Transient-evoked otoacoustic emissions and high-frequency acoustic trauma in the guinea pig

Transient-evoked otoacoustic emissions (TEOE) disappear when hearing loss exceeds 40 dB in the range 1-4 kHz, due to impairment of cochlear outer hair cells. Early screening of sensorineural hearing losses is based on this finding. However, little is known as to the frequency specificity of TEOE abnormalities in a damaged cochlea. TEOE were recorded in 18 normally hearing guinea pigs in a range from 1.5 to 5 kHz before and after exposure to loud pure tones (3-10 kHz, 95-105 dB SPL, 1-10 min). The thresholds of eighth-nerve compound action potentials (CAP) were monitored with a round-window electrode; it was confirmed that acoustic overexposure resulted in typical high-frequency threshold elevations (10 to 50 dB). Most TEOE components were found at lower frequencies at which CAP thresholds did not change. However, a significant linear regression was found between their amplitudes and the percentage of basal cochlea with unaltered CAP thresholds (r = 0.69, p < 0.0001). Two alternative hypotheses are proposed to account for this correlation. Either acoustic trauma induced some minute cochlear damage at places tuned to low frequencies and TEOE were more sensitive to it than CAP thresholds, or significant contributions to lower-frequency TEOE came from the most damaged cochlear places tuned to much higher frequencies.