Abstracts

Rationale: Fast oscillations seem to play an important role in normal and pathological brain function. For useful clinical applications, it would be preferable to record fast oscillations non-invasively. This study aims to identify gamma and ripple bands on scalp EEGs of epileptic patients and analyses the association between them and interictal and ictal discharges. Methods: Scalp EEG of 15 patients with focal epilepsy was low-pass filtered at 200Hz and sampled at 600Hz. Spikes and fast oscillations were visually marked during non-REM sleep samples of 30 min. High-pass filters at 40 and 80Hz were used to identify gamma (40-80Hz) and ripple band (80-200Hz) oscillations. Only events containing at least four consecutive oscillations and an amplitude clearly higher than the background were considered as fast oscillations. We analyzed the rates of gamma and ripples, their co-occurrence with spikes, the number of channels with fast oscillations inside and outside the seizure onset zone (SOZ), and the specificity, sensitivity and accuracy of gamma, ripples and spikes to determine the SOZ. SOZ was defined as the scalp area where the first ictal discharge prior or concomitant to the clinical onset was seen. Results: We identified a SOZ in 8/15 patients. Gamma was recorded in all patients and ripples in 12. Averaging all 439 channels (approximately 31 channels per patient), the rates were: spikes (1.7 1.13/min), gamma (0.35 0.35/min) and ripples (0.16 0.24/min). Gamma and ripples co-occurred with a spike in 77.5% and 63% (fig. 1 and 2), and spikes co-occurred with gamma in 14.5% and with ripples in 7%. In only two patients gamma and ripples were recorded on non-spiking channels. There was a positive correlation between rates of spiking and rates of gamma (p<0.0001) or ripples (p<0.0001). In the eight patients with a defined SOZ, 44 channels were inside the SOZ and 195 outside. For all events, the number of channels inside the SOZ was consistently higher than outside: spikes {44/44 (100%) vs. 137/195 (70%), p<0.0001}; gamma {36/44 (82%) vs. 63/195 (32%), p<0.0001}; and ripples {21/44 (48%) vs. 22/195 (11%), p<0.0001}. The number of channels in which spikes co-occurred with gamma {31/44 (70.5%) vs. 47/195 (24%), p<0.0001} or ripples {16/44 (36%) vs. 15/195 (8%), p<0.0001} was also significantly higher inside than outside the SOZ. For all events, the mean rates were higher inside than outside the SOZ: spikes (2.64 1.70 vs. 0.69 0.26, p=0.02); gamma (0.77 0.12 vs. 0.10 0.16, p=0.02); and ripples (0.08 0.12 vs. 0.04 0.09, p=0.04). The sensitivity, specificity and accuracy to identify the SOZ were 100%, 30% and 43% for spikes; 82%, 68% and 70% for gamma; and 48%, 89% and 81% for ripples.