Spiral ganglion loss outpaces inner hair cell loss in endolymphatic hydrops

Abstract

Neuronal toxicity is thought to be important in Meniere's disease and experimental endolymphatic hydrops (ELH). This study quantifies the relationship between neuronal degeneration and hair cell degeneration in ELH to evaluate the hypothesis that a primary neural insult would yield greater loss in the spiral ganglion than at the inner hair cell level. Following induction and histopathologic confirmation of endolymphatic hydrops in guinea pigs, the degree of hydrops, spiral ganglion loss, and hair cell degeneration were quantified and compared. Guinea pigs with surgically induced unilateral hydrops were sacrificed and their cochleas preserved. Hydrops severity and spiral ganglion density were quantified using automated methods. Hair cells were counted manually. Values were normalized against the contralateral ear to create loss indexes. Inner hair cell (IHC) loss at the apex is significantly lower than corresponding neuronal loss. IHC loss at the base is also lower than neuron loss, although not significantly. Regression analysis shows a significant, positive correlation between neuron loss severity and IHC loss severity at the apex, but not at the base. There is no correlation between hydrops severity and inner hair cell loss. By confirming that spiral ganglion loss is more severe than hair cell loss, and that hair cell loss appears to worsen with neuronal degeneration, this study supports the theory that neuronal toxicity is the primary insult in ELH-related disorders, such as Meniere's disease, and may provide the basis for designing treatment strategies.