Effect of vestibular rehabilitation on passive dynamic visual acuity

Ocular torsion induced by Coriolis stimulation

OBJECTIVE

The present study aimed to observe and analyze the ocular movements induced by Coriolis stimulation (eccentric pitch while rotating: PWR) that induces Coriolis forces on the vestibular apparatus of healthy human individuals.

METHODS

A total of 31 healthy subjects participated in the study. Eccentric PWR was performed on 27 subjects, by pitching the participants' heads forward and backward at an angle of 30° each on an axis parallel and 7 cm below inter-aural axis, at a frequency of 0.5 Hz while on a chair rotating at a constant angular velocity of 97.2°/s on the earth-vertical axis. Ocular movements during stimulation were recorded using three-dimensional video-oculography. As a subsidiary analysis, 0.5 Hz head roll tilt was used as another stimulus that also induced torsional ocular movements. The forces induced on the vestibular apparatus, and phases of ocular torsion against the stimulus were calculated from the observed data.

RESULTS

In the Coriolis stimulation during rightward yaw rotation, a rightward ocular torsion of 4.8° on average, was observed when the head pitched forward, and the direction of ocular torsion reversed when the head pitched backward. During leftward yaw rotation, these relationships were reversed with an average amplitude of 4.7° The phase of ocular torsion preceded that of Coriolis force by 0.2 s during rightward rotation and 0.14 s during leftward rotation. There were no significant differences in amplitude or phase between the directions of rotation. The phase lead of 0.5 Hz roll-tilt was significantly smaller than that of Coriolis stimulation (p < 0.01).

CONCLUSION

Coriolis stimulation induced a specific pattern of ocular torsion, where its direction and phase suggested that the mechanism likely involved both the otolith and semicircular canals. Further studies may provide a clue to the magnitude of the otolith and semicircular canal contributions.

Efficacy of vestibular rehabilitation program in children with balance disorders and sensorineural hearing loss

OBJECTIVE

Asses the efficacy of a Vestibular-balance rehabilitation program to minimize or reverse balance disability in children with sensorineural hearing loss.

METHOD

Forty-five hearing-impaired children with balance deficits (i.e., variable degrees of sensorineural hearing loss or auditory neuropathy). Thirty-five were rehabilitated with cochlear implants, and ten with hearing aids. Their age ranged from 4 to 10 years old. A Pre-rehab evaluation was done using questionnaires, neuromuscular evaluation, vestibular and balance office testing, and vestibular lab testing (using cVEMP and caloric test). Customized balances, as well as vestibular rehabilitation exercises, have been applied for three months. That was followed by post-rehab assessment, including the Arabic DHI questionnaire, PBS, BESS, HTT, and DVA test.

RESULTS

There was a statistically significant difference in all measured parameters (including the Arabic DHI questionnaire, PBS, BESS, HTT, and DVA test) after rehabilitation.

CONCLUSIONS

Vestibular-balance rehabilitation intervention positively impacts vestibular and balance functions in hearing-impaired children.

Adults are not older adolescents: comparing physical therapy findings among adolescents, young adults and older adults with persistent post-concussive symptoms

OBJECTIVE

Individuals with persistent post-concussive symptoms (PPCS) may present with a myriad of physical symptoms. There is limited research available comparing the presence of examination findings among individuals with PPCS from different age groups.

METHODS

Retrospective case-control chart review of 481 patients with PPCS and 271 non-trauma controls. Physical assessments were categorized as ocular, cervical, and vestibular/balance. Differences in presentation were compared between PPCS and controls as well as between individuals with PPCS in three age groups: adolescents, young adults, and older adults.

RESULTS

All three PPCS groups had more abnormal oculomotor findings than their age-matched counterparts. When comparing PPCS patients from different age groups, no differences were seen in prevalence of abnormal smooth pursuits or saccades; however, adolescents with PPCS had more abnormal cervical findings and a lower prevalence of abnormal NPC, vestibular and balance findings.

CONCLUSION

Patients with PPCS presented with a different constellation of clinical findings based on their age. Adolescents were more likely to demonstrate evidence of cervical injury compared to younger and older adults, and adults were more likely to present with vestibular findings and impaired NPC. Adults with PPCS were more likely to present with abnormal oculomotor findings compared to adults with non-traumatic causes of dizziness.

The minimal clinically important difference for gait speed in significant unilateral vestibular hypofunction after vestibular rehabilitation

Gait speed is a valid measure of both physical function and vestibular health. Vestibular rehabilitation is useful to improve gait speed for patients with vestibular hypofunction, yet there is little data to indicate how changes in gait speed reflect changes in patient-reported health outcomes. We determined the minimal clinically important difference in the gait speed of patients with unilateral vestibular hypofunction, mostly due to deafferentation surgery, as anchored to the Dizziness Handicap Index and the Activities Balance Confidence scale, validated using regression analysis, change difference, receiver-operator characteristic curve, and average change methods. After six weeks of vestibular rehabilitation, a change in gait speed from 0.20 to 0.34 m/s with 95% confidence was required for the patients to perceive a significant reduction in perception of dizziness and improved balance confidence.

