CLICK-EVOKED VESTIBULO-OCULAR REFLEX DISTINGUISHES POSTERIOR FROM SUPERIOR CANAL DEHISCENCE

Abstract

A 26-year old man had oscillopsia in response to loud sounds following head trauma in a skiing accident. His vision would blur when he heard loud music or when listening to the telephone with his right ear. His examination was unremarkable without sound- or pressure-induced eye movements. His symptoms suggested a pathologic third window into the inner ear labyrinth, most commonly due to a superior canal dehiscence.1 However, posterior canal dehiscence (PCD) due to jugular bulb abnormalities2,3 or cholesteatoma erosion4 has been described. We used binocular, 3-dimensional click-evoked vestibulo-ocular reflex (VOR),1 ocular vestibular evoked myogenic potentials (oVEMP),5 and CT imaging1,2,6 to identify the cause of his symptoms. Stimulation of the unaffected left ear with 5-click trains at 110 dB NHL yielded normal miniscule eye rotations,1 but stimulation of the symptomatic right ear yielded enlarged, downward, contraversive-torsional (counterclockwise, i.e., eye's upper pole rotated away from the right ear) and rightward eye rotations (figure 1A). Eye velocity magnitudes from left ear stimulation were ≤1.0 °/s; for right ear stimulation, they were 37.3 °/s for the ipsilateral eye and 49.3 °/s for the contralateral eye, with a left-right VOR asymmetry of 1:37 (ipsilateral eye) and 1:49 (contralateral eye). Figure 1 Click-evoked vestibulo-ocular reflex in a posterior canal dehiscence confirmed by CT imaging (A) Binocular, 3-dimensional 5-click evoked vestibulo-ocular reflex (VOR) from a right posterior canal dehiscence patient. Left ear 5-click evoked VOR at 110 dB normal hearing level (NHL) with miniscule eye rotations (x, y, z < ± 0.006)° and (x, y, z ≤ 1) °/s was normal.1 Right ear 5-click VOR shows enlarged downward, contraversive-torsional counterclockwise and small rightward eye rotations in positions and velocities from ipsilateral eye: (x = …