Round window membrane permeability during experimental purulent otitis media: altered Cortisporin ototoxicity.

Round window membrane (RWM) permeability is the most critical factor influencing cochlear function following otitis media. Because otic drops are frequently used during purulent otitis media (POM), we investigated RWM permeability and ototoxicity of Cortisporin otic suspension after inducing experimental POM. Unilateral POM was induced in eight chinchillas by inoculating type 7F Streptococcus pneumoniae into the right ears. Left ears were inoculated with phosphate-buffered saline (control). When POM resolved, the animals were divided into two groups. The round window niches of group 1 were covered with Cortisporin otic suspension. Compound action potentials were measured before and after drug application. The RWM permeability was measured in group 2 by use of tetraethylammonium (TEA) ions as tracers, and the arrival time of TEA and the slope of the potassium-selective microelectrode response were measured. Animals with otitis media exhibited less susceptibility to ototoxicity of Cortisporin otic suspension and reduced RWM permeability to the medium-sized molecule TEA.