Correlation between hearing loss and scala media area in guinea pigs with long-standing endolymphatic hydrops.

Abstract

Histologic analysis of the hydropic and normal guinea pig cochleae was undertaken to assess a potential correlation between the magnitude of endolymphatic hydrops and hearing loss. It was hypothesized that a greater correlation than previously reported might be found by looking at long-standing endolymphatic hydrops and high-frequency range hearing. Surgically induced endolymphatic hydrops in guinea pigs is the most widely used animal model for the study of human Ménière's Disease and recapitulates both endolymphatic hydrops and progressive sensorineural hearing loss. A strong correlation between the magnitude of hydrops and severity of hearing loss has been reported in the human condition, but not in the animal model. Nine albino guinea pigs were each subjected to surgical obstruction of the endolymphatic sac and duct of the right ear. The left ears remained as internal histologic controls. Hearing was assessed from 2 kHz to 32 kHz by auditory brain stem response testing for 16 to 25 weeks after surgery. Histologic morphometry after the animals were killed was used to quantify both turn-specific and weighted overall hydrops. These measures were correlated with hearing loss in each animal at all tested frequencies. A statistically significant correlation between the magnitude of hydrops and the severity of hearing loss was observed for 2 kHz and 16 kHz. These frequencies correlated with both turn-specific hydrops and overall hydrops. However, turn-specific hydrops did not reliably correlate with the magnitude of hearing loss at anatomically appropriate frequency ranges. Where such a correlation did exist, it might well have been simply part of an expression of an overall correlation between hydrops and hearing loss. There may be a greater correlation between hydrops and hearing loss in guinea pigs with long-standing surgically induced hydrops than has previously been reported in animals with less advanced disease. These findings help to validate continued use of the model for hearing loss related to Ménière's Disease, verify the rationale of treatment modalities aimed at reducing hydrops in the human condition, and may indicate that efforts at reducing hydrops in such patients has benefits toward long-term hearing preservation.