Galvanic ocular vestibular evoked myogenic potentials provide new insight into vestibulo-ocular reflexes and unilateral vestibular loss

Objective: Synchronous extraocular muscle activity can be recorded from around the eyes at the beginning of a vestibular-evoked eye movement (ocular vestibular evoked myogenic potentials, OVEMPs). As galvanic vestibular stimulation (GVS) evokes the vestibulo-ocular reflex, we wished to investigate GVS-evoked OVEMPs. Methods: We stimulated 10 normals and 6 patients with unilateral vestibular loss (uVL) with bi/unipolar 4 mA, 2 ms current steps at the mastoid. OVEMPs were recorded from electrodes placed Superior and inferior to the eyes. Results: OVEMPs were present beneath both eyes in all normal subjects: an initial positivity ipsilateral to the cathodal electrode (peak latency 9.9 ms, amplitude 1.3 mu V) and an initial negativity contralateral to the cathode (8.8 ms, 2.4 mu V). In the patients, stimulation of the affected side produced little or no response. Stimulation of the intact side produced only contralateral responses. Conclusions: The infra-orbital response is likely produced primarily by the inferior obliques, producing conjugate torsion away from the cathode. The projection to the ipsilateral eye depends upon normal vestibular function on the contralateral side. Significance: OVEMPs call be evoked by GVS. While bilateral effects are obtained with unilateral stimulation in normals, the primary vestibular pathway to the inferior oblique in humans is crossed. (C) 2008 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.