Passive Head Impulses Research Articles

Long-term dance training modifies eye-head coordination in response to passive head impulse.

Long-term dance training is known to improve postural control, especially in challenging postural tasks. However, the effect of dance training on the vestibulo-ocular reflex (VOR) has yet to be properly assessed. This study directly investigated whether VOR parameters are influenced by long-term dance training by testing dancers and controls using the video head impulse test. VOR gains using two of the most common methods (area ratio and instantaneous gains), latency and amplitude of the first saccade, if applicable, were computed. Results revealed a larger VOR gain as measured by area gain and instantaneous gain at 40 ms specifically for left-head impulses, but not right-head impulses. No significant differences in saccade frequency, amplitude, or latency were observed between groups. These differences appear to stem from a modified eye-to-head relationship during high-velocity head impulses in dancers. More specifically, the dancers' eyes lead head movement during passively applied head impulses, which result in higher VOR gain.NEW & NOTEWORTHY This study demonstrates, for the first time, that long-term dance training results in a nonlinear relationship between eye and head velocity within the first milliseconds following passive head impulse. The data also suggest a larger VOR gain in dancers. This finding suggests that dance training may modify eye-head relationship in passive high-frequency head movements. This is of particular interest for vestibular rehabilitation.

Unidirectional Vertical Vestibuloocular Reflex Adaptation in Humans Using 1D and 2D Scenes.

The vertical vestibuloocular reflex (VOR) in response to pitch head impulses can be optimally trained to increase in one direction using a two-dimensional (2D) visual training target with minimal effect on the horizontal VOR. We modified the incremental VOR adaptation (IVA) technique, shown to increase the horizontal VOR in patients with vestibular hypofunction, to drive vertical VOR adaptation in healthy control subjects. We measured the horizontal and vertical active (self-generated) and passive (imposed) head impulse VOR gains (eye velocity/head velocity) before and after 15 minutes of unidirectional downward IVA training. IVA training consisted of two sessions, one using a single-dot one-dimensional (1D) target, the other a grid-of-dots 2D target. The downward head impulse VOR gain significantly increased because of training by 13.3%, whereas the upward VOR gain did not change. The addition of extraretinal (2D) feedback did not result in greater adaptation, i.e., 1D and 2D gain increases were 15.5% and 10.6%, respectively. The vertical VOR gain increase resulted in a 3.2% decrease in horizontal VOR gain. This preliminary study is the first to show that physiologically relevant (high frequency) unidirectional increases in vertical VOR gain are possible with just 15 minutes of training. This study sets the basis for future clinical trials examining vertical IVA training in patients, which may provide the first practical rehabilitation treatment to functionally improve the vertical VOR.

Two conditions to fully recover dynamic canal function in unilateral peripheral vestibular hypofunction patients.

The crucial role of early vestibular rehabilitation (VR) to recover a dynamic semicircular canal function was recently highlighted in patients with unilateral vestibular hypofunction (UVH). However, wide inter-individual differences were observed, suggesting that parameters other than early rehabilitation are involved. The aim of the study was to determine to what extent the degree of vestibular loss assessed by the angular vestibulo-ocular reflex (aVOR) gain could be an additional parameter interfering with rehabilitation in the recovery process. And to examine whether different VR protocols have the same effectiveness with regard to the aVOR recovery. The aVOR gain and the percentage of compensatory saccades were recorded in 81 UVH patients with the passive head impulse test before and after early VR (first two weeks after vertigo onset: N = 43) or late VR (third to sixth week after onset: N = 38) performed twice a week for four weeks. VR was performed either with the unidirectional rotation paradigm or gaze stability exercises. Supplementary outcomes were the dizziness handicap inventory (DHI) score, and the static and dynamic subjective visual vertical. The cluster analysis differentiated two distinct populations of UVH patients with pre-rehab aVOR gain values on the hypofunction side below 0.20 (N = 42) or above 0.20 (N = 39). The mean gain values were respectively 0.07±0.05 and 0.34±0.12 for the lateral canal (p < 0.0001), 0.09±0.06 and 0.44±0.19 for the anterior canal (p < 0.0001). Patients with aVOR gains above 0.20 and early rehab fully recovered dynamic horizontal canal function (0.84±0.14) and showed very few compensatory saccades (18.7% ±20.1%) while those with gains below 0.20 and late rehab did not improve their aVOR gain value (0.16±0.09) and showed compensatory saccades only (82.9% ±23.7%). Similar results were found for the anterior canal function. Recovery of the dynamic function of the lateral canal was found with both VR protocols while it was observed with the gaze stability exercises only for the anterior canal. All the patients reduced their DHI score, normalized their static SVV, and exhibited uncompensated dynamic SVV. Early rehab is a necessary but not sufficient condition to fully recover dynamic canal function. The degree of vestibular loss plays a crucial role too, and to be effective rehabilitation protocols must be carried out in the plane of the semicircular canals.

