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

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.