Surgery of the inner ear with hearing preservation: serial histological changes.

Abstract

Surgery of the inner ear can result in hearing preservation under certain conditions, but the mechanisms responsible for hearing preservation or loss are not well understood. The specific aim of the study is to examine histological sections obtained at different time intervals after varying degrees of surgical entry into the inner ear, to understand how the cochlea is protected. The hypothesis is that internal partitioning occurs. Histologic examination of guinea pig inner ears by light microscopy. Guinea pigs underwent lateral semicircular canal transection and plugging, ampullectomy, or vestibulotomy, and tone-burst auditory brainstem response thresholds at 2, 8, and 24 kHz were measured at intervals before and after surgery. Animals were killed after 1, 3, 7, or 21 or more days, and temporal bones were examined histologically. The histological response to surgical trauma consists of fibrosis and varying amounts of inflammation near the site of surgical entry. Cochlear hair cells are nearly always preserved, even when hearing loss occurs. Extension of the inflammatory response to the cochlea is associated with greater degrees of hearing loss. The guinea pig inner ear is capable of withstanding surgical trauma to the semicircular canals and vestibule without complete loss of cochlear function. Fibrosis creates an effective partition between the site of surgical entry and the rest of the inner ear. Cochlear preservation might be enhanced if the inflammatory response can be contained.