Abstracts

Rationale: High Frequency Oscillations (HFOs; Ripples 80-250 Hz, and Fast Ripples 250-500 Hz) are biomarkers for the seizure onset zone (SOZ) in patients undergoing epilepsy surgery. Removal of fast ripples (FR) correlates with good postoperative outcome. Spontaneous FR are considered pathological. However, discrimination between spontaneous physiological, e.g. in mesiotemporal and occipital regions [1], and pathological Ripples (R) is under debate. HFOs can be evoked with Single Pulse Electrical Stimulation (SPES) during chronic ECoG [2]. Evoked HFOs are specific for the SOZ. We compared the occurrence of HFOs evoked by SPES to spontaneous HFOs and their ability to correctly indicate the SOZ, especially in the ripple range. Methods: We selected 2 out of 13 patients that underwent ECoG evaluation including SPES before epilepsy surgery. ECoG in 64 subdural grid-electrodes was sampled at 2048 Hz. SPES (10 pulses of 1ms, 8 mA, 0.2 Hz) was performed, stimulating adjacent electrode pairs. Time Frequency analysis of the SPES results was performed [2]. Significant averaged responses for each stimulation pair per channel were classified as an evoked R or FR. Spontaneous HFOs were analyzed for one minute during wake before SPES or >1h after SPES. R and FR were visually marked (MvtK, FIR high pass R>80Hz and 5uV, FR>250Hz and 1 uV, 0.4s/page, AVG-montage). Electrodes showing artifacts were discarded from analysis. The SOZ electrodes were marked by an experienced neurologist (FL). The total number of R and FR per electrode was calculated. We evaluated the percentage of the total number of spontaneous vs. evoked R and FR present in the SOZ. We also compared the proportion of SOZ electrodes with evoked vs. spontaneous events. Results: Patient 1 (F, 23y, TLE, Engel I) had six SOZ electrodes out of a total of 51 electrodes; patient 2 (M, 9y, extra-TLE, Engel IV; incomplete SOZ removal) had 28 SOZ electrodes out of 48 electrodes. We found spontaneous and evoked HFOs in both patients. When evoked, the proportion of R in the SOZ was consistently higher than the proportion of spontaneous R; for evoked FR this was lower than for spontaneous FR. The proportion of SOZ electrodes showing evoked HFOs was higher than for spontaneous HFOs. (table 1, fig 1)Conclusions: The percentage evoked R in the SOZ is higher than spontaneous R. We found the opposite effect for FR. SPES can help differentiate between physiological and pathological ripples. Possibly SPES specifically provokes the pathological epileptogenic network related to the SOZ, demonstrating the advantage of stimulus locked HFOs. We will analyze more patients to confirm these findings. References: [1] Nagasawa, T. et al. (2012). 'Spontaneous and visually driven high-frequency oscillations in the occipital cortex: intracranial recording in epileptic patients.' Hum Brain Mapp 33(3): 569-583. [2] van t Klooster, M.A. et al. (2011). Time-frequency analysis of single pulse electrical stimulation to assist delineation of epileptogenic cortex. Brain, Oct; 134:2855-66.