Do repetitive botulinum neurotoxin injections induce muscle fibrosis? Sonographic observation of the masseter muscle.

[OBJECTIVE] In the esthetic field, the masseter muscle is commonly targeted by botulinum neurotoxin for facial contouring. However, multiple botulinum neurotoxin injections have been reported to cause muscle fibrosis. Ultrasonography can be useful for clinical consideration in such cases.

[MATERIALS AND METHODS] This study presents nine cases of masseteric fibrosis caused by repeated botulinum neurotoxin injections with ultrasonographic analysis of full and partial masseteric fibrosis.

[RESULTS] Repetitive botulinum neurotoxin injections resulted in reduced masseter muscle volume, which frequently appeared hyperechoic on ultrasonography. The hyperechoic region was mostly located in the deep and posterior portions; however, in some cases, it was observed throughout the muscle, including the superficial, deep, or both areas.

[CONCLUSION] The fibrotic masseter muscles appear hyperechoic, and ultrasonography is necessary to analyze the degree and location of fibrosis. Predictions can be made for cases in which botulinum neurotoxin injections may have less of an effect after ultrasonography. Because muscle fibrosis can be localized, it is necessary to confirm the degree and location of fibrosis before determining the effective area of injection. In clinical practice, muscle fibrosis may be visible in a specific area where blind injections are administered.