Abstracts

We recently described techniques to perform longterm DC-coupled EEG recordings at the bedside, and have used these methods to demonstrate that inspection of infraslow ([lt] 0.5 Hz) activity can correctly lateralize temporal lobe seizures (Vanhatalo et al, Neurology.60:1098, 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. We address these issues with improvements in the DC-coupled EEG recording and analysis technique, and by extending our observation to seizures arising from a variety of cerebral regions. Recordings were performed on patients receiving presurgical monitoring for medically intractable localization related epilepsy with 20 seizures in 11 patients included in analysis. Recordings used a commercial system for DC-coupled recording, with sintered Ag/AgCl electrodes in a standard array defined by the 10:10 system, with additional anterior temporal electrodes, as well as subtemporal chains. Afterwards, ictal events were exported to an EEG analysis software package (BESA 5.0) to review the seizures with source montages. Recognition and localization of the infraslow signal was made easier i) by compressing the time scale (30 to 90 seconds per screen), ii) by filtering the signal in time domain (i.e. regular high pass/low pass or band pass filtering), iii) by filtering the signal in space (e.g. with a source montage), and iv) by using a moving average to smooth the recording. The spatial filtering properties of the source montage also makes it easier to differentiate the ictal change from physiological artifacts, such as eye movements, that arise from other head regions. In addition, the time frequency characteristics of some seizures were analysed by FFT and by use of Morlet wavelets. Infraslow signals were present with all focal seizures, sometimes with amplitude orders of magnitude higher than the ictal signals in the convention EEG frequency band (0.5 to 70 Hz). Analysis of these infraslow signals was a reliable localization tool. 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. DC-coupled EEG recordings can give additional information that is clinically useful to noninvasively localize the epileptogenic focus. The value of this method is increased by source analysis tools that can reveal highly localized changes more clearly than direct visual inspection. (Supported by The UW Regional Epilepsy Center.)