Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig

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.

Plasticity during Vestibular Compensation: The Role of Saccades

This paper is focused on one major aspect of compensation: the recent measures of saccadic responses to high acceleration head turns during human vestibular compensation and their possible implications for recovery after unilateral vestibular loss (UVL). New measurement techniques have provided additional insights into how patients recover after UVL and have given clues for vestibular rehabilitation. Prior to this it has not been possible to quantify the level of function of all the peripheral vestibular sense organs. Now it is. By using vestibular-evoked myogenic potentials to measure utricular and saccular function and by new video head impulse testing to measure semicircular canal function to natural values of head accelerations. With these new video procedures it is now possible to measure both slow phase eye velocity and also saccades during head movements with natural values of angular acceleration. The present evidence is that after UVL there is little or no restoration/compensation of slow phase eye velocity responses to natural head accelerations. It is doubtful as to whether the modest changes in slow phase eye velocity to small angular accelerations are functionally effective during compensation. On the other hand it is now clear that saccades can play a very important role in helping patients compensate and return to a normal lifestyle. Preliminary evidence suggests that different patterns of saccadic response may predict how well patients recover. Furthermore it may be possible to train patients to produce more effective saccadic patterns in the first days after their unilateral loss and possibly improve their compensation process. Some patients do learn new strategies, new behaviors, to conceal their inadequate vestibulo-ocular response but when those strategies are prevented from operating by using passive, unpredictable, high acceleration natural head movements, as in the head impulse test, the vestibular loss can be demonstrated. It is those very strategies which the tests exclude, which may be the cause of their successful compensation.

High-frequency skull vibration-induced nystagmus test in partial vestibular lesions

OBJECTIVES

To establish the effectiveness of the skull vibration-induced nystagmus test (SVINT) as a rapid high-frequency stimulation test, in the evaluation of partial unilateral vestibular lesions (pUVL).

METHODS

SVINT (30, 60, and 100 Hz), caloric, and head-shaking tests were performed in 99 patients with pUVL. These results were compared with those in 9 patients with symmetrical partial bilateral labyrinthine malformations, 131 patients with total unilateral vestibular lesions (tUVL), and 95 control subjects.

RESULTS

A skull vibratory nystagmus (SVN) was found in 75% of patients with pUVL and 98% with tUVL. In pUVL: SVINT revealed asymmetric responses in 20% of patients where other tests were normal; SVN direction at 100 Hz was opposite to the head-shaking nystagmus direction in 30% and opposite to SVN at 30 Hz in 10% of cases. At 100 Hz, SVN beat toward the safe side in 91% of cases; SVN values at 60 and 100 Hz were higher than those at 30 Hz (p < 0.005). SVN was found in unilateral superior canal dehiscences. Partial bilateral labyrinthine malformations revealed no nystagmus.

CONCLUSION

SVINT complements head-shaking and caloric tests in multifrequency assessment of patients with pUVL, as a global vestibular test. In contrast with tUVL results, SVINT does not always indicate the side of partial lesions, neither does it locate their level on the vestibulo-ocular pathway. This test is useful to reveal a vestibular asymmetry as a bedside examination test and may be used as a "vestibular Weber."

The basis for using bone-conducted vibration or air-conducted sound to test otolithic function

Extracellular single neuron recordings of primary vestibular neurons in Scarpa's ganglion in guinea pigs show that low-intensity 500 Hz bone-conducted vibration (BCV) or 500 Hz air-conducted sound (ACS) activate a high proportion of otolith irregular neurons from the utricular and saccular maculae but few semicircular canal neurons. In alert guinea pigs, and humans, 500 Hz BCV elicits otolith-evoked eye movements. In humans, it also elicits a myogenic potential on tensed sternocleidomastoid muscles. Although BCV and ACS activate both utricular and saccular maculae, it is possible to probe the functional status of these two sense organs separately because of their differential neural projections. Saccular neurons have a strong projection to neck muscles and a weak projection to the oculomotor system. Utricular afferents have a strong projection to eye muscles. So measuring oculomotor responses to ACS and BCV predominantly probes utricular function, while measuring neck muscle responses to these stimuli predominantly probes saccular function.

