Botulinum toxin type A and EMG: A key to the understanding of chronic tension-type headaches?

Background.-The pathogenesis of chronic tension-type headache remains unclear, and the role of muscle tension is especially controversial. Botulinum toxin type A, a potent inhibitor of muscle tone, has been used to treat chronic tension-type headache. Objective.-To determine whether clinical response to treatment of chronic tension-type headache with Botox A parallels changes in resting muscle activity recorded through serial electromyography (EMG). Methods.-We randomly assigned eight patients with chronic tension-type headache to pericranial injection of 500 MU Botox A versus placebo (isotonic saline). Results.-At 6 and 12 weeks following treatment, there were no significant differences in clinical outcome between the placebo and the Botox A groups. This occurred despite EMG evidence of a reduction in resting muscle activity in the Botox A-treated patients. Conclusion.-These results support the hypothesis that peripheral mechanisms such as increased muscle tone play, at most, a minor role in the pathophysiology of chronic tension-type headache.