Rehabilitation to Improve Gaze and Postural Stability in People With Multiple Sclerosis: A Randomized Clinical Trial

BACKGROUND

People with multiple sclerosis (PwMS) frequently experience dizziness and imbalance that may be caused by central vestibular system dysfunction. Vestibular rehabilitation may offer an approach for improving dysfunction in these people.

OBJECTIVE

To test the efficacy of a gaze and postural stability (GPS) retraining intervention compared to a strength and endurance (SAE) intervention in PwMS.

METHODS

About 41 PwMS, with complaints of dizziness or history of falls, were randomized to either the GPS or SAE groups. Following randomization participants completed 6-weeks of 3×/week progressive training, delivered one-on-one by a provider. Following intervention, testing was performed at the primary (6-weeks) and secondary time point (10-weeks). A restricted maximum likelihood estimation mixed effects model was used to examine changes in the primary outcome of the Dizziness Handicap Inventory (DHI) between the 2 groups at the primary and secondary time point. Similar models were used to explore secondary outcomes between groups at both timepoints.

RESULTS

Thirty-five people completed the study (17 GPS; 18 SAE). The change in the DHI at the primary time point was not statistically different between the GPS and SAE groups (mean difference = 2.33 [95% CI -9.18, 12.85]). However, both groups demonstrated significant improvement from baseline to 6-weeks (GPS -8.73; SAE -7.31). Similar results were observed for secondary outcomes and at the secondary timepoint.

CONCLUSIONS

In this sample of PwMS with complaints of dizziness or imbalance, 6-weeks of GPS training did not result in significantly greater improvements in dizziness handicap or balance compared to 6-weeks of SAE training.

Evaluating the Association Between Hearing Loss and Falls in Adults With Vestibular Dysfunction or Nonvestibular Dizziness

OBJECTIVES

Although emerging evidence suggests that hearing loss (HL) is an independent risk factor for falls, it is unclear how HL may impact falls risk in adults with vestibular dysfunction and nonvestibular dizziness. The purpose of this study was to characterize the impact of HL on falls in patients with vestibular dysfunction and nonvestibular dizziness relative to a group of patients without dizziness. In addition, this study aimed to evaluate whether there was an interactive effect between HL and vestibular dysfunction or nonvestibular dizziness on the odds of falling.

DESIGN

The authors conducted a retrospective cross-sectional study of 2,750 adult patients with dizziness evaluated at a tertiary care center vestibular clinic between June 1, 2015, and October 7, 2020. Only patients with available self-reported falls status, as extracted from the electronic medical record, were included. Patients were classified into the following diagnostic groups based on rotary chair testing and videonystagmography: benign paroxysmal positional vertigo (BPPV, n = 255), unilateral vestibular hypofunction (UVH, n = 456), bilateral vestibular hypofunction (BVH, n = 38), central dysfunction (n = 208), multiple diagnoses (n = 109), and dizzy, nonvestibular (n = 1,389). A control group of patients without dizziness (n = 295) was identified by a random sample of audiology patients. Degree of HL was characterized by the 4-frequency pure tone average (PTA) (0.5, 1, 2, and 4 kHz) of the better hearing ear. Demographic variables, comorbidities, cognitive impairment status, and falls-associated medications were extracted from the electronic medical record and included as covariates during analysis. Potential associations between PTA and falls status and possible interactions between diagnostic group and PTA were explored using multivariate logistic regression.

RESULTS

The BVH and central dysfunction groups had the highest rates of self-reported falls at 26.3% and 26.9%, respectively. The control group had the lowest rate of self-reported falls at 6.4%. With the exception of the multiple diagnoses group, all diagnostic groups had elevated odds of falling compared with the control group, when adjusting for demographics, comorbidities, cognitive impairment status, and falls-associated medications. There was no significant association between degree of HL and falls status (odds ratio [OR] = 1.02; 95% confidence interval [CI] = 0.93, 1.11; p = 0.713) when adjusting for diagnostic group and all other covariates. Furthermore, there were no significant interactions between diagnostic group and degree of HL on the odds of falling.

CONCLUSIONS

These results indicate that HL was not associated with falls in patients with vestibular dysfunction or nonvestibular dizziness, while adjusting for demographics, comorbidities, and falls-associated medications. There was no significant interactive effect observed between HL and vestibular dysfunction or nonvestibular dizziness on the odds of falling. As previously reported, vestibular dysfunction and nonvestibular dizziness were independently associated with falls relative to a group of patients without dizziness. A population-based study utilizing more robust falls data is needed to explore a potential association between HL and falls in those with vestibular dysfunction.

