Abstract
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 Rfast 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.
Highlights
Temporal lobe epilepsy is the most frequent cause for drugresistant seizures [1]
Most clinicians have focused on interictal epileptic spikes for decades and resection of spike-generating tissue correlates to some degree with post-surgical outcome in neocortical epilepsy [8]
More recent studies suggest that high-frequency oscillations (HFOs), divided into ripples (80–250 Hz) and fast ripples (250–500 Hz), might have additional value when it comes to understanding epileptic networks and identifying epileptic foci
Summary
Temporal lobe epilepsy is the most frequent cause for drugresistant seizures [1] These patients have a higher chance of achieving seizure freedom if treated by epilepsy surgery rather than prolonged medical therapy [2, 3] and surgical outcomes are better if imaging revealed a potentially epileptogenic lesion [4, 5]. Stereotactic electroencephalography (SEEG) may be helpful, but especially if only few seizures were captured, remaining uncertainty is considerable [7]—and patients rarely become seizure-free [1]. Even more in such scenarios, analysis of interictal activity may contribute substantially to presurgical evaluation. These tools enable us to analyze HFOs in a clinical routine setting
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