A Review of Neural Data and Modelling to Explain How a Semicircular Canal Dehiscence (SCD) Causes Enhanced VEMPs, Skull Vibration Induced Nystagmus (SVIN), and the Tullio Phenomenon.

Angular acceleration stimulation of a semicircular canal causes an increased firing rate in primary canal afferent neurons that result in nystagmus in healthy adult animals. However, increased firing rate in canal afferent neurons can also be caused by sound or vibration in patients after a semicircular canal dehiscence, and so these unusual stimuli will also cause nystagmus. The recent data and model by Iversen and Rabbitt show that sound or vibration may increase firing rate either by neural activation locked to the individual cycles of the stimulus or by slow changes in firing rate due to fluid pumping ("acoustic streaming"), which causes cupula deflection. Both mechanisms will act to increase the primary afferent firing rate and so trigger nystagmus. The primary afferent data in guinea pigs indicate that in some situations, these two mechanisms may oppose each other. This review has shown how these three clinical phenomena-skull vibration-induced nystagmus, enhanced vestibular evoked myogenic potentials, and the Tullio phenomenon-have a common tie: they are caused by the new response of semicircular canal afferent neurons to sound and vibration after a semicircular canal dehiscence.

A review of the geometrical basis and the principles underlying the use and interpretation of the video head impulse test (vHIT) in clinical vestibular testing.

This paper is concerned mainly with the assumptions underpinning the actual testing procedure, measurement, and interpretation of the video head impulse test-vHIT. Other papers have reported in detail the artifacts which can interfere with obtaining accurate eye movement results, but here we focus not on artifacts, but on the basic questions about the assumptions and geometrical considerations by which vHIT works. These matters are crucial in understanding and appropriately interpreting the results obtained, especially as vHIT is now being applied to central disorders. The interpretation of the eye velocity responses relies on thorough knowledge of the factors which can affect the response-for example the orientation of the goggles on the head, the head pitch, and the contribution of vertical canals to the horizontal canal response. We highlight some of these issues and point to future developments and improvements. The paper assumes knowledge of how vHIT testing is conducted.

A Single Fast Test for Semicircular Canal Dehiscence-oVEMP n10 to 4000 Hz-Depends on Stimulus Rise Time.

As previously reported, a single test measuring oVEMP n10 to 4000 Hz stimuli (bone-conducted vibration (BCV) or air-conducted sound (ACS)) provides a definitive diagnosis of semicircular canal dehiscence (SCD) in 22 CT-verified patients, with a sensitivity of 1.0 and specificity of 1.0. This single short screening test has great advantages of speed, minimizing testing time, and the exposure of patients to stimulation. However, a few studies of the 4000 Hz test for SCD have reported sensitivity and specificity values which are slightly less than reported previously. We hypothesized that the rise time of the stimulus is important for detecting the oVEMP n10 to 4000 Hz, similarly to what we had shown for 500 and 750 Hz BCV. We measured oVEMP n10 in 15 patients with CT-verified SCD in response to 4000 Hz ACS or BCV stimuli with rise times of 0, 1, and 2 ms. As a result, increasing the rise time of the stimulus reduced the oVEMP n10 amplitude. This outcome is expected from the physiological evidence of guinea pig primary vestibular afferents, which are activated by sound or vibration. Therefore, for clinical VEMP testing, short rise times are optimal (preferably 0 ms).

Vestibular semicircular canal function as detected by video Head Impulse Test (vHIT) is essentially unchanged in people with Parkinson's disease compared to healthy controls.

