Rethinking Paradoxical Bulging of the Masseter Muscle Following Botulinum Toxin Injection: An Ultrasound Evaluation.

Background: Paradoxical masseter bulge (PMB) is an uncommon complication following treatment with botulinum neurotoxin-A (BoNT-A). This is currently believed to be caused by the uneven distribution of BoNT-A within the masseter, resulting in stronger, compensatory, contraction of the superficial head relative to its deeper heads. Objectives: To visualize under ultrasound which part of the masseter muscle is chiefly responsible for PMB and to propose a framework for assessment and prevention of the complication. Methods: A case series of 6 patients presenting with PMB were identified in the practice of 2 experienced doctors over 24 months. Case notes, photographs, and videos were reviewed along with static and dynamic ultrasound examinations to assess masseter architecture and patterns of contraction. Results: In 7 of 8 affected masseter muscles, PMB was associated with retained movement of a deeper portion of the masseter bulging superficially during contraction. In all cases, the bulge appeared to originate from anterior or posterior compartments of the muscle. This supports the concept that masseter contraction can be classified functionally by anterior or posterior compartments, rather than anatomically by the traditional superficial, deep, and intermediate heads. Conclusions: Paradoxical muscle bulging of the masseter may originate from the deeper portion of the muscle bulging superficially. In addition, the authors propose that masseters can be considered functionally as having anterior and posterior functional units. This classification may enable clinical and ultrasound preassessment for anterior or posterior dominance and the injection technique to be adjusted to minimize the risk of PMB. Level of Evidence 5 Diagnostic: