What is the surgical management of hypoplasia of the depressor anguli oris muscle?

Surgical Management of Hypoplasia of Depressor Anguli Oris

Surgical intervention for congenital hypoplasia of the depressor anguli oris (DAO) muscle is generally not recommended, as the condition is primarily a cosmetic concern that typically does not require treatment and often improves with age.

Clinical Recognition and Diagnosis

The condition presents as asymmetric crying facies in newborns, with the affected side showing failure of the lower lip to depress during crying or smiling 1. Key diagnostic features include:

• Asymmetry is most pronounced during crying, smiling, or full mouth opening 1, 2
• The face appears symmetric at rest 1
• Other facial nerve functions remain intact (forehead wrinkling, eye closure, nasolabial fold depth) 1
• The unaffected side shows normal downward and lateral lip movement 1

Critical diagnostic pitfall: Ultrasound studies demonstrate that in the majority of cases (6 of 7 patients), the DAO muscle is actually well-developed on the affected side, suggesting the disorder may be neurogenic rather than purely muscular 3. This finding is crucial because it indicates that muscle resection procedures may not address the underlying pathophysiology.

Mandatory Associated Anomaly Screening

Before considering any intervention, comprehensive evaluation for associated congenital anomalies is essential 1:

• Cardiac evaluation with echocardiography (to exclude congenital heart disease, particularly ventricular septal defects and coarctation) 1
• Chromosomal analysis for 22q11 deletion syndrome 2
• MRI of temporal bones and internal auditory canals 1
• Auditory brainstem response testing 1
• Assessment for other craniofacial anomalies (cartilaginous tags, accessory tragi) 4

Surgical Options and Outcomes

When surgical intervention is pursued in severe cases persisting into adulthood, the evidence is limited but suggests:

Ineffective Approaches

• Resection of the depressor labii inferioris muscle has proven ineffective 2
• This failure supports the theory that the condition is not purely due to muscle hypoplasia 3

Potentially Effective Approach

• Bidirectional (horizontal and vertical) fascia lata grafting showed improvement in one adult case with severe asymmetry 2
• This technique improved lower lip symmetry during full mouth opening at 1-year follow-up 2
• The mechanism likely involves providing structural support rather than restoring muscle function 2

Conservative Management Strategy

The preferred approach is observation and reassurance, as:

• The condition is primarily cosmetic with no functional impairment 1
• Many cases show spontaneous improvement with facial growth and development
• Surgical outcomes are unpredictable and limited by the underlying neurogenic component 3

Role of Adjunctive Therapies

Contralateral botulinum toxin injection may be considered in select cases to achieve temporary symmetry by weakening the normal side 4. However:

• This requires precise anatomical knowledge of the DAO muscle boundaries 5
• The optimal injection site is the fan-shaped area bounded by specific anatomical landmarks (44.7±13.7 degrees from the sagittal line through the modiolus) 5
• Long-term efficacy data are lacking 4

Key Clinical Pitfalls

• Do not assume the condition is purely muscular hypoplasia - imaging often shows normal muscle development, suggesting neurogenic etiology 3
• Do not proceed with surgery without comprehensive screening for associated anomalies, particularly cardiac defects 1
• Avoid resection of the depressor labii inferioris muscle - this has been shown to be ineffective 2
• Recognize that preoperative DAO paralysis significantly impacts postoperative outcomes in patients undergoing macrostomia repair 4

When to Consider Surgical Intervention

Surgery should only be considered in:

• Adults with severe, persistent asymmetry causing significant psychosocial distress 2
• Cases where conservative management and observation have failed 2
• Patients with realistic expectations understanding that complete symmetry may not be achievable 4

The surgical approach should involve fascia lata grafting rather than muscle resection, performed by surgeons experienced in facial reanimation procedures 2.