The Effect of Different Head Movement Paradigms on Vestibulo-Ocular Reflex Gain and Saccadic Eye Responses in the Suppression Head Impulse Test in Healthy Adult Volunteers.

Objective: This study aimed to identify differences in vestibulo-ocular reflex gain (VOR gain) and saccadic response in the suppression head impulse paradigm (SHIMP) between predictable and less predictable head movements, in a group of healthy subjects. It was hypothesized that higher prediction could lead to a lower VOR gain, a shorter saccadic latency, and higher grouping of saccades.Methods: Sixty-two healthy subjects were tested using the video head impulse test and SHIMPs in four conditions: active and passive head movements for both inward and outward directions. VOR gain, latency of the first saccade, and the level of saccade grouping (PR-score) were compared among conditions. Inward and active head movements were considered to be more predictable than outward and passive head movements.Results: After validation, results of 57 tested subjects were analyzed. Mean VOR gain was significantly lower for inward passive compared with outward passive head impulses (p < 0.001), and it was higher for active compared with passive head impulses (both inward and outward) (p ≤ 0.024). Mean latency of the first saccade was significantly shorter for inward active compared with inward passive (p ≤ 0.001) and for inward passive compared with outward passive head impulses (p = 0.012). Mean PR-score was only significantly higher in active outward than in active inward head impulses (p = 0.004).Conclusion: For SHIMP, a higher predictability in head movements lowered gain only in passive impulses and shortened latencies of compensatory saccades overall. For active impulses, gain calculation was affected by short-latency compensatory saccades, hindering reliable comparison with gains of passive impulses. Predictability did not substantially influence grouping of compensatory saccades.

Short-Latency Covert Saccades - The Explanation for Good Dynamic Visual Performance After Unilateral Vestibular Loss?

Background: Functional head impulse test (fHIT) tests the ability of the vestibulo-ocular reflex (VOR) to allow visual perception during head movements. Our previous study showed that active head movements to the side with a vestibular lesion generated a dynamic visual performance that were as good as during movements to the intact side.Objective: To examine the differences in eye position during the head impulse test when performed with active and passive head movements, in order to better understand the role of the different saccade properties in improving visual performance.Method: We recruited 8 subjects with complete unilateral vestibular loss (4 men and 4 women, mean age 47 years) and tested them with video Head Impulse Test (vHIT) and Functional Head Impulse Test (fHIT) during passive and active movements while looking at a target. We assessed the mean absolute position error of the eye during different time frames of the head movement, the peak latency and the peak velocity of the first saccade, as well as the visual performance during the head movement.Results: Active head impulses to the lesioned side generated dynamic visual performances that were as good as when testing the intact side. Active head impulses resulted in smaller position errors during the visual perception task (p = 0.006) compared to passive head-impulses and the position error during the visual perception time frame correlated with shorter latencies of the first saccade (p < 0.001).Conclusion: Actively generated head impulses toward the side with a complete vestibular loss resulted in a position error within or close to the margin necessary to obtain visual perception for a brief period of time in patients with chronic unilateral vestibular loss. This seems to be attributed to the appearance of short-latency covert saccades, which position the eyes in a more favorable position during head movements.

