Abstracts

Partial seizures originate from focal regions of epileptogenesis. The dynamic of seizure onset and pattern of ictal discharges may reflect the cerebral region of seizure onset. To date, however, these observations have been largely derived from visual analysis or simple frequency characterization. We here report detailed comparative analysis of seizure dynamics at seizure onset. We analyzed data from multiple partial seizures from patients with mesial temporal onset epilepsy (MTLE; n=3) or with neocortical lesional epilepsy (NLE; n=6) monitored in the epilepsy monitoring unit for presurgical evaluation with intracranial subdural grid arrays. The time-frequency decomposition was obtained with the matching pursuit (MP) method. The Gabor atom density (GAD) (Jouny et al. 2003), derived from MP, provides a measure of signal complexity. Different features of the intracranial EEG (ICEEG) or of GAD can be used to synchronize seizures. Onset of complexity increase was used to synchronize the events. Time-frequency maps reconstructed from the MP decomposition were then re-aligned and averaged. GAD plots reveal that all mesial temporal onset seizures and neocortical onset seizures have an increase in complexity at seizure onset and have a reproducible pattern of onset. These consistent features can be used to synchronize the events. Because of the slight variability in timing between different phases of the seizure, one can enhance the visibility of any of these phases by choosing their feature as the synchronizing event for the re-alignment. Onset features which are not time-locked will not be enhanced by averaging. In this study, averaged seizure onsets exhibit distinguishable predominant frequency components in 5 out of 9 patients (either [sim]7Hz, [sim]15Hz, [sim]20Hz or [sim]40Hz) or activities with a broader spectral signature in 3 out of 9 patients. The assumption that multiple seizure onsets in a given patient have stereotypical features is based predominantly on clinical experience and visual analysis. We studied here the detailed time-frequency components of partial seizures and, by averaging their reconstructed maps, isolated the components that are common to all seizures. One of the advantage of the method was to reveal an onset pattern including high-frequency components ([gt]40Hz) with a decreasing main frequency which was not evident on ICEEG. High-frequency components during onset might go unnoticed - especially when embedded in electrodecrement pattern of onset. High-frequency recordings and detailed time-frequency analysis can reveal the dynamic evolution of onsets and provide important information regarding timing of onset pattern and localization. (Supported by NIH grant NS 33732.)