The effects of Botulinum Toxin type A on capsaicin-evoked pain, flare, and secondary hyperalgesia in an experimental human model of trigeminal sensitization.

The trigeminovascular system is involved in migraine. Efficacy of Botulinum Toxin type A (BoNT-A) in migraine has been investigated in clinical studies but the mechanism of action remains unexplored. It is hypothesized that BoNT-A inhibits peripheral sensitization of nociceptive fibers and indirectly reduces central sensitization. We examined the effect of intramuscular injection of BoNT-A on an experimental human model of trigeminal sensitization induced by intradermal capsaicin injection to the forehead. BoNT-A (BOTOX) or saline was injected intramuscularly in precranial, neck and shoulder muscles to 32 healthy male volunteers in a double blind-randomized manner. Intradermally capsaicin-induced pain, flare and secondary hyperalgesia were obtained before and 1, 4 and 8 weeks after the above treatments. A significant suppressive effect of BoNT-A on pain, flare and hyperalgesia area was observed. The pain intensity area was significantly smaller in BoNT-A group (9.16+/-0.83 cm x s) compared to saline group (15.41+/-0.83cm x s) (P=0.011). The flare area was also reduced significantly in BoNT-A group (29.81+/-0.69 cm2) compared to saline group (39.71+/-0.69 cm2) (P<0.001). Similarly, the mean area of secondary hyperalgesia was significantly smaller in BoNT-A group (4.25+/-0.91 cm2) compared to saline group (7.03+/-0.91 cm2) (P=0.040). Post hoc analysis showed significant differences across the trials with a remarkable suppression effect of BoNT-A on capsaicin-induced sensory and vasomotor reactions as early as week1 (P<0.001). BoNT-A presented suppressive effects on the trigeminal/cervical nociceptive system activated by intradermal injection of capsaicin to the forehead. The effects are suggested to be caused by a local peripheral effect of BoNT-A on cutaneous nociceptors.