Abstracts

Rationale: Interictal high frequency (80-600Hz) oscillations (iiHFO) have been described during invasive VEEG evaluations of patients with refractory focal epilepsy. Prior studies have demonstrated that iiHFO >250Hz are able to predict the location of the seizure onset zone (SOZ). The objective of this study is to establish if the presence and location of iiHFOs obtained with subdural macroelectrodes and frequencies 80-200Hz are useful in predicting the location of the seizure onset zone (SOZ) and post operative outcome in patients with focal epilepsy. Methods: All patients who underwent invasive EEG evaluation at our institution between 2007 and 2009 were identified. We performed a retrospective review of EEG data as well as chart data. iiHFOs were identified and localized to the specific electrodes. The SOZ was also identified. Sampling rate was 250Hz for one of the cases and 500Hz for the rest. The time between surgery and last follow up was 6 to 31 months. The concordance between the location of iiHFOs and the SOZ was analyzed in relation to the epilepsy type and post surgical outcome (Engel Classification). Results: 30 patients underwent Invasive VEEG evaluation at our institution between 2007 and 2009. 12 patients were excluded because of unreliable EEG data or because the source of the ictal activity was found to be non localizable. 10 (56%) of the 18 patients analyzed were female. Age range was 14-56 years. iiHFOs in a range of 83 to 134Hz were identified in 16 (89%) patients. One patient did not have resective surgery following the evaluation. 11 patients underwent temporal lobe resections, 9 of them had class I outcome at the time of last follow up. 5 of these patients had positive correlation between the location of iiHFO and SOZ. Two temporal lobe patients had outcome class II or III, one had positive iiHFO-SOZ correlation. 10 out of the 11 temporal cases had areas displaying iiHFOs outside of the SOZ. Six patients underwent extratemporal resections. Three had class I outcome, none of them showed positive iiHFO-SOZ correlation. From the remaining 3 patients with outcome class II or III all had positive iiHFO-SOZ correlation. All of the extratemporal cases had areas with iiHFO outside of the SOZ. Conclusions: iiHFOs in the range of 80-200Hz can be identified during VEEG evaluations using subdural macroelectrodes and sampling frequencies of 500Hz. In contrast to prior published data we did not find the presence of iiHFOs to be restricted to the epileptogenic zone as we did not find that a higher iiHFO-SOZ correlation was reflected on better postsurgical outcome both for temporal and extratemporal cases. We did not find the presence of iiHFO outside of the SOZ to be related with worse outcome in either group. iiHFOs in the range of 80-200 might not be as useful as faster iiHFOs (>250Hz) in identifying the epileptogenic zone.