Abstracts

Rationale: High frequency oscillations between 80-500 Hz are markers of epileptic areas in intracranial EEG. They most frequently occur in the seizure onset zone (SOZ) and surgical removal of HFO generating brain tissue correlates with a seizure free postsurgical outcome. Increasing evidence suggest that they can also be recorded with scalp electrodes. Simultaneous recordings of scalp and intracranial EEG provide a good opportunity to further investigate the origin and clinical relevance of scalp HFOs. In this project we hypothesized that occurrence and location of scalp HFOs can provide important additional clinical information about the extent of the epileptic zone in patients with refractory epilepsy. Methods: Inclusion of all patients which received chronic simultaneous intracranial and scalp EEG recordings at the Freiburg Epilepsy Center between 2011-2012. EEG was recorded with 2000Hz sampling rate and events were visually identified on a bipolar montage during 5 minutes of EEG. Rates of spikes, ripples (80-250Hz) and fast ripples (250-500Hz) in all intracranial and spikes and ripples in all scalp EEG channels were calculated. Analysis of correlation of event location between intracranial and scalp EEG as well as relationship between events and the SOZ and zone of surgical removal. Results: 24 patients could be included, 9 of which had subdural grid and 18 stereotactic depth electrodes. 23 showed epileptic spikes and 19 HFOs on scalp recordings (average ripple rate 1,8/min). In 15 /19 patients highest scalp HFO rate was located over the implantation site, with 13 patients having the highest scalp HFO rate located directly above the highest intracranial HFO rate (Fig 1). In these 13 patients highest rates were also located over the lobe of the SOZ. In 4 patients low rates of bilateral HFOs, in 2 HFOs only contralateral to the SOZ were observed. 17 patients underwent surgery, 7 became seizure free (Engel 1). In patients remaining to have seizure bilateral HFO occurrence was more common and HFOs were seen in significantly more scalp contacts than in those patients which became seizure free (Table 1). Conclusions: Scalp HFOs are visible in most patients undergoing simultaneous intracranial and scalp EEG recordings, but rates are much lower than for intracranial HFOs. Scalp HFOs are mostly located over the implantation site and the SOZ. Widespread and bilateral scalp HFO occurrence might be indicative of a larger epileptic network and associated with poor postsurgical outcome. Recording Simultaneous scalp EEG might help to overcome the spatial restriction of intracranial EEG by providing important information about contralateral HFO occurrence. The possibility of identifying HFOs on scalp EEG opens up opportunities of using them as a biomarker in a larger group of patients.