INTRAOPERATIVE ELECTROCOCHLEOGRAPHIC MONITORING OF INNER-EAR SURGERY FOR ENDOLYMPHATIC HYDROPS - A REVIEW OF CASES

The purpose of this review of cases was to determine whether or not the SP/AP amplitude ratio changes during unidirectional inner ear valved shunt surgery for the decompression of the hydropic labyrinth in Meniere's disease. A series of 62 patients underwent shunt surgery over a 20 month period. In 43 cases (43 ears), ECoG responses were successfully recorded throughout the duration of surgery. A pair of two-tailed paired t-tests were computed for SP/AP amplitude ratios acquired during intraoperative ECoG monitoring at the baseline and closing stage of surgery. The first t-test compared the mean SP/AP amplitude ratios at baseline and closing for those cases in which the SP/AP amplitude ratio at baseline was abnormal (baseline > 35 %). The t was significant, t(19) = 4.63, p < 0.01. The second t-test compared the mean SP/AP amplitude ratios at baseline and closing for those cases in which the SP/AP amplitude ratio at baseline was WNL (baseline less-than-or-equal-to 35 %). The t was insignificant, t(20) = 0.31, p > 0.05. Variations between baseline and closing measurements were categorized as either a reduction, no change (very stable over time), or an increase in the SP/AP amplitude ratio. A change in the SP/AP amplitude ratio from baseline to closing of greater-than-or-equal-to 7 % (X = 2 %, SD = 2 %) was considered statistically significant. Overall (43 cases), 49 % of the variations fell into the no change category, with 42 % and 9 % of the cases placed in the reduction and increase categories, respectively. Of the cases (22) in which the baseline measurement was outside the limits of normal, 64 % showed a reduction, 32 % showed no change, and 4 % showed an increase in the SP/AP amplitude ratio at closing. Changes in the SP/AP amplitude were observed at various stages of the surgical procedure. Reductions were found at each of the five surgical steps identified, with the majority of the changes almost evenly divided between the stages of mastoid drilling and opening sac. Increases in the SP/AP amplitude ratio were found in four cases. The increases were observed during drilling of the mastoid bone in two cases and sac decompression and opening the sac in the remaining cases.