Abstracts

Rationale: Infraslow Activity (ISA) is the term used to describe brain activity that occurs in frequencies below 0.1 Hz. Previously we reported ISA in 12 patients with ictal MEG studies and demonstrated large amplitude activity was detected in the MEG data up to 194 seconds prior to seizure onset. We now have used coherence, a measure of synchronicity of the oscillating neurons, to further localize the potential seizure onset zone based on these slow waves.Methods: An archival review of all epilepsy patients who had undergone simultaneous MEG and EEG was performed to identify patients with a seizure during the recording. MEG and EEG data were acquired with DC to 100 Hz. Data were decimated from 508 Hz to 12 Hz. The Gradiometer reference channels were then used to subtract any environmental noise from the data sets and the data were filtered from 0 0.01 Hz. For each run that contained a seizure the entire MEG and EEG data were displayed in one window in MEG-TOOLS, our software analysis package. Data were visually inspected for large amplitude (>10pT) and long latency (> 30 seconds) MEG signals. We selected a segment of the data just prior to the seizure onset for coherence localization of the slow waves.Results: We identified 13 patients (6 male, 7 female; age range 10 60 years (median 22 years, mean 24 +/-12.3 years) with an ictal MEG/EEG recording. The age at seizure onset ranged from 0.5 to 59 years (median 14 years; mean 14.1 + 14.8 years) and the duration of epilepsy from 0.5 24.5 years (median 7 years, mean 9.9 + 7.5 years). Table 1 summarizes the coherence localization of the ISA, seizure semiology, neuroimaging findings, and MEG spike dipole localization. Coherence was concordant with the seizure semiology, MRI findings, and/or the MEG dipole localization in 11 of the 13 patients. Ten of the 11 patients with concordant coherence displayed a structural lesion on MRI and coherence localized to the structural abnormality in 70% of these patients. Conclusions: Infraslow activity can be detected during an ictal MEG and may precede the seizure onset by up to 3 minutes. Coherence measurements can potentially further help localize the seizure onset zone and may be particularly helpful in patients with structural abnormalities. In patients who have had a recent seizure (within 12 hours) ifraslow wave activity may still be present. Localization of the ISA after a seizure may also aid in seizure localization when no epileptiform spikes are present or when a MEG study can be performed soon after a seizure.