gms | German Medical Science

Objective: Structural changes are a major cause of pharmaco-resistant focal epilepsy. Surgical therapeutic options are based on the identification of the epileptogenic focus and its structural connectivity. The intention of our study was to analyze structural epileptic connectivity and to approach a diagnostic tool based on quantitative global fiber tractography.

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 3T MRI scanner and whole brain global fiber tracking was performed. A connectivity matrix of the epileptogenic focus and propagation pattern was developed based on functional stereo-EEG data. Therefore, multiple temporal and extratemporal depth electrodes were placed (mean n=10/patient) in frame-based stereotactic surgery and the position confirmed by post-operative MRI. 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. Epileptic fiber tracts (n=318) were extracted and structural connectivity was analyzed along the functional propagation pathways and compared to those without functional connectivity. Based on graph and streamline representations a visualization tool was developed in a web-based visualization and study processing platform (http://www.nora-imaging.org).

Results: The exact localization of ictal epileptogenicity and delineation of fiber tracts by global fiber tracking provides evidence of increased structural connectivity (49.6%) along pathways of epileptic propagation compared to those without functional connectivity (41.1%; p=0.01) enabling the development of a web-based visualization tool.

Conclusion: The analysis of functional and structural connectivity based on a high spatial accuracy and global tractography method reveals an increase of structural connectivity along pathways of early epileptic propagation that can be visualized based on inferred network dynamics. In future, this approach may be advanced to a non-invasive diagnostic tool for presurgical evaluation, development of the surgical strategy and assessment of therapeutic prospects.