ELECTROCOCHLEOGRAPHIC STUDY OF EXPERIMENTALLY INDUCED ENDOLYMPHATIC HYDROPS

An endolymphatic hydrops was induced in the left ear of each of 32 guinea pigs by obliteration of the endolymphatic sac. Both the hydropic ear and the control ear were examined by electrocochleography 1, 2, 4 or 8 months after obliteration of the sac using electrodes on the apex and near the round window. One month after obliteration, the threshold of the compound action potential (AP) and the increase of the AP amplitude with sound pressure level (SPL) were the same as the results recorded from the control ears despite the presence of a histologically confirmed hydrops. In the 2-month group, small AP threshold differences (10-20 dB) were recorded, increasing up to 10-40 dB in the 4- and 8-month groups. In the latter animals we also found a more rapid increase of the AP amplitude with SPL, a finding suggestive of recruitment. In the 1-month group we recorded an enhanced negative SP for 2- and 4 kHz stimuli when the electrode was placed near the apex. The same was recorded for 4-kHz stimuli in the 2-month group. In the 4- and 8-month groups there was a tendency towards a decrease in the negative SP. The SP-AP ratio recorded from the apical position showed the same pattern as the SP amplitude, an increase after 1 month and a tendency to decrease in the following months. Near the round window there was no significant effect of hydrops on the SP amplitude or on the SP-AP ratio. A deviation in the AP-SP wave form was recorded in 69% (22/32) of the guinea pigs. This deviation most frequently consisted of an increased second peak (N2) in the AP wave form.