Detection of VOR dysfunction during the gaze stabilization test: Does target size matter?

BACKGROUND

The Gaze Stabilization Test (GST) identifies vestibulo-ocular reflex (VOR) dysfunction using a decline in target recognition with increasing head velocity, but there is no consensus on target (optotype) size above static visual acuity.

OBJECTIVE

To determine the optimal initial optotype size above static visual acuity to be used during the GST in subjects with unilateral vestibular dysfunction and healthy individuals.

METHODS

Eight subjects with unilateral vestibular dysfunction (UVD) and 19 age-matched, healthy control subjects were studied with the standard GST protocol using two optotype sizes, 0.2 and 0.3 logMAR above static visual acuity (ΔlogMAR). Maximal head velocity achieved while maintaining fixation on both optotypes was measured. Sensitivity, specificity and receiver-operator characteristic area under the curve (ROC AUC) analyses were performed to determine the optimal head velocity cut off point for each optotype, based on ability to identify the lesioned side of the UVD group from the control group.

RESULTS

There was a significant difference in maximal head velocity between the UVD group and control group using 0.2 ΔlogMAR (p = 0.032) but not 0.3 ΔlogMAR (p = 0.061). While both targets produced similar specificities (90%) for distinguishing normal from subjects with UVD, 0.2 ΔlogMAR targets yielded higher sensitivity (75%) than 0.3 logMAR (63%) and accuracy (86% vs 80%, respectively) in detecting the lesioned side in subjects with UVD versus controls with maximal head velocities≤105 deg/s (p = 0.017). Furthermore, positive likelihood ratios were nearly twice as high when using 0.2 ΔlogMAR targets (+ LR 10) compared to 0.3 ΔlogMAR (+ LR 6.3).

CONCLUSION

The 0.2 ΔlogMAR optotype demonstrated significantly superior identification of subjects with UVD, better sensitivity and positive likelihood ratios than 0.3 ΔlogMAR for detection of VOR dysfunction. Using a target size 0.2logMAR above static visual acuity (ΔlogMAR) during GST may yield better detection of VOR dysfunction to serve as a baseline for gaze stabilization rehabilitation therapy.

Rehabilitation to improve gaze and postural stability in people with multiple sclerosis: study protocol for a prospective randomized clinical trial

BACKGROUND

The use of vestibular rehabilitation principles in the management of gaze and postural stability impairments in people with multiple sclerosis (PwMS) has shown promise in pilot work completed in our lab and in a recently published randomized clinical trial (RCT). However, further work is needed to fully quantify the gaze and postural impairments present in people with multiple sclerosis and how they respond to rehabilitation.

METHODS/DESIGN

The study is a single blind RCT designed to examine the benefit of a gaze and postural stability (GPS) intervention program compared to a standard of care (SOC) rehabilitation program in dizzy and balance impaired PwMS. Outcomes will be collected across the domains of body structure and function, activity, and participation as classified by the World Health Organization International Classification of Functioning, Disability, and Health (ICF). Our primary outcomes are the Dizziness Handicap Inventory (DHI) and the Functional Gait Assessment (FGA). Secondary outcomes include other measures of gaze and postural stability, fatigue, and functional mobility. Participants who are interested and eligible for enrollment will be consented prior to completing a baseline assessment. Following the baseline assessment each participant will be randomized to either the GPS or SOC intervention group and will complete a 6 week treatment period. During the treatment period, both groups will participate in guided exercise 3x/week. Following the treatment period participants will be asked to return for a post-treatment evaluation and again for a follow-up assessment 1 month later. We anticipate enrolling 50 participants.

DISCUSSION

This study will be an innovative RCT that will utilize gaze and postural stability metrics to assess the efficacy of vestibular rehabilitation in PwMS. It will build on previous work by examining measures across the ICF and improve the current evidence base for treating PwMS.

TRIAL REGISTRATION

ClinicalTrials.gov, May 29th 2018, NCT03521557 .

Changes in Measures of Cervical Spine Function, Vestibulo-ocular Reflex, Dynamic Balance, and Divided Attention Following Sport-Related Concussion in Elite Youth Ice Hockey Players

BACKGROUND

Concussion is a commonly occurring injury. The extent to which the cervical spine, vestibulo-ocular reflex (VOR), dynamic balance, and divided attention are affected following concussion is not well understood.

OBJECTIVES

To evaluate acute changes in measures of (1) cervical spine function, (2) VOR function, (3) dynamic balance, and (4) tasks of divided attention in elite youth ice hockey players following a sport-related concussion.

