Histologic changes after semicircular canal occlusion in guinea pigs.

Hypothesis: Histologic changes occurring after varying degrees of surgical trauma to the inner ear in guinea pigs can reveal the mechanism of hearing preservation/loss.

Background: Surgical approaches to the inner ear that allow for hearing preservation have gained increasing acceptance in neurotologic surgery. The mechanisms responsible for hearing preservation and hearing loss after partial labyrinthectomy are as yet poorly understood.

Methods: Ten animals underwent semicircular canal occlusion, suctioning of perilymph, ampullectomy, or wide vestibulotomy. Tone-burst auditory brain stem response (ABR) thresholds were performed at weekly intervals after surgery. After 4 weeks, temporal bone specimens were processed to obtain 10-jim sections from plastic-embedded ears. The histologic findings were correlated with the initial and final ABR thresholds.

Results: After surgical occlusion of one or more semicircular canals, ABR thresholds were preserved, as the authors reported previously. Suctioning of inner ear fluid led to transient loss of thresholds with recovery. Ampullectomy produced dichotomous results, with some subjects preserving auditory function and others losing auditory function. Wide vestibulotomy resulted in permanent loss of auditory function in most cases. Histologically, there was intraluminal fibrosis and inflammation near the site of surgical entry. Most specimens showed normal cochlear architecture and hair cell counts, irrespective of the degree of hearing loss. Vestibular hair cells were also well preserved, even when they were close to the site of surgical injury.

Conclusions: These findings suggest that electromechanical changes, rather than cell death, are responsible for changes in auditory and vestibular function after partial labyrinthectomy.