Hemifacial Microsomia: Branchial Arch Involvement
Hemifacial microsomia primarily affects structures derived from the first and second branchial arches, resulting in unilateral hypoplasia of craniofacial hard and soft tissues. 1, 2, 3
Primary Branchial Arch Involvement
First Branchial Arch Structures
- Mandibular hypoplasia is the hallmark feature, ranging from mild asymmetry to complete absence of the mandibular ramus, condyle, and temporomandibular joint 1, 2, 3
- Maxillary involvement occurs secondarily due to inadequate mandibular vertical growth on the affected side, leading to failure of midface growth 1
- Masticatory muscle underdevelopment affects muscles of mastication derived from the first arch 3
Second Branchial Arch Structures
- Ear malformations are characteristic, ranging from minor deformities to complete absence (microtia or anotia) 3, 4
- Facial nerve defects may occur, occasionally presenting with facial nerve paralysis 3
- Facial muscle hypoplasia affects muscles of facial expression derived from the second arch 3
Clinical Spectrum and Severity
The condition demonstrates extremely variable expressivity, creating a spectrum from mild facial asymmetry to severe craniofacial malformation 1, 2. The severity classification directly correlates with treatment approach:
- Type I (mild): Minimal mandibular hypoplasia with facial asymmetry and ear deformity 1, 4
- Type IIA (moderate): More pronounced mandibular hypoplasia with functional impairment 1
- Type IIB and Type III (severe): Absent or severely hypoplastic mandibular structures including ramus, glenoid fossa, and temporomandibular joint 1
Associated Dental Manifestations
- Tooth agenesis occurs more frequently on the affected side, with severity correlating to the degree of hemifacial microsomia 5
- Third molars are most commonly missing teeth in these patients 5
- Dental inclusions may occur but show no relationship to severity 5
Important Clinical Distinction
While hemifacial microsomia is mentioned in the context of craniofacial malformations requiring imaging for treatment planning 6, it must be distinguished from other conditions causing progressive facial asymmetry. Hemifacial microsomia shows progressive asymmetry with growth over years, not acute onset 7. This distinguishes it from acute neurological conditions like DIPG, which would present with rapid symptom onset over weeks to months 7.
Treatment Implications Based on Arch Involvement
The dysfunctional matrix concept guides treatment: correcting the mandibular deficiency (first arch) minimizes secondary maxillary distortion and reduces the need for later maxillary and orbital osteotomies 1. Early mandibular reconstruction in severe cases (Type IIB and III) converts the deformity to a more functional Type IIA status, maximizing growth potential 1.