What is the function of the depressor anguli oris (DAO) muscle and how do neuromodulators like Botox (Botulinum toxin) affect it?

Depressor Anguli Oris Function and Effects of Botulinum Toxin

The depressor anguli oris (DAO) muscle functions to pull down the corner of the mouth, and botulinum toxin effectively weakens this muscle by blocking acetylcholine release at the neuromuscular junction, resulting in temporary flaccid paralysis that lasts for weeks to months. 1

Anatomy and Function of the Depressor Anguli Oris

The DAO is a thin, superficial facial muscle located below the corner of the mouth with the following characteristics:

• Originates from the mandible and inserts at the modiolus (corner of the mouth)
• Primary function is to pull the oral commissure downward and laterally
• When hyperactive, it creates a downturned mouth appearance that can make individuals look sad, tired, or angry 2
• Anatomically complex as its medial border overlaps with the depressor labii inferioris muscle, while its lateral border is adjacent to the risorius, zygomaticus major, and platysma muscles 3

Mechanism of Action of Botulinum Toxin on DAO

Botulinum toxin (Botox) affects the DAO through the following mechanism:

1. The toxin consists of a heavy chain (100,000 daltons) and light chain (50,000 daltons) 1
2. After injection, it enters peripheral cholinergic nerve terminals
3. Blocks acetylcholine transmission at the neuromuscular junction
4. Results in temporary flaccid paralysis of the muscle
5. Recovery occurs through sprouting of new nerve terminals over weeks to months 1

Clinical Applications for DAO Botulinum Toxin Injections

Botulinum toxin injections into the DAO are primarily used for:

• Treating downturned mouth corners that create a sad or angry appearance 2
• Improving facial symmetry in patients with synkinetic facial paralysis 4
• Enhancing lower face rejuvenation and creating a more harmonious jawline 5

Optimal Injection Technique

For optimal results when injecting botulinum toxin into the DAO:

• Use anatomical landmarks for precise targeting:

  • The labiomandibular fold (LMF) serves as a convenient landmark 5
  • Injection at the middle and lower thirds of the LMF at a 4-5 mm depth is recommended 5
  • The fan-shaped area bounded by the lateral and medial borders of the DAO and the mandibular border is the safest injection site 3

• Technical considerations:

  • EMG guidance may be used for precise targeting in complex facial musculature 1
  • Careful injection technique is essential to avoid complications
  • The skin-to-muscle depth varies from approximately 5.26 mm medially to 8.42 mm laterally 5

Outcomes and Effectiveness

Recent research demonstrates:

• Botulinum toxin effectively weakens the DAO, improving the appearance of downturned mouth corners 2
• In patients with facial asymmetry, DAO muscle block significantly improves resting symmetry, modiolus angle, and exposure of teeth during smile 4
• The effects are temporary, typically lasting several months before requiring repeat injection 1

Potential Pitfalls and Complications

Important considerations to avoid complications:

• Precise anatomical knowledge is crucial as the DAO has complex relationships with adjacent muscles
• Incorrect injection can lead to:
  • Asymmetrical smiles
  • Unintended weakening of adjacent muscles (depressor labii inferioris, risorius, or platysma)
  • Difficulty with certain facial expressions
• The DAO becomes thinner and wider from superior to inferior aspects, requiring adjusted dosing 5
• The overlapping of the DAO with the depressor labii inferioris increases from superior to inferior aspects 5

Clinical Decision Making

When considering botulinum toxin for DAO treatment:

1. Assess the patient's facial anatomy and muscle activity
2. Use ultrasound guidance when available to identify the precise location of the DAO
3. Consider that approximately one-third of patients may not have DAO muscle fibers in the upper portions of the labiomandibular fold 5
4. Start with appropriate dosing based on muscle bulk and desired effect
5. Monitor for asymmetry and adjust subsequent treatments accordingly