PhD Candidate Year 3, Neuroimaging Western University London, Ontario, Canada
Rationale: In patients with medically-refractory temporal lobe epilepsy (TLE), resective surgery may be beneficial. However, identification of suitable surgical candidates has proven difficult, often requiring stereoencephalography (SEEG) to delineate the epileptogenic zone (EZ). Currently, identification of the mesial temporal sources involves implantation of hippocampal electrodes. However, despite the complex cytoarchitecture and functionally distinct subregions of the hippocampus, the influence of local electrode positioning on the localization of the EZ remains unknown. This study aims to leverage the increased resolution of ultra-high field (7T) MRI to uncover the influence of local hippocampal electrode position on the electrophysiologic profile identified through SEEG. Methods: Seven patients with medically-refractory TLE undergoing clinical SEEG evaluation were imaged using a 7T MRI protocol. The positions of 26 early-ictal active contacts within the bounds of the hippocampus were first localized to the hippocampal head, body, or tail and subsequently hierarchically organized based on relative position in a three dimensional construct: 1) anterior-mid-posterior, 2) medial-central-lateral), and 3) inferior-level-superior. Contacts near the hippocampal bounds were also categorized (extra-lateral white matter, n = 11; inferior white matter, n = 6). Ictal SEEG onset abnormalities were identified across 35 seizures by an expert neurologist and correlated to position to uncover local differences in electrophysiologic profiles. Results: All contacts positioned within hippocampal boundaries and inferior white matter registered at least one early ictal event, while no extra-lateral contacts displayed ictal activity, despite similar proximity to the hippocampal boundaries. Three hippocampal contact positions exhibited the highest probability for recording seizures: 1) Head, mid-central-inferior (22/35; 62.9% of seizures); 2) Head, mid-mesial-inferior (15/35; 42.9%); 3) Head, mid-lateral-inferior (12/35; 34.3%). Conclusions: Local hippocampal electrode position variations across subregions of the hippocampus exhibited different samples of ictal onset electrical activity. Future research should aim to further elucidate the relationship between regional differences in SEEG electrode positioning and measurements of epileptiform activity. Funding: Please list any funding that was received in support of this abstract.: N/A
