Abstracts

Rationale: Stimulation studies are used during intracranial EEG investigations to delineate epileptogenic areas and seizure onset zones (SOZ) by provoking after-discharges (AD) or the patients’ typical seizure. High Frequency Oscillations (HFOs, 80-500Hz) are linked to seizure onset and lesional epileptogenic areas. This study was performed to investigate whether interictal HFOs and stimulation studies can locate SOZs and other epileptogenic areas. We hypothesized that interictal HFOs are more frequent in areas with a low threshold to provoke ADs or typical epileptic seizures. Methods: Intracranial EEGs were studied in patients with intractable epilepsy using depth macro-electrodes. EEG was filtered at 500Hz and sampled at 2000Hz. HFOs (Ripples (R) =80-250Hz, Fast Ripples (FR) =250-500Hz) were visually identified on bipolar montage in a five-minute segment of slow wave sleep using high-pass filters at 80Hz and 250Hz. Rates of events were calculated for each channel. For clinical indications, 21 patients underwent electrical stimulation. Adjacent contacts were stimulated consecutively using a biphasic 60Hz stimulus over 3-4s, with gradually increasing electrical currents from 0.2 to 2mA in temporal and 0.8 to 10mA in neocortical regions. Results were interpreted as agreeing or disagreeing with the intracranial study (typical clinical-EEG seizure onset defined the SOZ). Current thresholds provoking an AD or seizure were correlated with the rate of HFOs of each channel (Spearman coefficient, p<0.05). Results: Among the 21 patients, 11 had mesial temporal and 10 neocortical generators. In 12 patients a seizure, in 20, ADs, and in one no response could be provoked. The stimulation results were concordant with the intracranial investigation in 15 patients, and discordant or unclear in six. HFOs were found in all patients and rates were significantly higher in the SOZ than outside (p<0.001). In the 12 patients with provoked seizures, the threshold was negatively correlated with the rates of R in 2/7 temporal and 0/5 neocortical patients, and with the rates of FR in 5/7 temporal and 4/5 neocortical patients. In the 20 patients with ADs, the threshold was negatively correlated with the rates of R in 8/11 temporal and 4/9 neocortical patients, and with the rates of FR in 9/11 temporal and 5/9 neocortical patients (Fig 1&2). These correlations were not only observed in areas in which the stimulation results confirmed the SOZ but also in those with simulation response outside the SOZ.