Botulinum Toxin A reduces neurogenic flare but has almost no effect on pain and hyperalgesia in human skin.

Botulinum toxin A (BoNT/A) has been used therapeutically to treat muscular hypercontractions and sudomotor hyperactivity. There is increasing evidence that BoNT/A might also have analgesic properties, in particular in headache. In the present investigation we tested the often cited hypothesis that BoNT/A-induced analgesia can be attributed to inhibition of neuropeptide release from nociceptive nerve fibers. In 15 healthy volunteers BoNT/A (5, 10, 20 mouse units BOTOX) or saline (contralateral side) was injected intracutaneously on the volar forearm. On day zero, the day of injection, no further tests were performed. We repeatedly elicited pain, mechanical hyperalgesia and neurogenic flare by transcutaneous electrical stimulation simultaneously on the BoNT/A and saline treated side on day 1, 2, 3, 7 and 14 after injection. Before each session, sweating and local anhidrosis was assessed by iodine starch staining.BoNT/A suppressed sweating as early as from the second day after injection (p < 0.001). The size of electrically induced flare was smaller on the BoNT/A treated arm (BoNT/A side: 21.46 cm(2) +/- 3.58, saline side 24.80 +/- 3.46, p < 0.005) and BoNT/A reduced electrically-induced pain by about 10 % (p < 0.001). However, hyperalgesia to pin-prick and allodynia after electrical stimulation were unchanged. In conclusion our results indicate that peripheral neuropeptide release is attenuated by BoNT/A. In contrast, the analgesic effect of BoNT/A was very limited. Therefore we assume that other than neuropeptide mechanisms must be important for BoNT/A induced pain relief in clinical pain syndromes.