Effects of two different units of botulinum toxin type a evaluated by computed tomography and electromyographic measurements of human masseter muscle.

BACKGROUND: Masseteric hypertrophy is recognized as an asymptomatic enlargement of one or both masseter muscles. Botulinum toxin type A injection into the masseter muscle can be alternatively used as a noninvasive treatment for masseteric hypertrophy. The aim of this study was to evaluate the effects of two different quantities of botulinum toxin type A on the thickness and cross-sectional area and electromyographic changes in the masseter muscle using computed tomography and electromyographic measurement.

METHODS: Thirty-two Korean subjects were enrolled in this study. Twenty-five units of botulinum toxin type A was injected bilaterally in the masseter muscle of 16 subjects, and 35 U was injected bilaterally in the remaining 16. The thickness and cross-sectional area of the masseter muscle were measured at three positions before and 12 weeks after injection using computed tomography. The electromyographic changes in the masseter muscle during maximum voluntary clenching were also evaluated before and 2, 4, 12, and 24 weeks after injection. The difference in the results was statistically analyzed.

RESULTS: Statistically significant differences before and after injection were observed when the dimensions and electromyographic values of the masseter muscle were evaluated. However, there was no significant difference between the 25-U and 35-U injections of botulinum toxin type A. The least reduction of muscle thickness was found at the site farthest from the injection point. Several mild side effects were reported, all of which were temporary and localized.

CONCLUSIONS: Botulinum toxin type A is effective in treating hypertrophy of the masseter muscle, but the dose of the toxin is not associated with its effectiveness within the limits of this study.