Synergistic effects of noise and hand-arm vibration on distortion product otoacoustic emissions in healthy subjects

An experimental study has been performed on the effect of exposure to noise (N), vibration (V), and combined noise and vibration (NV), on the distortion product otoacoustic emission (DPOAE) level. The aim of the study is investigating the possible synergistic effect of noise and vibration, increasing the risk of damage to the hearing function due to noise only. Twelve normal-hearing volunteers were randomly exposed to hand arm vibration (HAV) only, noise only, and HAV and noise simultaneously. The exposure consisted of five consecutive sessions interspersed with quiet periods. DPOAEs were recorded during the quiet intervals following each exposure session, and during recovery, and compared to their baseline level, and among different test treatments. An ad hoc acquisition system developed in Labview was used to record high frequency-resolution DPOAE spectra in a suitably short time. Time-frequency filtering, based on the wavelet transform, was used to separate the DPOAE components coming from different generation mechanisms. The three test treatments, N, V, and NV, yielded significantly different results. The condition affecting most significantly all DPOAE component levels was the exposure to vibration only, causing a statistically significant increase of the signal level during all the exposure periods, with a decreasing trend during recovery. Also the exposure to noise only, N, caused a DPOAE level increase, statistically significant for the long latency (LL) DPOAE component. Only when exposure to noise was in combination with vibration, the zero-latency (ZL) component was clearly suppressed during the exposure. The results suggest a synergistic adverse effect of noise and vibration on the cochlear function.