We have previously demonstrated inspection of infraslow (< 0.5 Hz) activity on DC-coupled bedside EEG recordings can correctly lateralize focal temporal lobe seizures (Vanhatalo et al, 2003). However, even high-amplitude infraslow activity can sometimes be difficult to localize by simple visual inspection if there is considerable overlying faster EEG activity or obscuring slow artifact. Also, this earlier study did not investigate extratemporal seizures, which are often more difficult to localize. In the present study, we extend our observation to include seizures arising from extratemporal regions. Furthermore, we performed a methodical comparison of analysis tools by using the source montage and principal component analysis in a commercial EEG software package (BESA 5.0). Recordings were performed on patients receiving presurgical monitoring for medically intractable localization related epilepsy with 20 seizures in 11 patients included in analysis. Recordings were performed using a commercial system for DC-coupled recording, using sintered Ag(AgCl electrodes in a standard array defined by the 10:10 system, with additional anterior temporal electrodes, as well as subtemporal chains. We found that the most reliable method to localize these signals and distinguish them from physiologic artifacts was to inspect the seizures in a source montage after performing band pass filtering below 0.5 Hz. Five patients in this study received epilepsy surgery and had follow-up documenting significant seizure reduction, and analysis of infraslow signals correctly localized epileptic foci in all five, while conventional noninvasive EEG recording with visual analysis of seizures correctly localized only two.
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