Possible cues driving context-specific adaptation of optocollic reflex in pigeons (Columba livia)

Context-specific adaptation (Shelhamer M, Clendaniel R. Neurosci Lett 332: 200-204, 2002) explains that reflexive responses can be maintained with different "calibrations" for different situations (contexts). Which context cues are crucial and how they combine to evoke context-specific adaptation is not fully understood. Gaze stabilization in birds is a nice model with which to tackle that question. Previous data showed that when pigeons (Columba livia) were hung in a harness and subjected to a frontal airstream provoking a flying posture ("flying condition"), the working range of the optokinetic head response [optocollic reflex (OCR)] extended toward higher velocities compared with the "resting condition." The present study was aimed at identifying which context cues are instrumental in recalibrating the OCR. We investigated that question by using vibrating stimuli delivered during the OCR provoked by rotating the visual surroundings at different velocities. The OCR gain increase and the boost of the fast phase velocity observed during the "flying condition" were mimicked by body vibration. On the other hand, the newly emerged relationship between the fast-phase and slow-phase velocities in the "flying condition" was mimicked by head vibration. Spinal cord lesion at the lumbosacral level decreased the effects of body vibration, whereas lesions of the lumbosacral apparatus had no effect. Our data suggest a major role of muscular proprioception in the context-specific adaptation of the stabilizing behavior, while the vestibular system could contribute to the context-specific adaptation of the orienting behavior. Participation of an efferent copy of the motor command driving the flight cannot be excluded.

Click-evoked responses in vestibular afferents in rats

Sound activates not only the cochlea but also the vestibular end organs. Research on this phenomenon led to the discovery of the sound-evoked vestibular myogenic potentials recorded from the sternocleidomastoid muscles (cervical VEMP, or cVEMP). Since the cVEMP offers simplicity and the ability to stimulate each labyrinth separately, its values as a test of human vestibular function are widely recognized. Currently, the cVEMP is interpreted as a test of saccule function based on the assumption that clicks primarily activate the saccule. However, sound activation of vestibular end organs other than the saccule has been reported. To provide the neural basis for interpreting clinical VEMP testing, we employed the broadband clicks used in clinical VEMP testing to examine the sound-evoked responses in a large sample of vestibular afferents in Sprague-Dawley rats. Recordings were made from 924 vestibular afferents from 106 rats: 255 from the anterior canal (AC), 202 from the horizontal canal (HC), 177 from the posterior canal (PC), 207 from the superior vestibular nerve otolith (SO), and 83 from the inferior nerve otolith (IO). Sound sensitivity of each afferent was quantified by computing the cumulative probability of evoking a spike (CPE). We found that clicks activated irregular afferents (normalized coefficient of variation of interspike intervals >0.2) from both the otoliths (81%) and the canals (43%). The order of end organ sound sensitivity was SO = IO > AC > HC > PC. Since the sternocleidomastoid motoneurons receive inputs from both the otoliths and the canals, these results provide evidence of a possible contribution from both of them to the click-evoked cVEMP.

Cervical and ocular vestibular-evoked myogenic potentials in acute vestibular neuritis

OBJECTIVES

To clarify the origin and afferent pathways of short-latency ocular vestibular-evoked myogenic potential (oVEMP) in response to air-conducted sound (ACS), we evaluated cervical (cVEMP) and ocular VEMPs in patients with vestibular neuritis (VN).

METHODS

In response to air-conducted tone burst, the oVEMP and cVEMP were measured in 60 healthy controls and in 41 patients with acute VN. The VN selectively involved the superior vestibular nerve (superior VN) in 30 patients, affected the inferior vestibular nerve only (inferior VN) in three and damaged both superior and inferior vestibular nerve branches in eight.

RESULTS

All 30 patients with superior VN presented normal cVEMPs, indicating preservation of the saccular receptors and their afferents in the inferior vestibular nerve. However, the oVEMP was abnormal in all patients with superior VN. By contrast, the patients with inferior VN showed normal oVEMP and abnormal cVEMP.

