Compensatory Masseteric Bulging: A Novel Observation and Its Implications for Botulinum Neurotoxin Injection Techniques.

[AIMS] Compensatory masseteric bulging, a newly identified complication, arises from repeated botulinum neurotoxin injections targeting the lower mid-masseter in East Asians. This phenomenon occurs when untreated upper muscle layers hypertrophy to compensate for weakened lower regions, disrupting facial symmetry. Traditional injection strategies, focused on the lower muscle bulk, overlook the masseter's complex three-layered anatomy (superficial, middle, deep), increasing asymmetry risks.

[METHODS] To prevent compensatory bulging, a retrograde, layered injection technique is proposed, distributing botulinum neurotoxin evenly across the upper, middle, and lower masseter. Ultrasound guidance enhances precision, ensuring toxin delivery to targeted layers while avoiding diffusion into adjacent muscles (e.g., risorius). Personalized dosing, adjusted for muscle thickness, activity, and treatment history, minimizes localized over-atrophy.

[RESULTS] A 34-year-old female developed upper masseter bulging after four lower mid-masseter botulinum toxin sessions over two years. Ultrasound revealed upper hypoechoic hypertrophy (12 mm thickness) contrasting with lower hyperechoic atrophy (5 mm). Injecting 50 units of LetibotulinumtoxinA into the upper masseter reduced hypertrophy (8 mm post-treatment), restoring facial symmetry.

[DISCUSSION] Compensatory bulging underscores the need for holistic treatment addressing the entire muscle. Layered injections, guided by ultrasound and tailored dosing, mitigate asymmetry risks. Clinicians must adopt comprehensive strategies, integrating anatomical insights and advanced imaging, to optimize aesthetic outcomes in masseter hypertrophy management.