CHANGES IN DP-AMPLITUDES AFTER ACUTE NOIS E IMPACT

Background: The inner ear responds to acute impact of high-intensity n oise with a temporary threshold shift (TTS), which represents a reacti on of the outer hair cells. That is why TTS should be detectable as ch anges of distortion-product otoacoustic emissions (DPOAE). Methods: DP OAE were measured in 102 ears in 59 test subjects with normal hearing before and after definitive noise exposure (20min of ''white noise'' a t 90 dB HL). We also registrated the TTS (4 kHz) immediately after noi se and measured DP changes during a 30-min recovery period. Results: T ypically we found a reduction of DP amplitude. It averaged between 2.0 and 2.5 dB HL in the frequency range from 2 to 5 kHz. There was no co rrelation between TTS acid DP amplitude reduction at 4 kHz. During the recovery period, individual ears showed an erratic pattern of DP ampl itudes. However a tendency toward incremental DP amplitude regeneratio n appeared after averaging. As many as 10% of the measured ears showed an atypical reaction with an increase of DP amplitudes. Whether the l atter is due to errors in the test procedure or is evidence of a parti cularly stable inner ear remains to be discussed. Conclusions: The res ults are further proof for the reaction of the outer hair cells on acu te noise impact, which apparently leads to an increase of the stiffnes s of basilary membrane due to contraction processes. It may be possibl e to differentiate vulnerable inner ears from stable ones.