Abstracts

Rationale: The distinction of hypersynchronous (HYP) and low-voltage fast (LVF) onset seizures in mesial temporal lobe epilepsy (MTLE) is well established, but classifying individual seizures and patients is often challenging. Experimental work indicates a strong association of HYP with fast ripples (250-500 Hz) and of LVF with ripples (80-250 Hz). We aimed to investigate whether analysis of high-frequency oscillations can be useful for characterizing the process of seizure generation in human MTLE patients. Methods: We retrospectively compared 19 HYP and 14 LVF onset clinical seizures from ten and six consecutive MTLE patients with a predominance of the respective pattern. Five second intervals of stereotactic EEGs from the seizure onset zone were selected, representing pre-ictal and initial ictal periods and, after the large spikes of HYP onsets, the LVF-like pattern that frequently follows. Results: Looking at frequencies below 80Hz, frequency and periodicity of the two LVF patterns (at onset and after HYP spikes) varied considerably within, but not significantly between groups. However, pre-ictal fast ripple density and rate were higher in HYP than in LVF patients. This association was also found for initial ictal segments. Furthermore, fast ripple density and rate were higher during the LVF-like pattern after HYP spikes than during LVF without preceding HYP. Ripple density and rate in contrast did not differ significantly. Fast ripple and ripple amplitude was higher during the LVF-like pattern after HYP spikes when compared to LVF without preceding HYP. Conclusions: We confirm experimental findings regarding the association of fast ripples with HYP onsets, but question that ripples are specifically linked to the LVF pattern in human MTLE patients. LVF occurring after the HYP pattern and “pure” LVF at onset, though appearing visually similar, have different HFO subtype profiles. Our findings thus indicate a clear connection between experimental findings and human epilepsy, and that the mechanisms underlying seizure generation differ substantially between human MTLE patient subgroups. During and immediately before seizures, fast ripple analysis may facilitate classification. Funding: This work was supported by the German Research Foundation (Research Fellowship to J. S.) and by the Canadian Institute of Health Research (grant FDN-143208 to J. G.).