Cochlear morphology in relation to loss of behavioural, electrophysiological, and middle ear reflex thresholds after exposure to noise

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Abstract
Loss of auditory function was correlated with different pathological findings in the inner ear of the rabbit after 15 or 30 min exposure to high level broad band (2-7 kHz) noise. The ears from animals with post-exposure times ranging from 1 to 23 months were analysed by scanning electron microscopy (SEM). Complete cochleograms with quantification of loss of hair cells and damage to stereocilia were produced. Loss of function was defined by determining behavioural threshold, threshold of auditory brainstem response (ABR) to narrow-band stimuli and threshold of the middle ear muscle reflex (MER). A small scattered loss of OHCs and slight disarray of stereocilia were found in non-exposed ears of young animals, but fusion of IHC cilia was not observed. Normal auditory function was not found in any noise exposed animal with abnormal morphology. A loss of threshold sensitivity of up to about 30-40 dB was noted without loss of hair cells in the corresponding region of the cochlea. The mildest structural damage that correlated to a functional alteration consisted of damage to stereocilia of IHC. This correlated well to the MER threshold shift. In two animals a shift of MER threshold of about 20 dB was the only functional abnormality. In ears with more extensive IHC damage loss of OHCs was also found and these animals had primarily an increase in auditory threshold shift with only a small additional change in MER threshold. It is concluded that in the rabbit and with the methods used the IHC stereocilia are the structures most susceptible to damage by the noise used and that the degree of injury can be fairly well assessed by functional tests of a type that is used also routinely in the audiological clinic.