Determining the Position, Morphology, and Location of the Depressor Anguli Muscle Via Ultrasound in the Caucasian Population

Abstract Background The Depressor Anguli Oris (DAO) muscle plays a key role in expressing sadness by lowering the oral commissures. Botulinum toxin (BT) is frequently used to weaken DAO hyperactivity, but results remain inconsistent due to reliance on cadaveric studies and surface landmarks, often derived from Asian populations. Ethnic anatomical variability may compromise injection accuracy. Objectives Evaluate DAO morphology in Caucasian patients using high-frequency ultrasound. Methods 290 Caucasian patients seeking DAO treatment with BT were assessed. Exclusion criteria were neuromuscular disease or previous BT in the area. Ultrasound evaluated DAO position relative to the mid-pupillary line, depth from the skin, overlap with adjacent muscles. Results DAO localization showed significant variability: 20.2% were aligned with the classical mid-pupillary landmark, 79.8% exhibited lateral displacement (2–5 mm). Mean DAO depth was 3.1 mm, greater in overweight/obese patients (p<0.001). Overlap with the depressor labii inferioris occurred in 12%. Conclusions DAO topography in Caucasians differs significantly from that reported in standard references. Blind injections may reduce the treatment efficacy and increase the side-effect rate. High-frequency ultrasound enables precise DAO mapping, individualized injection planning, and improved treatment predictability. Pre-injection ultrasound should be considered for safe and effective DAO neuromodulation.