The Ototoxicity of Methyl Mercury

Abstract

To provide understanding of deafness associated with methylmercury poisoning, we studied the pathological effects of methyl mercury hydroxide (MMH) on the nervous tissue and sensory epithelium of the inner ear. Young Hartley guinea pigs (250–500 g) were treated with daily subcutaneous doses of 2 mg Hg/kg body weight MMH, and the cochleas were examined by phase-contrast and electron microscopy. By the use of cochleographic reconstructions of surface preparations of the organ of Corti, the outer hair cells (OHC) were significantly damaged (P<0.05) at the level of 212 turns from the cochlear base. Pathology also occurred at 312, 112, and 12 turns but was not significant. Cellular pathology included derangement of orderly pattern of stereocilia, cytoplasmic vesiculation, and collapse of cell membranes. Study of the stria vascularis and auditory nerve and its spiral ganglion did not reveal any pathology. Thus, MMH exerts an ototoxic effect directly on the OHC and is a unique ototoxic agent, since aminoglycosidic antibiotics and ethacrynic acid cause initial pathology in the basal turns. The sensorineural hearing loss occurring in methyl mercury poisoning, which was presumed secondary to diffuse CNS pathology, now can be considered secondary to a primary cochlear lesion from organic mercurials.