CONCLUSION

These dissociations in the abnormalities of cVEMP and oVEMP in patients with VN selectively involving the superior or inferior vestibular nerve suggest that the origin of the vestibular nerve afferents of oVEMP differ from those of cVEMP.

SIGNIFICANCE

The oVEMP in response to ACS may be mediated by the superior vestibular nerve, probably due to an activation of the utricular receptors.

Bone conducted vibration activates the vestibulo-ocular reflex in the guinea pig

The aim of the study was: (a) to test whether short duration (6 ms) 500 Hz bone-conducted vibration (BCV) of the skull in alert head free guinea pigs would elicit eye movements; (b) to test whether these eye movements were vestibular in origin; and (c) to determine whether they corresponded to human eye movements to such stimuli. In this way we sought to establish the guinea pig as an acceptable model for testing the mechanism of the effect BCV on the vestibulo-ocular reflex. Consistent short-latency stimulus-locked responses to BCV were observed. The magnitude of eye displacement was directly related to stimulus intensity as recorded by accelerometers cemented onto the animal's skull. The strongest and most consistent response component was intorsion of both eyes. In lateral-eyed animals intorsion is produced by the combined contraction of the inferior rectus and superior oblique muscles. In humans the same pair of muscles acts to cause depression of the eye. To test whether the movements were vestibular we selectively ablated the vestibular endorgans: 3 of the 8 animals underwent a bilateral intratympanic injection of gentamicin, an ototoxic aminoglycoside antibiotic, to ablate their vestibular receptors. After ablation there was an overall reduction in the magnitude of eye displacement, as well as a reduction in the effectiveness of the BCV stimulus to elicit eye movements. The animals' hearing, as measured by the threshold for auditory brainstem responses, remained unchanged after gentamicin, confirming that the cochlea was not affected. The reduced magnitude of responses after vestibular receptor ablation demonstrates that the eye-movement responses to BCV are probably caused by the stimulation of vestibular receptors, which in turn activate the vestibulo-ocular reflex.

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.

Direction-dependent excitatory and inhibitory ocular vestibular-evoked myogenic potentials (oVEMP) produced by oppositely directed accelerations along the midsagittal axis of the head [corrected]

Oppositely directed displacements of the head need oppositely directed vestibulo-ocular reflexes (VOR), i.e. compensatory responses. Ocular vestibular-evoked myogenic potentials (oVEMPs) mainly reflect the synchronous extraocular muscle activity involved in the process of generating the VOR. The oVEMPs recorded beneath the eyes when looking up represent electro-myographic responses mainly of the inferior oblique muscle. We aimed: (1) to study the properties of these responses as they were produced by head acceleration impulses to the forehead and to the back of the head; (2) to investigate the relationships between these responses and the 3-D linear head accelerations that might reflect the true stimulus that acts on the vestibular hair cells. We produced backward- and forward-directed acceleration stimuli in four conditions (positive and negative head acceleration impulses to the hairline and to the inion) in 16 normal subjects. The oVEMPs produced by backward- and forward-directed accelerations of the head showed consistent differences. They were opposite in the phase. The responses produced by backward accelerations of the head began with an initial negativity, n11; conversely, those produced by accelerations directed forward showed initially a positive response, p11. There was a high inter-subject correlation of head accelerations along the head anteroposterior and transverse axes, but almost no correlation of accelerations along the vertical axis of the head. We concluded that backward-directed head accelerations produced an initial excitatory response, and forward-directed accelerations of the head were accompanied by an initial inhibitory response. These responses showed dependence on acceleration direction in the horizontal plane of the head. This could be consistent with activation of the utricle.

[Recording cervical and ocular vestibular evoked myogenic potentials. Part 2: influencing factors, evaluation of findings and clinical significance]

VEMP measurements are subject to various influencing factors: patient age, threshold, sound intensity and frequency. Using air (AC) and bone conduction (BC) the vestibular receptors and afferents of the otolith organs can be activated to varying degrees. Recordings of cervical (cVEMP) and ocular VEMP (oVEMP) are clinically possible. AC-cVEMP are primarily an indicator of the sacculocollic reflex pathway. Together with findings on the vestibuloocular reflex (VOR) and complimentary otolith tests, VEMP enable otolith function analysis of each side separately. In addition, the distinction between combined or isolated canal and otolith dysfunction in terms of subtyping and patterns of damage in mono- and bilateral disorders, such as vestibular neuritis or bilateral vestibulopathy, is possible. Moreover, VEMP is relevant in terms of prognostic and therapeutic considerations as well as expert assessments.