BACKGROUND: Parkinson's disease (PD) is a common multi-system neurodegenerative disorder with possible vestibular system dysfunction, but prior vestibular function test findings are equivocal. OBJECTIVE: To report and compare vestibulo-ocular reflex (VOR) gain as measured by the video head impulse test (vHIT) in participants with PD, including tremor dominant and postural instability/gait dysfunction phenotypes, with healthy controls (HC). METHODS: Forty participants with PD and 40 age- and gender-matched HC had their vestibular function assessed. Lateral and vertical semicircular canal VOR gains were measured with vHIT. VOR canal gains between PD participants and HC were compared with independent samples t-tests. Two distinct PD phenotypes were compared to HC using Tukey's ANOVA. The relationship of VOR gain with PD duration, phenotype, severity and age were investigated using logistic regression. RESULTS: There were no significant differences between groups in vHIT VOR gain for lateral or vertical canals. There was no evidence of an effect of PD severity, phenotype or age on VOR gains in the PD group. CONCLUSION: The impulsive angular VOR pathways are not significantly affected by the pathophysiological changes associated with mild to moderate PD.

Enhanced Eye Velocity in Head Impulse Testing-A Possible Indicator of Endolymphatic Hydrops.

Introduction: On video head impulse testing (vHIT) of semicircular canal function, some patients reliably show enhanced eye velocity and so VOR gains >1.0. Modeling and imaging indicate this could be due to endolymphatic hydrops. Oral glycerol reduces membranous labyrinth volume and reduces cochlear symptoms of hydrops, so we tested whether oral glycerol reduced the enhanced vHIT eye velocity. Study Design: Prospective clinical study and retrospective analysis of patient data. Methods: Patients with enhanced eye velocity during horizontal vHIT were enrolled (n = 9, 17 ears) and given orally 86% glycerol, 1.5 mL/kg of body weight, dissolved 1:1 in physiological saline. Horizontal vHIT testing was performed before glycerol intake (time 0), then at intervals of 1, 2, and 3 h after the oral glycerol intake. Control patients with enhanced eye velocity (n = 4, 6 ears) received water and were tested at the same intervals. To provide an objective index of enhanced eye velocity we used a measure of VOR gain which captures the enhanced eye velocity which is so clear on inspecting the eye velocity records. We call this measure the initial VOR gain and it is defined as: (the ratio of peak eye velocity to the value of head velocity at the time of peak eye velocity). The responses of other patients who showed enhanced eye velocity during routine clinical testing were analyzed to try to identify how the enhancement occurred. Results: We found that oral glycerol caused, on average, a significant reduction in the enhanced eye velocity response, whereas water caused no systematic change. The enhanced eye velocity during the head impulses is due in some patients to a compensatory saccade-like response during the increasing head velocity. Conclusion: The significant reduction in enhanced eye velocity during head impulse testing following oral glycerol is consistent with the hypothesis that the enhanced eye velocity in vHIT may be caused by endolymphatic hydrops.

Activation of Guinea Pig Irregular Semicircular Canal Afferents by 100 Hz Vibration: Clinical Implications for Vibration-induced Nystagmus and Vestibular-evoked Myogenic Potentials.

HYPOTHESIS: Bone-conducted vibration (BCV) at 100 Hz causes endolymph displacement at hair cell stereocilia in semicircular canal (SCC) ducts of the intact bony labyrinth resulting in activation of irregularly discharging afferent neurons. BACKGROUND: Suprathreshold 100 Hz BCV is employed in the clinic to evoke skull vibration-induced nystagmus, an indicator for peripheral vestibular asymmetry. Recently, this stimulus has also been used in vestibular-evoked myogenic potentials, a selective test for otolithic function. METHODS: We performed extracellular recordings from utricular and SCC afferents in guinea pigs during application of suprathreshold BCV stimuli (100-500 Hz) to the animal's skull. Vibration was administered in a way that the animal, the vibrator, and the recording electrode moved as one. RESULTS: In summary, 19 of 43 recorded SCC afferents displayed a stimulus- and phase-locked increase in firing during stimulation at 100 Hz BCV with no perstimulatory adaptation and no poststimulatory silencing. All of the 19 activated SCC afferents had an irregular resting discharge. Neuronal activation of SCC afferents was less pronounced at 200 Hz and largely absent at 500 Hz. On the contrary, a stimulus- and phase-locked increase in firing was observed for irregularly discharging utricular neurons at all frequencies tested. CONCLUSIONS: At intensities usually applied in the clinic, 500 Hz BCV is a largely selective otolithic stimulus, while 100 Hz BCV can activate both otolith and SCC afferents. Therefore, while 100 Hz BCV is ideally suited for evoking skull vibration-induced nystagmus in peripheral vestibular asymmetry, it is not recommended for vestibular-evoked myogenic potentials, as it lacks otolithic specificity.