Absence of a vergence-mediated vestibulo-ocular reflex gain increase does not preclude adaptation

The gain (eye-velocity/head-velocity) of the angular vestibuloocular reflex (aVOR) during head impulses can be increased while viewing near-targets and when exposed to unilateral, incremental retinal image velocity error signals. It is not clear however, whether the tonic or phasic vestibular pathways mediate these gain increases. Determine whether a shared pathway is responsible for gain enhancement between vergence and adaptation of aVOR gain in patients with unilateral vestibular hypofunction (UVH). 20 patients with UVH were examined for change in aVOR gain during a vergence task and after 15-minutes of ipsilesional incremental VOR adaptation (uIVA) using StableEyes (a device that controls a laser target as a function of head velocity) during horizontal passive head impulses.A 5 % aVOR gain increase was defined as the threshold for significant change. 11/20 patients had >5% vergence-mediated gain increase during ipsi-lesional impulses. For uIVA, 10/20 patients had >5% ipsi-lesional gain increase. There was no correlation between the vergence-mediated gain increase and gain increase after uIVA training. Vergence-enhanced and uIVA training gain increases are mediated by separate mechanisms and/or vestibular pathways (tonic/phasic).The ability to increase the aVOR gain during vergence is not prognostic for successful adaptation training.

Once-Daily Incremental Vestibular-Ocular Reflex Adaptation Training in Patients With Chronic Peripheral Vestibular Hypofunction: A 1-Week Randomized Controlled Study

This was a double-blinded randomized controlled study to investigate the effects of once-daily incremental vestibulo-ocular reflex (VOR) training over 1 week in people with chronic peripheral vestibular hypofunction. A total of 24 patients with peripheral vestibular hypofunction were randomly assigned to intervention (n = 13) or control (n = 11) groups. Training consisted of either x1 (control) or incremental VOR adaptation exercises, delivered once daily for 15 minutes over 4 days in 1 week. Primary outcome: VOR gain with video-oculography. Secondary outcomes: Compensatory saccades measured using scleral search coils, dynamic visual acuity, static balance, gait, and subjective symptoms. Between-group differences were analyzed with a linear mixed-model with repeated measures. There was a difference in the VOR gain increase between groups (P < 0.05). The incremental training group gain increased during active (13.4% ± 16.3%) and passive (12.1% ± 19.9%) head impulse testing (P < 0.02), whereas it did not for the control group (P = 0.59). The control group had reduced compensatory saccade latency (P < 0.02). Both groups had similarly improved dynamic visual acuity scores (P < 0.05). Both groups had improved dynamic gait index scores (P < 0.002); however, only the incremental group had improved scores for the 2 walks involving head oscillations at approximately 2 Hz (horizontal: P < 0.05; vertical: P < 0.02), increased gait speed (P < 0.02), and step length (P < 0.01) during normal gait, and improved total Dizziness Handicap Inventory (P < 0.05). Our results suggest incremental VOR adaptation significantly improves gain, gait with head rotation, balance during gait, and symptoms in patients with chronic peripheral vestibular hypofunction more so than conventional x1 gaze-stabilizing exercises.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A336).

Vestibular Functioning in Children with Neurodevelopmental Disorders Using the Functional Head Impulse Test.

Several studies in children with neurodevelopmental disorders (NDDs) including autism spectrum disorders (ASDs), reading impairment, or attention deficit/hyperactive disorder (ADHD) pointed toward a potential dysfunction of the vestibular system, specifically in its complex relationship with the cerebellum. The aim of the present study was to test the functional vestibulo-ocular reflex (VOR) responses in children with NDDs to measure functional performance of the vestibular system. The VOR is specifically involved in this stabilization of the image on the retina during rapid movements of the head. To perform this study, four groups of children with ASD, ADHD, reading impairment, and with neurotypical development (TD) were enrolled (n = 80). We performed the functional head impulse test (fHIT), which measured the percentage of correct responses by asking the child to identify an optotype briefly presented during passive head impulse in each direction of each semicircular canal plane. We observed significantly lower correct answers in children with NDDs compared with those with TD (p < 0.0001). Surprisingly, there was no significant difference between the three groups of children with NDDs. Our study fostered preliminary evidence suggesting altered efficiency of vestibular system in children with NDDs. VOR abnormalities estimated using the fHIT could be used as a proxy of NDD impairments in children, and represent a potential biomarker.

