Posture, Gait, Quality of Life, and Hearing with a Vestibular Implant

Abstract

Background: Bilateral vestibular hypofunction is associated with chronic disequilibrium, postural instability, and unsteady gait owing to failure of vestibular reflexes that stabilize the eyes, head, and body. A vestibular implant may be effective in alleviating symptoms.

Methods: Persons who had had ototoxic (7 participants) or idiopathic (1 participant) bilateral vestibular hypofunction for 2 to 23 years underwent unilateral implantation of a prosthesis that electrically stimulates the three semicircular canal branches of the vestibular nerve. Clinical outcomes included the score on the Bruininks-Oseretsky Test of Motor Proficiency balance subtest (range, 0 to 36, with higher scores indicating better balance), time to failure on the modified Romberg test (range, 0 to 30 seconds), score on the Dynamic Gait Index (range, 0 to 24, with higher scores indicating better gait performance), time needed to complete the Timed Up and Go test, gait speed, pure-tone auditory detection thresholds, speech discrimination scores, and quality of life. We compared participants' results at baseline (before implantation) with those at 6 months (8 participants) and at 1 year (6 participants) with the device set in its usual treatment mode (varying stimulus pulse rate and amplitude to represent rotational head motion) and in a placebo mode (holding pulse rate and amplitude constant).

Results: The median scores at baseline and at 6 months on the Bruininks-Oseretsky test were 17.5 and 21.0, respectively (median within-participant difference, 5.5 points; 95% confidence interval [CI], 0 to 10.0); the median times on the modified Romberg test were 3.6 seconds and 8.3 seconds (difference, 5.1; 95% CI, 1.5 to 27.6); the median scores on the Dynamic Gait Index were 12.5 and 22.5 (difference, 10.5 points; 95% CI, 1.5 to 12.0); the median times on the Timed Up and Go test were 11.0 seconds and 8.7 seconds (difference, 2.3; 95% CI, -1.7 to 5.0); and the median speeds on the gait-speed test were 1.03 m per second and 1.10 m per second (difference, 0.13; 95% CI, -0.25 to 0.30). Placebo-mode testing confirmed that improvements were due to treatment-mode stimulation. Among the 6 participants who were also assessed at 1 year, the median within-participant changes from baseline to 1 year were generally consistent with results at 6 months. Implantation caused ipsilateral hearing loss, with the air-conducted pure-tone average detection threshold at 6 months increasing by 3 to 16 dB in 5 participants and by 74 to 104 dB in 3 participants. Changes in participant-reported disability and quality of life paralleled changes in posture and gait.

Conclusions: Six months and 1 year after unilateral implantation of a vestibular prosthesis for bilateral vestibular hypofunction, measures of posture, gait, and quality of life were generally in the direction of improvement from baseline, but hearing was reduced in the ear with the implant in all but 1 participant. (Funded by the National Institutes of Health and others; ClinicalTrials.gov number, NCT02725463.).

Conflict of interest statement

Dr. Della Santina and Johns Hopkins University report holding royalty interests in pending and awarded patents related to forms of technology discussed in this article, as well as equity interest in Labyrinth Devices, of which Dr. Della Santina is the founder and chief executive officer. Terms of this arrangement are managed in accordance with Johns Hopkins University policies on conflicts of interest.

Copyright © 2021 Massachusetts Medical Society.

Figures

Figure 1.. Components and Mechanism of a Vestibular Implant.

To bypass damaged hair cells in dysfunctional semicircular canals, a vestibular implant electrically stimulates vestibular nerve branches with the use of electrical current pulses that vary in rate and amplitude depending on head rotation speed and axis. The multichannel vestibular implant system (Panel A) comprises an implanted stimulator with electrodes designed for insertion in the semicircular canals; an external head-worn unit, which senses head rotation and transmits power and command signals to the stimulator with the use of a transcutaneous inductive link; and a power and control unit that houses a battery and a microprocessor that stores stimulus settings and controls pulse timing. To mimic nerve activity patterns that normally represent the speed and three-dimensional axis of head rotation, the system separates head rotational velocity into three components, each aligned with the axis of one semicircular canal, and then stimulates each canal separately (Panel B).

Figure 2.. Changes in Posture and Gait.

Panel A shows within-participant changes in posture tests during both treatment-mode stimulation and placebo-mode stimulation relative to baseline values measured before implantation. Total scores on the balance subtest of the Bruininks–Oseretsky Test of Motor Proficiency range from 0 to 36, with higher scores indicating better performance. Time to failure on the modified Romberg test ranges from 0 to 30 seconds, with longer durations indicating better performance. Panel B shows within-participant changes in gait tests during both treatment-mode stimulation and placebo-mode stimulation relative to baseline values measured before implantation. Scores on the Dynamic Gait Index range from 0 to 24, with higher scores indicating better gait performance. For the Timed Up and Go test, shorter durations indicate better performance. For gait speed, a higher speed indicates better performance. In both panels, data are shown for all eight participants at 6 months after implantation and for six participants (all except Participants 7 and 8) at 1 year after implantation. Gray shading denotes ±1 minimally important difference. Brackets depict the 95% confidence intervals of the medians, which are equivalent to the ranges. The widths of the confidence intervals have not been adjusted for multiple comparisons and cannot be used to infer a treatment effect. The y axis for the Timed Up and Go test is inverted so that a relative increase in performance is upward for all the outcomes.

Figure 3.. Changes in Dizziness, Disability, and Quality of Life.

Panel A shows within-participant changes in assessments of dizziness and disability relative to preimplantation scores. Scores on the Dizziness Handicap Inventory range from 0 to 100, with lower scores indicating a lower respondent-perceived dizziness handicap. Scores on the Vestibular Disorders Activities of Daily Living assessment range from 1 to 10, with lower scores indicating lower respondent-perceived disability. Panel B shows within-participant changes in quality-of-life assessments relative to preimplantation scores. Scores on the health utility index derived from data from the 36-Item Short-Form Health Survey (SF-36 utility) range from 0.3 (worst) to 1 (best). Scores on the Health Utilities Index Mark 3 range from 0 (death) to 1 (perfect health). Data are shown individually for each of the eight participants at 6 months after implantation and for all but two participants (those who most recently underwent implantation) at 1 year. Gray shading denotes changes within ±1 minimally important difference as defined by published standards for each test. Brackets depict the 95% confidence intervals of the medians, which are equivalent to the ranges. The widths of the confidence intervals have not been adjusted for multiple comparisons and cannot be used to infer a treatment effect.