Combined MRI morphometry and source imaging guide placement of stereo-EEG electrodes in focal epilepsy with subtle or absent lesions

Introduction: Planning stereo-electroencephalography (sEEG) for focal drug-resistant epilepsy with subtle or absent lesions requires accurate non-invasive spatial information about the hypothetical organization of the epileptic focus. The targeting of individual trajectories for a limited number of invasive depth electrodes is particularly challenging in patients who have undergone prior epilepsy surgery. This study investigated how information from multimodal imaging can guide sEEG planning and enable successful epilepsy surgery in patients with non-lesional focal epilepsy. Methods: We studied 15 patients who appeared non-lesional on conventional MRI and were suspected to have mono-focal epilepsy. These patients underwent sEEG implantation between October 2019 and October 2022, based on findings from non-invasive video-EEG monitoring and multimodal imaging. Among the participants, four had undergone prior epilepsy surgery, including three who had previously undergone invasive EEG. All patients underwent high-resolution 3 T MRI and MRI morphometry (MAP) as part of their non-invasive presurgical diagnostics. Electric and magnetic source imaging were performed in patient subgroups. sEEG planning incorporated findings from the available imaging methods registered within the stereotactic planning system. Results: A median of nine sEEG electrodes (range: 7-11) were implanted in each patient, targeting both primary and secondary hypotheses about the epileptic focus location. sEEG recordings revealed a monofocal seizure onset in 12 out of 15 patients, all of whom subsequently underwent epilepsy surgery. No bleeding complications occurred. Of these patients, nine achieved Engel 1 postsurgical outcomes, while three had Engel ≥2 outcomes. Surgery was not performed in three patients due to multifocal epilepsy (n = 2) or an unidentified seizure onset zone (SOZ, n = 1). Concordance across multiple imaging modalities was associated with favorable surgical outcomes. Conclusion: In patients with focal epilepsy and subtle or absent lesions, sophisticated sEEG diagnostics guided by advanced multimodal imaging can successfully identify the seizure onset zone. When focal onset is confirmed and multifocal epilepsy is excluded through sEEG, subsequent epilepsy surgery often results in seizure-free outcomes.