Computing Endolymph Hydrodynamics During Head Impulse Test on Normal and Hydropic Vestibular Labyrinth Models.

Hypothesis: Build a biologic geometry based computational model to test the hypothesis that, in some circumstances, endolymphatic hydrops can mechanically cause enhanced eye velocity responses during clinical conditions of the head impulse test. Background: Some recent clinical and experimental findings had suggested that enhanced eye velocity responses measured with the video head impulse test could not only be caused by recording artifacts or central disfunction but also could be directly caused by the mechanical effect of endolymphatic hydrops on horizontal semicircular canal receptor. Methods: Data from clinical video head impulse test was computed in three biologic-based geometry models governed by Navier-Stokes equations; six head impulses of incrementally increasing peak head velocity were computed in each one of the three different geometric models, depending on absence, canal or utricular hydrops. Results: For all computed head impulses an increased endolymphatic pressure was measured at the ampullar region of the horizontal semicircular canal on both canal and utricular hydrops models. The mean of aVOR gain was 1.01 ± 0.008 for the no-hydrops model, 1.14 ± 0.010 for the canal hydrops model was, and 1.10 ± 0.007 for the utricular hydrops model. Conclusion: The results of the physical computation models support-the hypothesis that in endolymphatic hydrops conditions, which are affecting horizontal semicircular canal and utricular region on moderate dilatations, the eye velocity responses output-by the aVOR will be enhanced by a 1.14 factor and aVOR gain values will be enhanced by over 1.1 for impulses to the right side.

The Evidence for Selective Loss of Otolithic Function.

Recent advances in vestibular testing now permit functional testing of all peripheral vestibular sense organs (all three semicircular canals, utricle, and saccule). This makes it possible to identify patients with isolated dysfunction of the utricle or saccule, even though parallel pathways for vestibular information are ultimately integrated centrally. Selective, isolated unilateral loss of utricular function as measured by ocular vestibular-evoked myogenic potentials (VEMPs) has been observed in patients with normal semicircular canal function as measured by the video head impulse test of all six semicircular canals, and normal bilateral saccular function as determined by symmetrical cervical VEMPs. How these patients present clinically and how they recover is discussed and contrasted with acute vestibular neuritis. In some patients, the unilateral loss of otolith organ (utricle or saccule) function persists and yet the patient recovers functionally to their usual lifestyle. Until the testing of all peripheral vestibular sense organs is routine, the frequency of isolated loss of otolith function cannot be gauged.

A review of mechanical and synaptic processes in otolith transduction of sound and vibration for clinical VEMP testing.

Older studies of mammalian otolith physiology have focused mainly on sustained responses to low-frequency (<50 Hz) or maintained linear acceleration. So the otoliths have been regarded as accelerometers. Thus evidence of otolithic activation and high-precision phase locking to high-frequency sound and vibration appears to be very unusual. However, those results are exactly in accord with a substantial body of knowledge of otolith function in fish and frogs. It is likely that phase locking of otolith afferents to vibration is a general property of all vertebrates. This review examines the literature about the activation and phase locking of single otolithic neurons to air-conducted sound and bone-conducted vibration, in particular the high precision of phase locking shown by mammalian irregular afferents that synapse on striolar type I hair cells by calyx endings. Potassium in the synaptic cleft between the type I hair cell receptor and the calyx afferent ending may be responsible for the tight phase locking of these afferents even at very high discharge rates. Since frogs and fish do not possess full calyx endings, it is unlikely that they show phase locking with such high precision and to such high frequencies as has been found in mammals. The high-frequency responses have been modeled as the otoliths operating in a seismometer mode rather than an accelerometer mode. These high-frequency otolithic responses constitute the neural basis for clinical vestibular-evoked myogenic potential tests of otolith function.