Human Vestibulo-Ocular Reflex Adaptation Reduces when Training Demand Variability Increases.

One component of vestibular rehabilitation in patients with vestibulo-ocular reflex (VOR) hypofunction is gaze-stabilizing exercises that seek to increase (adapt) the VOR response. These prescribed home-based exercises are performed by the patient and thus their use/training is inherently variable. We sought to determine whether this variability affected VOR adaptation in ten healthy controls (× 2 training only) and ten patients with unilateral vestibular hypofunction (× 1 and × 2 training). During × 1 training, patients actively (self-generated, predictable) move their head sinusoidally while viewing a stationary fixation target; for × 2 training, they moved their outstretched hand anti-phase with their head rotation while attempting to view a handheld target. We defined the latter as manual × 2 training because the subject manually controls the target. In this study, head rotation frequency during training incrementally increased 0.5-2Hz over 20min. Active and passive (imposed, unpredictable) sinusoidal (1.3-Hz rotations) and head impulse VOR gains were measured before and after training. We show that for controls, manual × 2 training resulted in significant sinusoidal and impulse VOR adaptation of ~ 6% and ~ 3%, respectively, though this was ~two-thirds lower than increases after computer-controlled × 2 training (non-variable) reported in a prior study. In contrast, for patients, there was an increase in impulse but not sinusoidal VOR response after a single session of manual × 2 training. Patients had more than double the variability in VOR demand during manual × 2 training compared to controls, which could explain why adaptation was not significant in patients. Our data suggest that the clinical × 1 gaze-stabilizing exercise is a weak stimulus for VOR adaptation.

Retinal Image Slip Must Pass the Threshold for Human Vestibulo-Ocular Reflex Adaptation.

We sought to determine whether repeated vestibulo-ocular reflex (VOR) adaptation training to increase the VOR gain (eye/head velocity) had a lasting effect in normal subjects and whether there was a retinal image slip tolerance threshold for VOR adaptation. We used the unilateral incremental VOR adaptation technique and horizontal active (self-generated, predictable) head impulses as the vestibular stimulus. Both active and passive (imposed, unpredictable) head impulse VOR gains were measured before and after unilateral incremental VOR adaptation training. The adapting side was pseudo-randomized for left or right. We tested ten normal subjects over one block (10 sessions over 12days) of VOR adaptation training and testing, immediately followed by a second block (5 sessions over 19days) of testing only without training. Our findings show robust short-term VOR adaptation of ~ 10% immediately after each 15-min training session, but that the daily pre-adaptation gain was most different on days 1 and 2, and for subsequent training days before saturating to ~ 5% greater than the pre-adaptation gain on day 1. This increase was partially retained for 19days after regular training stopped. The data suggest that stable vision in normal subjects is maintained when there is < 5% deviation in VOR gain from the original baseline, which corresponds to < 9°/s retinal image slip. Below this threshold, there is poor adaptive drive to return the gain to its original baseline value.

Horizontal Vestibulo-Ocular Reflex Gain Measures During Convergence Using a Monocular Video Technique.

Vestibulo-ocular reflex (VOR) response measures during convergence, which are clinically important to measure peripheral vestibular organ function during rotational and translational rapid head movements, can be implemented using existing clinically available monocular video-oculography (VOG) systems. We have developed and validated a monocular VOG technique that allows for accurate measurement of the convergence angle immediately before a rapid translational or rotational head movement. We recorded binocular eye movements while subjects performed active or passive horizontal head impulses while viewing near and far targets. We calculated the convergence angles and VOR gains using monocular and binocular methods and compared them with a geometric model. The monocular VOG technique resulted in convergence angle and VOR gain (eye velocity/head velocity) calculations that differed by ∼10% compared with values calculated using the binocular data. The monocular VOG technique can be clinically implemented using any unmodified, commercially available, monocular VOG system, provided its camera can be positioned to track either eye. Many vestibular clinics already have access to such systems. This method makes possible reliable measurement of the near-viewing horizontal angular VOR during the head impulse test, the translational VOR during the head heave test in patients, and the clinical measurement of convergence insufficiency.

