A randomized, double-blind, placebo controlled study on analgesic effects of botulinum toxin A.

OBJECTIVE: Botulinum toxin type A (BTXA) is used to treat neurologic disorders associated with increased muscle tone. Its use is often associated with pain relief.

METHODS: A possible direct analgesic effect of BTXA on C and Adelta fibers was studied on 16 healthy volunteers receiving 30 U BTXA into one forearm and pure saline into the other. To exclude the secondary effect due to muscular tone reduction, BTXA was injected intradermally. Thermal sensory testing of heat pain (threshold and tolerance) and neuroselective current sensory testing of current pain threshold/tolerance were performed at baseline and 3, 14, and 28 days after treatment. Thereafter, on day 28, capsaicin was administered simultaneously into both forearms to evaluate a possible peripheral effect and central effect on pain processing and on the axon reflex flare.

RESULTS: The authors observed no significant difference in any of the perception outcome measures between BTXA and placebo pretreated areas. Flare areas as a result of the release of neuropeptides after capsaicin application showed no differences.

CONCLUSIONS: The results suggest that pain reduction after BTXA treatment is mediated through its effect on muscle tone rather than a direct analgesic effect.