Superior semicircular canal dehiscence mimicking otosclerotic hearing loss (original) (raw)

A puzzling aspect of middle ear surgery is the presence of an air-bone gap in a small number of cases with no apparent cause. We believe that some of these cases are due to unrecognized superior semicircular canal dehiscence (SSCD). We have now gathered experience from 20 patients with SSCD presenting with apparent conductive hearing loss without vestibular symptoms. All affected ears had SSCD on high-resolution CT scan. The common findings in these patients were: (1) the air-bone gaps occurred in the lower frequencies below 2,000 Hz, and ranged from 10 to 60 dB; (2) bone conduction thresholds below 2,000 Hz were sometimes negative (-5 dB to -15 dB); (3) the acoustic (stapedial) reflex was present; (4) measurement of umbo velocity by laser Doppler vibrometry showed slight hypermobility of umbo motion; (5) the vestibular-evoked myogenic potential response was present, with thresholds that were abnormally low, and (6) the middle ear was normal at exploratory tympanotomy, including nor...