Partial labyrinthectomy with hearing preservation: frequency-specific data using tone-burst auditory brain stem response.

Surgical approaches to the inner ear with hearing preservation have valuable implications for neurotologic surgery. In a previous study in guinea pigs, we demonstrated that click-evoked auditory brain stem responses (ABRs) were preserved after transection and plugging of 1 or more semicircular canals but were lost after entering the vestibule (Smouha EE, et al. Otolaryngol Head Neck Surg 1996; 114:777-784). A limitation of that study was that click-evoked ABR might not represent the function of the entire cochlea. In this study we used tone-burst ABR to determine thresholds across a broad range of frequencies (2 to 24 kHz) before and after surgical entry into the labyrinth at the lateral semicircular canal, ampulla, and vestibule. Serial measurements were made, and a sham surgery group was used as a control. The results obtained with tone-burst ABR generally agreed with those previously obtained with click stimuli. Toneburst ABR thresholds were similar across most frequencies tested. Transection of the lateral semicircular canal resulted in preservation of ABR thresholds. Ampullectomy had a variable effect on ABR thresholds. Vestibulotomy usually resulted in substantial hearing loss. We conclude that hearing can be preserved across a range of frequencies after selective surgery of the inner ear. Surgical entry into the membranous labyrinth near the vestibule is the critical factor contributing to hearing loss after partial labyrinthectomy.