Effect of botulinum toxin on pressure pain threshold and EMG power spectrum of masseter muscle during sustained fatiguing contraction.

Objectives: The aims of this study were to test the effect of botulinum toxin on a reversible, prolonged, experimental pain and on muscle fatigue in human masseter muscle, both caused by a sustained clench.

Design: The masseter muscles were injected with botulinum toxin in 19 subjects and with isotonic saline in 16 subjects. We measured electromyographic activity at maximum voluntary contraction, pressure pain threshold before and after a 70% maximum voluntary contraction clench sustained to pain tolerance, and the median frequency of the electromyographic power spectrum during that clench at baseline and at 1, 2, and 3 mos after botulinum toxin or saline injection.

Results: After botulinum toxin injection, the botulinum toxin group had a reduced maximum voluntary contraction for the ensuing 3 mos and smaller decreases in pressure pain threshold from before to after the sustained clench. Also, the change in median frequency from before to after the sustained clench did not significantly differ during the postinjection sessions. However, postinjection, preclench median frequency was lower in the group injected with botulinum toxin.

Conclusions: We interpret the reduced change in pressure pain threshold with botulinum toxin as a clinically modest but statistically significant analgesic effect on this model of acute muscle pain.