Basal cochlear lesions result in increased amplitude of otoacoustic emissions

We have measured the changes in transient otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) during and after ototoxic amikacin treatment in an animal (chinchilla) model. TEOAE and DPOAE were recorded from 6 adult chinchillas over a 6-week time course starting just before a 5-day or 7-day treatment period with amikacin sulphate (400 mg/kg/day, i.m.). After final recordings, cochlear morphology was assessed by scanning electron microscopy. Generally, both DPOAE and TEOAE amplitudes change during and after treatment in a systematic fashion. High-frequency components change first, followed by lower-frequency components. We note that there is often a long latency to the onset of changes in otoacoustic emissions (OAE), and that these changes can continue for weeks after treatment. Most importantly we report that when the basal region of the cochlea is damaged in the frequency region above the OAE recording bandwidth (0.6-6 kHz for TEOAE; 1-6.7 kHz for DPOAE), we often find an increase in OAE amplitudes. More specifically, we note that as a cochlear lesion progresses apically, there is often a transient increase in a frequency-specific OAE before it reduces or is lost. Our results suggest that the increase in OAE amplitudes precedes the expression of detectable cochlear pathology.