METHODS

In this prospective cohort study, elite 13- to 17-year-old ice hockey players completed cervical spine measures (cervical flexor endurance test, head perturbation test, anterolateral strength, cervical flexion rotation test, joint position error), VOR function tests (head thrust test, dynamic visual acuity [clinical and computerized]), dynamic balance tests (Functional Gait Assessment), and divided-attention tasks (walking-while-talking test) both in the preseason and following concussion.

RESULTS

At least 1 test was completed by 69 of 97 (71%) players (a maximum of 55 for any 1 test) at both preseason and immediately following concussion (median, 4 days post concussion). After Bonferroni corrections (α = .00625), using Wilcoxon signed-rank tests, cervical spine measures were significantly worse following concussion compared to baseline (cervical flexor endurance test: z = -5.20, P<.001; anterolateral neck strength: z_left = -5.36, P<.001 and z_right = -5.45, P<.001; and head perturbation test: z = -4.36, P<.001). Time taken to complete a complex task of divided attention relative to normal walking speed was faster (improved) compared to the preseason (z = -2.59, P<.01). There was no change in VOR or dynamic balance following concussion.

CONCLUSION

Measures of cervical spine function and divided attention were altered following concussion. However, tests of VOR and dynamic balance were not significantly different from baseline. Future research to evaluate the mechanism underlying these changes is warranted. J Orthop Sports Phys Ther 2018;48(12):974-981. Epub 27 Jul 2018. doi:10.2519/jospt.2018.8258.

Head-Movement-Emphasized Rehabilitation in Bilateral Vestibulopathy

Objective: Although there is evidence that vestibular rehabilitation is useful for treating chronic bilateral vestibular hypofunction (BVH), the mechanisms for improvement, and the reasons why only some patients improve are still unclear. Clinical rehabilitation results and evidence fromeye-head control in vestibular deficiency suggest that headmovement is a crucial element of vestibular rehabilitation. In this study, we assess the effects of a specifically designed head-movement-based rehabilitation program on dynamic vision, and explore underlying mechanisms.

Methods: Two adult patients (patients 1 and 2) with chronic BVH underwent two 4-week interventions: (1) head-movement-emphasized rehabilitation (HME) with exercises based on active head movements, and (2) eye-movement-only rehabilitation (EMO), a control intervention with sham exercises without head movement. In a double-blind crossover design, the patients were randomized to first undergo EMO (patient 1) and–after a 4-week washout–HME, and vice-versa (patient 2). Before each intervention and after a 4-week follow-up patients’ dynamic vision, vestibulo-ocular reflex (VOR) gain, as well as re-fixation saccade behavior during passive headmotion were assessed with the head impulse testing device–functional test (HITD-FT).

Results: HME, not EMO, markedly improved perception with dynamic vision during passive head motion (HITD-FT score) increasing from 0 to 60% (patient 1) and 75% (patient 2). There was a combination of enhanced VOR, as well as improved saccadic compensation.

Conclusion: Head movement seems to be an important element of rehabilitation for BVH. It improves dynamic vision with a combined VOR and compensatory saccade enhancement.

The interacting effects of treadmill walking and different types of visuospatial cognitive task: Discriminating dual task and age effects

OBJECTIVE

The objective of this study is to examine the influence that visuospatial cognitive tasks have on gait function during DT treadmill walking, and as a function of age. Conversely, to examine the influence that walking has on executive functions involving visuospatial processing.

METHODS

Twenty-five young (26±6.1years) and 25 older adults (76±3.9) performed different types of computerized visuomotor (VM) tracking and visuospatial cognitive tasks (VCG) while standing and treadmill walking. Spatiotemporal gait variables, average values and co-efficient of variation (COV) were obtained from 40 consecutive steps during single- and dual-task walk trials. Performance-based measures of the VM and VCG task were obtained during standing and walking.

RESULTS

VM dual-task walking had a significant effect on gait measures in the young age group (YG), but no DT effect was observed in the old age group (OG). Visuomotor tracking performance, however, was significantly reduced in the OG as compared to the YG when tested in both standing and walking. The opposite was true for VCG; a significant DT effect on gait performance was observed in the OG, but no DT effect was observed in the YG. Success rate of the VCG task decreased during walking, but only for OG.

CONCLUSION

Controlling gait speed and objective evaluation of the visuospatial cognitive tasks helps to determine the level of engagement in the DT tasks. This is important in order to determine the strategies used during the DT test protocols, i.e. cross-domain interference.

