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

Area of Science:

• Medical imaging
• Ultrasound technology
• Facial nerve disorders

Background:

• Synkinesis of the depressor anguli oris (DAO) muscle is a common complication of facial palsy, affecting facial symmetry.
• This condition impairs both dynamic and resting facial appearance.
• Objective assessment of DAO muscle changes is crucial for effective management.

Purpose of the Study:

• To evaluate the morphologic features of the DAO muscle using high-resolution ultrasound (HRUS).
• To quantify bilateral differences in DAO cross-sectional diameter (CSD) in patients with unilateral synkinesis.
• To determine if HRUS can objectively differentiate between affected and unaffected DAO muscles during facial animation.

Main Methods:

• A prospective study involving 30 patients with diagnosed unilateral synkinesis.
• High-resolution ultrasound (HRUS) was used to measure DAO cross-sectional diameter (CSD) bilaterally.
• Measurements were taken 1 cm inferior to the modiolus in both resting and smiling positions.

Main Results:

• Synkinetic DAO muscles showed an increased CSD (0.64 ± 0.38 mm) during smiling, unlike control sides which decreased (-0.19 ± 0.43 mm).
• This pattern of increased CSD in the affected DAO and decreased CSD in the control DAO during animation was observed in 70% of patients.
• DAO CSD at rest was 2.41 ± 0.67 mm on the control side and 2.66 ± 0.98 mm on the affected side.

Conclusions:

• HRUS effectively demonstrates increased CSD in synkinetic DAO muscles during active smiling.
• Control side DAO muscles show decreased CSD with animation.
• HRUS is a valuable preoperative tool for objectively assessing DAO dysfunction and guiding therapy in facial palsy patients.