Low intensity vagal nerve stimulation lowers human thermal pain thresholds

The effect of vagal nerve stimulation (VNS) on thermal pain sensation was s tudied in eight subjects who had vagal nerve stimulators surgically implant ed for purposes of seizure control. Prior to their involvement in the study , all subjects had the intensity of their VNS (30 Hz, 0.5 ms, 1.0-2.75 mA) adjusted upwards until achieving their desired clinical effect of reduced s eizures. Thermal pain thresholds were determined using a Medoc TSA-2001 wit h a thermode applied to the skin of the forearm. During VNS at settings 100 % of those used clinically to control their seizures, subjects showed a sta tistically significant decrease in their thermal pain threshold of 1.1 +/- 0.4 degrees C. Acute effects of graded VNS on thermal pain thresholds were determined in seven of the subjects after cessation of chronic VNS. Two the rmal threshold measurements were obtained while the subject received sham s timulation (0 mA intensity), during tactile control stimulation and during 30 s of VNS at intensities approximately 33, 66 and 100% of the settings ut ilized to control their seizures. Tactile control stimulation was provided by electrical stimulation of the skin of the ankle with the intensity adjus ted by the patient to match the intensity of any sensations felt in the nec k during VNS, Subjects were not aware of the settings employed. Their stimu lator was adjusted with each trial and an ascending/descending ordering of intensity was utilized with an inter-trial interval of 2 min. Thermal pain thresholds were significantly decreased in relation to tactile control stim ulation at all intensities of VNS tested with the greatest effect occurring at the 66% level. Subjects were also monitored non-invasively and hemodyna mic responses to VNS were determined. No significant alterations in hemodyn amic variables were observed. The findings of this human study are consiste nt with experiments in non-human animals which demonstrate a pronociceptive effect of low intensity VNS. (C) 2000 International Association for the St udy of Pain. Published by Elsevier Science B.V. All rights reserved.