Deep-plane paradoxical masseteric bulging after dual-plane botulinum neurotoxin injection: a case report.

Paradoxical masseteric bulging (PMB) is a distressing complication typically attributed to compensatory contraction of untreated superficial muscle fibers. However, the anatomical mechanism of bulging that occurs despite dual-plane injection remains poorly defined. We report a case of deep-plane paradoxical compensatory masseteric bulging in which the protrusion originated from the inner functional compartment of the superficial layer of the masseter muscle following planned dual-plane botulinum neurotoxin type A injection. Dynamic ultrasonography demonstrated complete paralysis of the outer sub-layer with paradoxical thinning during clenching, while the inner sub-layer exhibited marked compensatory thickening beneath a distinct internal aponeurosis. Targeted deep-plane reinjection under ultrasound guidance resulted in complete resolution. This case highlights the compartmentalized anatomy of the masseter muscle and emphasizes the importance of anatomical and sonographic evaluation in refractory PMB.