Scalp injection of
botulinum toxin type A (BT-A) into the superficial musculature has evoked interest in the management of
migraine headache. In clinical trials, prevention of
migraine attacks for 3 months or more has been seen in some patients following BT-A scalp
injections. In the majority of
pain syndromes where BT-A is effective, inhibition of
muscle spasms appears to be an important component of its activity. A direct or independent and prolonged
analgesic action unrelated to skeletal muscle relaxation is believed to underlie the prophylactic efficacy of BT-A in
migraine; peripheral and central modulation of
pain impulses by BT-A has also been proposed. A direct peripheral antinociceptive effect was not seen in three controlled studies of BT-A in normal human volunteers. Experimental evidence for BT-A-induced
analgesia in rats is suggestive but dose-dependent and lasts only 2 weeks. In
migraine patients, a consistent or dose-dependent response to BT-A treatment has not been seen. Peak responses to BT-A in
migraine patients are seen at 8-12 weeks, whereas BT-A-affected nerve endings in mice fully recover function between 63 and 91 days; the difference in species limits the interpretation of this dissonance. As BT-A does not normally cross the intact blood-brain barrier, meningeal nociceptors appear unlikely to be influenced by scalp
injections of BT-A; the possibility of antidromic transfer of BT-A in the trigeminovascular system should be considered. The extended period for which
migraine prophylaxis might be required, the antigenic and
headache-provoking potential of BT-A, the inability of BT-A to affect central neuronal processes significantly, including the
aura of
migraine, the possible placebo effect of
needling, and purely subjective outcome measures in
headache studies are additional concerns in evaluating this treatment strategy. The clinical utility of BT-A has not been compared against established
migraine prophylactic agents. The efficacy of BT-A in preventing
migraine headache attacks remains controversial and the underlying scientific rationale is debatable.