To the Editor:—
Recently, Byas-Smith et al. 1reported that tourniquet constriction expands and exacerbates pain during intradermal injection of capsaicin in humans. The underlying mechanism was unclear. It is suggested herein that excitation of paravascular nociceptors is involved in expansion of pain.
Capsaicin has been shown to evoke pain from skin, 2muscle, 3and paravascular tissue, but not from veins. 4In the latter study, one of the authors had a disconcerting experience. Capsaicin was perfused through a vascularly isolated hand vein segment to test capsaicin for its property to excite vascular nociceptors. It definitively did not, but strong pain occurred distant from the perfusion site and spread to the entire forearm. In fear of spreading pain to the entire body, a tourniquet was installed quickly to the upper arm, which, however, increased pain further, up to an unbearable intensity. It was determined that capsaicin solution had drained via a previously unnoticed side branch of the isolated vein segment into the venous system. From there, capsaicin apparently had gained access to the paravascular space (capsaicin does not evoke pain in veins). The substantial increase in capsaicin-induced pain during tourniquet inflation is unknown. A recruitment of myelinated fibers during ischemia has been discussed;5however, fostering by venous congestion of transendothelial crossing of capsaicin to the paravascular tissue also may play a role. Thus, the spread of capsaicin from the site of application to the paravascular space may have contributed, at least in part, to the observations made by Byas-Smith et al. 1