Abstracts

Rationale: Dual pathology patients with mesial temporal lobe epilepsy (MTLE) have two potentially epileptogenic lesions: One in the hippocampus and another one in the neocortex. If epilepsy surgery is considered in drug-resistant subjects, stereotactic electroencephalography (SEEG) may reveal which of the lesions is primarily seizure-generating. Frequently, however, some uncertainty remains, especially if only few seizures were captured. We aimed to investigate whether interictal high-frequency oscillations, which are a promising biomarker of epileptogenicity, are associated with the primary focus. Methods: We retrospectively analyzed 13 consecutive patients with MTLE and dual pathology who had been admitted to the Freiburg Epilepsy Center for SEEG. Post-implantation MRI was used to identify electrode contacts located inside the hippocampal or neocortical lesion. We first grouped patients according to their primarily seizure-generating lesion, as revealed by ictal SEEG. A support vector machine-based detector was then applied to determine rates of interictal epileptic spikes, ripples (80-250 Hz), ripples co-occurring with spikes (IES-ripples) and fast ripples (250-500 Hz) during a 1-hour episode of slow-wave sleep. We computed a ratio R of mean rates (hippocampus - neocortex) / (hippocampus + neocortex) for each of these events to obtain an indicator of whether events were more frequent in the hippocampal lesion (R close to 1), more or less similar (R close to 0) or more frequent in the neocortical lesion (R close to -1). Results: Seizures originated exclusively in the hippocampal lesion in 5 patients (group 1), in both lesions in 5 patients (group 2), and exclusively in the neocortical lesion in 3 patients (group 3). We found a significant correlation between the group to which the patient had been assigned, i.e. his primary focus, and the ratio R, i.e. the proportion of interictal fast ripples detected in this lesion (p = 0.006, Spearman’s rank order correlation). No significant correlation was observed for interictal epileptic spikes (p = 0.38), ripples (p = 0.34), and IES-ripples (p = 0.99). In general, ripple and fast ripple rate were significantly higher in the hippocampal lesion than in the neocortical lesion (p = 0.001, Mann-Whitney U test). Conclusions: We report a strong correlation between interictal fast ripple rate and the primary focus, which was not found for epileptic spikes. Application of our tool could provide helpful information for generating a hypothesis on seizure-generating networks, especially in cases with few or no recorded seizures. Whether this improves surgical outcome will have to be demonstrated by a prospective study. Funding: Berta-Ottenstein-Programme, Faculty of Medicine, University of Freiburg; German Research Foundation (DFG; JA 1725/4-1)