Dehiscence of bone overlying the superior canal as a cause of apparent conductive hearing loss

Objective: To identify patients with superior semicircular canal dehiscence and apparent conductive hearing loss and to define the cause of the air-bone gap. Study Design: Prospective study of patients with superior canal dehiscence. Setting: Tertiary referral center. Patients: Vestibular and/or auditory findings indicative of canal dehiscence and demonstration of superior canal dehiscence on computed tomography of the temporal bone. Intervention: Vestibular-evoked myogenic potentials, three-dimensional eye movement recordings, and surgical resurfacing of the superior canal. Outcome Measure: Association of superior canal dehiscence with an air-bone gap on audiometry. Results: Four patients with dehiscence of bone overlying the superior canal were found to have air-bone gaps in the affected ears that were greatest at lower frequencies and averaged 24 +/- 7 dB over the frequency range of 250 to 4,000 Hz. Three of these patients had undergone stapedectomy before the identification of superior canal dehiscence. The air-bone gap was unchanged postoperatively. Each patient had an intact vestibular-evoked myogenic potential (VEMP) response from the affected ear, a finding that would not have been expected based on a middle ear cause of conductive hearing loss. One patient underwent resurfacing of the superior canal through a middle fossa approach. Postoperatively, his vestibular symptoms were relieved, and his air conduction thresholds were improved by 20 dB. Conclusions: Superior canal dehiscence can result in apparent conductive hearing loss. The third mobile window created by the dehiscent superior canal results in dissipation of acoustic energy and is a cause of inner ear conductive hearing loss.