Botulinum toxin type A for the treatment of hypertrophy of the masseter muscle.

Background: The authors investigated retrospectively the long-term treatment effects of botulinum toxin by analyzing the follow-up data of masseter hypertrophy patients at the Gyalumhan Plastic Aesthetic Clinic, located in Seoul, Korea, from March of 2001 to September of 2007. This is a second follow-up study following the previous study report in 2005. Methods: A total of 121 patients treated for more than 1 year with injection of botulinum toxin type A were included in this analysis. At every patient's visit, masseter muscle thickness was measured using ultrasonography. The dose of injection was 100 to 140 U of Dysport for each side based on the muscle thickness. Results: Of a total 121 patients, six patients received two injections, 28 patients received three injections, 41 patients received four injections, 23 patients received five injections, 16 patients received six injections, six patients received seven injections, and one patient received eight injections. Overall masseter muscle size was reduced from 13.32 mm at the baseline visit to 9.94 mm at the last visit on average. As the number of visits increased through two to eight visits, the mean muscle size was decreased. According to the increase in the number of visits, the mean dose was decreased. There was no significant difference in muscle reduction effect analyzed by age subgroup. The muscle reduction effect after botulinum toxin treatment was better in patients with thicker masseter muscles. Conclusions: Botulinum toxin type A injections have a long-term effect on masseter muscle hypertrophy. A positive correlation was found between the number of injections and the decrease of muscle volume.