ELECTROCOCHLEOGRAPHY IN ENDOLYMPHATIC HYDROPS USING TONE-PIP AND CLICK STIMULI

The use of electrocochleography (ECochG) in the identification of endo lymphatic hydrops in Meniere's disease, using either trans-tympanic or extra-tympanic recording techniques, has become very popular. The pre sence of an enhanced summating potential (SP) component is considered to be a diagnostic indicator of hydrops. The response evoked by a clic k stimulus is widely used in this investigation. The aim of this study was to assess the value of tone-pip stimuli for measurement of the SP component in patients with suspected Meniere's disease.Extra-tympanic ECochG was recorded from a surface electrode positioned on the poster o-inferior quadrant of the ear canal close to the tympanic annulus. Fo rty patients referred to the Evoked Potentials Clinic at the Queen's M edical Centre were studied. The combined summating potential and actio n potential waveform (SP/AP) were recorded using click and tone-pip st imuli. The pips had tone frequencies of 1 and 4 kHz and all stimuli we re presented with alternating phase at a repetition rate of 10 per sec ond. The percentage SP (%SP), width and latency characteristics of the SP/AP waveform were calculated for each stimulus condition. There wer e four patients (10%) with an enhanced tone-pip %SP when the click %SP was within normal limits. Width measurement of the tone-pip waveform at the onset point of the response was abnormal in 13 patients (33%) w hen a similar measurement of the click-evoked response was normal. The re were nine of these patients identified by an abnormal width measure ment at the 50% amplitude point of the SP/AP waveform. Both tone-pip f requencies were required in order to identify all these abnormal measu rements. In conclusion, the tone-pip response provides valuable additi onal information to the click stimulus in support of the diagnosis of endolymphatic hydrops.