Objectifying the Role of the Depressor Anguli Oris Muscle Using High-Resolution Ultrasound: A Prospective Study

Background: In the setting of facial palsy, synkinesis of the depressor anguli oris (DAO) muscle is a common pathology, impairing dynamic and resting facial symmetry. In this prospective study, the authors used high-resolution ultrasound (HRUS) to evaluate the morphologic features of DAO and to quantify bilateral differences in DAO cross-sectional diameter (CSD) in individuals with unilateral synkinesis. Methods: From June of 2020 to May of 2021, 30 patients (19 women, 11 men) with clinically diagnosed unilateral synkinesis underwent evaluation with HRUS. DAO CSD was measured bilaterally, 1 cm inferior to the modiolus, in both the resting and smiling positions. Results: The Sunnybrook Facial Grading System Synkinesis Score was 6.20 ± 2.48 (range, 0 to 10). DAO CSD at rest measured 2.41 ± 0.67 mm (range, 1.40 to 4.00 mm) on the control side and 2.66 ± 0.98 mm (range, 1.60 to 5.10 mm) on the affected side. On the control side, DAO CSD was reduced by −0.19 ± 0.43 mm (range, −1.10 to 1.12 mm) during smiling. In contrast, the synkinetic DAO CSD increased by 0.64 ± 0.38 mm (range, 0.00 to 1.59 mm) (P < 0.001) with animation. The pattern of increased CSD in synkinetic DAO muscles and decreased or unchanged CSD on the control side during smiling was identified in 70% of patients. Conclusions: HRUS demonstrates increased CSD in synkinetic DAO muscles during active smiling. The opposite is true for DAO muscles on the control side, which exhibit decreased CSD with animation. HRUS can be performed preoperatively to objectify DAO dysfunction and guide targeted therapy. CLINICAL QUESTION/LEVEL OF EVIDENCE: Diagnostic, IV.