Evidence Missed

Welgampola and Carey have missed evidence that shows how utricular and saccular function can be differentiated, and here the authors note that evidence and report a new result that further substantiates the differentiation.

Vestibular primary afferent responses to sound and vibration in the guinea pig

This study tested whether air-conducted sound and bone-conducted vibration activated primary vestibular afferent neurons and whether, at low levels, such stimuli are specific to particular vestibular sense organs. In response to 500 Hz bone-conducted vibration or 500 Hz air-conducted sound, primary vestibular afferent neurons in the guinea pig fall into one of two categories--some neurons show no measurable change in firing up to 2 g peak-to-peak or 140 dB SPL. These are semicircular canal neurons (regular or irregular) and regular otolith neurons. In sharp contrast, otolith irregular neurons show high sensitivity: a steep increase in firing as stimulus intensity is increased. These sensitive neurons typically, but not invariably, were activated by both bone-conducted vibration and air-conducted sound, they originate from both the utricular and saccular maculae, and their sensitivity underpins new clinical tests of otolith function.

Test-retest reliability and age-related characteristics of the ocular and cervical vestibular evoked myogenic potential tests

OBJECTIVE

To determine the test-retest reliability and age-related trends of the cervical and ocular vestibular evoked myogenic potential (cVEMP and oVEMP, respectively) responses to air-conducted sound and bone-conducted vibration stimulation.

STUDY DESIGN

Prospective study.

SETTING

Tertiary referral center.

PATIENTS

Fifty-three healthy adults with no hearing or vestibular deficits.

INTERVENTION(S)

All subjects underwent cVEMP and oVEMP testing in response to sounds (0.1-ms clicks and 500-Hz tone bursts) and vibration (midline forehead taps at the hairline, Fz, with a reflex hammer and a Brüel & Kjaer Mini-Shaker Type 4810). Twelve subjects underwent an additional testing session that was conducted at a mean of 10 weeks after the first one.

MAIN OUTCOME MEASURE(S)

Test-retest reliability for VEMP response parameters (latency, peak-to-peak amplitude, and asymmetry ratio) were assessed using the intraclass correlation coefficient (ICC).

RESULTS

: oVEMP amplitudes had excellent test-retest reliability (ICC > 0.75) for all 4 stimuli; cVEMP amplitudes had excellent reliability for hammer taps and fair-to-good reliability for other stimuli. oVEMP asymmetry ratios had excellent reliability for clicks and fair-to-good reliability (ICC = 0.4-0.75) for other stimuli; cVEMP asymmetry ratios had fair-to-good reliability for clicks and hammer taps. Older subjects (>50 years old) were found to have significantly decreased cVEMP amplitudes in response to clicks, tones, and taps with a Mini-Shaker and significantly decreased oVEMP amplitudes in response to clicks, tones, and taps with a reflex hammer. No age-related changes were found for latencies or asymmetry ratios.

CONCLUSION

Overall, oVEMP response parameters demonstrated better test-retest reliability than cVEMP response parameters, but oVEMPs and cVEMPs had similar age-related changes.

Lack of otolith involvement in balance responses evoked by mastoid electrical stimulation

