Pain relief in complex regional pain syndrome due to spinal cord stimulation does not depend on vasodilation

Background: Spinal cord stimulation (SCS) is known to relieve pain in patie nts with complex regional pain syndrome (CRPS) and, in general, to cause va sodilation. The vasodilatory effect of SCS is hypothesized to be secondary to inhibition of sympathetically mediated vasoconstriction, or through anti dromic impulses resulting in release of vasoactive substances. The aim of t he present study was to assess whether pain relief in CRPS after SCS is, in fact, dependent on vasodilation. In addition, we tried to determine which of the potential mechanisms may cause the vasodilatory effect that is gener ally found after SCS. Methods: Twenty-four of 36 patients with unilateral CRPS responded to the t est of SCS. Twenty-two of these 24 responders (hand, n = 14; foot, n = 8) w ho had undergone previous sympathectomy were enrolled for the study. In add ition, 20 control subjects (10 controls for each extremity) mere studied. B y means of laser Doppler flowmetry, the skin microcirculation of the patien ts was measured bilaterally while the SCS system was switched off and while it was activated, Control subjects (n = 20) were tested once only. The rat io of the rest flow at heart level and the dependent position was defined a s the vasoconstriction index. Results: Both. in affected hands and feet, patients mere found to have lowe r vasoconstriction indices (P < 0.01) as compared with controls, indicating a decreased sympathetic tone, Applying SCS did not result in any microcirc ulatory change as compared with baseline or the contralateral clinically un affected side. Conclusions: The current study failed to show that SCS influences skin micr ocirculation in patients with CRPS and a low sympathetic tone. Therefore, w e may conclude that pain relief in CRPS due lo SCS is possible without vaso dilation. Because sympathetic activity was greatly decreased in our patient s, these results support the hypothesis that the vasodilation that is norma lly found with SCS is due to an inhibitory effect on sympathetically mainta ined vasoconstriction.