Abstracts

Rationale: Localizing and delineating the epileptogenic zone (EZ) is crucial for successful epilepsy surgery. When the electro-clinical and imaging data are insufficient to delineate the EZ, invasive EEG (iEEG) can be used. In three experienced European centers, we systematically compared patient characteristics, results of presurgical workup, risk profiles, diagnostic yield and outcome after implantation between subdural grids and stereoelectroencephalography (sEEG). Methods: We retrospectively included 264 consecutive patients undergoing iEEG between 2010 and 2016 at the University Medical Centre Utrecht, Great Ormond Street Hospital for Children, and National Hospital for Neurology and Neurosurgery in London. Three groups were delineated based on the iEEG method: 1) subdural grids with/without ≤2 depth electrodes, 2) sEEG, 3) subdural grids with >2 depth electrodes. The hypothesis based on the pre-implantation workup was compared to the hypothesis after iEEG. A change in hypothesis was defined as either “sublobar discordance” (when the iEEG defined EZ was not part of the pre-implantation hypothesis), “extent discordance” (when the EZ was more extensive than suspected), or “multifocality discordance” (when an initial focal EZ was suspected, but iEEG revealed mutifocal sources). Any unexpected adverse event that was either symptomatic or required medical treatment or surgical intervention was considered a complication. Seizure outcome was defined according to the Engel classification. Results: We included 173 adults and 91 children (range 2-60 years). Of the 264 patients, 144 (55%) were in the group 1 (exclusively grids: n=96, 36%), 85 (32%) in the group  2 and 35 (13%) in group 3. The percentage of MRI-negatives differed significantly between the three groups (group 1: n=29%, group 2: 42%, group 3: 20%. X2=7.3, p < 0.05). The iEEG could not define a clear EZ in 15 patients. In 53 (22%) patients the iEEG led to a change in hypothesis, this rate did not significantly differ between the three iEEG groups. (Sub)lobar discordance was seen in 27 patients, a more extended EZ in 13 and multifocality in 13. There was a significant difference in the percentage of patients who did not proceed to resection between the sEEG group (n=30, 35%) and the grid group (n=30, 21%) (X2=22, p<0.05), remaining significant when correcting for MRI-negativity (p<0.05). Seizure outcome after at least 1 year follow-up was available for 166 patients: 84 (51%) had an Engel 1 score, there was no significant difference between the groups (X2=1.5). The complication rate did not significantly differ between the groups (group 1: n=25 (17%), group 2: n=10 (12%), group 3: n=10 (29%)). Conclusions: We found no difference in discordance rates between the pre-implantation hypothesis and the iEEG hypothesis or complication rates between the three groups.  We found a difference in the percentage of patients who did not proceed to resection after iEEG. This is probably due to the selection of patients for a certain iEEG method: subdural grids when a clear hypothesis is already available and the EZ or functional zones need to be delineated, and sEEG when the hypothesis is less clear or when there are multiple possible EZs. There appears to be a clear role for each technique, and consequently each surgical center should offer both, with indications depending on individual patient characteristics and pre-implantation hypotheses, rather than on individual centers customs and beliefs. Funding: WKZ Fonds, NVKN