Passing current through mastoid electrodes (conventionally termed galvanic vestibular stimulation; GVS) evokes a balance response containing a short- and a medium-latency response. The origins of these two responses are debated. Here we test the hypotheses that they originate from net signals evoked by stimulation of otolith and semi-circular canal afferents, respectively. Based on anatomy and function, we predicted the directions of the stimulus-evoked net head rotation vector from the canals and the linear acceleration net vector from the otoliths. We tested these predictions in healthy adults by obtaining responses with the head in strategic postures to alter the relevance of the signals to the balance system. Cross-covariance between a stochastic waveform of stimulating current and motor output was used to assess the balance responses. Consistent with the canal hypothesis, with the head pitched down the medium-latency EMG response was abolished while the short-latency EMG response was maintained. The results, however, did not support the otolith hypothesis. The direction of the linear acceleration signal from the otoliths was predicted to change substantially when using monaural stimuli compared to binaural stimuli. In contrast, short-latency response direction measured from ground-reaction forces was not altered. It was always directed along the inter-aural axis irrespective of whether the stimulus was applied binaurally or monaurally, whether the head was turned in yaw through 90 deg, whether the head was pitched down through 90 deg, or combinations of these manipulations. We conclude that a net canal signal evoked by GVS contributes to the medium-latency response whilst a net otolith signal does not make a significant contribution to either the short- or medium-latency responses.

Responses of the ear to low frequency sounds, infrasound and wind turbines

Infrasonic sounds are generated internally in the body (by respiration, heartbeat, coughing, etc) and by external sources, such as air conditioning systems, inside vehicles, some industrial processes and, now becoming increasingly prevalent, wind turbines. It is widely assumed that infrasound presented at an amplitude below what is audible has no influence on the ear. In this review, we consider possible ways that low frequency sounds, at levels that may or may not be heard, could influence the function of the ear. The inner ear has elaborate mechanisms to attenuate low frequency sound components before they are transmitted to the brain. The auditory portion of the ear, the cochlea, has two types of sensory cells, inner hair cells (IHC) and outer hair cells (OHC), of which the IHC are coupled to the afferent fibers that transmit "hearing" to the brain. The sensory stereocilia ("hairs") on the IHC are "fluid coupled" to mechanical stimuli, so their responses depend on stimulus velocity and their sensitivity decreases as sound frequency is lowered. In contrast, the OHC are directly coupled to mechanical stimuli, so their input remains greater than for IHC at low frequencies. At very low frequencies the OHC are stimulated by sounds at levels below those that are heard. Although the hair cells in other sensory structures such as the saccule may be tuned to infrasonic frequencies, auditory stimulus coupling to these structures is inefficient so that they are unlikely to be influenced by airborne infrasound. Structures that are involved in endolymph volume regulation are also known to be influenced by infrasound, but their sensitivity is also thought to be low. There are, however, abnormal states in which the ear becomes hypersensitive to infrasound. In most cases, the inner ear's responses to infrasound can be considered normal, but they could be associated with unfamiliar sensations or subtle changes in physiology. This raises the possibility that exposure to the infrasound component of wind turbine noise could influence the physiology of the ear.

The ocular vestibular-evoked myogenic potential to air-conducted sound; probable superior vestibular nerve origin

OBJECTIVE

Intense air-conducted sound (ACS) elicits an ocular vestibular-evoked myogenic potential (oVEMP), and it has been suggested that it does so by stimulating saccular receptors and afferents in the inferior vestibular nerve and so activating a crossed sacculo-ocular pathway. Bone conducted vibration (BCV) also elicits an oVEMP probably by activating utricular receptors and a crossed utriculo-ocular pathway. Are there two separate pathways mediating oVEMPs for ACS and BCV? If saccular receptors and afferents are primarily responsible for the oVEMP to ACS, then the oVEMP to ACS should be normal in patients with reduced or absent utricular function--unilateral superior vestibular neuritis (SVN). If utricular receptors and afferents are primarily responsible for oVEMP n10, then oVEMP to ACS should be reduced or absent in SVN patients, and in these patients there should be a close relationship between the size of the oVEMP n10 to BCV and to ACS.

METHODS

The n10 component of the oVEMP to 500 Hz BCV and to 500 Hz ACS was recorded in 10 patients with unilateral SVN but who had saccular and inferior vestibular nerve function preserved, as shown by their normal cVEMP responses to ACS.

RESULTS

In SVN patients with normal saccular and inferior vestibular nerve function, the oVEMP n10 in response to ACS was reduced or absent. Across SVN patients there was a very close correspondence between the size of oVEMP n10 for ACS and for BCV.

CONCLUSIONS

The n10 component of the oVEMP to ACS is probably mediated predominantly by the superior vestibular nerve and so most likely by utricular receptors and afferents.

