■ OBJECTIVES: The aim of this study is to disclose the underlying mechanisms of hearing loss due to transient cochlear ischemia in an animal model. The results are correlated to an audiogram typically seen in idiopathic sudden sensorineural hearing loss (ISSHL).

■ METHODS: Mongolian gerbils weighing 60-80g were used. The animal lacks the posterior communicating arteries and the cochlea is nourished solely by the vertebral arteries. Cochlear ischemia was introduced by occluding the bilateral vertebral arteries for 15-min. Hearing was assessed by recording an ABR or an Electrocochleogram. Dysfunction of the stria vascularis wasevaluated by recording an endocochlear potential (EP) and by  observing the specimens with TEM. Damages of the Corti’s organ were evaluated by staining the specimens with rhodamine-phalloidin and Hoechst 33342. The spiral ganglion was also observed in specimens stained with Hematoxylin-Eosine. The data were obtained before, 1, 4, and 7 days of ischemia.

■ RESULTS: Post-ischemic processes differed by the sites of the cochlea. Damage of the stria vascularis was severe immediate after the ischemia, which recovered gradually in a week. In the organ of Corti, the hair cells underwent apoptotic cell death that went on for 3 days. Degeneration of the spiral ganglion neurons progressed slowly, lasting for more than a week. They were supposed to undergo secondary degeneration following death of the hair cells in the organ of Corti.

■ CONCLUSIONS: The present study suggested that various mechanisms work at respective time courses in the background of hearing loss due to cochlear ischemia, which is supposed to be one of the major causes of ISSHL. In the early phase, hearing loss is mainly due to stria vascularis. In the following phase, degeneration of the hair cells and the spiral ganglion neurons progress; the latter progresses longer than the former. As a result, hearing loss may recover gradually corresponding to recovery of the stria vascularis, while the hair cells and the neurons of the spiral ganglion undergo irreversible degeneration.