Pattern of hearing loss following cochlear implantation.

Cochlear implantation is associated with deterioration in hearing. Despite the fact that the damage is presumed to be of sensory origin, residual hearing is usually assessed by air-conduction thresholds alone. This study sought to determine if surgery may cause changes in air- and bone-conduction thresholds producing a mixed-type hearing loss. The sample included 18 patients (mean age 37 years) with an air-bone gap of 10 dB over three consecutive frequencies and measurable masked and reliable bone-conduction thresholds of operated and non-operated ears who underwent cochlear implant surgery. All underwent comprehensive audiologic and otologic assessment and imaging before and after surgery. The air-bone gap in the treated ears was 17-41 dB preoperatively and 13-59 dB postoperatively over 250-4,000 Hz. Air-conduction thresholds in the treated ears significantly deteriorated after surgery, by a mean of 10-21 dB. Bone-conduction levels deteriorated nonsignificantly by 0.8-7.5 dB. The findings indicate that the increase in air-conduction threshold after cochlear implantation accounts for most of the postoperative increase in the air-bone gap. Changes in the mechanics of the inner ear may play an important role. Further studies in larger samples including objective measures of inner ear mechanics may add information on the source of the air-bone gap.