Superior Canal Dehiscence Syndrome: Relating Clinical Findings With Vestibular Neural Responses From a Guinea Pig Model.

HYPOTHESIS: In superior canal dehiscence (SCD), fluid displacement of the endolymph activates type I vestibular hair cells in the crista of the affected canal and thus irregular superior canal (SC) neurons in Scarpa's ganglion, which provides the neurophysiological basis for the clinical presentation of SCD. BACKGROUND: Patients with SCD display sound- and vibration-induced vertigo/nystagmus and increased amplitudes of vestibular evoked myogenic potentials. METHODS: Extracellular recordings from n = 25 primary vestibular neurons of 16 female guinea pigs were analyzed. We recorded from the same vestibular neuron before, during and after creating the dehiscence and after closing the dehiscence. Neurobiotin labeling was employed in n = 11 neurons. RESULTS: After SCD, previously unresponsive irregular SC neurons displayed a stimulus-locked increase in discharge during application of air-conducted sound (ACS) or bone-conducted vibration (BCV) for a broad range of frequencies (ACS: 200-4000 Hz; BCV: 500-1500 Hz). This typical response was only observed for irregular SC neurons (n = 19), but not regular SC neurons, or irregular/regular horizontal canal neurons (n = 2 each), and was abolished after closing the dehiscence. Eleven irregular SC neurons responsive to ACS and/or BCV were traced back to calyx synapses in the central crista of the affected superior canal by neurobiotin labeling. CONCLUSIONS: Stimulus-locked activation of irregular SC neurons by ACS and BCV is the neurophysiological basis for sound- and vibration-induced vertigo/nystagmus and increased VEMP amplitudes in SCD. The results of the present study help to improve vestibular diagnostics in patients with suspected SCD.

Phase-locking of irregular guinea pig primary vestibular afferents to high frequency (>250 Hz) sound and vibration.

Phase-locking of cochlear neurons to sound has been of great value in understanding cochlear transduction. Phase-locking has also been reported previously in irregular vestibular afferents, but detailed information about it is sparse. We measured the phase-locking of guinea pig irregular otolithic neurons and canal neurons (after a semicircular canal dehiscence allowed them to respond) to both sound and vibration stimuli. Irregular vestibular afferents from both otoliths and canals have a range of preferred phase angles which systematically increase as frequency is increased from 250 Hz to above 1000 Hz. Surprisingly vestibular afferents show more precise phase-locking than comparable auditory afferents as reported by Palmer and Russell (1986), and they do so up to higher frequencies. This high precision implies a very sharp, fast threshold for evoking an action potential with minimal variability, and so has implications for the current controversy about hair-cell-afferent transmission in the vestibular system. Following recent evidence, we suggest that potassium in the unique type I-calyx synapse may be a major factor in generating this very precise phase-locking.

Enhanced Vestibulo-Ocular Reflex Responses on vHIT. Is It a Casual Finding or a Sign of Vestibular Dysfunction?

In current clinical practice, when in response to vHIT testing the observed slow-phase eye-velocity responses are significantly higher than head velocity, the most probable cause is considered to be an inadequate collection method or a recording artifact. We present two cases with clinical diagnoses of Menière's Disease: for both cases, enhanced eye velocity responses were measured with a rigorous vHIT testing protocol. In the first case we measured these enhanced responses on each test performed during a 5 year time series; in the second case multiple measurements were obtained from a patient after the radiologic diagnosis of vestibulo-cochlear hydrops. The two cases presented and the new evidence reported by other researchers suggest that owing to the low probability of artifact and the high consistency of the vHIT measurements, we should consider the hypothesis of a physio-pathologic cause for the enhanced eye responses to vHIT testing of some patients with vestibular dysfunction.

Otolithic Receptor Mechanisms for Vestibular-Evoked Myogenic Potentials: A Review.

