Abstracts

Rationale: High frequency oscillations (HFOs) called ripples (80-250 Hz) and fast ripples (FR, 250-500Hz) can be recorded from intracerebral macroelectrodes in patients with intractable epilepsy. HFOs occur predominantly in the seizure onset zone (SOZ) but their relationship to the underlying pathology is unknown. In this study, we analyzed the occurrence of interictal HFOs in 10 patients with focal epilepsy and different pathologies.Methods: Five patients had mesial temporal atrophy, 3 cortical focal dysplasia (CD) and 2 nodular heterotopia (NH). Intracerebral macroelectrodes had 9 contacts, each with a 0.7mm surface. The EEG was filtered at 500Hz and sampled at 2000Hz. Between 7 and 13 channels were selected in each patient, including all lesional channels and a comparable number of non-lesional channels. We also selected additional channels in the SOZ when the SOZ was outside the lesion. HFOs were identified visually during a 10-min period of slow wave sleep, using high-pass filters at 80Hz for ripples and 250Hz for FRs. Rates/min were statistically compared between lesional and non-lesional channels, and between SOZ lesional and non-SOZ lesional channels. Comparisons were made between the different types of lesion.Results: In all patients with mesial temporal atrophy, the SOZ was located in the lesion (2 had additional lesional areas, and one also had seizures originating from the contralateral non-atrophic hippocampus). Several channels were within the CDs, but not all were identified as being in the SOZ. The 2 patients with NH had multiple lesions but the SOZ was in the mesial temporal structures. HFOs were significantly more frequent in lesional than nonlesional areas (ripples, p= 0.03; FRs, p=0.01, table 1 and fig. 1) and, in general, FRs were a better indicator of the SOZ than ripples. In mesial temporal lesions, HFOs were most frequent when lesion and SOZ were congruent (ripples, p< 0.001; FRs, p= 0.005). Nonlesional SOZ in these patients showed fewer HFOs. In patients with CDs, the lesional channels within the SOZ always showed the highest rate of HFO followed by the lesional channels outside the SOZ. In these patients, FRs were significantly more common in lesional versus non-lesional areas (p=0.04). In patients with NH, the nonlesional SOZ channels in the hippocampus were the most active. Five of 10 lesional NH channels showed rare HFOs; the others showed none.Conclusions: Interictal HFOs predominate in lesional areas except in heterotopia. Within a lesion, they are most common inside the SOZ, which is relevant for the identification of highly epileptogenic areas like in CDs or mesial temporal sclerosis. In patients where the SOZ is outside the lesion, such as the two patients with NH, HFOs are more closely related to the SOZ than to the lesion. We conclude that HFOs are more indicative of the SOZ than of the lesion, particularly FRs. The localizing value of HFOs within lesions will have to be definitively evaluated using postsurgical results.