Focal corticothalamic sources during generalized absence seizures: a MEG study

Magnetoencephalography (MEG) was used to determine cortical and subcortical contributions to the formation of spike and wave discharges in twelve newly diagnosed, drug naïve children during forty-four generalized absence seizures. Previous studies have implicated various cortical areas and thalamic nuclei in the generation of absence seizures, but the relative timing of their activity remains unclear. Beamformer analysis using synthetic aperture magnetometry (SAM) was used to confirm the presence of independent thalamic activity, and standardized Low Resolution Brain Electromagnetic Topography (sLORETA) was used to compute statistical maps indicating source locations during absence seizures. Sources detected in the 50ms prior to the start of the seizure were more likely to be localized to the frontal cortex or thalamus. At the time of the first spike on EEG, focal source localization was seen in the lateral frontal cortex with decreased thalamic localization. Following the spike, localization became more widespread throughout the cortex. Comparison of the earliest spike and wave discharge (SWD) (Ictal Onset) and a SWD occurring 3s into the seizure (mid-Ictal) revealed significant differences during the slow wave portion of the SWDs. This study of MEG recordings in childhood absence seizures provides additional evidence that there are focal brain areas responsible for these seizures which appear bilaterally symmetric and generalized with a conventional 10-20 placement scalp EEG.