Abstracts

Seizures recorded with subdural grids have demonstrated high frequency activities up to 130Hz in contacts close to the seizure focus. We report localized changes in high frequency activity during seizures in patients with temporal and extra-temporal epilepsy implanted with depth macroelectrodes. Intracranial EEGs were obtained from stereotaxically-placed depth and epidural electrodes filtered at 500Hz and sampled at 2000Hz in 8 consecutive patients. The epileptic zone was well-localized in 6 patients with 4 having mesial temporal (MT) and 2 having frontal neocortical foci with dysplastic lesions, and was regional or poorly localized in 2 others. Characteristic EEG sections lasting 3-10 s were selected within the seizures of each patient to reflect ictal onset, propagation and termination. Distinct high frequency ([gt]100 Hz) segments were identified visually in the background and in the seizure sections by analyzing the expanded record dually with 50Hz and 100 Hz high-pass filters. Focal changes in high frequency activity during the seizure were also identified using spectral analysis by comparing section band powers (i.e. slow (0-40Hz), high frequency (HF, 100-200Hz), very high frequency (VHF, 250-500Hz)) with the background. 12 seizures were analyzed from the 4 patients with MT foci, 7 from the 2 patients with neocortical foci, and 4 from the 2 patients with no definitive epileptogenic zone. Discrete, focal VHF segments were visually identified at seizure onset in 4/4 MT patients, involving contacts in the epileptic zone. Increased HF activity compared to the background was distinguished both visually and with spectral analysis later in the seizures of 3/4 MT patients, involving contacts in the epileptic zone and, in one patient, areas of contralateral peri-hippocampal propagation. The 2 patients with neocortical foci had increased HF activity confined to the epileptic zone late in their seizures, identified by visual and spectral analysis. No discrete HF or VHF activity was present in the poorly-localized seizures. VHF segments (fast ripples?) were consistently present in the epileptic zone early in seizures of patients with MT epilepsy. In contrast, HF activity was seen more prominently later in seizures, involving the epileptogenic areas of MT and neocortical patients, as well as areas of mesial temporal propagation. Changes in high frequency activity (100 to 500Hz) were absent in patients whose seizures were poorly-localized.This study demonstrates that changes in high frequency activity occur in regions of primary epileptogenesis and secondary spread in anatomically different foci. These activities may reflect focal synchronization of neuronal ensembles that are important in initiating or maintaining ictal activity. Moreover, absent high frequency activity seems to indicate poor localization. (Supported by Canadian Institutes of Health Research.)