Abstracts

Outcome of callosotomy depends on whether the transcallosal pathway is involved in the generation of bilateral synchronized discharges (BS). For true transcallosal propagation, the time lag between the leading and following hemispheres must be longer than the minimal transcallosal transfer time of 20 ms. We have demonstrated the superior definition of the leading spike focus by MEG compared to scalp EEG. The goal of this study was to find out whether MEG can provide an additional information for callosotomy cases. Pre- and postoperative EEG and MEG were simultaneously recorded in a 15 yrs old F with Sturge-Weber syndrome (Case 1) and a 9 yrs old M with non-lesional epilepsy (Case 2) who presented with drop attacks (DAs). All discharges were classified into BS and unilateral independent discharges (UI). Interhemispheric time lag (ITL) was calculated for BS with [ldquo]primary peaks[rdquo] (most prominent peak for a single spike or first prominent peak for a burst) defined in each hemisphere by MEG or EEG. Case 1. UI discharges did not show laterality before callosotomy. ITL was calculated in 62% of BS by MEG and in 36% by EEG. BS appeared first in the right hemisphere in 74% by MEG, but in 52% by EEG. Average ITL in MEG was 25.9 ms, 7.1 ms longer than by EEG. The frequency of DAs reduced and MEG showed no BS after callosotomy. The proportion of right UI increased. Case 2. Right and left UI accounted for 11% and 6%, respectively, before callosotomy. ITL was calculated in 88% by MEG and in 32% by EEG. BS appeared first in the left hemisphere in 50% by MEG and in 87% by EEG. Average ITL was 8.3 ms by MEG and 12.7 ms by EEG. DAs reappeared two months after surgery. MEG also indicated persistent BS. ITL was calculated in 38% by MEG and in 22% by EEG. Average ITL increased to 109.1 ms by MEG and to 95.8 ms by EEG. BS appeared first in the right hemisphere in 84% by MEG and which hemisphere was leading in generating of BS was impossible to clarify by EEG because of inconsistent values. ITL of BS before surgery in Case 1 corresponded to the minimal transcallosal transfer time and most BS appeared first in the right hemisphere. Marked decrease of DAs and the absence of BS after surgery also suggested the dominant role of the transcallosal pathway in drop attacks. In contrast, ITL before callosotomy in Case 2 was too short to be explained by a simple transcallosal transfer and no hemisphere was dominant in the generation of BS. Recurrence of DAs and persistent BS after surgery indicated an underlying non-callosal mechanism in Case 2. However, the postoperatively prolonged ITL suggested that the corpus callosum was partially involved in synchronization before surgery. MEG showed better correlation with the clinical outcome than EEG. MEG is useful for evaluation of patients with DAs and BS.