Acute single photon emission computed tomographic study of vagus nerve stimulation in refractory epilepsy

Purpose: Left-sided vagus nerve stimulation (VNS) is an efficacious treatme nt for patients with refractory epilepsy. The precise mechanism of action r emains to be elucidated. Only limited data on VNS-induced changes in region al cerebral blood flow (rCBF) are available. The aim of this study was to i nvestigate rCBF changes during initial VNS with single-photon emission comp uted tomography (SPECT). Methods: In 12 patients (8 women, 4 men) with mean age of 32 years and mean duration of epilepsy of 19 years, VNS-induced rCBF changes were studied by means of a Tc-99m-ethyl cysteinate dimer activation study with a single-da y split-dose protocol before and immediately after initial stimulation. Ima ges were acquired on a triple-head camera with fan-beam collimators and wer e reconstructed with scatter and attenuation correction. After coregistrati on to a standardized template, both a semiquantitative analysis using prede fined volumes-of-interest (VOIs) as well as voxel-by-voxel analysis of the intra-subject activation were performed. During follow-up, efficacy of VNS in terms of seizure-frequency reduction was studied. Results: The semiquantitative analysis, with reference to the total counts in all VOIs, revealed a significant decrease of activity in the left thalam us immediately after the initial stimulation train. These results agreed wi th voxel-by-voxel analysis. In our study ipsilateral thalamic hypoperfusion was the most significant finding. Mean frequency of complex partial seizur es was reduced from 30 per month before implantation to six per month after implantation. Conclusions: VNS induces rCBF changes immediately after initial stimulation that can be studied with SPECT. VNS-induced changes in the thalamus may pl ay an important role in suppression of seizures. However, no significant re lation between the level of hypoperfusion and subsequent clinical efficacy was found.