Abstracts

Rationale: Childhood absence epilepsy (CAE) is characterized by seizures consisting of brief impairments of consciousness with bilaterally synchronous 3 Hz spike and wave discharges (SWD) on EEG. This study was undertaken to examine the feasibility of using magnetoencephalography (MEG) to measure SWDs in patients with newly diagnosed and untreated CAE. The corticothalamic system plays a major role in the underlying pathophysiology of CAE. The specific areas of thalamus and cortex that are crucial for the initiation, propagation, and termination of SWDs are still unclear. This is in part because it has been difficult to couple the temporal resolution of epileptic discharges with the spatial resolution needed to identify specific groups of neurons. The purpose of this study was to determine if the fast activity component of SWDs could be recorded within thalamic nuclei. Methods: Children, aged 8 to 11 years old, with newly diagnosed and untreated CAE were recruited for the study. Protocols for MEG recording were similar to those used for conventional EEG. MEG recordings were completed in 10 minute time blocks for a total of 40 minutes, with hyperventilation used as provocation if needed. MEG signal analysis was completed using the beamformer technique for the time periods of several milliseconds before and after the appearance of SWDs. Data was analyzed at frequency bandwidths of 5 Hz to 70 Hz and 20 Hz to 70Hz. Virtual sensors were placed within the thalamus as well as in the frontal, parietal, temporal, and occipital cortices. MEG recordings were conducted on a 275 channel CTF magnetometer and MEG signal was analyzed with source localization software including the beamformer technique. Results: Four children, aged 8 to 11 years old, with newly diagnosed and untreated CAE were recruited for the study. A total of ten absence seizures occurred during MEG recording. High frequency MEG activity was seen within the frontal, temporal, and occipital cortices as well as the thalamus during all seizures. Activity was diffuse and bilateral within the thalamus whereas cortical activity was more focal and bilateral. The thalamic bursting pattern did not mirror the activity within the cortex, suggesting that it may be an independent generator. Conclusions: Using MEG, we have been able to detect focal areas of activity within the thalamus and cortex during absence seizures in patients with untreated CAE. Activity within the thalamus did not mirror that found within the frontal, temporal, and occipital cortices in either amplitude or timing, suggesting that thalamic activity is independent of that within the cortex. These data indicate that MEG may be a useful tool for studying generalized epileptiform discharges that involve both cortical and subcortical regions of the brain. The high temporal resolution and very good spatial resolution of MEG could provide more information regarding the initiation and propagation of generalized seizures than has been possible to date with other imaging modalities.