Abstracts

Rationale: To describe the cortical cortico-thalamic circuit involvement during the evolvement of MEG spike-and-wave discharges (SWDs) in order to identify a focal cortical onset zone in human absence epilepsy using MEG. Methods: MEG recordings were performed for 21 patients (6-15 years of age) with absence seizures, ranging from patients with childhood (CAE) or juvenile (JAE) absence epilepsy who can be treated quite successfully with anti-epileptics, to patients with more complex atypical forms of absence epilepsy and children who both had absences and complex partial clinical features. For each seizure of these children a non-linear association analysis was performed for time windows moving through the SWDs present in the MEG, similar to the analyses as used in the study of Meeren et al. (2002). Next, an average association strength was obtained based on all association values calculated for each possible combination of MEG sensors. This function was used for monitoring the time varying spatial distributions of the SWDs. Results: Studying in detail the changes in time and space of the association strength revealed three distinct phases of the SWDs. The generalized 3 Hz SWDs showed, like in the study of Westmijse et al. (2009), a recurrent pattern of focal regional activity during the spikes alternated by generalized activity during the slow-wave phase of the SWDs, indicating most likely the involvement of a cortico-thalamic network. The focal regions that are present during the spikes of the SWDs, also appear to be present during the period from the first spike visible in the MEG to the first generalization and indicate the involvement of the bilateral frontal and parietal regions. Furthermore, the association analysis enabled the identification of a driving node of the SWDs in the period before generalization or even before the first visible spike in the MEG for 18 out of the 21 patients studied. These results will be discussed in relation with the source analysis results of the driving node activity, using either dipole analysis or beamforming source analysis in the time- and frequency domain. Conclusions: Spatiotemporal association analysis is helpful in identifying the driving cortical node of the SWDs of patients with generalized absence epilepsy. The location of the driving node appears to differ for the clinically distinct patient populations, whereas the activity underlying the generalized SWDs seems to share a common network during the evolvement of these discharges for all patients studied. References: Meeren HK, Pijn JP, van Luijtelaar EL, Coenen AM, Lopes da Silva FH. (2002) Cortical focus drives widespread corticothalamic networks during spontaneous absence seizures in rats. J Neurosci. 22: 1480-1495. Westmijse I, Ossenblok P, Gunning B, van Luijtelaar G. (2009) Onset and propagation of spike and slow wave discharges in human absence epilepsy: A MEG study..Epilepsia 50 (12): 2538-48. Acknowledgments: Funded by the Dutch Organization for Scientific Research.