Vestibular Rehabilitation and Otolith Dysfunction

Recent studies suggest that otolith dysfunction is a common finding in individuals with a history of head trauma/blast exposure and/or noise-induced hearing loss. Therefore, otolith dysfunction may be a significant health concern for the Veteran population, and determining optimal intervention strategies for otolith dysfunction is important for VA healthcare. The purpose of this project is to identify optimum stimulus parameters of a novel treatment, off-axis rotation (centrifugation) for otolith dysfunction, in healthy participants.

The primary function of the vestibular (inner ear balance) system is to maintain gaze and postural stability. The vestibular system is comprised of two types of sensory organs (semicircular canals and otolith organs) each with unique contributions to balance. Vestibular Rehabilitation (VR) is the treatment of choice for patients experiencing dizziness, imbalance, and mobility impairments related to vestibular dysfunction. VR typically includes gaze stability exercises, gait and balance training, and general conditioning. Gaze stability exercises were developed based on the concepts of adaptation and substitution with the goal of improving gaze stability by facilitating vestibular compensation of the semicircular canal-mediated vestibulo-ocular reflex (VOR). Many factors that might influence recovery, such as the involvement of different vestibular sensory organs (semicircular canals versus otolith organs) have not been examined to determine their impact on recovery. Most studies examining the effectiveness of VR have used only tests of VOR function (caloric and rotational tests) that measure horizontal semicircular canal to determine vestibular loss. Thus, little is known about interventions to facilitate vestibular compensation of the otolith organs. Recent studies have demonstrated adaptation following otolith organ stimulation using centrifugation (or linear acceleration), but there is no data regarding optimum stimulus parameters. The concept of using centrifugation for otolith adaptation may be similar to using gaze stability exercises for VOR adaptation. Healthy control subjects (n = 5 per experiment) will participate in three separate experiments to determine: (1) the optimum off-axis distance of the rotary chair, (2) the optimum duration of off-axis rotation (OAR), and (3) the optimum OAR stimulation/training schedule.

To determine optimal stimulus parameters for off-axis rotation (OAR), participants progressed through the study starting with off-axis distance (3.5 vs 7.0 cm), off-axis duration (1 vs 3 minutes) and finally training schedule (daily vs biweekly). SVV was measured immediately before and after training. To determine optimal distance, OAR was first performed at 3.5 cm and then 7.0 cm off-axis for 1 minute and the change in SVV compared. Participants trained daily for 5 consecutive days at 3.5 cm followed by a washout period of two weeks, then trained at 7.0 cm. To determine optimal duration, participants were rotated at 3.5 cm off-axis (determined to be optimal) for 3 minutes and the change in SVV was compared to 3.5 cm off-axis for 1 minute. To determine optimal training schedule, participants were rotated at 3.5 cm off-axis for 3 minutes (determined to be optimal) on a biweekly schedule for a total of 5 sessions and change in SVV was compared to the daily training schedule.

Three experiments were performed to identify optimal centrifugation parameters: (1) distance off-axis (3.5 vs 7 cm); (2) duration (1 min vs 3 mins); (3) schedule (daily vs biweekly). The comparisons were all within subjects; i.e., each subject was tested systematically for each centrifugation parameter under both conditions. The change in the outcome measure SVV (from pre- to post-off-axis rotation) for each condition (e.g., 3.5 vs 7 cm) within a parameter was compared.

To determine optimal distance off-axis, participants were rotated in a darkened rotary chair booth with 1 ear positioned 3.5 cm off-axis and the other ear positioned on-axis for 1 minute. Participants received 5 sessions in a 1-week period. Following a 2-week washout period, participants were rotated in a darkened rotary chair booth with 1 ear positioned 7.0 cm off-axis and the other ear positioned on-axis for 1 minute. Participants received 5 sessions in a 1-week period.

To determine optimal duration, after a two week washout period, participants were rotated off-axis at 3.5 cm (determined to be optimal in Exp 1) for 3 minutes. Participants received 5 sessions in a 1-week period.

To determine optimal schedule, after a two week washout period, participants were rotated off-axis at 3.5 cm (determined to be optimal in Exp 1) for 3 minutes (determined to be optimal in Exp 2). Participants received biweekly sessions for a total of 5 sessions.

Static subjective visual vertical (SVV) assesses spatial perception and is influenced by otolith function. Perception of vertical is measured in a darkened room with subject seated upright. The test assesses an individual's ability to adjust a laser line to be parallel with true vertical in the absence of any other visual cues. The start position of line for SVV testing is randomized and participants are instructed to use the track ball to position the line in a vertical position. Five trials are completed, and the software calculates the distance (in degrees) from vertical. The average of the trials is calculated and used for data analysis.

Inclusion Criteria: At least 18 years of age Documented balance or mobility problems, or healthy control without imbalance Otolith dysfunction or healthy control without vestibular dysfunction Exclusion Criteria: Progressive neurological disorders and central vestibular abnormalities Benign paroxysmal positional vertigo Superior semicircular canal dehiscence Middle-ear pathology with conductive hearing loss Lower extremity joint replacement Cognitive impairment (Mini Mental Status Exam < 24/30) Severe depression (geriatric depression scale 10) Severe anxiety (geriatric anxiety inventory 11/30) Best-corrected visual acuity worse than 20/40 in the better eye