Immediate effects of intravenous tobramycin and gentamicin on human cochlear function.

Immediate electrocochleographic changes have been studied in a series of patients following intravenous infusion of either tobramycin or gentamicin. In patients receiving tobramycin, as soon as peak serum levels of antibiotic were reached, a dramatic decrease occurred in the magnitude of the compound VIII nerve action potential (AP), and of the cochlear microphonic (CM). The shape of the AP also changed. The N1 component of the waveform became very small, and N2 increased in size; this is the dissociated pattern of basal coil cochlear damage. The speed of onset of these electrical changes and their apparent reversibility suggests a temporary metabolic block caused by tobramycin. No such changes were observed following infusion of gentamicin.