Computerized Dual-Task Testing of Gait and Visuospatial Cognitive Functions; Test-Retest Reliability and Validity

The common occurrence of age decline in mobility and cognition does cause a decrease in the level of physical activity and an increased falls risk. Consequently, dual -task (DT) assessment that simultaneously addresses both mobility skills and cognitive functions are important because, continued difficulties and fall injuries will have a sizable impact in this population. The first objective of the present study was to assess test-retest reliability of a computerized DT treadmill walking protocol and concurrent outcome measures of gait and visuospatial executive function in a group of healthy older adults. Secondly, discriminative validity was evaluated by examining the effect of DT conditions (single task vs. dual-task) on; (a) spatiotemporal gait measures (average and coefficient of variation) and (b) visuomotor and visuospatial executive performance measures. Twenty-five community-dwelling individuals median age 65 (range 61-67) were recruited from a Fitness Facility. Participants performed a computerized visuomotor tracking task and a visuospatial executive game task in standing and while treadmill walking. Testing was conducted on two occasions, 1 week apart. Moderate to high test-retest reliability (ICC values of 0.65-0.88) were observed for spatiotemporal gait variables. No significant differences between the group means were observed between test periods in any gait variable. Moderate test-retest reliability (ICC values of 0.6-0.65) was observed for measures of visuomotor and visuospatial executive performance during treadmill walking. Significant DT effects were observed for both spatiotemporal gait variables and visuospatial executive performance measures. This study demonstrates the reliability and reproducibility of the computer-based assessment tool for dual task treadmill walking. The high to moderate ICC values and the lack of systematic errors in the measures indicate that this tool has the ability to repeatedly record reliable data from community-dwelling older adults. The present computerized dual-task protocols broaden the types of standardized visuomotor and visuospatial executive activities for use with DT treadmill walking that has previously been reported.

A new method to improve the imbalance in chronic unilateral vestibular loss: the organization of refixation saccades

CONCLUSION

VOR adaptation and organization of refixation saccades in a gathered pattern is a process that can be artificially induced in patients with unilateral vestibular loss who have not developed it naturally, improving imbalance and vestibular disability.

OBJECTIVE

To test that temporary grouping of refixation saccades should be linked to better clinical status without gain recovery.

METHODS

A training to induce the refixation saccades into gathered fashion is performed. The outcome measures are handicap level measured by the dizziness handicap index (DHI) and refixation saccades organization pattern measured by a numeric score called 'PR' given by a software developed by the authors. Analysis is done before the training and 1 and 3 months after ending, Non-parametric tests were used for statistical analysis.

RESULTS

This study has included 10 healthy subjects (four males, six females), and 16 patients with chronic unsteadiness due to unilateral vestibular loss (nine vestibular neuritis, four post-surgical vestibular schwannoma, and three cases after intra-tympanic gentamycin in patients with Ménière's disease). The reduction in the DHI score was significant at 1 (p = 0.028) and 3 months (p = 0.042) post-treatment. Also, statistically significant differences were found between the PR score before and PR score 1 (p = 0.005) and 3 months after the treatment (p = 0.003).

Games and Telerehabilitation for Balance Impairments and Gaze Dysfunction: Protocol of a Randomized Controlled Trial

BACKGROUND

Digital media and gaming have received considerable interest from researchers and clinicians as a model for learning a broad range of complex tasks and facilitating the transfer of skills to daily life. These emerging rehabilitation technologies have the potential to improve clinical outcomes and patient participation because they are engaging, motivating, and accessible. Our research goal is to develop preventative and therapeutic point-of-care eHealth applications that will lead to equivalent or better long-term health outcomes and health care costs than existing programs. We have produced a novel computer-aided tele-rehabilitation platform that combines computer game-based exercises with tele-monitoring.

OBJECTIVE

Compare the therapeutic effectiveness of an in-home, game-based rehabilitation program (GRP) to standard care delivered in an outpatient physical therapy clinic on measures of balance, gaze control, dizziness, and health-related quality of life.

METHODS

A randomized, controlled, single-blind pilot trial will be conducted. Fifty-six participants with a diagnosis of peripheral vestibular disorder will be randomly assigned to either usual physical therapy (comparator group) or to a game-based intervention (experimental group). Measures to be assessed will include gaze control, dynamic balance, and self-reported measures of dizziness.

RESULTS

The project was funded and enrollment was started in August 2014. To date, 36 participants have been enrolled. There have been 6 drop-outs. It is expected that the study will be completed January 2016 and the first results are expected to be submitted for publication in Spring of 2016.

CONCLUSIONS

A successful application of this rehabilitation program would help streamline rehabilitation services, leverage therapist time spent with clients, and permit regular practice times at the client's convenience.

TRIAL REGISTRATION

Clinicaltrials.gov: NCT02134444; https://clinicaltrials.gov/ct2/show/NCT02134444 (Archived by WebCite at http://www.webcitation.org/6cE18bqqY).

