Hearing loss in relation to round window membrane morphology in experimental chronic otitis media.

The present study was performed to test the effect of single and repeated Pseudomonas aeruginosa exotoxin A (PaExoA) instillations in the middle ear of the rat. The hearing level was examined by the ABR technique, round window membrane (RWM) thickness was measured and morphology was studied by light microscopy. The results showed both reversible and permanent hearing loss (HL). In animals that received a single dose of PaExoA, the RWM thickness doubled initially and remained thickened during the observation period. When PaExoA was instilled on several occasions, RWM thickness doubled, before decreasing to near-control levels. This study confirms the toxicity of PaExoA and the partially reversible HL occurring after a single application of the toxin. The diminished effect of repeated toxin instillations--despite the decreasing thickness of the RWM--is discussed. Copyright 2001 S. Karger AG, Basel