Bulging of the Masseter Muscle Following Botulinum Neurotoxin Type A Injection.

Botulinum neurotoxin toxin type A is widely used in the management of facial paralysis to reduce synkinesis and improve facial symmetry. Although the masseter muscle is not innervated by the facial nerve, aberrant reinnervation may lead to abnormal activity requiring treatment. We report the case of a 48-year-old woman with long-standing, partially regenerated right-sided facial palsy who developed paradoxical masseter muscle bulging following botulinum neurotoxin toxin type A injection. After a single injection targeting masseter hypertonicity, the patient exhibited a visible bulge during clenching. Ultrasound imaging revealed superficial muscle bulging associated with intramuscular septations. Uneven toxin diffusion caused by internal tendinous barriers within the masseter muscle was identified as the most likely mechanism. This case highlights the importance of understanding masseter anatomy and applying precise injection techniques to minimize adverse effects and optimize therapeutic outcomes.