Vestibular-evoked myogenic potential thresholds normalize on plugging superior canal dehiscence

Background: Diagnosis of the superior canal dehiscence syndrome (SCDS) relies on symptoms such as sound-or pressure-induced vertigo or oscillopsia, demonstration of sound or pressure-evoked vertical/torsional eye movements, and the presence of a defect in the bony roof overlying the superior semicircular canal. Lowered thresholds for eliciting vestibular-evoked myogenic potentials (VEMPs) provide additional conformation. Objective: To examine VEMP characteristics before and after canal plugging for SCDS. Methods: VEMPs evoked by air-and bone-conducted tones were measured from the sternocleidomastoid muscles (cVEMP) and periocular sites (oVEMP) of 20 normal volunteers, 10 newly diagnosed subjects with SCDS, and 12 subjects who underwent successful superior canal plugging. Results: In all SCDS ears, thresholds for evoking VEMP using air-conducted tones were pathologically lowered, with average values of 83.85 +/- 1.40 dB sound pressure level (SPL) for cVEMP and 85.38 +/- 1.32 dB SPL for oVEMP, 20 to 30 dB below those of controls. Successful canal plugging resulted in normal reflex thresholds. For bone vibration, average thresholds in SCDS ears were 114.62 +/- 1.54 dB FL (force level) for cVEMP and 116.0 +/- 1.52 dB FL for oVEMP, 10 to 20 dB below controls, yet three SCDS ears had normal thresholds. Conclusions: Ocular and cervical vestibular-evoked myogenic potentials evoked by air-conducted sound are equally useful in the diagnosis and follow-up of superior canal dehiscence syndrome. Stimulus thresholds are consistently lowered upon presentation and normalize after corrective surgery. Thresholds for bone vibration, in contrast, have a lower diagnostic yield.