Postural compensation for vestibular loss and implications for rehabilitation

Abstract

Purpose: This chapter summarizes the role of the vestibular system in postural control so that specific and effective rehabilitation can be designed that facilitates compensation for loss of vestibular function.

Methods: Patients with bilateral or unilateral loss of peripheral vestibular function are exposed to surface perturbations to quantify automatic postural responses. Studies also evaluated the effects of audio- and vibrotactile-biofeedback to improve stability in stance and gait.

Results: The most important role of vestibular information for postural control is to control orientation of the head and trunk in space with respect to gravitoinertial forces, particularly when balancing on unstable surfaces. Vestibular sensory references are particularly important for postural control at high frequencies and velocities of self-motion, to reduce trunk drift and variability, to provide an external reference frame for the trunk and head in space; and to uncouple coordination of the trunk from the legs and the head-in-space from the body CoM.

Conclusions: The goal of balance rehabilitation for patients with vestibular loss is to help patients 1) use remaining vestibular function, 2) depend upon surface somatosensory information as their primary postural sensory system, 3) learn to use stable visual references, and 4) identify efficient and effective postural movement strategies.

Figure 1

Hypermetria of postural responses to surface translations in cats and humans with bilateral vestibular loss is illustrated. A. Example of semitendinosis EMG response and CoM and CoP response from 3 trials of forward-right diagonal translations in a cat before and after bilateral labyrinthectomy (adapted from Inglis and Macpherson, 1995). B. Group average of scaling surface reactive responses to increasing backward translation velocities from 7 subjects with chronic, bilateral vestibular loss due to ototoxicity and 7 age-matched control subjects and examples of surface reactive torques from a subject with bilateral labyrinthectomy compared with an age-matched control subject. Schematic of potential explanations for postural hypermetria after bilateral vestibular loss. Hypermetric proprioceptive-triggered postural responses could result either from synaptogenesis or increased efficacy of somatosensory inputs to the vestibular nucleus after loss of vestibular inputs or from loss of vestibular input to the cerebellum, resulting in loss of inhibitory drive to proprioceptive pathways involved in automatic postural responses.

Figure 2

A. Comparison of upper trunk orientation variability during sinusoidal surface translations with eyes open in 3 well-compensated, 3-poorly compensated vestibular loss subjects, and 10 control subjects. B. Comparison between 3 well-compensated and 3-poorly compensated vestibular loss subjects in head, trunk and leg orientation variation during sinusoidal surface translations with eyes open. Control subjects all showed head, trunk and leg orientation variation below 1 cm. (Adapted from Buchanan and Horak, 2000).

Figure 3

Effects of light (<100 grams) finger tip touch on postural stability (gain-CoM displacement/surface displacement) during.01, .03, .10, .20, .40 Hz sinusoidal surface rotation in seven subjects with bilateral vestibular loss and seven age-matched control subjects (adapted with permission from Creath, et al, 2002). Fingertip touch reduces postural sway more in subjects with vestibular loss than controls and subjects with vestibular loss benefit most at high frequencies of surface rotation whereas controls benefit most at low frequencies.