Abstracts

Rationale: Rationale: Invasive EEG (iEEG) monitoring is used when the seizure-onset zone (SOZ) is not evident or is discorcordant with a non-invasive pre-surgical assessment in patients with drug-resistant focal epilepsy. Subdural electrodes are frequently used for localization and functional mapping in complex patients. We reviewed iEEG with respect to application and outcome. Methods: Methods: 84 patients with drug-resistant focal epilepsy underwent iEEG with placement of subdural electrodes as part of a comprehensive pre-surgical evaluation. Post-surgical follow-up was at least 1 year and resection performed at a single center with 1 epilepsy surgeon. Patient demographics, implanted sites, electrode array, number of contacts/site, and type of surgery were correlated with outcome. Surgery was individualized and tailored to the anticipated SOZ. Hypothesis proportions testing and Fisher s Exact test (p=<0.05) determined significance of electrode designs with the student-t test used for case-control comparison. Results: Results: 84 patients with focal epilepsy (49 female) aged 31.9 years had an MRI lesion in 25/84 (29.8%) and a VNS in 28/84 (33.3%). In total, 65/84 (77%) underwent resective surgery. iEEG was composed of strip electrodes alone in 59.5%, grids alone (4.8%), or a combination of both in 35.7%. Bihemispheric electrode placement was most prevalent in 61%. Strip electrodes were the most common electrode type in 57% of patients with suspected TLE. Grid use with more contacts/lobe was more prevalent in suspected extratemporal lobe epilepsy. Targets were bitemporal in 49%, unihemispheric in 31%, and unilateral extratemporal in 7%. 23/84 (27.4%) of implanted patients became seizure free (SF) with resective surgery. 25/84 (25%) were rejected or not improved. The use of iEEG portended a non-seizure-free (NSF) outcome compared with a historical control of 46/80 (58%) obtained from the epilepsy surgery RCT (p= <0.0005). Electrode design including the array, EEG localization, EEG lateralization, and surgery performed were similarly not predictive of outcome (p=NS). Of those with lesions on MRI, 11/18 (61.1%) were seizure free v 4/18 (22%) that had VNS pre-operation (p= .0405). Conclusions: Conclusions: iEEG is a negative predictor for a SF outcome, still 3/4th of patients offered iEEG undergo a definitive procedure. Electrode design for iEEG was not predictive of a SF outcome. Lateralization of the SOZ is the most common indication for iEEG with bilateral strip use greatest in suspected TLE and grid use greatest in ETE. While outcomes differ, individual center use of iEEG in the absence of a lesion should include the reduced expectation for a SF outcome during pre-surgical counseling.