Air-conducted sound and bone-conduced vibration activate otolithic receptors and afferent neurons in both the utricular and saccular maculae, and trigger small electromyographic (EMG) responses [called vestibular-evoked myogenic potentials (VEMPs)] in various muscle groups throughout the body. The use of these VEMPs for clinical assessment of human otolithic function is built on the following logical steps: (1) that high-frequency sound and vibration at clinically effective stimulus levels activate otolithic receptors and afferents, rather than semicircular canal afferents, (2) that there is differential anatomical projection of otolith afferents to eye muscles and neck muscles, and (3) that isolated stimulation of the utricular macula induces short latency responses in eye muscles, and that isolated stimulation of the saccular macula induces short latency responses in neck motoneurons. Evidence supports these logical steps, and so VEMPs are increasingly being used for clinical assessment of otolith function, even differential evaluation of utricular and saccular function. The proposal, originally put forward by Curthoys in 2010, is now accepted: that the ocular vestibular-evoked myogenic potential reflects predominantly contralateral utricular function and the cervical vestibular-evoked myogenic potential reflects predominantly ipsilateral saccular function. So VEMPs can provide differential tests of utricular and saccular function, not because of stimulus selectivity for either of the two maculae, but by measuring responses which are predominantly determined by the differential neural projection of utricular as opposed to saccular neural information to various muscle groups. The major question which this review addresses is how the otolithic sensory system, with such a high density otoconial layer, can be activated by individual cycles of sound and vibration and show such tight locking of the timing of action potentials of single primary otolithic afferents to a particular phase angle of the stimulus cycle even at frequencies far above 1,000 Hz. The new explanation is that it is due to the otoliths acting as seismometers at high frequencies and accelerometers at low frequencies. VEMPs are an otolith-dominated response, but in a particular clinical condition, semicircular canal dehiscence, semicircular canal receptors are also activated by sound and vibration, and act to enhance the otolith-dominated VEMP responses.

A new saccadic indicator of peripheral vestibular function based on the video head impulse test.

OBJECTIVE: While compensatory saccades indicate vestibular loss in the conventional head impulse test paradigm (HIMP), in which the participant fixates an earth-fixed target, we investigated a complementary suppression head impulse paradigm (SHIMP), in which the participant is fixating a head-fixed target to elicit anticompensatory saccades as a sign of vestibular function. METHODS: HIMP and SHIMP eye movement responses were measured with the horizontal video head impulse test in patients with unilateral vestibular loss, patients with bilateral vestibular loss, and in healthy controls. RESULTS: Vestibulo-ocular reflex gains showed close correlation (R(2) = 0.97) with slightly lower SHIMP than HIMP gains (mean gain difference 0.06 ± 0.05 SD, p < 0.001). However, the 2 paradigms produced complementary catch-up saccade patterns: HIMP elicited compensatory saccades in patients but rarely in controls, whereas SHIMP elicited large anticompensatory saccades in controls, but smaller or no saccades in bilateral vestibular loss. Unilateral vestibular loss produced covert saccades in HIMP, but later and smaller saccades in SHIMP toward the affected side. Cumulative HIMP and SHIMP saccade amplitude differentiated patients from controls with high sensitivity and specificity. CONCLUSIONS: While compensatory saccades indicate vestibular loss in conventional HIMP, anticompensatory saccades in SHIMP using a head-fixed target indicate vestibular function. SHIMP saccades usually appear later than HIMP saccades, therefore being more salient to the naked eye and facilitating vestibulo-ocular reflex gain measurements. The new paradigm is intuitive and easy to explain to patients, and the SHIMP results complement those from the standard video head impulse test. CLASSIFICATION OF EVIDENCE: This case-control study provides Class III evidence that SHIMP accurately identifies patients with unilateral or bilateral vestibulopathies.

The response of guinea pig primary utricular and saccular irregular neurons to bone-conducted vibration (BCV) and air-conducted sound (ACS).

