Abstracts

Rationale:
Failure of first epilepsy surgery and continued seizures occur in nearly one-third of the patients with medically refractory epilepsy (MRE). Reasons for initial surgical failure are commonly attributed to residual epileptogenic zone, incomplete resection, diffuse epileptogenic processes and emergence of new or secondary epileptogenesis. Underlying changes in brain connectivity and network characteristics after resection leading to new network configurations and epileptogenicity has been hypothesized as a complex mode of seizure recurrence, although this remains to be further elucidated. We evaluate brain connectivity changes on routine scalp electroencephalogram (EEG) in a pediatric cohort with repeat epilepsy surgery and describe its effect on seizure generation in relation to changes in seizure semiology and seizure frequency.
Method:
We examined serial scalp EEG data from children with MRE who underwent reoperations after failed surgery. We retrieved scalp EEG data (19-24 channels) that were collected longitudinally from three different periods (see Figure): 1) before first surgery, 2) after first surgery and before reoperation, and 3) after reoperation. Only sleep EEG segments free of any epileptiform activity were used for the analysis (Figure). We computed amplitude envelope correlation between all EEG electrodes as an estimate of underlying functional connectivity; a pairwise orthogonalization approach was used to attenuate the cross-talk between signals and the effect of volume conduction. A minimum spanning tree was built to determine the centrality of each EEG electrode within the child’s brain network at each time period (1,2 and 3). Surgery-related changes in the network were identified comparing the centrality of each electrode before and after each surgery (two-sample t-tests) in relation to the resection. We also explored whether EEG connectivity changes were linked to changes in seizure semiology over the course of epilepsy surgeries.
Results:
We examined 18 EEGs from six children (Table). Following the first (failed) surgery, an increase in connectivity was seen around the resection in four patients (83%, #2,3,4,6): at least one of the EEG electrodes overlaying the resected lobe showed significant increase in centrality (p< 0.05). In two patients (#1,5), who showed changes in seizure semiology post-surgery, the connectivity within the resected EEG electrodes was unchanged (p>0.05) but increased far from the resection cavity. In patient #2, who also showed changes in semiology post-surgery, connectivity increased not only around the resection but also far from it. Reoperations consisted of extension of the previous resection in all patients, plus partial corpus callosotomy in one case (#3) and hippocampectomy(#4) in another. When reoperations led to seizure-freedom (n=3; 50% of cases), a significant decrease in connectivity was seen over the resected area (p< 0.05). In two cases (#1, #6) where reoperations lead to seizure-freedom, decreased connectivity (compared to the first surgery) was seen in the EEG electrodes overlaying the resected regions.
Conclusion:
We demonstrate first evidence using routine scalp EEG to describe serial connectivity changes in children following repeat epilepsy surgery. Our data suggests that successful reoperation is associated with decreased EEG connectivity in the resected areas. Changes in seizure semiology post-epilepsy surgery may be linked to an increase in connectivity far from the resected areas. Plotting brain connectivity changes using scalp EEG (free of epileptiform activity) can be used as an additional tool for understanding epileptogenic networks in patients undergoing evaluation for repeat surgery.
Funding:
:None