Patient and Normal Three-dimensional Eye-Movement Responses to Maintained (DC) Surface Galvanic Vestibular Stimulation

Abstract

That disease or dysfunction of vestibular end organs in human patients will reduce or eliminate the contribution of the affected end organs to the total eye-movement response to DC surface galvanic vestibular stimulation (GVS). It was assumed that DC GVS (at current of 5 mA) stimulates all vestibular end organs, an assumption that is strongly supported by physiological evidence, including the activation of primary vestibular afferent neurons by galvanic stimulation. Previous studies also have described the oculomotor responses to vestibular activation. Stimulation of individual semicircular canals results in eye movements parallel to the plane of the stimulated canal, and stimulation of the utricular macula produces changes in ocular torsional position. It was also assumed that the total three-dimensional eye-movement response to GVS is the sum of the contributions of the oculomotor drive of all the vestibular end organs. If a particular vestibular end organ were to be diseased or dysfunctional, it was reasoned that its contribution to the GVS-induced oculomotor response would be reduced or absent and that patients thus affected would have a systematic difference in their GVS-induced oculomotor response compared with the response of normal healthy individuals. Three-dimensional video eye-movement recording was carried out in complete darkness on normal healthy subjects and patients with various types of vestibular dysfunction, as diagnosed by independent vestibular clinical tests. The eye-movement response to long-duration bilateral and unilateral surface GVS was measured. The pattern of horizontal, vertical, and torsional eye velocity and eye position during GVS of patients independently diagnosed with bilateral vestibular dysfunction, unilateral vestibular dysfunction, CHARGE syndrome (semicircular canal hypoplasia), semicircular canal occlusion, or inferior vestibular neuritis differed systematically from the responses of normal healthy subjects in ways that corresponded to the expectations from the conceptual approach of the study. The study reports the first data on the differences between the normal response to GVS and those of patients with a number of clinical vestibular conditions including unilateral vestibular loss, canal block, and vestibular neuritis. The GVS-induced eye-movement patterns of patients with vestibular dysfunction are consistent with the reduction or absence of oculomotor contribution from the end organs implicated in their particular disease condition.