Abstracts

Rationale: Focal cortical dysplasia (FCD) is an important cause of focal seizures in children[1] and can often lead to epilepsy that is refractory to medical treatments. Previous studies have shown that quantitative measures of network connectivity differ between adult patients with focal cortical dysplasia and healthy controls[2]. Furthermore, tools used in quantitative network analysis may be help improve localization of the seizure onset zone, thereby improving outcomes in respective epilepsy surgery[3]. The goal of this study was to develop a quantitative method for comparing network characteristics in pediatric patients with FCD, with a future goal of applying similar methods to pre-operative intracranial EEG in these patients. Methods: In order to quantify and compare global measures of network connectivity, interictal scalp EEG from 30 patients with FCD and 30 controls without FCD was analyzed using a previously described method[4]. Twenty-minute epochs free from movement artifact were identified visually and then signal from that epoch was band-pass filtered (0.5 to 55 Hz). We then used a cross-correlation method from the HERMES toolbox to identify correlations maximum at non-zero time lag in one-second segments of the 20-minute EEG[5]. Statistical significance of each value was calculated using the method of repeated surrogate data available in HERMES. Finally we calculated the proportion of 1-second segments of the 20-minute EEG that showed statistical significance. This proportion was used as the estimate of the strength of each node-to-node network connection. We then the Brain Connectivity Toolbox to calculate and compare network measures[6]. Results: Mean clustering coefficient (CC), mean node degree (ND), mean betweenness centrality (BC), and mean global efficiency (GE) were calculated for the network for each subject. Mean clustering coefficient was significantly different between the two groups. Controls had mean CC of 0.098 (SD 0.032) and cases had mean CC 0.13 (SD 0.062). Comparison of these means with a 2-sided student’s t-test shows that they are significantly different, p-value 0.02. Similarly, mean GE for controls (0.12, SD 0.035) and for cases (0.15, SD 0.065) were significantly different, p-value 0.01. Node degree and betweenness centrality were not different between these two groups using this method. Conclusions: Quantitative network measures derived from interictal scalp EEG in pediatric patients with FCD were significantly different from healthy controls. These findings are similar to findings previously published for adult subjects. Further research is needed, using pre-operative intracranial EEG data, to understand whether similar quantitative methods can be used to localize seizure onset zone in pediatric patients with FCD. This analysis implies that similar quantitative methods can be used in pediatric patients to analyze epileptic functional networks and, potentially, aid in identifying the seizure onset zone. Funding: No funding was received in support of this abstract.