gms | German Medical Science

Objective: Focal epilepsy due to structural changes is a major cause of pharmaco-resistant epilepsy often treated by epilepsy surgery. Stereo-EEG recording enables the detection of the epileptogenic focus and propagation patterns of the epileptic activity. The intention of our study was to precisely delineate structural pathways of epileptogenic propagation by global fiber tracking.

Methods: Fifteen patients with focal epilepsy undergoing invasive epileptological assessment were included in the study. Pre-operative high-angular diffusion weighted images were acquired with 61 directions on a 3Tesla MRI scanner and whole brain global fiber tracking was performed. Multiple temporal and extratemporal depth electrodes were placed (mean n=10/patient) in frame-based stereotactic surgery. The electrode position was confirmed by post-operative MRI and the epileptogenic focus and early propagation pattern was evaluated by stereo-EEG recording. The exact electrode contact positions (total n=128) detecting the epileptogenic focus and the target points of early propagation were identified on the stereotactic treatment plan and were transferred into a MCP-based coordinate system. MRI T1w3D sequences were superimposed on color encoded DTI images, postprocessed for global fiber tracking and transferred into a common space using an inhouse software. Fiber tracts (n=318) were extracted and connectivity was analyzed along the electroencephalographic propagation pathway. For control, fiber tracts originating in the epileptogenic lesion, but without functional connectivity to randomly chosen contacts (n=90) or corresponding tracts contralaterally (n=318), were analyzed for structural connection.

Results: The exact localization of ictal epileptogenicity and delineation of fiber tracts by global fiber tracking provides evidence of increased structural connectivity (49.6%) of the epileptogenic focus to regions of early propagation (41.1%) compared to those without detected propagation (p=0.01) or the corresponding contralateral tracts (p=0.01).

Conclusion: The analysis of functional and structural connectivity based on a high spatial accuracy and global tractography method, reveals an increase of connectivity along pathways of early propagation of epileptic activity. This method proves to be promising for further investigation regarding the connectivity of epileptogenic lesions and may offer an additional diagnostic method for profound presurgical assessment and subsequent operative therapy.