A prospective study of the effect of botulinum toxin A on masseteric muscle hypertrophy with ultrasonographic and electromyographic measurement.

We evaluated the effect of botulinum toxin A on masseteric muscle hypertrophy by using ultrasound and electromyography. Five patients (four with bilateral and one with unilateral masseteric muscle hypertrophy) were studied prospectively. In each patient, ultrasound-guided percutaneous intramuscular injection of botulinum toxin A was carried out. The change in muscle bulk was evaluated using serial ultrasonography and the electrical activity was assessed with electromyography. All five patients (nine hypertrophic muscles) showed a good response, with the maximal effect of a 31% reduction in muscle bulk seen 3 months after treatment. The effect remained stable one year after injection for six of the hypertrophic muscles, whereas three muscles needed a second injection to maintain the atrophy. This preliminary prospective study suggests that botulinum toxin A is a safe alternative method of treating masseteric muscle hypertrophy. However, the effect may be temporary and further intramuscular injection may be required to maintain atrophy. Copyright 2001 The British Association of Plastic Surgeons.