Abstracts

Rationale: High frequency oscillations (HFOs) in the ripple (80-250Hz) and fast ripple (250-500Hz) ranges are hypothesized to reflect increases in neural synchrony. In epileptic syndromes, this increase is pathological, and localized to the seizure onset zone. Interictal HFOs have been proposed as a marker for the identification of the seizure onset zone. Recently, the graph index complexity (GIC) of visibility graphs has been introduced as a measure of HFOs, and has been shown to increase ictally at the seizure focus. However, the properties of the GIC have yet to be investigated interictally. Methods: The interictal intracranial electroencephalograms (iEEGs) of 5 patients with focal epilepsy were analyzed using a bipolar montage during 5-10 minutes of continuous slow-wave sleep. The data were bandpass filtered for ripples and labelled using an automated detection algorithm: the filtered data were normalized by the root-mean-square of a sliding 1 second epoch, and ripples were identified wherever the signal exceeded a threshold of 3 standard deviations from the mean during a minimum of six consecutive half-cycles. A visibility graph was constructed for each 1 second epoch of the filtered time-series, with a one-half overlap. The GIC was calculated from each visibility graph, resulting in a time-series of GIC values for each channel. The electrodes corresponding to the seizure onset zone were identified clinically. The mean GIC and number of ripples per minute (ripple rate) for all electrodes within the seizure onset zone, as well as the mean for all electrodes without, were calculated for each patient. A Wilcoxon signed-rank test was performed to determine whether the GIC and ripple rate were greater within the seizure onset zone across patients. Results: The mean ripple rate for each channel group ranged from 1.26 to 21.33, and was significantly greater for channels within the seizure onset zone in all patients (Figure 1), with a median difference of 8.46 (p = 0.0312; Wilcoxon signed-rank test). The mean GIC values for each channel group ranged from 2.28 to 7.22, and was significantly greater for channels within the seizure onset zone in all patients (Figure 2), with a mean difference of 2.25 (p = 0.0312; Wilcoxon signed-rank test). Conclusions: Both the ripple rate and GIC-SD are significantly greater in channels within the seizure onset zone than without, during slow-wave sleep. Both markers of high-frequency oscillatory activity convey information that may be useful in the localization of the seizure onset zone in patients with focal epilepsy.