Focal Hippocampal Onset and its Effect on Surgical Outcomes: A Case Report and Case Series

SUMMARY: In mesial temporal lobe epilepsy, resection or ablation-typically of anterior temporal structures-is highly successful in achieving seizure freedom. However, in a substantial proportion of patients, seizures recur for unclear reasons. Invasive hippocampal recordings suggest that, in some patients, hippocampal seizures are highly focal, often restricted to the anterior or posterior hippocampus. This led us to hypothesize that resection or ablation limited to the hippocampal subregion(s) involved at seizure onset is more likely to control seizures than surgery that does not address the specific onset locations. We present a case in which seizures were successfully controlled after posterior hippocampal ablation, as well as a retrospective case series of 18 patients in which 10 cases demonstrated focal onsets within the anterior or posterior hippocampus. Better outcomes occurred when surgery was concordant with the onset region(s) versus discordant (n = 7 concordant vs. 11 discordant patients, P = 0.046, Barnard Exact test). These data suggest that mesial temporal lobe epilepsy can be subdivided according to whether the hippocampal onset pattern is focal or nonfocal, and that targeting localized seizure onset zones within the hippocampus can be effective for seizure control, while limiting surgical morbidity and potentially the risk of adverse cognitive outcomes.