Test-Retest Reliability and Response Stability of Gaze Stabilization, Postural Sway, and Dynamic Balance Tests in Persons with Multiple Sclerosis and Controls.

Psychometric properties of tests that assess the angular vestibulo-ocular reflex (aVOR) and vestibulospinal reflex function are currently unknown. This study investigated the test-retest reliability and response stability of gaze stabilization, postural sway, and dynamic balance measures in persons with multiple sclerosis (MS) and controls. Nineteen adults with MS and 14 controls performed passive horizontal head impulses, quiet standing, and dynamic balance tests on two separate occasions. Gaze stabilization measures included aVOR gain, number of compensatory saccades (CSs) per head rotation, CS latency, and gaze position error. Postural sway included sway amplitude and total sway path. Dynamic balance measure included the Functional Gait Assessment. Intraclass correlation coefficient, standard error of measurement (SEM, SEM%), and minimal detectable difference at 95% confidence level were calculated. Test-retest reliability for aVOR gain, CSs per head rotation, and gaze position error was moderate and for each postural sway and dynamic balance measure was good. Low error (SEM, SEM%) for aVOR gain, CS latency, postural sway, and dynamic balance variables and low minimal detectable difference values for aVOR gain and Functional Gait Assessment scores were seen, suggestive of acceptable response stability. These results support the utility of some of the gaze and postural measures for examination and treatment efficacy purposes in people with MS.

New advances regarding adaptation of the vestibulo-ocular reflex.

This is a review summarizing the development of vestibulo-ocular reflex (VOR) adaptation behavior with relevance to rehabilitation over the last 10 years and examines VOR adaptation using head-on-body rotations, specifically the influence of training target contrast, position and velocity error signal, active vs. passive head rotations, and sinusoidal vs. head impulse rotations. This review discusses optimization of the single VOR adaptation training session, consolidation between repeated training sessions, and dynamic incremental VOR adaptation. Also considered are the effects of aging and the roles of the efferent vestibular system, cerebellum, and otoliths on angular VOR adaptation. Finally, this review examines VOR adaptation findings in studies using whole body rotations.

Functional Head Impulse Testing Might Be Useful for Assessing Vestibular Compensation After Unilateral Vestibular Loss.

Background: Loss of the vestibulo-ocular reflex (VOR) affects visual acuity during head movements. Previous studies have shown that compensatory eye-saccades improve visual acuity and that the timing of the saccade is important. Most of the tests involved in testing VOR are made with passive head movement, that do not necessarily reflect the activities of daily living and thus not being proportionate to symptoms and distresses of the patients.Objective: To examine differences between active (self-generated) or passive (imposed by the examiner) head rotations while trying to maintain visual focus on a target.Method: Nine subjects with unilateral total vestibular loss were recruited (4 men and 5 women, mean age 47) and tested with video Head Impulse Test (vHIT) and Head Impulse Testing Device-Functional Test (HITD-FT) during passive and active movements while looking at a target. VOR gain, latencies of covert saccades, frequency of covert saccades and visual acuity were measured and analyzed.Results: Active head-impulses toward the lesioned side resulted in better visual acuity (p = 0.002) compared to conventional passive head-impulses and generated eye-saccades with significantly shorter latencies (p = 0.004). Active movements to the lesioned side generated dynamic visual acuities that were as good as when testing the intact side.Conclusion: Actively generated head impulses resulted in normal dynamic visual acuity, even when performed toward the side of total vestibular loss. This might be attributed to the appearance of short-latency covert saccades. The results show a strong relationship between self-generated movements, latencies of covert saccades and outcome in HITD-FT, i.e., a better dynamic visual function with less retinal slip which is the main function of the VOR. The method of active HITD-FT might be valuable in assessing vestibular compensation and monitoring ongoing vestibular rehabilitation.

Enhanced vestibulo-ocular reflex suppression in dancers during passive high-velocity head impulses.

