Sendhil Govender

The effect of gaze direction on the ocular vestibular evoked myogenic potential produced by air-conducted sound.

OBJECTIVE To assess the effects of vertical and horizontal gaze, head rotation, body position, and vision on the ocular vestibular evoked myogenic potential (OVEMP) produced by air-conducted (AC) sound. METHODS Ten normal subjects were stimulated by 500 Hz 2 ms AC tone bursts at 136-142 dB peak SPL. OVEMPs were recorded from electrodes placed beneath the eyes. Angles of vertical gaze ranged from maximal downward to upward gaze in increments of 5-10 degrees . Horizontal gaze was measured during elevation and ranged from 20 degrees adduction to 20 degrees abduction. RESULTS Increasing vertical gaze increased OVEMP amplitude, especially for the contralateral eye (neutral vs maximal upward gaze; contra: 1.0 vs 2.6 microV; ipsi: 0.8 vs 0.9 microV; P<0.001). OVEMPs from the contralateral eye peaked significantly earlier in the upward gaze positions (contra: 9.2 ms; ipsi: 10.4 ms; P<0.001), but peaked later during downward gaze (contra: 14.2 ms; ipsi: 11.4 ms; P=0.014). There were small effects of horizontal gaze and supine body position, but no effects of head rotation or vision. CONCLUSIONS OVEMP amplitudes are strongly modulated by gaze position. Truncal position also affects OVEMP amplitude. SIGNIFICANCE This study quantifies the effect of gaze on the OVEMP and demonstrates the importance of controlling for gaze in clinical and experimental studies.

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.

Vestibular neuritis has selective effects on air- and bone-conducted cervical and ocular vestibular evoked myogenic potentials

OBJECTIVE To characterise the changes in cervical (cVEMP) and ocular (oVEMP) vestibular evoked myogenic potentials to different stimuli in patients with vestibular neuritis (VN). METHODS cVEMPs and oVEMPs were recorded using air-conducted (AC; clicks and short tone bursts) and bone-conducted (BC; lateral impulses and taps) stimuli in VN patients (n=23) and normals (n=40). RESULTS AC evoked cVEMPs revealed few abnormalities, significantly less than for AC evoked oVEMPs (cVEMP: 22% vs oVEMP: 68%, P<0.001). Lateral impulses showed high rates of abnormalities (74% vs 70%, P>0.05) for both reflexes. Although forehead taps produced low rates of abnormalities for both reflexes (33% vs 13%, P>0.05), response amplitudes were smaller from the affected ear (P<0.05). CONCLUSIONS AC stimuli were associated with low abnormality rates of cVEMPs, consistent with sparing of inferior nerve function in VN, but frequent abnormalities of oVEMPs. The high rates of abnormalities shown for lateral impulses suggest a dependence on superior nerve (i.e. utricular) afferents for both oVEMPs and cVEMPs. SIGNIFICANCE Lateral impulses behave as expected for utricular function and AC cVEMPs for saccular function. The AC evoked oVEMP seems to depend on the integrity of the superior vestibular nerve, possibly due to saccular afferents travelling in it.

Vestibular evoked myogenic potentials (VEMPs) evoked by air- and bone-conducted stimuli in vestibular neuritis.

OBJECTIVE To compare and characterise abnormalities for short latency vestibular evoked myogenic potentials (VEMPs) elicited by air- (AC) and two differing types of bone-conducted (BC) stimuli during vestibular neuritis (VN). METHODS AC (500Hz short tone bursts) and two BC stimuli (500Hz at the forehead and impulses at the mastoids) were used to evoke cervical and ocular potentials (cVEMPs and oVEMPs) in VN patients (n=22) and healthy subjects. RESULTS More abnormalities were observed for the oVEMP than the cVEMP when using either AC 500Hz or BC 500Hz. The AC stimulus showed slightly more abnormalities than the BC 500Hz stimulus. In contrast, BC impulses produced frequent abnormalities for both oVEMPs and cVEMPs. The findings were modelled, based upon presumed selective lesions of the superior nerve. CONCLUSIONS AC 500Hz stimulation was slightly better than BC 500Hz in demonstrating abnormalities in patients with VN. BC impulses behave as expected for a predominantly utricular stimulus. The relative contributions of saccular and utricular fibres differ for stimulus type and target reflex. SIGNIFICANCE AC 500Hz is as effective as BC 500Hz for investigating VN. BC impulses act most strongly on utricular afferents.

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.

Ocular vestibular evoked myogenic potentials produced by impulsive lateral acceleration in unilateral vestibular dysfunction

OBJECTIVE To deduce the connectivity underlying ocular vestibular evoked myogenic potentials (OVEMPs) recorded from two sites and produced by lateral transmastoid stimulation in patients with unilateral vestibular dysfunction. METHODS OVEMPs were recorded using lateral impulsive stimuli delivered by a hand-held minishaker placed at the mastoid. Twelve patients were tested using the typical OVEMP recording montage placed inferior to the eyes. In a subset of 6 patients, recordings were also made using a lateral electrode montage. The majority of patients were tested following surgery for inner ear disease. Patient responses were compared to those in normal subjects under similar recording conditions. RESULTS For the inferior montage, regardless of which mastoid was stimulated, deficits were observed only from the eye opposite the affected ear. In contrast, OVEMPs recorded using the lateral electrode montage showed changes on both sides. CONCLUSIONS OVEMPs produced using lateral transmastoid stimulation and recorded from beneath the eyes are generated by a crossed vestibulo-ocular pathway while the projections underlying the lateral responses are likely to be bilateral. SIGNIFICANCE The vestibular-ocular connectivity underlying the OVEMPs recorded from inferior and lateral recording sites differs. For clinical use, the inferior recording site is the simplest to interpret.

