Abstracts

Rationale: High Frequency oscillations (HFOs) known as ripples (80-250Hz) and Fast Ripples (FRs, 250-500Hz) can be recorded from macroelectrodes inserted in patients with intractable focal epilepsy. HFOs have been linked to epileptogenesis in rats and seen in human interictal and ictal recordings. While ripples occur in normal and pathological tissue, it is hypothesized that FRs reflect pathological hypersynchronous events. Interictal HFOs may have a localizing value, as they seem to occur predominantly in the seizure onset zone (SOZ). HFOs, however, frequently occur at the same time as EEG spikes and it is not clear if they have an independent localizing value.Methods: Intracerebral EEGs were studied in 10 patients with intractable focal epilepsy using macroelectrodes with 9 contacts, each with a 0.7 mm surface. The EEG was filtered at 500Hz and sampled at 2000Hz. In each patient, between 7 and 13 channels were selected according to the presence or absence of spikes and to their location within or outside the SOZ. Spikes and HFOs were visually identified independently, in two separate copies of the EEG, in a 10-min slow wave sleep sample. High-pass filters at 80Hz and 250Hz were used to identify ripples and FRs, respectively. The co-occurrence of HFOs and spikes was determined using the timing of the events. The percentage of spikes containing HFOs and the rate of ripples and FRs were evaluated for each channel. A one-way ANOVA was performed to detect significant differences in HFOs according to anatomical region (temporal vs. extratemporal), spiking activity and SOZ.Results: In total, 81 channels were analyzed (57 spiking, 24 non-spiking; 29 in SOZ, 52 outside SOZ). HFOs could be observed in spiking channels (ripples 81%; FRs 61%) as well as non-spiking channels (ripples 63%; FRs 63%). A high proportion of HFOs occurred independently of spikes. The rate of occurrence of ripples and FRs was significantly higher inside than outside SOZs (p < 0.001). Also, the proportion of spikes containing ripples and FRs was significantly higher in SOZ channels than in channels outside the SOZ (table 1, p< 0.001).In all anatomical structures, HFOs were more frequent in the SOZ than outside the SOZ. HFOs were also more frequent in the SOZ located in the hippocampus and the amygdala than in other cortical areas (p< 0.001)(fig.1). However, especially in the hippocampus, ripples seemed to be less specific to the SOZ than FRs and could also be seen in the contralateral hippocampus.Conclusions: HFOs occur frequently with spikes but also often in non-spiking channels. They are more frequent in mesial temporal structures than other cortical areas. Independently of the anatomical structure, areas of seizure onset present a higher rate of HFOs and a higher percentage of spikes containing HFOs. Therefore, HFOs are likely to provide additional information on epileptogenicity independently of spiking activity. Whether these findings have predictive value for the areas of seizure onset and post surgical outcome in individual patients will have to be evaluated in future studies.