Postural compensation for vestibular loss

Abstract

To what extent can remaining sensory information and/or sensory biofeedback (BF) compensate for loss of vestibular information in controlling postural equilibrium? The primary role of the vestibulospinal system is as a vertical reference for control of the trunk in space, with increasing importance as the surface becomes increasingly unstable. Our studies with patients with bilateral loss of vestibular function show that vision or light touch from a fingertip can substitute as a reference for earth vertical to decrease variability of trunk sway when standing on an unstable surface. However, some patients with bilateral loss compensate better than others, and found that those with more complete loss of bilateral vestibular function compensate better than those with measurable vestibulo-ocular reflexes. In contrast, patients with unilateral vestibular loss (UVL) who reweight sensory dependence to rely on their remaining unilateral vestibular function show better functional performance than those who do not increase vestibular weighting on an unstable surface. Light touch of <100 grams or auditory biofeedback can be added as a vestibular vertical reference to stabilize trunk sway during stance. Postural ataxia during tandem gait in patients with UVL is also significantly improved with vibrotactile BF to the trunk, beyond improvements due to practice. Vestibular rehabilitation should focus on decreasing hypermetria, decreasing an overdependence on surface somatosensory inputs, increasing use of any remaining vestibular function, substituting or adding alternative sensory feedback related to trunk sway, and practicing challenging balance tasks on unstable surfaces.

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.