The vestibulo-ocular reflex (VOR) is responsible for stabilizing images on the fovea during head movements. However, in some situations, one needs to suppress the VOR to be able to follow a target moving along with the head. Evidence suggests that the visual mechanism underlying VOR suppression can be modulated by experience. Unfortunately, the non-visual mechanism underlying VOR suppression has never been examined in dancers and, consequently, it is still unsure whether dance training can enhance eye-head tracking accuracy. The goal of the present study was to look at the influence of dance training on the VOR suppression during passive head impulses. Twenty-four individuals participated, 12 controls and 12 dancers. VOR and VOR suppression were assessed using a head impulse paradigm as well as a suppression head impulse test paradigm (SHIMP) with video head impulse test, respectively. The results suggest that dancers display a significantly reduced VOR gain during the SHIMP at 60ms in comparison to controls. Moreover, dancers with more than 10years of dance training exhibited a significantly reduced VOR gain during the SHIMP at 60ms. Overall, the results suggest that dance training improves VOR suppression, but also modulates VOR suppression abilities. Although studies are needed to shed light on the possible mechanisms involved in the modulation of the VOR gain, the observed changes in dancers' vestibulo-cerebellum and its role in the modulation of the VOR gain makes the cerebellar-vestibular nuclei pathway a possible model to explain the present results.

The functional head impulse test: preliminary data.

The functional head impulse test is a new test of vestibular function based on the ability to recognize the orientation of a Landolt C optotype that briefly appears on a computer screen during passive head impulses imposed by the examiner over a range of head accelerations. Here, we compare its results with those of the video head impulse test on a population of vestibular neuritis patients recorded acutely and after 3months from symptoms onset. The preliminary results presented here show that while both tests are able to identify the affected labyrinth and to show a recovery of vestibular functionality at 3months, the two tests are not redundant, but complementary.

New insights into vestibular-saccade interaction based on covert corrective saccades in patients with unilateral vestibular deficits.

In response to passive high-acceleration head impulses, patients with low vestibulo-ocular reflex (VOR) gains often produce covert (executed while the head is still moving) corrective saccades in the direction of deficient slow phases. Here we examined 23 patients using passive, and 9 also active, head impulses with acute (< 10 days from onset) unilateral vestibular neuritis and low VOR gains. We found that when corrective saccades are larger than 10°, the slow-phase component of the VOR is inhibited, even though inhibition increases further the time to reacquire the fixation target. We also found that 1) saccades are faster and more accurate if the residual VOR gain is higher, 2) saccades also compensate for the head displacement that occurs during the saccade, and 3) the amplitude-peak velocity relationship of the larger corrective saccades deviates from that of head-fixed saccades of the same size. We propose a mathematical model to account for these findings hypothesizing that covert saccades are driven by a desired gaze position signal based on a prediction of head displacement using vestibular and extravestibular signals, covert saccades are controlled by a gaze feedback loop, and the VOR command is modulated according to predicted saccade amplitude. A central and novel feature of the model is that the brain develops two separate estimates of head rotation, one for generating saccades while the head is moving and the other for generating slow phases. Furthermore, while the model was developed for gaze-stabilizing behavior during passively induced head impulses, it also simulates both active gaze-stabilizing and active gaze-shifting eye movements.NEW & NOTEWORTHY During active or passive head impulses while fixating stationary targets, low vestibulo-ocular gain subjects produce corrective saccades when the head is still moving. The mechanisms driving these covert saccades are poorly understood. We propose a mathematical model showing that the brain develops two separate estimates of head rotation: a lower level one, presumably in the vestibular nuclei, used to generate the slow-phase component of the response, and a higher level one, within a gaze feedback loop, used to drive corrective saccades.

Stability of the aVOR to Repeat Head Impulse Testing.

The angular vestibulo-ocular reflex (aVOR) is known to be influenced by factors such as arousal and cognition during traditional vestibular function testing. However, the inherent variability of the aVOR to head impulse testing has not been explicitly examined. The purpose of this study was to determine the variability of the aVOR to active and passive head impulses using the gold standard scleral search coil method to record head and eye rotation. Descriptive. Tertiary referral center. Twenty six healthy control subjects agreed to active and passive horizontal head impulse testing on at least two separate sessions from two unique institutions. An additional 27 individuals with cochlear implantation (CI) underwent passive horizontal and vertical semicircular canal plane head impulse testing. Test sessions were separated from 3 to 210 days in the normal subjects and from 49 to 537 days in the subjects with CI. Reliability of the angular VOR gain (eye velocity/head velocity) over time. In the healthy control subjects, there was no difference in aVOR gain between right and left ears, between session one and session two, or between active (self-generated, 0.99 ± 0.08) or passive (imposed, 1.0 ± 0.08) head impulses. In the patients, we also found the aVOR gain very stable over time. However, the aVOR gains of the patients were different across the semicircular canal planes tested (p < 0.001) with the four vertical semicircular canals having lower aVOR gains than the two horizontal canals. Our data suggest the aVOR gain is quite stable when tested across unique days in healthy controls and patients with auditory-only inner ear pathology.

