for clinical vestibular testing by means of vestibular-evoked myogenic potentials (VEMPs). Recent research is elaborating the specificity of ACS and BCV on vestibular receptors. The evidence that saccular afferents can be activated by ACS has been mistakenly interpreted as showing that ACS only activates saccular afferents. That is not correct – ACS activates both saccular and utricular afferents, just as BCV activates both saccular and utricular afferents, although the patterns of activation for ACS and BCV do not appear to be identical. The otolithic input to sternocleidomastoid muscle appears to originate predominantly from the saccular macula. The otolithic input to the inferior oblique appears to originate predominantly from the utricular macula. Galvanic stimulation by surface electrodes on the mastoids very generally activates afferents from all vestibular sense organs. This review summarizes the physiological results, the potential artifacts and errors of logic in this area, reconciles apparent disagreements in this field. The neurophysiological results on BCV have led to a new clinical test of utricular function – the n10 of the oVEMP. The cVEMP tests saccular function while the oVEMP tests utricular function.     

Ocular vestibular evoked myogenic potentials (OVEMPs) produced by air- and bone-conducted sound

ObjectiveTo determine the origin and properties of short latency extraocular potentials produced by activation of the vestibular apparatus using two modes of acoustic stimulation.MethodsExtraocular potentials were measured in 10 normal subjects using a bipolar montage to increase selectivity. Three dimensional eye movements were also recorded in five subjects. The subjects were stimulated with both air-conducted (AC) and bone-conducted (BC) sound using a single cycle of a 500 Hz sine wave.ResultsShort latency positive and negative potentials that peaked at 8.1–12.7 ms for AC and 7.5–13.9 ms for BC stimulation were recorded, which were distinct for the two eyes and for the two modes of stimulation. The extraocular potentials began prior to the onset of eye movements, which peaked at 16.5–20.1 ms for AC, 17.8–25.0 ms for BC stimulation.ConclusionsThe pattern of short latency eye movements and extraocular potentials induced by AC and BC vestibular stimulation are distinct. As the potentials preceded the eye movements and were not correlated morphologically with them, the source of the observed potentials is not an eye movement and thus we refer to them as ocular vestibular evoked myogenic potentials (OVEMPs).SignificanceThe potentials had properties consistent with modulation of the electromyogenic activity of the extraocular muscles and if interpreted as originating from displacement of the eye will give misleading results. AC and BC acoustic stimulation are likely to activate differing profiles of vestibular end organs.     

Why and when to refer patients for vestibular evoked myogenic potentials: A critical review

Cervical and ocular vestibular evoked myogenic potentials (cVEMPs and oVEMPs respectively) are now used by an increasing number of laboratories to evaluate otolith inner ear function and their pathways through the central nervous system. However, the literature is incomplete or unclear as to what information both c- and oVEMPs can add beyond what a good clinical examination can provide, and what other paramedical tests can provide also, and the present review aims to clarify what is known so far. The following review will describe what is known with regards to both c- and oVEMPs and their use. MEDLINE (accessed by PubMed, years 1994–2018) was searched with the following string: (“vestibular evoked myogenic potentials” [all fields]). Only articles published in English were evaluated. Both c- and oVEMPs are useful not only for confirming the presence of superior semicircular canal dehiscence (SSCD), but also for confirming the presence of acoustic neuromas when MRI is not available, bilateral vestibulopathies, inferior vestibular neuritis and vestibular dysfunction in inherited neuropathies. Further work is required, especially with respect to oVEMPs. The usefulness of both c- and oVEMPs goes beyond the confirmation of SSCDs, and is useful in many clinical cases.     

