Abstracts

To date, Vagus Nerve Stimulation (VNS) is the only FDA approved effective electrical stimulation treatment of epilepsy. Although recent studies hypothesize how VNS effectively reduces seizure frequency in many medication refractory patients, its mechanism of action remains unclear.
Human studies of scalp recorded EEG have shown variable results with respect to VNS related changes. Although some EEG studies demonstrate changes in occurrence of interictal epileptiform discharges with VNS, corresponding electrocorticography (ECoG) studies are virtually nonexistent. To our knowledge, there have been no frequency analyses of ECoG with VNS. Power spectra demonstrate the relative power of regional cortical activity at a broad range of frequencies. Coherence is the cross-correlation of frequency content between regions.
In this study, we analyze the effects of VNS on power spectra and coherence of interictal ECoG. Two patients with medication refractory seizures and functioning VNS were admitted to our epilepsy monitoring unit for intracranial EEG monitoring for epilepsy surgery evaluation. ECoG was recorded with subdural electrodes placed over the most active cortical regions, as determined by video-EEG scalp recordings. 12 hours of continuous video-ECoG were recorded, 6 hours prior to turning VNS ON, and 6 hours after. Two hours of ECoG were analyzed, one hour with VNS ON and one hour with VNS OFF. Power and coherence spectra during these two epochs were compared for regions with greatest interictal epileptiform activity. In patient 1 a broad reduction in ECoG power occurred with VNS ON. For patient 2 the opposite effect was observed. For patient 1, the coherence between adjacent electrode pairs was decreased at lower frequencies (3-7 Hz) with VNS ON, but was unchanged at the remaining frequencies. A broad peak in coherence at 7-9 Hz (the interictal spike rate) occurred with VNS ON for patient 2. Above 11 Hz there was a broad decrease in coherence with VNS ON for patient 2. VNS had opposite effects on ECoG power spectra from the two patients.
VNS resulted in opposite changes in ECoG coherence at lower frequencies, and a broad decrease in ECoG coherence in one patient at higher frequencies.
Different patterns of ECoG frequency response to VNS can occur both within and between regions. This may reflect different mechanisms of action for VNS.
Further frequency analyses of ECoG in these and other patients will be performed to determine the different patterns of change with VNS in regions adjacent to and remote from the epileptic focus. This could elucidate what response patterns can be predictive indices of VNS efficacy.