Asymmetry of balance responses to monaural galvanic vestibular stimulation in subjects with vestibular schwannoma

Abstract

Objective: We investigated the potential of galvanic vestibular stimulation (GVS) to quantify lateralised asymmetry of the vestibulospinal pathways by measuring balance responses to monaural GVS in 10 subjects with vestibular schwannoma and 22 healthy control subjects.

Methods: Subjects standing without vision were stimulated with 3 s, 1 mA direct current stimuli delivered monaurally. The mean magnitude and direction of the evoked balance responses in the horizontal plane were measured from ground-reaction forces and from displacement and velocity of the trunk. Vestibular-evoked myogenic potentials (VEMPs) to 500 Hz air and bone-conducted tones were also recorded.

Results: In healthy subjects, the magnitudes of the force, velocity and displacement responses were not significantly different for left compared to right ear stimulation. Their individual asymmetry ratios were always < 30%. Subjects with vestibular schwannoma had significantly smaller force, velocity and displacement responses to stimulation of the affected compared with non-affected ear. Their mean asymmetry ratios were significantly elevated for all three measures (41.2 ± 10.3%, 40.3 ± 15.1% and 21.9 ± 14.6%).

Conclusions: Asymmetry ratios of balance responses to monaural GVS provide a quantitative and clinically applicable lateralising test of the vestibulospinal pathways.

Significance: This method offers a more clinically relevant measure of standing balance than existing vestibular function tests which assess only vestibuloocular and vestibulocollic pathways.

Keywords: Balance; Galvanic vestibular stimulation; Vestibular schwannoma; Vestibular system; Vestibulospinal.

Fig. 1

Horizontal displacement, velocity and shear force responses during and after a 3 s pulse of 1 mA GVS. A right cathodal stimulus evokes leftward sway after stimulus onset “On response” and a return to the baseline at stimulus offset “Off response”.

Fig. 2

Horizontal displacement, velocity and shear force responses in a subject with a large left sided schwannoma affecting the left side and causing brainstem compression. All three responses were attenuated upon stimulation of the affected side.

Figure 3

Reflex asymmetry ratios for healthy controls (black) and subjects with vestibular schwannoma (white) for force (F), velocity (V) and displacement (D) responses. Group mean values shown by thick horizontal bars.