Abstracts

Rationale: In patients with drug-resistant focal epilepsy (DRFE), surgical resection of the epileptogenic zone (EZ) can represent a viable and curative treatment when the EZ is accurately identified and removed. To do so, a comprehensive multimodal non-invasive presurgical evaluation is warranted and, in a number of patients, invasive EEG methods (e.g. stereo-EEG [SEEG]) remain necessary. Previous reports have shown that high-density EEG recordings combined with electric source imaging (HD-ESI) can often accurately localize the EZ, especially in temporal lobe cases. Here, we present preliminary findings of our study aimed to determine the clinical validity and utility of HD-ESI in localizing the EZ in patients with suspected extra-temporal DRFE, using SEEG as a validating method.Methods: Fifty-three consecutive patients with suspected extra-temporal DRFE scheduled to undergo a SEEG investigation where enrolled from June 2013 to May 2015. All patients underwent a 60-minute 256-electrode high-density EEG recording in addition to the standard multimodal non-invasive presurgical evaluation. Interictal discharges (IID) (i.e. spikes, sharp waves, etc.) were marked during offline EEG analysis and exported for HD-ESI. The early component (upslope) of individual and/or averaged IID was modeled with both multiple signal classification (MUSIC) and standardized low-resolution brain electromagnetic tomography (sLORETA) methods. A realistic head model using the boundary element method based on individual high-resolution MRI was used to compute the forward solution. HD-ESI analysis was performed prior to SEEG interpretation and therefore blinded to its result. SEEG was performed and analyzed by independent epileptologists who delineated the EZ. For each method, the concordance between the HD-ESI source maximum and SEEG-defined EZ was determined. HD-ESI results were considered concordant when the source maximum was located inside the EZ.Results: To date, 23 of 53 recruited patients have undergone both HD-ESI and stereo-EEG analysis (age range: 7-50 years). In 3 patients (13%), no IIDs were recorded and therefore, HD-ESI was not performed. Location of the EZ in these 3 patients were: right fronto-polar region, right precuneus and bottom of left intraparietal sulcus. Source localization methods were applied to the remaining 20 patients. HD-ESI localization was concordant with SEEG findings in 85% of cases (17/20) using the MUSIC method and in 80% (16/20) using the sLORETA method. After SEEG analysis, EZ location was shown to be extra-temporal in 17 patients and confined to the temporal lobe in the remaining 6 patients (Table 1).Conclusions: Our preliminary results demonstrate a high concordance between the HD-ESI source maximum and the SEEG-defined EZ when IID are recorded. These findings suggest an added value of HD-ESI during the presurgical evaluation of patients with suspected extra-temporal DRFE that, in the future, might improve both S-EEG electrode placement and SEEG sensitivity.