Why do oVEMPs become larger when you look up? Explaining the effect of gaze elevation on the ocular vestibular evoked myogenic potential

ObjectivesThe ocular vestibular evoked myogenic potential (oVEMP) is a vestibular reflex recorded from the inferior oblique (IO) muscles, which increases in amplitude during eye elevation. We investigated whether this effect of gaze elevation could be explained by movement of the IO closer to the recording electrode.MethodsWe compared oVEMPs recorded with different gaze elevations to those recorded with constant gaze position but electrodes placed at increasing distance from the eyes. oVEMPs were recorded in ten healthy subjects using bursts of skull vibration.ResultsoVEMP amplitude decreased more with decreasing gaze elevation (9 μV from 24° up to neutral) than with increasing electrode distance (2.7 μV from baseline to 6.4 mm; P < 0.005). The oVEMP recorded with gaze 24° down had delayed latency (by 4.5 ms).ConclusionThe effect of gaze elevation on the oVEMP cannot be explained by changes in position of the muscle alone and is likely mainly due to increased tonic contraction of the IO muscle in up-gaze. The oVEMP recorded in down-gaze (when the IO is inactivated, but the IR activated) likely originates in the adjacent IR muscle.SignificanceOur results suggest that oVEMP amplitudes in extraocular muscles scale in response to changing tonic muscle activity.     

Selective effects of head posture on ocular vestibular-evoked myogenic potential (oVEMP) by bone-conducted vibration

ObjectiveBy altering head postures from sitting, supine to head hanging, this study investigated the effects of gravitational force on ocular vestibular-evoked myogenic potential (oVEMP) via either air-conducted sound (ACS) or bone-conducted vibration (BCV) stimuli.MethodsTwenty healthy volunteers underwent the oVEMP test via ACS or BCV stimuli with the sitting, supine, and head hanging positions on the same day in a randomized order.ResultsAll subjects had clear BCV oVEMPs in the three head postures. No significant differences existed in terms of mean nI and pI latencies, the nI–pI interval, and asymmetry ratio regardless of various positions. However, the mean nI–pI amplitude with the head hanging position (15.9 ± 6.4 μV) was significantly larger than that with the sitting position (13.8 ± 6.0 μV), but not significantly larger than that with the supine position (14.7 ± 6.1 μV). Nevertheless, such a difference in reflex amplitude does not exist in oVEMPs elicited by ACS stimuli. With the sitting position, mean linear acceleration at the mastoids in response to BCV stimuli was −0.06 ± 0.02, 0.20 ± 0.04 and −0.04 ± 0.02 g along the x-, y-, and z-axis, respectively, which did not differ significantly from those with the head hanging position.ConclusionBy altering head postures from sitting to head hanging, gravitational force can exert a selective effect on the reflex amplitude of oVEMPs elicited by BCV stimuli, but not by ACS stimuli.SignificanceCompared to ACS mode, BCV mode can provoke higher response rate, generate earlier and larger waveforms, and be influenced by both dynamic shearing force and static gravitational force to enlarge the reflex amplitude of oVEMPs.     

Effects of repetition rate of bone-conducted vibration on ocular and cervical vestibular-evoked myogenic potentials

Objective

This study applied bone-conducted vibration (BCV) stimuli at various repetition rates to investigate the effects of repetition rate on both ocular and cervical vestibular-evoked myogenic potentials (oVEMPs and cVEMPs).

Methods

Twenty-five healthy subjects underwent oVEMP tests in BCV mode at repetition rates of 1, 5, 10, 20, 30 and 40 Hz. The optimal repetition rates (5, 10 and 20 Hz) for oVEMPs were also adopted to elicit cVEMPs, and 20 Hz stimuli were further evaluated in pathological ears.

Results

At repetition rates of 1, 5, 10, 20, 30 and 40 Hz, the prevalence of clear oVEMPs were 100% in groups of 5, 10 and 20 Hz, with no significant differences in the mean nI latency, but the mean nI–pI amplitude of the 20 Hz group showed significantly larger. For the BCV–cVEMPs, 5, 10 and 20 Hz stimuli yielded similar information. Clinically, the BCV mode at 20 Hz stimuli was also appropriate for evaluating VEMPs in ears of vestibular schwannoma.

Conclusions

The BCV mode at a repetition rate of 20 Hz is recommended for the mass detection of VEMPs.

Significance

Eliciting VEMPs in BCV mode using 20 Hz stimuli takes a short time and may trigger a high prevalence with large amplitude.     

