Abstracts

Rationale: High frequency oscillations (HFOs, 80-500Hz) are EEG biomarkers for epileptogenic areas. HFOs predominantly occur in the seizure onset zone (SOZ) and the surgical removal of HFO generating tissue correlates with the postsurgical seizure outcome. New studies suggest that HFO can also measure the actual epileptogenicity of the underlying tissue, as HFO rates increase after reduction of antiepileptic medication. High frequency electrical stimulation (HFS) can be used for diagnostic purposes as well as therapy in patients with refractory epilepsy. This study investigates the occurrence and changes of HFOs during and after HFS performed for diagnostic purposes in patients with refractory neocortical epilepsy. Methods: All patients which were recorded with a sampling rate of 1024 Hz and underwent cortical stimulation for the purpose of localizing functional or epileptic tissue at the Epilepsy Centre Freiburg were included. Ripples (80-200Hz) and fast ripples (200-500Hz) were visually marked in a baseline EEG segment prior to HFS, after each period of HFS as well as after the end of HFS, in patients in whom the stimulation triggered a seizure a pre- and postictal segment was marked. Rates of HFOs were compared for the different time periods using Wilcoxon rank sum test (p<0.05). Areas of seizure onset were analysed separately. Results: 12 patients with 911 EEG channels were analysed, 7 patients experienced seizures from untypical and one from typical location. Rates of ripples and fast ripples were significantly higher in the SOZ than outside (p<0.001). Ripple (chi-squared = 54.41; df = 9; p<0.001) as well as fast ripple (chi-squared = 19.30; df = 9; p = 0.02) rates decreased significantly over the course of stimulation. This was the case in SOZ (figure 1) as well as non-SOZ channels. Rates were reduced in patients which experienced provoked seizures (chi-squared = 101.49; df = 9; p <0,001) as well as those which did not (chi-squared = 33.50, df = 9, p<0,001) (figure 2). Conclusions: HFS resulted in a gradual decrease of HFO-Rates over time in all analysed patients. This was the case for patients with provoked seizures as well as for those without. The decrease of HFOs was not limited to SOZ areas. The observed seizures mainly occurred outside the spontaneous SOZ and might result from high current stimulation. If HFOs are considered as markers of epileptogenicity the reduction in HFO-rates has to be interpreted as a reduction of epileptogenicity during the period of HFS. This observation fits well with the latest studies in whom longterm HFS results in a reduction of seizure frequency and can be used for antiepileptic therapy.