Treatment of Menière's Disease

OPINION STATEMENT

Diagnosis of Menière's disease is made with a characteristic patient history, including discrete episodes of vertigo lasting 20 min or longer, accompanied by sensorineural hearing loss, which is typically low frequency at first, aural fullness, and tinnitus. Workup includes audiometry, a contrast enhanced MRI of the internal auditory canals, and exclusion of other diseases that can produce similar symptoms, like otosyphilis, autoimmune inner ear disease, perilymphatic fistula, superior semicircular canal syndrome, Lyme disease, multiple sclerosis, vestibular paroxysmia, and temporal bone tumors. A history of migraine should be sought as well because of a high rate of co-occurrence (Rauch, Otolaryngol Clin North Am 43:1011-1017, 2010). Treatment begins with conservative measures, including low salt diet, avoidance of stress and caffeine, and sleep hygiene. Medical therapy with a diuretic is the usual next step. If that fails to control symptoms, then the options of intratympanic (IT) steroids and betahistine are discussed. Next tier treatments include the Meniett device and endolymphatic sac surgery, but the efficacy of both is controversial. If the above measures fail to provide symptomatic control of vertigo, then ablative therapies like intratympanic gentamicin are considered. Rarely, vestibular nerve section or labyrinthectomy is considered for a patient with severe symptoms who does not show a reduction in vestibular function with gentamicin. Benzodiazepines and anti-emetics are used for symptomatic control during vertigo episodes. Rehabilitative options for unilateral vestibular weakness include physical therapy and for unilateral hearing loss include conventional hearing aids, contralateral routing of sound (CROS) and osseointegrated hearing aids.

The effect of vestibulo-ocular reflex deficits and covert saccades on dynamic vision in opioid-induced vestibular dysfunction

Patients with bilateral vestibular dysfunction cannot fully compensate passive head rotations with eye movements, and experience disturbing oscillopsia. To compensate for the deficient vestibulo-ocular reflex (VOR), they have to rely on re-fixation saccades. Some can trigger “covert” saccades while the head still moves; others only initiate saccades afterwards. Due to their shorter latency, it has been hypothesized that covert saccades are particularly beneficial to improve dynamic visual acuity, reducing oscillopsia. Here, we investigate the combined effect of covert saccades and the VOR on clear vision, using the Head Impulse Testing Device – Functional Test (HITD-FT), which quantifies reading ability during passive high-acceleration head movements. To reversibly decrease VOR function, fourteen healthy men (median age 26 years, range 21–31) were continuously administrated the opioid remifentanil intravenously (0.15 µg/kg/min). VOR gain was assessed with the video head-impulse test, functional performance (i.e. reading) with the HITD-FT. Before opioid application, VOR and dynamic reading were intact (head-impulse gain: 0.87±0.08, mean±SD; HITD-FT rate of correct answers: 90±9%). Remifentanil induced impairment in dynamic reading (HITD-FT 26±15%) in 12/14 subjects, with transient bilateral vestibular dysfunction (head-impulse gain 0.63±0.19). HITD-FT score correlated with head-impulse gain (R = 0.63, p = 0.03) and with gain difference (before/with remifentanil, R = −0.64, p = 0.02). One subject had a non-pathological head-impulse gain (0.82±0.03) and a high HITD-FT score (92%). One subject triggered covert saccades in 60% of the head movements and could read during passive head movements (HITD-FT 93%) despite a pathological head-impulse gain (0.59±0.03) whereas none of the 12 subjects without covert saccades reached such high performance. In summary, early catch-up saccades may improve dynamic visual function. HITD-FT is an appropriate method to assess the combined gaze stabilization effect of both VOR and covert saccades (overall dynamic vision), e.g., to document performance and progress during vestibular rehabilitation.

Compensatory saccade differences between outward versus inward head impulses in chronic unilateral vestibular hypofunction

The horizontal head impulse test (HIT) is a valuable clinical tool that can help identify peripheral vestibular hypofunction by the refixation (compensatory) saccade that returns the eyes to the target of interest after the head has stopped. We asked if there were differences in the compensatory saccade responses during the HIT when the head was rotated away or toward straight ahead (outward versus inward). We also investigated the influence of a fixation target. Using scleral search-coils, we tested five patients with chronic unilateral vestibular hypofunction (UVH) and three healthy control subjects. In UVH patients, the latencies of both overt and covert saccades were longer when the head was rotated inward from an initially eccentric position, regardless of a visual target. The proportion of HIT with covert saccades was independent of a visual target. In control subjects no compensatory saccades were observed and there were no differences in either angular vestibulo-ocular reflex gain or latency between inward and outward HIT. Our data suggest that inward applied HIT in chronic UVH is more likely to include an overt compensatory saccade based on its lengthened latency. Neither latency nor the occurrence of covert compensatory saccades during HIT depended on a visual target, suggesting they have become a learned behavior in response to chronic UVH.

