ELECTROCOCHLEOGRAPHIC MONITORING IN ENDOLYMPHATIC SAC SURGERY FOR MENIERES-DISEASE

Objective: To determine the efficacy of intraoperative electrocochleog raphic monitoring during endolymphatic sac-ballooning surgery for Meni ere's disease. Design: At each of five surgical steps, the eighth nerv e action potential (AP) amplitude and latency, summating potential (SP ) amplitude, and SP/AP amplitude ratio were recorded. Data were analyz ed using repeated measure analysis of variance. Patients: Electrocochl eographs were successfully recorded in 97 patients during endolymphati c sac-ballooning surgery. Forty patients had an abnormal baseline rati o (group 1), and 57 patients had a normal baseline ratio. Results: Dat a showed that the SP/AP amplitude ratio reductions were significant on ly in group 1 patients. The SP/AP amplitude ratio was reduced in two t hirds of the group 1 patients and unchanged in roughly the same propor tion of group 2 patients. A significant reduction of SP/AP amplitude r atios was found to be always associated with prominent SP/AP ratios at the baseline, irrespective of glycerol test results, and only in case s of classic Meniere's disease, indicating that this dominant ratio is a very reliable indicator of Meniere's disease and endolymphatic hydr ops. Importantly, reductions of SP/AP amplitude ratios were observed a t every step to be consistently due to AP increase rather than SP decr ease. Conclusions: It is theorized that the dominant SP/AP amplitude r atio found in patients with Meniere's disease and endolymphatic hydrop s is caused by the inhibition of AP activity rather than by the enhanc ement of the SP resulting from the displacement of the basilar membran e toward the scala tympani owing to hydropic endolymph, as is generall y believed. We conclude that electrocochleograpic monitoring is useful during endolymphatic sac-ballooning surgery to provide a definitive d elineation of the endolymphatic sac and its lumen; this delineation is the key to the success of such surgery.