SIGNIFICANCE

The n10 component of the oVEMP to either ACS or BCV probably indicates mainly superior vestibular nerve function.

Waiting for the evidence: VEMP testing and the ability to differentiate utricular versus saccular function

The advent of cervical vestibular evoked myogenic potentials (CVEMPs) marked a milestone in clinical vestibular testing because they provided a simple means of assessing human otolith function. The availability of air-conducted (AC) sound and bone-conducted vibration (BCV) to evoke CVEMPs and development of a new technique of recording ocular vestibular-evoked myogenic potentials (OVEMPs) have increased the complexity of this simple test, yet extended its diagnostic capabilities. Here we highlight the evidence-based assumptions that guide interpretation of AC sound- and BCV-evoked VEMPs and the gaps in VEMP research thus far.

Ocular vestibular-evoked myogenic potentials to bone-conducted vibration in superior vestibular neuritis show utricular function

OBJECTIVE

To determine whether the first negative component (n10) of the ocular vestibular-evoked myogenic potential (oVEMP) to bone-conducted vibration (BCV) is due primarily to activation of the utricular macula.

STUDY DESIGN

The n10 was recorded in response to brief BCV at the midline of the forehead at the hairline (Fz). If the n10 is due primarily to utricular activation, then diseases that affect only the superior division of the vestibular nerve in which all utricular afferents course (i.e., superior vestibular neuritis [SVN]) should reduce or eliminate n10 beneath the contralesional eye, whereas the n10 beneath the ipsilesional eye and the sacculo-collic cervical vestibular-evoked myogenic potential (cVEMP) on the ipsilesional side should be preserved.

SETTING

A prospective study at a tertiary neurotological referral center.

SUBJECTS AND METHODS

The n10 component of the oVEMP was measured in 133 patients with unilateral SVN but with inferior vestibular nerve function preserved, as shown by ipsilesional cVEMPs.

RESULTS

The n10 to Fz BCV of 133 SVN patients was reduced beneath the contralesional eye relative to the ipsilesional eye so that there was an n10 asymmetry that was significantly greater than the n10 asymmetry in the 50 healthy subjects. In terms of predicting the affected side (shown by canal paresis), using an n10 asymmetry ratio (asymmetry ratio for the relative size of the n10 of the oVEMPs for the two eyes [AR]) of 46.5 percent, the n10 AR has a diagnostic accuracy of 94 percent.

CONCLUSION

The n10 component of the oVEMP to BCV is probably mediated by the superior vestibular nerve and so mainly by the utricular receptors. The n10 AR is almost as good as canal paresis in identifying the affected side in patients.

Single trial detection of human vestibular evoked myogenic potentials is determined by signal-to-noise ratio

Vestibular reflexes in humans can be assessed by means of acoustically evoked responses of myogenic origin. For the vestibular-collic pathway this is termed the vestibular evoked myogenic potential (or VEMP) and for the vestibular-ocular pathway the ocular VEMP (or OVEMP). Usually VEMPs require an averaging process to obtain a clear response against the background myogenic activity, but depending on the combination of target reflex and stimulus mode, in some cases clear responses can be observed in single trials without averaging. We aimed to test whether this difference in detectability was simply related to signal-to-noise ratio (SNR), or a manifestation of some other difference in the reflex pathways. In four healthy subjects we recorded VEMPs and OVEMPs in response to 2-ms, 500-Hz sound pips and 10-ms, 100-Hz transmastoid vibrations at four intensity levels, and also determined thresholds. A plot of probability of detection P vs. SNR for all subjects and conditions fell onto a single sigmoid curve. When fitted by a logistic function after linearization a regression yielded an R(2) of 0.89 (n = 64, p < 0.001), with parameter estimates of mu = 2.9 and sigma = 2.0. Three patients with superior canal dehiscence, characterized by significantly lowered thresholds for sound-activated responses, exhibited a similar detection curve. We conclude that single trial detection of evoked myogenic potentials is a property mainly determined by SNR. Thus vestibular reflexes, differing in both their response magnitude and in their levels of myogenic activity by more than an order of magnitude, can be described by a single relationship when their magnitude is expressed relative to background activity, demonstrating the fundamental importance of the SNR.

