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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.

Objective: To evaluate the effect of botulinum toxin type A on nocturnal bruxism., Design: Twelve subjects reporting nocturnal bruxism were recruited for a double-blind, randomized clinical trial. Six bruxers were injected with botulinum toxin in both masseters, and six with saline. Nocturnal electromyographic activity was recorded in the subject's natural sleeping environment from masseter and temporalis muscles before injection, and 4, 8, and 12 wks after injection and then used to calculate bruxism events. Bruxism symptoms were investigated using questionnaires., Results: Bruxism events in the masseter muscle decreased significantly in the botulinum toxin injection group (P = 0.027). In the temporalis muscle, bruxism events did not differ between groups or among times. Subjective bruxism symptoms decreased in both groups after injection (P < 0.001)., Conclusions: Our results suggest that botulinum toxin injection reduced the number of bruxism events, most likely mediated its effect through a decrease in muscle activity rather than the central nervous system. We controlled for placebo effects by randomizing the interventions between groups, obtaining subjective and objective outcome measures, using the temporalis muscle as a control, and collecting data at three postinjection times. Our controlled study supports the use of botulinum toxin injection as an effective treatment for nocturnal bruxism.