[Extratympanic electrocochleography during glycerol dehydration test in unilateral Ménière's diseases]

Extratympanic electrocochleography and pure-tone audiometry were performed in unilateral Ménière's disease patients before and 1, 2 and 3 hours after oral administration of glycerol. The mean electrocochleographic response from 12 recordings at 80 dBnHL click stimuli was determined to obtain stable values. The electrocochleographic response during the glycerol dehydration test was compared with that in normal subjects (28 ears). A positive result on a pure-tone audiogram was determined by an improvement of at least 10 dB in two octave bands. In 25 involved ears with Ménière's disease, the action potential (AP) amplitudes gradually decreased during the test, but there were no significant differences. The summating potential (SP) amplitudes decreased during the test and there was a significant difference at 2 hours after administration. The negative SP/AP ratios were unchanged. The Ménière's disease patients were classified into two groups; one with a positive response on the audiogram and the other negative responses including deterioration. According to multiple comparison testing, there were significant differences in AP amplitudes and SP amplitudes between the positive group and normal controls at 2 and 3 hours after administration, and between the positive group and the negative group at 3 hours. Electrocochleographic changes in the positive audiogram group were significant. Then, the patients were classified into three groups by negative SP/AP ratio before administration. The first group consisted of negative ears whose negative SP/AP ratios were less than 0.36, which was the upper 95% confidence limit for 40 normal hearing ears, the second consisted of slightly positive ears (0.36 < or = negative SP/AP ratio < 0.40), the last consisted of severely positive ears whose ratios were more than 0.4, which was the upper 99% confidence limit. There was a significant difference for the SP amplitudes between the slightly positive group and normal controls at 2 hours after administration. Electrocochleographic changes in the slightly positive group with a negative SP/AP ratio were significant at this SP amplitude. In 18 normal ears with Ménière's disease, electrocochleographic changes showed no significant difference. However, the SP amplitudes in 3 ears and the negative SP/AP ratios in 4 ears decreased to less than 80% of the pre-administration value, although they had not fallen below that in the normal controls. These decreases indicate that some normal ears with Ménière's disease already had the distinctive features of involved ears. Whether the fluctuation in hearing will appear in the normal ears with decreasing SP amplitudes or negative SP/AP ratios, as in the involved ears, remains to be determined.