Abstracts

Traditional review of scalp and intracranial subdural video-EEG studies rely primarily upon the visual analysis of the interictal and ictal patterns. In many patients, particularly those with multifocal or neocortical epilepsy, seizures are difficult to localize due to rapid spread of ictal activity and high frequency rhythms that may elude detection, potentially affecting the surgical outcome. Time-frequency analysis is a conceptually simple technique that, when applied to intracranial EEG recordings, may reveal information that is not apparent to visual inspection alone. To facilitate its use, an easily interpreted visual presentation was created that provides an overview of the activity of the complete EEG montage. This case series examines the role of spectral time-frequency analysis in five patients with medically refractory epilepsy who underwent invasive subdural electrode video-EEG studies. Interictal and ictal recordings were studied from five patients who underwent intracranial subdural electrode studies for seizure localization as part of a presurgical evaluation. Time-frequency spectral analysis was performed on all channels over sequential one-second time windows. The spectrogram over the 1-200 Hz frequency range was displayed using a colormap with intensity corresponding to the logarithm of the FFT of the input signal over a one-second time window; the colormap is fixed for a given sequence to allow comparisons over time and between channels. EEG was recorded on a Nicolet Bravo system with a sampling rate of 400 Hz/channel and bandpass filtered (4^th order Butterworth) at 0.5 [ndash] 125 Hz. All computations were implemented in Matlab. Spectral analysis of seizure onsets are shown for the five patients and compared to the clinical interpretation. In four cases, the additional information provided confirmatory support for the clinical localization, while in the fourth case the analysis supported an alternative candidate seizure focus, an observation consistent with the surgical outcome. This case series illustrates the complementary role of spectral time-frequency analysis in the setting of intracranial EEG recordings, as well as the importance of a unified presentation that permits direct comparisons between channels. The role of this technique in providing information not apparent on visual EEG interpretation and its contribution to the evaluation of complex neocortical epilepsies, as well as its potential effect on surgical outcome, warrants further assessment. (Supported by FACES.)