Effect of cortisporin otic suspension on cochlear function and efferent activity in the guinea pig

To predict cochlear receptor cell insult from therapeutic aminoglycoside antibiotics, we sought to determine whether the presence of subtoxic concentrations of Cortisporin otic suspension in the cochlea can be detected by measuring changes in efferent suppression of compound action potentials recorded from the round window. Olivocochlear efferent suppression in an earlier study was reversibly inhibited by injections of gentamicin in guinea pigs. Using that same technique, we found that 2.0 microliter of Cortisporin otic suspension (neomycin, polymyxin B, hydrocortisone) placed on the round window of guinea pigs in varying concentrations causes a rapid, permanent alteration in thresholds. Solutions of Cortisporin otic suspension diluted to a concentration of 1:20 yield a reversible loss of efferent suppression without measurable changes in compound action potential. These data suggest that the presence of Cortisporin otic suspension can be detected functionally in the cochlea at very low concentrations prior to the development of a threshold shift.