Modulation of neural oscillations by vagus nerve stimulation in posttraumatic multifocal epilepsy: case report

The putative mechanism of vagus nerve stimulation (VNS) for medically refractory epilepsy is desynchronization of hippocampal and thalamocortical circuitry; however, the nature of the dose-response relationship and temporal dynamics is poorly understood. For greater elucidation, a study in a nonepileptic rat model was previously conducted and showed that rapid-cycle (RC) VNS achieved superior desynchrony compared to standard-cycle (SC) VNS. Here, the authors report on the first in-human analysis of the neuromodulatory dose-response effects of VNS in a patient with posttraumatic, independent, bilateral mesial temporal lobe epilepsy refractory to medications and SC-VNS who was referred as a potential candidate for a responsive neurostimulation device. During stereotactic electroencephalography (SEEG) recordings, the VNS device was initially turned off, then changed to SC-VNS and then RC-VNS settings. Spectral analysis revealed a global reduction of power in the theta (4-8 Hz) and alpha (8-15 Hz) bands with both SC- and RC-VNS compared to the stimulation off setting (p < 0.001). Furthermore, in the alpha band, both SC- and RC-VNS were associated with greater global desynchrony compared to the off setting (p < 0.001); and, specifically, in the bilateral epileptogenic hippocampi, RC-VNS further reduced spectral power compared to SC-VNS (p < 0.001). The dose-response and temporal effects suggest that VNS modulates regional and global dynamics differently.