Effect of an initial noise induced hearing loss on subsequent noise induced hearing loss.

The effect of previous noise induced hearing loss (NIHL) on subsequent NIHL was studied in rats. Three groups of animals were initially exposed to different durations of 113 dB SPL broad band noise (21 days, 3 days or 0 days--unexposed). Their permanent threshold shifts (PTS) from this exposure (PTS1) were evaluated using auditory nerve-brainstem evoked responses (ABR). All the animals were then noise-exposed for an additional 12 days, and the incremental PTS following this exposure (PTS2) was also assessed. The 21 day group showed the greater PTS1 [mean +/- SD: 27.03 +/- 6.78 dB, compared with 11.67 +/- 10.47 dB (3 day group)] and the lowest PTS2 [9.84 +/- 8.19 dB, compared with 13.33 +/- 14.60 dB (3 day group) and 24.04 +/- 12.4 dB (0 day group)]. This group also showed the highest total PTS and lowest SD following the two noise exposures [36.88 +/- 6.29 dB, compared with 25.00 +/- 12.68 dB (3 day group) and 26.35 +/- 11.93 dB (0 day group)]. The results may be explained by the lower effective intensity of the second noise exposure for the animals with a large PTS1 compared to those with little or no NIHL from the first noise exposure.