Sleep deprivation has no effect on dynamic visual acuity in military service members who are healthy

BACKGROUND

The risk of traumatic brain injury (TBI) and comorbid posttraumatic dizziness is elevated in military operational environments. Sleep deprivation is known to affect a service member's performance while deployed, although little is known about its effects on vestibular function. Recent findings suggest that moderate acceleration step rotational stimuli may elicit a heightened angular vestibulo-ocular reflex (aVOR) response relative to low-frequency sinusoidal stimuli after 26 hours of sleep deprivation. There is concern that a sleep deprivation-mediated elevation in aVOR function could confound detection of comorbid vestibular pathology in service members with TBI. The term "dynamic visual acuity" (DVA) refers to an individual's ability to see clearly during head movement and is a behavioral measure of aVOR function. The Dynamic Visual Acuity Test (DVAT) assesses gaze instability by measuring the difference between head-stationary and head-moving visual acuity.

OBJECTIVE

The purpose of this study was to investigate the effects of 26 hours of sleep deprivation on DVA as a surrogate for aVOR function.

DESIGN

This observational study utilized a repeated-measures design.

METHODS

Twenty soldiers with no history of vestibular insult or head trauma were assessed by means of the DVAT at angular head velocities of 120 to 180°/s. Active and passive yaw and pitch impulses were obtained before and after sleep deprivation.

RESULTS

Yaw DVA remained unchanged as the result of sleep deprivation. Active pitch DVA diminished by -0.005 LogMAR (down) and -0.055 LogMAR (up); passive pitch DVA was degraded by -0.06 LogMAR (down) and -0.045 LogMAR (up).

LIMITATIONS

Sample homogeneity largely confounded accurate assessment of test-retest reliability in this study, resulting in intraclass correlation coefficients lower than those previously reported.

CONCLUSIONS

Dynamic visual acuity testing in soldiers who are healthy revealed no change in gaze stability after rapid yaw impulses and subclinical changes in pitch DVA after sleep deprivation. Findings suggest that DVA is not affected by short-term sleep deprivation under clinical conditions.

Getting ahead of oneself: anticipation and the vestibulo-ocular reflex

Compensatory counter-rotations of the eyes provoked by head turns are commonly attributed to the vestibulo-ocular reflex (VOR). A recent study in guinea pigs demonstrates, however, that this assumption is not always valid. During voluntary head turns, guinea pigs make highly accurate compensatory eye movements that occur with zero or even negative latencies with respect to the onset of the provoking head movements. Furthermore, the anticipatory eye movements occur in animals with bilateral peripheral vestibular lesions, thus confirming that they have an extra vestibular origin. This discovery suggests the possibility that anticipatory responses might also occur in other species including humans and non-human primates, but have been overlooked and mistakenly identified as being produced by the VOR. This review will compare primate and guinea pig vestibular physiology in light of these new findings. A unified model of vestibular and cerebellar pathways will be presented that is consistent with current data in primates and guinea pigs. The model is capable of accurately simulating compensatory eye movements to active head turns (anticipatory responses) and to passive head perturbations (VOR induced eye movements) in guinea pigs and in human subjects who use coordinated eye and head movements to shift gaze direction in space. Anticipatory responses provide new evidence and opportunities to study the role of extra vestibular signals in motor control and sensory-motor transformations. Exercises that employ voluntary head turns are frequently used to improve visual stability in patients with vestibular hypofunction. Thus, a deeper understanding of the origin and physiology of anticipatory responses could suggest new translational approaches to rehabilitative training of patients with bilateral vestibular loss.

Effects of biphasic current pulse frequency, amplitude, duration, and interphase gap on eye movement responses to prosthetic electrical stimulation of the vestibular nerve

An implantable prosthesis that stimulates vestibular nerve branches to restore sensation of head rotation and vision-stabilizing reflexes could benefit individuals disabled by bilateral loss of vestibular (inner ear balance) function. We developed a prosthesis that partly restores normal function in animals by delivering pulse frequency modulated (PFM) biphasic current pulses via electrodes implanted in semicircular canals. Because the optimal stimulus encoding strategy is not yet known, we investigated effects of varying biphasic current pulse frequency, amplitude, duration, and interphase gap on vestibulo-ocular reflex (VOR) eye movements in chinchillas. Increasing pulse frequency increased response amplitude while maintaining a relatively constant axis of rotation. Increasing pulse amplitude (range 0- 325 μA) also increased response amplitude but spuriously shifted eye movement axis, probably due to current spread beyond the target nerve. Shorter pulse durations (range 28- 340 μs) required less charge to elicit a given response amplitude and caused less axis shift than longer durations. Varying interphase gap (range 25- 175 μs) had no significant effect. While specific values reported herein depend on microanatomy and electrode location in each case, we conclude that PFM with short duration biphasic pulses should form the foundation for further optimization of stimulus encoding strategies for vestibular prostheses intended to restore sensation of head rotation.

