Mechanisms in noise-induced permanent hearing loss: an evoked otoacoustic emission and auditory brainstem response study

In this study 22 patients (44 ears) with noise-induced permanent hearing lo ss were audiologically evaluated using transient-evoked otoacoustic emissio ns (TEOAE) and auditory brain-stem response (ABR). Twenty-one normal subjec ts (42 ears) without exposure to occupational noise were used as controls. Based upon the hearing loss at 4, 3, 2 and 1 kHz on the pure-tone audiogram , they were classified into four groups. In group I (eight ears), emissions were present in all ears but their TEOAE-noise level and their reproducibi lity (percentage) proved to be weak. The auditory brain-stem response (ABR) indicated that the I/V amplitude ratio, the latency values of wave V and t he I-V intervals fell within the normal range in all ears. In Group 2 (14 e ars), 40 per cent had no emissions, whereas the remaining ears showed weak emissions. The ABR revealed that in all ears the I/V amplitude ratio became small while wave V peak latency as well as I-V intervals were within the n ormal range. In Group 3 (10 ears), emissions were absent in 50 per cent, wh ile in the other ears the emissions were very weak. The ABR revealed that t he W amplitude ratio, which could be calculated in the 60 per cent in which wave I was present, was smaller than in Group 2. Wave V latency as well as I-V intervals were within the normal range. In Group 4 (12 ears), none of the ears showed emissions. The ABR indicated that the IN amplitude ratio wa s much smaller when wave I was present (27 per cent) as well as I-V interva l values being within the normal range. Wave V absolute latency value (Delt a V index) indicated a positive index in 17 per cent of this group (two ear s) when wave I was absent. In the present study a dynamic process from coch lear outer hair cells to cochlear neurons was seen, correlating with an inc reasing hearing loss.