Ocular and cervical vestibular evoked myogenic potentials produced by air- and bone-conducted stimuli: Comparative properties and effects of age

Objective

To compare amplitudes, latencies, symmetry and the effects of age for both ocular and cervical vestibular evoked myogenic potentials (oVEMPs and cVEMPs) produced by different types of air- (AC) and bone-conducted (BC) stimuli.

Methods

Sixty-one normal subjects aged 18–80 years participated. Both reflexes were recorded in response to AC clicks, AC and BC 500 Hz tone bursts, forehead taps and lateral mastoid accelerations.

Results

AC tone bursts, clicks and BC tone bursts evoked oVEMPs in 81%, 59% and 65% of ears, respectively. The AC stimuli had higher thresholds for oVEMPs than for cVEMPs and all three stimuli produced higher asymmetry for the oVEMP than for the cVEMP. Forehead taps and lateral pulses evoked oVEMPs in 96% and 92% of cases. AC click- and BC tone burst-evoked oVEMPs showed a significant decline with age.

Conclusions

AC stimulation and BC tone bursts delivered to the mastoid are less effective in evoking oVEMPs than in evoking cVEMPs, have high degrees of asymmetry in normals and appear to decline with age. Forehead taps and lateral accelerations produce more symmetrical effects and showed no significant decline with age.

Significance

Stimulus properties need to be considered when deciding the most appropriate way to investigate vestibular function using oVEMPs.     

Mastoid and vertex low-frequency vibration-induced oVEMP in relation to medially directed acceleration of the labyrinth

ObjectiveTo explore the stimulus site and stimulus configuration dependency for bone-conducted low-frequency vibration-induced ocular vestibular evoked myogenic potentials (oVEMPs).MethodsoVEMPs were tested in response to 125 Hz single cycle bone-conducted vibration in healthy subjects (n = 12) and in patients with severe unilateral vestibular lesions (n = 10). The stimulus sites were the mastoids and vertex. Both directions of initial stimulus motion were used.ResultsAt mastoid stimulation, the oVEMP to initial laterally directed acceleration of the labyrinth was delayed approximately the length of time of a stimulus half-cycle, as compared with the response to initial medially directed acceleration. At vertex stimulation, the oVEMP to positive initial acceleration was similar to the oVEMP to mastoid stimulation causing lateral initial acceleration. Likewise, the oVEMP to vertex negative initial acceleration was similar to mastoid stimulation causing initial medial acceleration. Further, patients with unilateral vestibular loss had, compared to healthy subjects, similar oVEMP from the healthy labyrinth.ConclusionsA fundamental dependency on medially directed accelerations of the labyrinth, based on the latency differences revealed, may theoretically account for oVEMP in response to low-frequency stimulation.SignificanceLow-frequency bone vibration stimulation at vertex might serve for simultaneous oVEMP testing of both ears.     

The role of the superior vestibular nerve in generating ocular vestibular-evoked myogenic potentials to bone conducted vibration at Fz

ObjectiveThe n10 component (n10) of the ocular vestibular evoked myogenic potential (oVEMP) to brief bone conducted vibration (BCV) of the forehead at Fz is probably caused by the vibration selectively activating vestibular otolithic receptors. If the n10 is due primarily to utricular activation then diseases which affect only the superior division of the vestibular nerve (SVN) should reduce or eliminate n10.MethodsThe n10 component of the oVEMP was measured in 13 patients with unilateral SVN but with inferior vestibular nerve function preserved.ResultsWe compared the n10 to BCV of these 13 SVN patients to previously published data for healthy subjects and patients after complete unilateral vestibular loss. We found that in 12 out of the 13 patients with SVN, n10 was markedly reduced or absent under the contralesional eye.ConclusionSince all utricular afferents course in the superior vestibular nerve and in 12/13 of these patients the n10 was reduced we conclude that the n10 component of the oVEMP to BCV is probably mediated by the superior vestibular nerve and probably due to activation of mainly utricular receptors.SignificanceThe n10 appears to be a simple new test of superior vestibular nerve and probably mainly utricular function.     

