CHANGES WITH AGING IN ECOCHG

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Abstract
The present study was designed to determine the standard values of AP, adapted AP and CM (0.5, 1, 2, and 4kHz) of the electrocochleogram using subjects with normal hearing (20 ears, mean subject age 30.4 years). The input-output function, the latency and the threshold were measured in AP, whereas only an input-output curve was made for the adapted AP and the input-output function and the threshold were measused in CM. The CM threshold had the highest positive correlation with the threshold of the conventional pure tone audiometry at 1kHz. These standard curves will be very useful for analysing clinical data. In order to discuss the aging changes affecting electrocochleograms, the test subjects were divided into two groups, i.e. one group was composed of subjects under 30 years of age (mean: 19.7 y/o, 10 ears) and the other of those 30 years old or over (mean: 41.1y/o, 10 ears). There were no differences in hearing threshold or the threshold of the electrocochleogram between the two groups. The CM threshold exhibited the highest positive correlation with the threshold of conventional audiometry at 1kHz in both groups. AP and adapted AP showed no difference in any item between the two groups. Although there was no difference in the CM threshold, there was an obvious difference in the CM input-output curves between these two groups at high stimulus intensities of 2 and 4kHz. It is known that the EP decrement makes CM smaller. However, it is unlikely that this decreased CM response in the older group was due to EP suppression, since this phenomenon was only observed in response to high intensity stimulo and there was no difference in responses to low intensity stimulo. Another possible explanation for this phenomenon is aging changes in the basilar membrane, tectorial membrane and hair cells. Further studies are necessary to elucidate the etiology of the CM suppression revealed in the present study.

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