UNLABELLED: This study sought to characterize the response of mammalian primary otolithic neurons to sound and vibration by measuring the resting discharge rates, thresholds for increases in firing rate and supra-threshold sensitivity functions of guinea pig single primary utricular and saccular afferents. Neurons with irregular resting discharge were activated in response to bone conducted vibration (BCV) and air conducted sound (ACS) for frequencies between 100 Hz and 3000 Hz. The location of neurons was verified by labelling with neurobiotin. Many afferents from both maculae have very low or zero resting discharge, with saccular afferents having on average, higher resting rates than utricular afferents. Most irregular utricular and saccular afferents can be evoked by both BCV and ACS. For BCV stimulation: utricular and saccular neurons show similar low thresholds for increased firing rate (around 0.02 g on average) for frequencies from 100 Hz to 750 Hz. There is a steep increase in rate change threshold for BCV frequencies above 750 Hz. The suprathreshold sensitivity functions for BCV were similar for both utricular and saccular neurons, with, at low frequencies, very steep increases in firing rate as intensity increased. For ACS stimulation: utricular and saccular neurons can be activated by high intensity stimuli for frequencies from 250 Hz to 3000 Hz with similar flattened U-shaped tuning curves with lowest thresholds for frequencies around 1000-2000 Hz. The average ACS thresholds for saccular afferents across these frequencies is about 15-20 dB lower than for utricular neurons. The suprathreshold sensitivity functions for ACS were similar for both utricular and saccular neurons. Both utricular and saccular afferents showed phase-locking to BCV and ACS, extending up to frequencies of at least around 1500 Hz for BCV and 3000 Hz for ACS. Phase-locking at low frequencies (e.g. 100 Hz) imposes a limit on the neural firing rate evoked by the stimulus since the neurons usually fire one spike per cycle of the stimulus. CONCLUSION: These results are in accord with the hypothesis put forward by Young et al. (1977) that each individual cycle of the waveform, either BCV or ACS, is the effective stimulus to the receptor hair cells on either macula. We suggest that each cycle of the BCV or ACS stimulus causes fluid displacement which deflects the short, stiff, hair bundles of type I receptors at the striola and so triggers the phase-locked neural response of primary otolithic afferents.

Bone conducted vibration to the mastoid produces horizontal, vertical and torsional eye movements.

INTRODUCTION: We have previously shown that bone-conducted vibration to each mastoid in humans results in small stimulus-locked horizontal and vertical eye movements suggesting that the ipsilateral superior oblique and the contralateral inferior rectus are activated. However, as torsional responses were not measured we have repeated the study by measuring torsional as well as horizontal and vertical responses in seven adult subjects during different directions of gaze. METHODS: We measured stimulus-locked horizontal (H), vertical (V) and torsional (T) eye movements during brief stimulation to the mastoid unilaterally and bilaterally using a high-speed, high-resolution video technique. Bone-conducted vibrations were delivered to each mastoid separately as well as bilaterally using a Radioear B71 bone oscillator while the subjects viewed targets 2 metres away situated straight ahead and 10° to the right and left. RESULTS: For unilateral stimulation the eyes moved downwards when directed away from the stimulated mastoid, along with bilateral counter-rolling away from the side of stimulation. Horizontal movements were small, with little effect of gaze position. Bilateral stimulation enhanced the downward responses. CONCLUSIONS: Bone conducted vibration to the mastoid in humans produces vertical and torsional eye movements consistent with activation of the ipsilateral superior oblique muscle, and the contralateral inferior oblique muscle to produce counter rolling of the eyes away from the stimulated mastoid.

Maintaining Balance when Looking at a Virtual Reality Three-Dimensional Display of a Field of Moving Dots or at a Virtual Reality Scene.