The effect of alcohol on cervical and ocular vestibular evoked myogenic potentials in healthy volunteers

ObjectiveWe investigated the effect of alcohol on the cervical and ocular vestibular evoked myogenic potentials (cVEMPs and oVEMPs). As alcohol produces gaze-evoked nystagmus (GEN), we also tested the effect of nystagmus independent of alcohol by recording oVEMPs during optokinetic stimulation (OKS).MethodsThe effect of alcohol was tested in 14 subjects over multiple rounds of alcohol consumption up to a maximum breath alcohol concentration (BrAC) of 1.5‰ (mean 0.97‰). The effect of OKS was tested in 11 subjects at 5, 10 and 15 deg/sec.ResultsoVEMP amplitude decreased from baseline to the highest BrAC level by 27% (range 5–50%, P < 0.001), but there was no significant effect on oVEMP latency or cVEMP amplitude or latency. There was a significant negative effect of OKS on oVEMP amplitude (16%, P = 0.006).ConclusionsWe found a selective effect of alcohol on oVEMP amplitude, but no effect on the cVEMP. Vertical nystagmus elicited by OKS reduced oVEMP amplitude.SignificanceAlcohol selectively affects oVEMP amplitude. Despite the effects of alcohol and nystagmus, both reflexes were reliably recorded in all subjects and conditions. An absent response in a patient affected by alcohol or nystagmus indicates a vestibular deficit.     

Feasibility of the simultaneous ocular and cervical vestibular-evoked myogenic potentials in unilateral vestibular hypofunction

ObjectiveThis study compared the results of combined and individual ocular vestibular-evoked myogenic potential (oVEMP) and cervical VEMP (cVEMP) tests in healthy subjects and patients with unilateral vestibular hypofunction to confirm the effectiveness of the combined oVEMP and cVEMP test.MethodsTwenty healthy volunteers and 12 patients with unilateral vestibular hypofunction underwent individual oVEMP and cVEMP tests in a random order, and combined oVEMP and cVEMP test on another day.ResultsTwenty healthy subjects had 100% response rates for oVEMPs and cVEMPs in both individual and combined stimulation modes. Significant positive correlations exist between individual and combined oVEMPs/cVEMPs in terms of latencies and amplitudes. In 12 patients with unilateral vestibular hypofunction, differences in abnormal percentages of oVEMPs or cVEMPs were non-significant. Additionally, the characteristic parameters of oVEMP/cVEMP among ears of healthy subjects, good and pathological ears of patients with unilateral vestibular hypofunction did not differ significantly, regardless of whether the individual or combined mode was used.ConclusionsThe combined oVEMP and cVEMP test generates similar information to that obtained by individual mode, regardless of whether subjects are healthy or have unilateral vestibular hypofunction.SignificanceSimultaneous oVEMP and cVEMP tests may be a convenient screening tool for assessing crossed vestibulo-ocular reflex and ipsilateral sacculo-collic reflex, which definitely shortens the diagnostic test time. Thus, it may favor the large diffusion of these techniques.     

Vestibular-evoked extraocular potentials by air-conducted sound: Another clinical test for vestibular function

OBJECTIVE: The purpose of this study was to identify an appropriate way to detect vestibular-evoked extraocular potentials (oVEMPs) produced by air-conducted sound stimulation in healthy subjects and to apply this test clinically in patients with various vestibular disorders. METHODS: Ten healthy subjects were included in this study. Surface electromyographic (EMG) activity was recorded from active electrodes placed on the face just inferior to each eye. Stimulation with 0.1 ms clicks and 500 Hz short tone bursts was used to activate the vestibular end-organs in healthy subjects. We also tested 12 patients with unilateral vestibular disorders using 500 Hz short tone bursts. RESULTS: In healthy subjects, negative-positive biphasic responses with short latency by air-conducted click (the first negative peak latency=8.8 ms and the following positive peak latency=14.5 ms on the average) (oVEMP) were only identified beneath the eye contralateral to the stimulating ear. On the other hand, stimulation with 500 Hz short tone bursts evoked negative-positive biphasic responses (the first negative peak latency=10.5 ms and the following positive peak latency=15.9 ms on the average) on both ipsilateral and contralateral eyes, while responses were contralateral eye-dominant. Contralateral eye responses by 500 Hz short tone bursts had higher response prevalence and larger amplitudes than clicks. In patients, oVEMPs evoked by the affected side stimulation tended to decrease or lacked a response. The presence of oVEMPs and cVEMPs coincided well in patients when 500 Hz short tone bursts were presented. CONCLUSIONS: oVEMPs can be evoked using air-conducted 500 Hz tone burst and are best recorded contralaterally on upgaze. SIGNIFICANCE: oVEMPs by air-conducted sounds could be a useful alternative clinical test for patients with vestibular lesions.     

