Abstracts

Interictal short oscillations between 100-500Hz have been reported in EEG acquired with implanted microelectrodes in epileptic rats and patients. Oscillations between 250-500Hz, Fast Ripples, appeared related to the focus. We report high frequency oscillations recorded with intracranial macroelectrodes, characterize their relation to the seizure focus, and quantify their concordance with epileptiform transients., Five patients with intractable epilepsy were evaluated. EEGs from depth electrodes were filtered at 500Hz and sampled at 2000Hz. Patients were selected if the implantation included electrodes from within and outside the seizure onset zone. The recordings, including the first slow wave sleep stage of the night, were high-pass filtered at 80Hz to visually identify the fast oscillations, which were then classified in 4 categories: ripples (R, 80-200Hz), isolated or during transients, and fast ripples (FR, 250-500Hz), isolated or during transients. The rate of occurrence was studied and the power in the R and FR bands was computed with wavelet analysis. Power differences were assessed between events in seizure onset electrodes (SOE) and those not in seizure onset electrodes (NSOE), and between oscillations with or without transients., One patient had no R nor FR. [underline]Isolated oscillations[/underline]. R were seen in SOE and NSOE in all 4 patients. In the SOE the R rate was 0.090 events/s and in the NSOE, 0.042. In one patient the rate in NSOE was higher than in SOE. The power in the R band was higher in SOE in 2 patients, lower in one and not significantly different in the fourth. FR were seen in 3 patients in the SOE; two of them also had FR in NSOE. The rate of FR in the SOE was 0.015 and in the NSOE was 0.017. The power in the FR band was higher in the SOE in the 2 patients with FR in both SOE and NSOE.
[underline]Oscillations with transients[/underline]. R and FR were seen in all four patients. The rate of R and FR was higher in the SOE than in the NSOE. The power in the R band was higher in SOE in 2 patients, lower in one and not significantly different in the fourth. In 3 patients the power in the FR band was higher in SOE than in NSOE and in one there was no difference.
[underline]Isolated vs Transients[/underline]. R and FR were more frequent with transients than when isolated. In the one patient whose rate of FR in NSOE was higher than in SOE, the rate of FR associated to transients was very high in the SOE and low in the NSOE. In the R and FR bands, the power of oscillations associated with transients and those without were not significantly different in 3 patients., Interictal R and FR can be recorded with depth macroelectrodes in patients. Most occur at the time of transients but some are isolated. R do not show a clear differentiation between the seizure onset zone and remote areas, whereas FR have clearly a higher rate (whether or not associated with transients) and a higher power in the seizure onset zone., (Supported by Canadian Institutes of Health Research, Basque Govt Dept of Education.)