Pilot study of a new rehabilitation tool: improved unilateral short-term adaptation of the human angular vestibulo-ocular reflex.

Unilateral vestibulo-ocular reflex (VOR) short-term adaptation training causes some increase toward the nonadapting side (~30% of increase on adapting side). We conducted a pilot study to determine if the increase could be reduced by providing a visual stimulus during rotations to the nonadapting side. Unilateral vestibular short-term adaptation is a technique that could increase the ipsilesional VOR response of vestibular patients with unilateral hypofunction. However, this technique results in the VOR response increasing for rotations toward the nonadapting (normal) side, which is undesirable because the VOR will be overcompensatory (causing nonstable vision) during head rotations toward the normal side. We built a portable helmet device that sensed horizontal angular head velocity to generate a visual target that required a preset VOR gain (eye velocity/head velocity) for optimal image stabilization that could be set differently for leftward and rightward head rotations. We tested 10 subjects (six controls and four patients with vestibular hypofunction). We measured the active and passive VOR gain during high-peak-acceleration, unilateral, transient head rotations (head impulses) before and after unilateral VOR adaptation training. In control subjects, for rotations toward the adapting side (target gain = 1.5), the VOR gain increased because of training by 26.1% ± 23.4% during active head impulses and by 14.6% ± 13.0% during passive head impulses. In contrast, for rotations toward the nonadapting side, there were no statistically significant increases. A visual stimulus driving the VOR gain to unity toward the nonadapting side aids unilateral adaptation more so than no visual stimulus.

F01 Vestibular Ocular Reflex Dynamics During Passive Head-impulses: Slow Phase And Quick-eye Movements In Huntington's Disease

Background Angular vestibular ocular reflex (VOR) can be non-invasively and easily assessed with the video head-impulse test (vHIT), allowing the quantification of VOR dynamic parameters (latency and gain) at a physiological frequency domain, as well as the characterisation of quick eye movements (QEM) triggered with head impulses. Here we characterise these findings in HD patients. Methods We explored 18 genetically confirmed HD patients (44.7 ± 8.1 years; male=9), classified as Shoulson and Fahn severity stages 1 (n = 5; pre-symptomatic), 2 (n = 4), 3 (n = 8) and 4 (n = 1) based on their Total Functional Capacity (TFC) scores, and 40 healthy controls (39.9 ± 16.5 years; male=20). We calculated the VOR latency and gain and determined the latency, peak-velocity and occurrence rate of the QEM triggered during and after head impulses. All patients had an oculomotor evaluation (spontaneous nystagmus, smooth pursuit, gaze fixation and saccades) before vHIT testing. Results VOR latency and VOR gain were not different from controls (p > 0.29 for both comparisons). No differences were found between presymptomatic and symptomatic patients. QEM were present in 11 patients and 18 controls, always after the head impulse and compensatory (overt saccades). QEM latency (174.5 ± 41.2 ms), peak velocity (71.58 ± 34.48 o/s) and occurrence rate (0.45 ± 0.29) were not different from controls (p > 0.11 for all comparisons). Though not realising it, 5 of the symptomatic patients majorly failed to direct gaze in darkness, both horizontal as vertically, this correlating with TFC scores (Spearman r = 0.65, p = 0.005). Conclusions VOR seems to be preserved at physiological frequency domains in HD patients, even in more advanced stages of the disease. Although both voluntary saccades and quick phases of nystagmus are known to be slower in HD, quick eye movements triggered with impulses showed no differences in comparison to controls. Gaze failure in darkness may prove beneficial as a biological marker for HD.