Abstracts

Our recent study (Epilepsia 2002;43:1536-1542) of intraoperative electrocorticogram (ECoG) and callosal compound action potential in patients performed callosotomy revealed the interhemispheric recruitment of bilateral epileptogenesis; i.e., callosal and other cortical neurons are simultaneously recruited in both hemispheres, and thereby result in bilaterally synchronous spike and wave discharges (BSSWs). Interhemispheric delays between bilateral spikes measured by ECoG were not constant but fluctuated in those patients. However, the individual distribution of interhemispheric delays, i.e., the degree of synchrony, has not been investigated in association with the presumed background pathology and surgical outcome of callosotomy. In this study, authors analyzed BSSWs on ECoGs in terms of degree of synchrony, and correlated surgical results. In 21 patients undergoing callosotomy for intractable epilepsy, intraoperative ECoGs from both hemispheres were recorded prior to callosal section, with patient consent. One-second long epochs of all BSSWs were extracted twice with reference to the ipsilateral and contralateral spike peak (ipsi-ref-spike and contra-ref-spike) respectively. Then, through an averaging of so collected spikes, degree of synchrony (presented as synchrony index, SI) was assessed with fractional rate of contra-ref-spike amplitude (i.e., SI = contra-ref-spike / ipsi-ref-spike). Finally, SI and postoperative outcome were compared. EEG and seizure outcome after callosotomy were evaluated one month postoperatively. Seizure reduction of at least 80% was obtained in 10 patients (excellent outcome group, Group E). In the remaining 11 patients, reduction of seizures was unsatisfactory at less than 80% (unsatisfactory outcome group, Group UNS). Group UNS had slightly high SI than group E (0.49 [plusmn] 0.06 vs. 0.38 [plusmn] 0.05, p = 0.20). As to interictal EEG outcome assessed by visual inspection of long-term EEGs, dramatic modifications of preoperatively observed BSSWs were commonly shown. Spikes were largely confined to one hemisphere in 10 (unilateral discharge group, Group U), and bilateral independent epileptiform spikes were evident in 11 patients (bilateral discharge group, Group B). Group B had significantly higher SI than Group U (0.35 [plusmn] 0.05 vs. 0.51 [plusmn] 0.05, p = 0.04). High SI shows that BSSWs frequently occurs with smaller or no interhemispheric delay, and low SI means BSSWs generation requires certain degree of time differences between hemispheres. Post-callosotomy unilateral or bilateral independent spikes may depend upon presumed hemispheric epileptogenisity, i.e., unilaterally dominant or bilaterally potent susceptibility. Therefore, following two situations of bilateral synchrony are suggested. (1) If both hemispheres have sufficient epileptogenisity, bilateral spikes are invoked almost at the same moment. (2) Otherwise, they require certain degree of delays to build up.