VNS induced desynchronization in gamma bands correlates with positive clinical outcome in temporal lobe pharmacoresistant epilepsy.

The vagus nerve stimulation (VNS) represents a diffuse non-pharmacological low-risk surgical option for epilepsy treatment. The aim of this study is to investigate the correlation between variations of global EEG synchronization and the clinical outcome in pharmacoresistant epileptic subjects implanted with VNS. Ten subjects affected by pharmacoresistant epilepsy were recruited on the basis of a clear-cut successful or unsuccessful outcome of the VNS add-on treatment. After five years from VNS surgery we examined the EEG in five subjects in each group. The investigation was led with the method of the phase lag index (PLI), which allows for the study of the global rate of synchronicity among the EEG signals before and after VNS implantation. The results of this study show that after five years from VNS surgery, in subjects whose seizures show a significant reduction, the desynchronization in the gamma frequency band is statistically decreased in comparison with patients who failed to show variations in the frequency and characteristics of their seizures. The other frequency bands are unaffected. This finding suggests that long lasting variations in gamma band desynchronization can be a new tool in assessing the efficacy of VNS. The possibility that GABA-mediated VNS-induced effects can also play a role in this result is discussed.