Superior Canal Dehiscence Effect on Hearing Thresholds

Abstract

Objective. Superior semicircular dehiscence syndrome is associated with vestibular symptoms and an air–bone gap component in the audiogram, apparently caused by the creation of a pathological bony “third window” in the superior semicircular canal. The aim of this study was to evaluate changes in auditory air- and bone-conduction thresholds to low- and high-frequency stimuli in an animal model of a bony fenestration facing the aerated mastoid cavity. Study Design. Anatomic, audiological. Setting. Tertiary university-affiliated medical center. Animals. A small hole was drilled in the bony apical portion of the superior semicircular canal facing the mastoid bulla/cavity, with preservation of the membranous labyrinth, in 5 adult-size fat sand rats. Main Outcome Measures. Auditory brain stem responses to clicks and 1-kHz tone bursts delivered by air and bone conduction before surgery, after opening the bulla, and after fenestration. Results. After fenestration, a significant air–bone gap was measured in response to clicks (mean ± standard deviation, 37 ± 5.8 dB) and bursts (mean ± standard deviation, 34 ± 14.5 dB). The gap was attributable solely to the significant deterioration in air-conduction thresholds, in the absence of a significant change in bone conduction thresholds. The pattern of auditory brain response changes closely resembled that reported for middle ear dysfunction, namely, an increase in absolute latency of waves I, III, and V without significant alterations in interpeak latency differences. Conclusions. Bony fenestration of the superior semicircular canal toward an aerated cavity in a rodent model mimics the auditory loss pattern of patients with superior semicircular dehiscence syndrome. The dehiscent membrane accounts for the auditory changes.