Low-frequency tuning in the human vestibular–ocular projection is determined by both peripheral and central mechanisms

We recently reported that a major contribution to the low-frequency tuning and sensitivity of the human vestibular system is the biomechanical properties of the vestibular end-organs. In the current paper, we investigate the contribution of additional mechanisms to low-frequency tuning. We compared the response properties of the vestibular system in 6 human volunteers to trains of 2 ms pulses of sound and transmastoid vibration using pulse repetition frequencies of 12.5, 25, 50, 100, 200 and 400 Hz. Measurements were made using two separate pathways arising from the vestibular apparatus: to the neck using vestibular evoked myogenic potentials (VEMPs), and to the eyes using ocular vestibular evoked myogenic potentials (OVEMPs). For both sound and vibration the two response pathways produced different tuning to pulse trains. The vestibulo-ocular pathway was characterised by a band-pass tuning with best frequency of 100 Hz whereas the vestibulo-collic pathway showed a peak at 400 Hz with sound only. These results suggest that properties of the vestibulo-ocular pathway also contribute to the low-frequency tuning that occurs for the OVEMP, in addition to previously reported end-organ effects.

Tuning of the ocular vestibular evoked myogenic potential to bone-conducted sound stimulation

Ocular vestibular evoked myogenic potentials (oVEMPs) are a recently described clinical measure of the vestibulo-ocular reflex. Studies demonstrating differences in frequency tuning between air-conducted and bone-conducted (BC) oVEMPs suggest a separate vestibular (otolith) origin for each stimulus modality. In this study, 10 healthy subjects were stimulated with BC stimuli using a hand-held minishaker. Frequencies were tested in the range of 50-1,000 Hz using both a constant-force and constant-acceleration method. Subjects were stimulated at the mastoid process and the forehead. For constant-force stimulation at both sites, maximum acceleration occurred around 100 Hz, in differing axes. Both forms of stimulation had low-frequency peaks of oVEMP amplitudes (constant force: mastoid, 80-150 Hz; forehead, 50-125 Hz; constant acceleration: mastoid, 100-200 Hz; forehead, 80-150 Hz), for both sites of application, despite differences in the magnitude and direction of evoked head acceleration. For mastoid stimulation, ocular responses changed from out of phase to in phase for 400 Hz and above. Our results demonstrate that BC stimuli show tuning around 100 Hz, independent of stimulus site, that is not due to skull properties. The findings are consistent with an effect on a receptor with a resonance around 100 Hz, most likely the utricle.

Two distinct patterns of VEMP changes with age

the stimulus) are similar to the emotionally-evoked theta hypersynchrony described in the pediatric literature; however, to our knowledge, this has never been reported in adulthood. Hence, it is unclear whether the concentration required in playing the word game or the psychological response to winning or losing, or some combination, was the provocative stimulus in this case. Nevertheless, modern technology, such as laptop computers and smartphones, enables patients to engage in an increasingly broader variety of social, recreational and work-related activities while on the video EEG unit. Consequently, perhaps we will begin to see evoked theta rhythms more frequently. We hope this report may lead electroencephalographers to identify and report such findings, to better clarify their significance in the adult and pediatric populations and in the greater context of understanding intrinsic rhythms of the brain.

Vestibular-dependent spinal reflexes evoked by brief lateral accelerations of the heads of standing subjects

An impulsive acceleration stimulus, previously shown to activate vestibular afferents, was applied to the mastoid. Evoked EMG responses from the soleus muscles in healthy subjects (n = 10) and patients with bilateral vestibular dysfunction (n = 3) were recorded and compared with the effects of galvanic stimulation (GVS). Subjects were stimulated while having their eyes closed, head rotated, and while tonically activating their soleus muscles. Rectified EMG responses were recorded from the leg contralateral to the direction of head rotation. Responses were characterized by triphasic potentials that consisted of short-latency (SL), medium-latency (ML), and long-latency (LL) components beginning at (mean ± SD) 54.2 ± 4.8, 88.4 ± 4.7, and 121 ± 7.1 ms, respectively. Mean amplitudes for the optimum stimulus rise times were 9.05 ± 3.44% for the SL interval, 16.70 ± 4.41% for the ML interval, and 9.75 ± 4.89% for the LL interval compared with prestimulus values. Stimulus rise times of 14 and 20 ms evoked the largest ML amplitudes. GVS evoked biphasic responses (SL and ML) with similar latencies. Like GVS, the polarity of the initial interval was determined by the polarity of the stimulus and the evoked EMG response was attenuated when subjects were seated. There was no significant EMG response evoked when subjects were stimulated using 500-Hz vibration or in patients with bilateral vestibular dysfunction. Our study demonstrates that a brief lateral acceleration, likely to activate the utricle, can evoke spinal responses with properties similar to those previously shown for vestibular activation by GVS. The triphasic nature of the responses may allow the nervous system to respond differently to short compared with long-duration linear accelerations, consistent with their differing significance.