Study of functional cortical connectivity in patients affected by drug resistant epilepsy treated with vagal nerve stimulation.

Objectives To explore functional brain connectivity [namely Phase Lag Index (PLI), Clustering Coefficient (Cw) and Characteristic path length (Lw)] in patients affected by drug-resistant epilepsy (DRE) treated with vagal nerve stimulation (VNS) by means of high density EEG. Materials Seven patients (3 females, 4 males, 27-63 y) waiting for VNS implantation, with drug-resistant epilepsy, non-eligible for epilepsy surgery, good compliance to electroencephalographic (EEG) recordings and no psychiatric comorbidity were selected for this study. Method Patients underwent VNS surgery; scalp signals were recorded using a 64 channel EEG system and was performed before and 1 year (12-14 months) after the VNS surgery. Signals were acquired during a 40 min eyes-closed resting state, at a distance from seizures or post-ictal state. For each subject ten artifact-free epochs using average reference were selected for analysis, analyzed separately in four bands (alfa: 8-12 Hz; beta: 12-30 Hz; theta: 3-8 Hz; delta: 0,5-3 Hz) and exported. Phase Lag Index (PLI), Clustering Coefficient (Cw) and Characteristic path length (Lw) were derived using Brain-wave software. Cw and Lw value computed for every patient in every frequency band before and after VNS substitution were compared using Student's t-test. Each EEG was compared to the same patient’s recording performed before VNS surgery. Results Every band shows an upward trend for clustering coefficient and a downward trend for characteristic path length after the substitution with the exception of theta band which shows an opposite trend. Ratio between Cw or Lw value after and before VNS surgery wasn’t statistically significant for any of the rhythms examined (p value 0,0721 - 0,9287). Discussion The peculiar trend observed in theta band is a marker of reduced synchronization. Previous studies demonstrated that VNS induce gamma band desynchronization in temporal lobe drug-resistant epilepsy. Theta band is related to encoding of new information and episodic memory; hippocampal theta band closely correlates with Long-Term Potentiation process and neuronal plasticity. Therefore, the peculiar trend of theta band could be related (if confirmed by larger studies) to hippocampal neuronal plasticity. Conclusions This preliminary study paves the way for various hypotheses on the effects of VNS on neuronal plasticity. It will be necessary to expand the study to a larger number of patients before first VNS implant in order to value functional connectivity before and after VNS.