Paradoxical bulging of mentalis after botulinum toxin type A injection

Background Botulinum toxin type A (BTA) injection is a widely-used procedure to correct cosmetic problems caused by muscular hyperactivity. "Cobblestone chin" refers to the appearance of remarkable chin creases caused by mentalis contraction. Aims The aim of this case report is to raise the awareness of paradoxical bulging after BTA injection in mentalis. Patient/methods A 26-year-old female asked for cosmetic correction of the "cobblestone chin". BTA injection was performed routinely to the mentalis muscle. However, on the second day after injection, an unusual paradoxical bulging at chin was presented. Results We speculated that the imbalanced relaxation of mentalis caused by improper injection and uneven diffusion of BTA is responsible for the bulging. After a supplementary injection of BTA in the remaining hyperactive muscle fibers based on the ultrasound examination, the bulging was relieved and chin contour improved significantly. Conclusion Injection of BTA in mentalis should cover the whole thickness of the muscle to prevent imbalanced paralysis and paradoxical chin bulging, especially in severe mentalis hyperactivity cases.