[Summating potential evoked by long-tone burst stimuli in Menière's disease]

It is now well documented that in cases of Menière's disease/endolymphatic hydrops, the electrocochleography (ECochG) evoked by a broad-band click is often characterized by an enlarged summating potential (enlarged-SP) and a large negative summating potential to action potential ratio (large -SP/AP) but these mechanism have not been clarified. The aim of this study was to find clues to the mechanism of enhanced -SP and large -SP/AP in Menière's disease, by using long-tone burst stimuli. We also sought to determine the significance of SP measurement for evaluating the cochlear function in Menière's disease/endolymphatic hydrops. ECochG was performed on 29 ears of patients with Menière's disease and 12 ears of adults with normal hearing as a control group. Extratympanic ECochG was carried out using an HN-7 electrode. The sound intensity used for measuring the SP and SP/AP was 90dBnHL. Both SP and AP were elicited by clicks and long-tone bursts of 50 ms duration with a 1-ms rise-fall time. The clicks and long-tone bursts with frequencies of 0.5, 1, 2, and 4 kHz were delivered by the tubal transducer (NC-3). In most of the patients with Menière's disease, SP evoked by the long tone burst stimuli showed negative polarity (-SP) at each frequency. Analysis of variance (X2test) indicated a significant difference in -SP at 0.5 and 1 kHz between Menière's disease and the control group (0.5 kHz: p < 0.001, 1 kHz: p < 0.02). When setting the normal upper limit of the -SP/AP to 0.4, we found that the number of cases over 0.4 which showed -SP at 0.5 and 1 kHz was much larger than those below 0.4 in the 29 ears of patients with Menière's disease. There was no difference between the types of Menière's disease or glycerol test and SP polarity elicited by long-tone burst stimuli. The enhanced -SP and large -SP/AP obtained by the broad band clicks seem to reflect a component of negative SP polarity in the low and middle frequency ranges. It was assumed that the reverse phenomena of SP polarity in the low and middle frequency ranges were related to changes in hydrodynamic characteristics in the upper cochlear turns due to abnormal endolymphatic pressure. Our findings suggest that SP recording by long-tone burst stimuli is useful in detecting changes in cochlear partition due to abnormal intracochlear pressure, most likely in the early phase of Menière's disease based on the wide frequency information of SP.