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Rationale:  Patients with dual pathology have two potentially epileptogenic lesions: One in the hippocampus and one in the neocortex. If epilepsy surgery is considered, stereotactic electroencephalography (SEEG) may reveal which of the lesions is seizure-generating, but frequently, some uncertainty remains. We aimed to investigate whether interictal high-frequency oscillations (HFOs), which are a promising biomarker of epileptogenicity, are associated with the primary focus.   Methods:  We retrospectively analyzed 16 patients with dual pathology. They were grouped according to their seizure-generating lesion, as suggested by ictal SEEG. An automated detector was applied to identify interictal epileptic spikes, ripples (80–250 Hz), ripples co-occurring with spikes (IES-ripples) and fast ripples (250–500 Hz). We computed a ratio R to obtain an indicator of whether rates were higher in the hippocampal lesion (R close to 1), higher in the neocortical lesion (R close to −1), or more or less similar (R close to 0).   Results:  Spike and HFO rates were higher in the hippocampal than in the neocortical lesion ( p  < 0.001), particularly in seizure onset zone channels. Seizures originated exclusively in the hippocampus in 5 patients (group 1), in both lesions in 7 patients (group 2), and exclusively in the neocortex in 4 patients (group 3). We found a significant correlation between the patients' primary focus and the ratio R fast ripples , i.e., the proportion of interictal fast ripples detected in this lesion ( p  < 0.05). No such correlation was observed for interictal epileptic spikes ( p  = 0.69), ripples ( p  = 0.60), and IES-ripples ( p  = 0.54). In retrospect, interictal fast ripples would have correctly “predicted” the primary focus in 69% of our patients ( p  < 0.01).   Conclusions:  We report a correlation between interictal fast ripple rate and the primary focus, which was not found for epileptic spikes. Fast ripple analysis could provide helpful information for generating a hypothesis on seizure-generating networks, especially in cases with few or no recorded seizures.

Interictal Fast Ripples Are Associated With the Seizure-Generating Lesion in Patients With Dual Pathology