EXPERIMENTAL OBJECTIVE: To provide a safe, simple, relatively inexpensive, fast, accurate way of quantifying balance performance either in isolation, or in the face of challenges provided by 3D high definition moving visual stimuli as well as by the proprioceptive challenge from standing on a foam pad. This method uses the new technology of the Wii balance board to measure postural stability during powerful, realistic visual challenges from immersive virtual reality. LIMITATIONS OF CURRENT TECHNIQUES: Present computerized methods for measuring postural stability are large, complex, slow, and expensive, and do not allow for testing the response to realistic visual challenges. PROTOCOL: Subjects stand on a 6 cm thick, firm, foam pad on a Wii balance board. They wear a fast, high resolution, low persistence, virtual reality head set (Oculus Rift DK2). This allows displays of varying speed, direction, depth, and complexity to be delivered. The subject experiences a visual illusion of real objects fixed relative to the world, and any of these displays can be perturbed in an unpredictable fashion. A special app (BalanceRite) used the same procedures for analyzing postural analysis as used by the Equitest. POWER OF THE TECHNIQUE: Four simple "proof of concept" experiments demonstrate that this technique matches the gold standard Equitest in terms of the measurement of postural stability but goes beyond the Equitest by measuring stability in the face of visual challenges, which are so powerful that even healthy subjects fall. The response to these challenges presents an opportunity for predicting falls and for rehabilitation of seniors and patients with poor postural stability. SIGNIFICANCE FOR THE FIELD: This new method provides a simpler, quicker, cheaper method of measurement than the Equitest. It may provide a new mode of training to prevent falls, by maintaining postural stability in the face of visual and proprioceptive challenges similar to those encountered in life.

The Video Head Impulse Test (vHIT) of Semicircular Canal Function - Age-Dependent Normative Values of VOR Gain in Healthy Subjects.

BACKGROUND/HYPOTHESIS: The video Head Impulse Test (vHIT) is now widely used to test the function of each of the six semicircular canals individually by measuring the eye rotation response to an abrupt head rotation in the plane of the canal. The main measure of canal adequacy is the ratio of the eye movement response to the head movement stimulus, i.e., the gain of the vestibulo-ocular reflex (VOR). However, there is a need for normative data about how VOR gain is affected by age and also by head velocity, to allow the response of any particular patient to be compared to the responses of healthy subjects in their age range. In this study, we determined for all six semicircular canals, normative values of VOR gain, for each canal across a range of head velocities, for healthy subjects in each decade of life. STUDY DESIGN: The VOR gain was measured for all canals across a range of head velocities for at least 10 healthy subjects in decade age bands: 10-19, 20-29, 30-39, 40-49, 50-59, 60-69, 70-79, 80-89. METHODS: The compensatory eye movement response to a small, unpredictable, abrupt head rotation (head impulse) was measured by the ICS impulse prototype system. The same operator delivered every impulse to every subject. RESULTS: Vestibulo-ocular reflex gain decreased at high head velocities, but was largely unaffected by age into the 80- to 89-year age group. There were some small but systematic differences between the two directions of head rotation, which appear to be largely due to the fact that in this study only the right eye was measured. The results are considered in relation to recent evidence about the effect of age on VOR performance. CONCLUSION: These normative values allow the results of any particular patient to be compared to the values of healthy people in their age range and so allow, for example, detection of whether a patient has a bilateral vestibular loss. VOR gain, as measured directly by the eye movement response to head rotation, seems largely unaffected by aging.

Horizontal Eye Position Affects Measured Vertical VOR Gain on the Video Head Impulse Test.

BACKGROUND/HYPOTHESIS: With the video head impulse test (vHIT), the vertical VOR gain is defined as (vertical eye velocity/vertical head velocity), but compensatory eye movements to vertical canal stimulation usually have a torsional component. To minimize the contribution of torsion to the eye movement measurement, the horizontal gaze direction should be directed 40° from straight ahead so it is in the plane of the stimulated canal plane pair. HYPOTHESIS: as gaze is systematically moved horizontally away from canal plane alignment, the measured vertical VOR gain should decrease. STUDY DESIGN: Ten healthy subjects, with vHIT measuring vertical eye movement to head impulses in the plane of the left anterior-right posterior (LARP) canal plane, with gaze at one of five horizontal gaze positions [40°(aligned with the LARP plane), 20°, 0°, -20°, -40°]. METHODS: Every head impulse was in the LARP plane. The compensatory eye movement was measured by the vHIT prototype system. The one operator delivered every impulse. RESULTS: The canal stimulus remained identical across trials, but the measured vertical VOR gain decreased as horizontal gaze angle was shifted away from alignment with the LARP canal plane. CONCLUSION: In measuring vertical VOR gain with vHIT the horizontal gaze angle should be aligned with the canal plane under test.