Abstracts

Rationale: Interictal high frequency (100-700Hz) oscillations (iiHFO) have been described during invasive EEG evaluations of patients with refractory temporal lobe epilepsy. Prior studies have shown there is a spatial relation of iiHFOs to the seizure onset zone and that the resection of iiHFO areas correlates with better surgical outcome. Intraoperative Electrocorticography (ECoG) is used to identify interictal spikes (iiS) and tailor the extent of resection. The presence of iiHFOs in ECoG and its potential use to define the epileptogenic zone has not been reported. The goal of this study is to identify iiHFO during ECoG in patients undergoing temporal lobe resections. The frequency and distribution of iiHFO in relation to iiS before and after resection is analyzed. Methods: This study included 10 patients who underwent temporal lobectomy (6 right, 4 left) guided in part by iiS in ECoG obtained with sampling frequency of at least 1000Hz (2 with 1000Hz, 8 with 2000Hz) and postoperative follow up of at least 6 months. Retrospective review of ECoG data as well as chart review was performed. iiHFO and iiS were identified by visual analysis and localized to the specific electrodes before and after resection. The presence, location and frequency of iiHFO and iiS were analyzed as well as its relation to pathology findings and post surgical outcome.Results: iiS were seen in 9 patients before resection and in 5 after. The number of electrodes displaying iiS decreased in 7 patients after resection, did not change in 2 and increased in 1. The frequency of iiS decreased in 7, remained unchanged in 1 and increased in 2 patients. iiHFO were seen in all patients before resection and in 1 after. The number of electrodes displaying iiHFO decreased in all patients and only 1 showed iiHFO after resection. Before resection, iiS and iiHFO showed a partially overlapping distribution in 6 patients. iiHFO area was a subset of the iiS area in 2 patients. iiHFO generating area included electrodes which did not display iiS in 7 patients and 1 had iiHFO but no iiS. Pathology reported hippocampus sclerosis in 5 patients and astrogliosis in 5, these results had no significant correlation to the presence of iiS or iiHFO before or after resection. Outcome at 6 to 18 months after surgery, reported in 9 patients (one died), was good (Engel class Ia-Ib) in 8. One patient had poor outcome (Engel class III), this patient had iiHFOs post resection, and increased iiS and iiHFOs frequency after resection.Conclusions: iiHFOs in the range of 100-700Hz can be identified in ECoG in patients with temporal lobe epilepsy who undergo resection under general anesthesia. The distribution of iiHFOs is not entirely concordant with the distribution of iiS. The presence of iiHFOs following temporal lobe resections, according to this very limited sample, has a better correlation to postoperative outcome than the presence of post resection iiS. This may be useful during the interpretation of ECoG in order to decide the location, extension and margins of resection even in the absence of recorded seizures.