Stereociliary pathology and noise-induced threshold shift: a scanning electron microscopic study.

Cats were exposed unilaterally to narrow-band noise at an intensity of 118-120 dB SPL for two hours. After long survival times thresholds of single auditory units were measured and the organ of Corti examined with a scanning electron microscope (SEM). The purpose of the study was to correlate pathologies of the hair cell stereocilia seen with the SEM with permanent hearing loss measured by elevated thresholds of single auditory nerve units. Pathological changes in inner hair cell (IHC) stereocilia were more severe and more numerous near the lesion focus diminishing progressively with increasing distance from it. Increasingly elevated thresholds correlated with increased damage to stereocilia of IHCs. Thresholds of nerve fibers innervating IHCs with missing or fused stereocilia were greatly elevated. Thresholds of fibers to IHCs with disarrayed stereocilia also seemed to be elevated but to a lesser degree.