Effects of intense noise exposure on fetal sheep auditory brain stem response and inner ear histology

Objective: To evaluate, in two separate experiments, the effects of intense noise exposures delivered to fetal sheep in utero during a time of rapid a uditory development, Design: In the first experiment, auditory brain stem response (ABR) thresho lds to clicks and tone bursts were recorded from chronically instrumented f etal sheep in utero before and after exposure of pregnant ewes to intense b roadband noise. A single 16 hr exposure was delivered at 113 days gestation al age, a time when the ABR is just emerging, Thresholds were compared with an age-matched, nonexposed control group. In the second experiment, fetal sheep at the same gestational age were exposed four times to broadband nois e and their cochleae were harvested 20 days later for histological analysis by the use of scanning electron microscopy, Comparisons were made with an age-matched, nonexposed control group. Results: Experiment One: ABR thresholds recorded between 10 to 20 days afte r the exposure were not as sensitive as thresholds obtained from control fe tuses, There was a tendency for thresholds to 0.5 kHz tone bursts to be mor e affected than thresholds to clicks, Experiment Two: Scanning electron mic roscopy of the organ of Corti from fetuses exposed to noise from 111 to 114 days gestational age revealed significant damage to inner and outer hair c ells in the middle and apical turns of cochleae, Similar hair cell damage w as not present in control fetuses. Conclusions: Intense exogenous noise penetrated the uterus of pregnant shee p and resulted in elevations in ABR thresholds 2 to 3 wk after exposure. In fetuses repeatedly exposed to noise, the middle and apical turns of the co chlea showed greater hair cell damage than found at the same locations in c ontrol cochlea. The basal turn of the cochlea was not damaged.