Toxic effects on inner ear of noxious agents passing through the round window membrane.

The effects on inner ear of Xylocain, EMLA, phenol, endotoxin and hydrocortisone were analysed before treatment and 24 h-6 months after instillation of each drug into the round window (RW) niche, where the substance was left. Inner ear function was determined by repeated auditory brainstem recordings (ABR). All cochleae were analysed morphologically at the light microscopic level. Hydrocortisone, Xylocain and endotoxin caused functional changes without morphological correlates, even at long-term follow-up. In contrast, phenol and EMLA caused both functional impairment and graded morphological damage to the organ of Corti in the basal coil. Outer hair cells (OHC) were more vulnerable than inner hair cells (IHC).