The effect of electrode positioning on the ocular vestibular evoked myogenic potential to air-conducted sound

ObjectiveTo assess the effect of electrode position on the amplitude and latency of ocular vestibular evoked myogenic potentials (oVEMPs) produced by air-conducted (AC) sound with a view to optimisation of the recording paradigm.MethodsEight otologically normal subjects (16 ears) were stimulated by 500 Hz AC tone bursts at 95 dBnHL; oVEMP traces were recorded below the eye contralateral to the acoustic stimulation. Five independent oVEMP measurements were recorded with the active electrode in equally spaced positions in the infra-orbital plane relative to a reference electrode positioned 2 cm below the lower lid in the orbital midline. These measurements included the accepted standard-montage in which the electrodes were positioned vertically above and below each other in the orbital midline. A further recording was made using a belly-tendon montage with reference to the inferior oblique muscle.ResultsOf the six recording paradigms tested the largest amplitude oVEMP response was found using the belly-tendon montage with an n₁₀ average of 5.67 ± 3.42 μV (sd). This was significantly larger than the amplitude recorded using the standard-montage (p < 0.01). With the reference electrode in the orbital midline, the position of the active electrode in the infra-orbital plane was found to significantly alter the response magnitude. As the active electrode was moved laterally the response reduced in amplitude, however when moved medially the response polarity reversed indicating the existence of a null-point at which no response was present.ConclusionsThe location of oVEMP recording electrodes significantly alters the response amplitude. Whilst the standard-montage provides a reasonable method for recording oVEMPs, the belly-tendon montage results in a significantly larger amplitude response. Furthermore medial and lateral variations in the position of the active electrode using the standard-montage significantly affect the magnitude and polarity of the response.SignificanceThe standard-montage used for recording oVEMPs is sensitive to the placement of the active electrode. Small variations in position result in significant changes in the n₁₀ amplitude and this may account for the variability reported in the literature. Using the belly-tendon montage, larger amplitude responses can be elicited which may improve the robustness with which oVEMPs can be collected. However this enhancement in response amplitude must be balanced against the increased possibility of signal contamination from neighbouring extraocular muscles.     

Delayed vestibular evoked responses to the eyes and neck in a patient with an isolated brainstem lesion.

OBJECTIVE: Two recently described tests of the vestibular system, vestibular evoked myogenic potentials (VEMPs) and ocular vestibular evoked myogenic potentials (OVEMPs), test the descending and ascending vestibular brainstem pathways, respectively. We describe a case of a patient in whom these investigations localised the lesion and suggested its nature. METHODS: VEMPs (to clicks and short duration galvanic stimulation) and OVEMPs (to clicks) were recorded. RESULTS: Click- and galvanic-evoked VEMPs were delayed on the left side (by approximately 5-6 ms), and click-evoked OVEMPs were similarly delayed (by approximately 4 ms) following left-sided stimulation. Repeat testing 21 months later showed partial resolution. CONCLUSIONS: The observed delays in evoked potentials suggested a demyelinating lesion. Furthermore, the similarity in delayed responses to neck and extraocular muscles was suggestive of a lesion at the root entry zone of the vestibulocochlear nerve. SIGNIFICANCE: VEMPs and OVEMPS may thus provide information about the location and nature of lesions affecting central vestibular pathways.