Electrocochleographic Monitoring in Endolymphatic Sac Surgery for Meniere's Disease

Objective: To determine the efficacy of intraoperative electrocochleographic monitoring during endolymphatic sac–ballooning surgery for Meniere's disease. Design: At each of five surgical steps, the eighth nerve action potential (AP) amplitude and latency, summating potential (SP) amplitude, and SP/AP amplitude ratio were recorded. Data were analyzed using repeated measure analysis of variance. Patients: Electrocochleographs were successfully recorded in 97 patients during endolymphatic sac–ballooning surgery. Forty patients had an abnormal baseline ratio (group 1), and 57 patients had a normal baseline ratio. Results: Data showed that the SP/AP amplitude ratio reductions were significant only in group 1 patients. The SP/AP amplitude ratio was reduced in two thirds of the group 1 patients and unchanged in roughly the same proportion of group 2 patients. A significant reduction of SP/AP amplitude ratios was found to be always associated with prominent SP/AP ratios at the baseline, irrespective of glycerol test results, and only in cases of classic Meniere's disease, indicating that this dominant ratio is a very reliable indicator of Meniere's disease and endolymphatic hydrops. Importantly, reductions of SP/AP amplitude ratios were observed at every step to be consistently due to AP increase rather than SP decrease. Conclusions: It is theorized that the dominant SP/AP amplitude ratio found in patients with Meniere's disease and endolymphatic hydrops is caused by the inhibition of AP activity rather than by the enhancement of the SP resulting from the displacement of the basilar membrane toward the scala tympani owing to hydropic endolymph, as is generally believed. We conclude that electrocochleograpic monitoring is useful durig endolymphatic sac–ballooning surgery to provide a definitive delineation of the endolymphatic sac and its lumen; this delineation is the key to the success of such surgery.