Characterizing high-velocity angular vestibulo-ocular reflex function in service members post-blast exposure

Blasts (explosions) are the most common mechanism of injury in modern warfare. Traumatic brain injury (TBI) and dizziness are common sequelae associated with blasts, and many service members (SMs) report symptoms worsen with activity. The purpose of this study was to measure angular vestibulo-ocular reflex gain (aVOR) of blast-exposed SMs with TBI during head impulse testing. We also assessed their symptoms during exertion. Twenty-four SMs recovering from TBI were prospectively assigned to one of two groups based on the presence or absence of dizziness. Wireless monocular scleral search coil and rate sensor were used to characterize active and passive yaw and pitch head and eye rotations. Visual analog scale (VAS) was used to monitor symptoms during fast walking/running. For active yaw head impulses, aVOR gains were significantly lower in the symptomatic group (0.79 ± 0.15) versus asymptomatic (0.87 ± 0.18), but not for passive head rotation. For pitch head rotation, the symptomatic group had both active (0.915 ± 0.24) and passive (0.878 ± 0.22) aVOR gains lower than the asymptomatic group (active 1.03 ± 0.27, passive 0.97 ± 0.23). Some SMs had elevated aVOR gain. VAS scores for all symptoms were highest during exertion. Our data suggest symptomatic SMs with TBI as a result of blast have varied aVOR gain during high-velocity head impulses and provide compelling evidence of pathology affecting the vestibular system. Potential loci of injury in this population include the following: disruption of pathways relaying vestibular efference signals, differential destruction of type I vestibular hair cells, or selective damage to irregular afferent pathways-any of which may explain the common discrepancy between reports of vestibular-like symptoms and laboratory testing results. Significantly reduced pitch aVOR in symptomatic SMs and peak symptom severity during exertional testing support earlier findings in the chronic blast-exposed active duty SMs.

High-velocity angular vestibulo-ocular reflex adaptation to position error signals

BACKGROUND AND PURPOSE

Vestibular rehabilitation strategies including gaze stabilization exercises have been shown to increase gain of the angular vestibulo-ocular reflex (aVOR) using a retinal slip error signal (ES). The identification of additional ESs capable of promoting substitution strategies or aVOR adaptation is an important goal in the management of vestibular hypofunction. Position ESs have been shown to increase both aVOR gain and recruitment of compensatory saccades (CSs) during passive whole body rotation. This may be a useful compensatory strategy for gaze instability during active head rotation as well. In vestibular rehabilitation, the imaginary target exercise is often prescribed to improve gaze stability. This exercise uses a position ES; however, the mechanism for its effect has not been investigated. We compared aVOR gain adaptation using 2 types of small position ES: constant versus incremental.

METHODS

Ten subjects with normal vestibular function were assessed with unpredictable and active head rotations before and after a 20-minute training session. Subjects performed 9 epochs of 40 active, high-velocity head impulses using a position ES stimulus to increase aVOR gain.

RESULTS

Five subjects demonstrated significant aVOR gain increases with the constant-position ES (mean, 2%; range, -18% to 12%) compared with another 5 subjects showing significant aVOR gain increases to the incremental-position ES (mean, 3.7%; range, -2% to 22.6%). There was no difference in aVOR gain adaptation or CS recruitment between the 2 paradigms.

DISCUSSION AND CONCLUSION

These findings suggest that some subjects can increase their aVOR gain in response to high-velocity active head movement training using a position ES. The primary mechanism for this seems to be aVOR gain adaptation because CS use was not modified. The overall low change in aVOR gain adaptation with position ES suggests that retinal slip is a more powerful aVOR gain modifier.

Vestibulo-ocular reflex responses to a multichannel vestibular prosthesis incorporating a 3D coordinate transformation for correction of misalignment

There is no effective treatment available for individuals unable to compensate for bilateral profound loss of vestibular sensation, which causes chronic disequilibrium and blurs vision by disrupting vestibulo-ocular reflexes that normally stabilize the eyes during head movement. Previous work suggests that a multichannel vestibular prosthesis can emulate normal semicircular canals by electrically stimulating vestibular nerve branches to encode head movements detected by mutually orthogonal gyroscopes affixed to the skull. Until now, that approach has been limited by current spread resulting in distortion of the vestibular nerve activation pattern and consequent inability to accurately encode head movements throughout the full 3-dimensional (3D) range normally transduced by the labyrinths. We report that the electrically evoked 3D angular vestibulo-ocular reflex exhibits vector superposition and linearity to a sufficient degree that a multichannel vestibular prosthesis incorporating a precompensatory 3D coordinate transformation to correct misalignment can accurately emulate semicircular canals for head rotations throughout the range of 3D axes normally transduced by a healthy labyrinth.