Vestibular schwannoma with absent vestibular evoked myogenic potentials to clicks but normal ABR, caloric responses and vestibular evoked myogenic potentials to 500 Hz tone bursts

We report a case of vestibular schwannoma with absent vestibular evoked myogenic potentials (VEMPs) to clicks but normal auditory brainstem responses, caloric responses, and vestibular evoked myogenic potentials to 500 Hz tone bursts. This patient visited our clinic with complaint of sudden right hearing loss. This was the third episode of hearing loss in his right ear. Due to atypical medical history, he underwent further neuro-otological and neuroradiological examinations. Among the neurophysiological tests, only VEMPs to clicks showed abnormal findings (absent responses on the right). MRI revealed a small vestibular schwannoma in the right internal auditory meatus, which was considered to be of inferior vestibular nerve origin. This case suggested that VEMPs to clicks should still be included in the test batteries for the diagnosis of vestibular schwannoma.

Ocular and cervical vestibular-evoked myogenic potentials to bone conducted vibration in Ménière's disease during quiescence vs during acute attacks

OBJECTIVE

Two indicators of otolithic function were used to measure dynamic otolith function in the same patients both during an acute attack of Ménière's disease (MD) and in the quiescent period between attacks.

METHODS

The early negative component (n10) of the ocular vestibular-evoked myogenic potential (the oVEMP) to brief 500 Hz bone conducted vibration (BCV) stimulation of the forehead, in the midline at the hairline (Fz) was recorded by surface EMG electrodes just beneath both eyes while the patient looked up. It has been proposed that the n10 component of the oVEMP to 500 Hz Fz BCV indicates utricular function. It has been proposed that the early positive component (p13) of the cervical vestibular-evoked myogenic potential (the cVEMP) recorded by surface electrodes on both tensed SCM neck muscles to 500 Hz Fz BCV indicates saccular function.

RESULTS

Sixteen healthy control subjects tested on two occasions showed no detectable change in the symmetry of oVEMPs or cVEMPs to 500 Hz Fz BCV. In response to 500 Hz Fz BCV 15 early MD patients tested at both attack and quiescent phases showed a dissociation: there was a significant increase in contralesional of n10 of the oVEMP during the attack compared to quiescence but a significant decrease in the ipsilesional p13 of the cVEMP during the attack compared to quiescence.

CONCLUSIONS

During an MD attack, dynamic utricular function in the affected ear as measured by the n10 of the oVEMP to 500 Hz Fz BCV is enhanced, whereas dynamic saccular function in the affected ear as measured by the p13 of the cVEMP to 500 Hz Fz BCV is not similarly affected.

SIGNIFICANCE

The MD attack appears to affect different otolithic regions differentially.

Vestibular evoked myogenic potentials: past, present and future

Since the first description of sound-evoked short-latency myogenic reflexes recorded from neck muscles, vestibular evoked myogenic potentials (VEMPs) have become an important part of the neuro-otological test battery. VEMPs provide a means of assessing otolith function: stimulation of the vestibular system with air-conducted sound activates predominantly saccular afferents, while bone-conducted vibration activates a combination of saccular and utricular afferents. The conventional method for recording the VEMP involves measuring electromyographic (EMG) activity from surface electrodes placed over the tonically-activated sternocleidomastoid (SCM) muscles. The "cervical VEMP" (cVEMP) is thus a manifestation of the vestibulo-collic reflex. However, recent research has shown that VEMPs can also be recorded from the extraocular muscles using surface electrodes placed near the eyes. These "ocular VEMPs" (oVEMPs) are a manifestation of the vestibulo-ocular reflex. Here we describe the historical development and neurophysiological properties of the cVEMP and oVEMP and provide recommendations for recording both reflexes. While the cVEMP has documented diagnostic utility in many disorders affecting vestibular function, relatively little is known as yet about the clinical value of the oVEMP. We therefore outline the known cVEMP and oVEMP characteristics in common central and peripheral disorders encountered in neuro-otology clinics.

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.

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.