Abstracts

Although spike- and- wave (SW) discharges in generalized epilepsy are bilaterally synchronous to eyeball examination, interhemispheric small time difference analysis ( STDA), might identify subgroups of asynchronous SW discharges. We believe this is an important finding, since small time differences in synchronization may reflect pathophysiological mechanism involving either subcortical or cortical structures.
Small time difference analysis was performed in 102 EEG and/or MEG generalized and bilaterally synchronous SW discharges recorded in 4 patients ( range of 11-38 discharges/patient ) with generalized epilepsy ( 1 female, age range : 3-53 years ). Simultaneous MEG ( 148 channels Magnes ) and EEG ( 32 channels ) were performed in all patients. EEG and MEG were digitized at 678 samples/seconds, time resolution error of 1.4 msecs.
We performed STDA for the spike component of generalized SW discharges ( MEG and EEG ) recorded from homologous frontal regions ( mid frontal EEG channels and corresponding MEG tracings).
102 EEG and/or MEG generalized SW discharges were studied ( 100 % ) . In 71 / 102 discharges, simultaneous MEG and EEG epileptic activity was recorded ( 69.6 % ). In 9 /102 discharges, only EEG SW activity was recorded ( 8.8 % ) and in 8 / 102 discharges, the SW activity was represented only in the MEG tracing (7.8%). The remaining 14/ 102 discharges (13.7 % ) were generalized in the EEG tracing but they were clearly lateralized in the MEG recording. Three populations of SW discharges were identified: Perfectly synchronized SWs: 0-3 msec lead -lag; asynchronous SWs: 3-12 msec lead- lag, and asynchronous SWs: 12-20 msec lead- lag (See Table).
Our results, clearly indicate that only a fraction of SW discharges in generalized epilepsy ( 29,5-33.8 % , see table) show a perfect hemispheric synchronization, suggesting a midline subcortical ( probably thalamic ) generator. The remaining generalized SW discharges ( approximately 70 % ) show small time differences in hemispheric synchronization. Of these, only a small proportion ( 15,9-19,7%, see table ) show a synchronization time difference [gt] 12 msec, a finding which is compatible with a cortical SW generation and callosal propagation. The exact mechanism of synchronization remains unknown in a larger subgroup of asynchronous ( 3-12 msecs) SW discharges ( 46,4-54,9 %, see table ). We postulate that synchronization in this group could be due to either by a cortical midline generator ( cingular cortex? ) or mediated by a subcortical synchronizing mechanism comprising non-midline thalamic or basal gangliae structures. Only recording from subcortical structures using MEG technology would clarify the latter.[table1]