Evaluation of Interhemispheric Time Difference by Magnetoencephalography Before and After Total Callosotomy-Two Case Reports-

Abstract

Interhemispheric time difference (ITD) measured by electroencephalography (EEG) and magnetoencephalography (MEG) was compared to seizure outcome after callosotomy. Two patients with frequent drop attacks underwent simultaneous EEG and MEG before and after total callosotomy. ITDs in 30 bilateral synchronized (BS) discharges were calculated independently by EEG and MEG. As minimum transcallosal conduction time was suggested to be approximately 20 msec, BS discharges were classified into five categories according to ITD and side: left-or right-leading long (300 to 80 msec), left-or right-leading moderate (80 to 20 msec), and negligible (º20 msec). In Case 1 before callosotomy, EEG detected 77% negligible and 23% right-leading moderate BS discharges, whereas MEG detected 30% and 63%, respectively. After callosotomy, drop attacks reduced remarkably and EEG and MEG detected no BS discharges. In Case 2 before callosotomy, EEG detected 77% negligible and 23% moderate BS discharges, whereas MEG detected 80% and 20%, respectively. After callosotomy, drop attacks recurred 2 months later and EEG and MEG detected left-and right-leading long BS discharges (63% by EEG and 56% by MEG). MEG detected a large number of BS discharges with moderate ITD before surgery in Case 1, suggesting that the transcallosal pathway was the main pathway for the synchronization, whereas the negligible ITD in Case 2 excludes transcallosal propagation. BS discharges with longer ITD after surgery in Case 2 suggest a persistent poly-synaptic non-transcallosal pathway. MEG with higher spatial resolution than EEG may provide surgical indications for callosotomy.

Generalized 3-Hz spike-and-wave complexes emanating from focal epileptic activity in pediatric patients

“…The high temporal resolution of MEG also clearly showed the time latency of bilateral spikes. Based on a recent study revealing that the corpus callosum has an important role in seizure activity propagation [12,13], seizures originating from left temporal lobe might propagate through the corpus callosum, midline structures, or both.…”

Magnetoencephalography using gradient magnetic field topography (GMFT) can predict successful anterior corpus callosotomy in patients with drop attacks

“…The control of drop attacks was not related with the longer ITDs noted in the interictal generalized spikes following ACC. Two case reports have demonstrated ITD of MEG before and after total callosotomy (Salayev KA et al, 2006). The postoperative elongation of ITD was seen in the second patient with residual seizure.…”

“…Salayev et al have evaluated ITD of spike itself on MEG using three categories: i.e. negligible (< 20 ms), moderate (20-80 ms) and long (80-300ms) duration in two patients with drop attacks (Salayev et al, 2006). Although our MEG sampling rate is 600.615Hz, we employ maximum sensor signals of GMFT with a 10-msec interval to evaluate details of predominant spike distribution on the volume rendered brain surface.…”

Reiterating the role of corpus callosum in generalization of interictal and ictal epileptiform discharges: a case report with post-callosotomy intracranial electroencephalography in Lennox–Gastaut syndrome

“…Drop attacks are the most disabling feature of LGS and are known to be caused by secondary bilateral synchrony. This rapid spread via the CC explains the efficacy of callosotomy in reducing drop attacks in LGS patients [1,[4][5][6][7][8][9][10].…”

“…Several hypotheses regarding how callosotomy reduces seizure frequency have been proposed. The CC, through its rich connectivity between the hemispheres, enhances bilateral synchronization and generalization of IEDs and IDs [1,[4][5][6][7][8][9][10]. Callosotomy disconnects the pathways between homologous areas of two hemispheres, disrupting the synchronized IEDS and IDs and revealing the primary focus (focal discharges) [4][5][6].…”