Abstracts

Rationale: Extraoperative cortical stimulation is used to identify eloquent cortex in patients undergoing invasive investigations. However, it is not standardized across centers. In some, adjacent pairs of electrodes are stimulated (bipolar stimulation), requiring to stimulate each electrode twice with different adjacent electrodes to identify the function underlying each electrode. In other centers, one electrode is stimulated, referenced against a distant electrode (monopolar stimulation). We aim to compare the two methods for mapping results, tolerability and ease of application. Methods: Four patients with refractory frontal lobe epilepsy (3 left, 1 right) were enrolled. The study was approved by the hospital ethics committee and informed consent obtained. Stimulation followed reinstatement of antiepileptics, starting away from the ictal onset zone, with stepwise increase of stimulus intensities (in 2 mA increments, maximum of 15mA). Bipolar stimulation was performed with first horizontal adjacent electrodes, then vertical as is practice in our center Results: A total of 417 electrodes/electrode pairs were stimulated (279 bipolar, 138 monopolar). 249 cortical stimulations resulted in clinical signs, with comparable stimulation intensity (Table1). Anatomical mapping results were similar but not identical: 71 electrodes were over eloquent cortex using bipolar stimulation, 76 using monopolar. 7 electrodes showed motor signs using monopolar stimulation only, two with bipolar stimulation only. The percentage of electrodes/ pairs exhibiting afterdischarges (ADs) and their duration was comparable. Stimulation sessions were performed over 2 days in 3 patients; in one patient it was completed in one day, with an increase in ADs in the third hour of stimulation (monopolar). When analyzing the 3 patients with sessions on separate days, monopolar stimulation showed significantly fewer ADs (Chi Square test, P<0.05). Bipolar stimulation required more time (124 34min versus 57 17min) and a brief final analysis, monopolar results are available immediately. Conclusions: Results of mapping identified similar anatomical areas as eloquent cortex, but were not identical. Monopolar stimulation identified a slightly larger area as eloquent. Some possible advantages of monopolar stimulation emerged: 1. Results are available quicker; 2. Our small sample did not reveal any definite differences in percentage or duration of afterdischarges; prolonged stimulation in one session may have marred results in one patient. In patients stimulated on 2 days, the percentage of afterdischarges appears lower using monopolar stimulation. This needs to be confirmed in a larger sample.