Temporary and Permanent Level Shifts in Distortion Product Otoacoustic Emissions following Noise Exposure in an Animal Model

Background: Noise-induced hearing loss (NIHL) is one of the most common occupational illnesses. Most of the studies on NIHL were conducted at high noise levels that people are rarely exposed to but in industries. The function of the outer hair cells (OHCs) is impaired after exposure to industrial noise. Distortion product otoacoustic emissions (DPOAEs) are useful in examination of noise-induced level shifts. Objectives: To assess the function of OHCs by DPOAE temporary and permanent level shifts (TLSdp and PLSdp) in rabbits exposed to white noise at realistic levels typically found in industrial settings over a broad range of frequencies. Methods: 12 albino rabbits were divided into two groups: the experimental group rabbits which were exposed to 95 dB SPL white noise at 500–8000 Hz for 8 hrs/day for 5 consecutive days, and the control group rabbits with no exposure to noise. The function of OHCs was examined by DPOAE level (Ldp) in different occasions. The study groups were compared for DPOAE temporary and permanent level shifts (TLSdp and PLSdp) to assess the effect of noise on OHCs function. Results: Noise-induced DPOAE levels (Ldp) were decreased up to 20.65 dB (on day 8) and 18.93 dB (on day 11) at 5888.50 Hz (p=0.081). TLSdp and PLSdp were significantly decreased up to 17.99 dB and 16.27 dB, respectively in the experimental group. The most and least Ldp were significantly different (p<0.05); they occurred at 5888.50 and 588.00 Hz, respectively. There were significant differences between temporary and permanent threshold shift at various frequencies (p<0.05). These differences were mainly related to 5888.50 Hz compared to other frequencies in each ear (p<0.05). Conclusion: DPOAEs are an attractive tool for obtaining information about small temporary or permanent threshold shifts, even when the pure tone audiogram is normal.