Ocular vestibular evoked myogenic potentials (OVEMPs) in superior canal dehiscence

Abstract

Objective: Patients with superior canal dehiscence (SCD) have large sound-evoked vestibular reflexes with pathologically low threshold. We wished to determine whether a recently discovered measure of the vestibulo-ocular reflex, the ocular vestibular evoked myogenic potential (OVEMP), produces similar high amplitude, low threshold responses in SCD, and can differentiate SCD patients from normals. Methods: Nine patients with CT-confirmed SCD and 10 normal controls were stimulated with 500 Hz, 2 ms tone bursts and 0.1 ms clicks at intensities up to 142 dB peak SPL. Conventional VEMPs were recorded from the ipsilateral sternocleidomastoid muscle to determine threshold, and OVEMPs were recorded from electrode pairs placed superior and inferior to the eyes. Three-dimensional eye movements were measured with scleral dual-search coils. Results: In SCD patients OVEMP amplitudes were significantly larger than normal (P < 0.001) and thresholds were pathologically low. The n10 OVEMP in the contralateral inferior electrode became particularly large with increasing stimulus intensity (up to 25 μV) and with up-gaze (up to 40 μV). Sound-evoked (slow phase) eye movements were present in all SCD patients (vertical: upward; torsional: upper pole away from the affected side; and horizontal: towards or away from the affected side), but began only as the OVEMP response became maximal, consistent with the surface potentials being produced by activation of the extraocular muscles that generated the eye movements. Conclusions: OVEMP amplitude and threshold (particularly the contralateral inferior n10 response) differentiated SCD patients from normals. Our findings suggest that both the OVEMPs and induced eye movements in SCD are a result of intense saccular activation in addition to superior canal stimulation.