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Hemifacial microsomia is a congenital craniofacial anomaly characterized by underdevelopment of structures derived from the first and second pharyngeal arches, resulting in unilateral facial asymmetry. It represents the second most common facial birth defect after cleft lip and palate, with an incidence of approximately 1 in 3,500 to 5,600 live births.
The condition results from disrupted development of neural crest cells during the 6th to 8th weeks of gestation, leading to inadequate formation of first and second pharyngeal arch derivatives. This developmental anomaly affects multiple structures including the maxilla, mandible, temporal bone, ear, facial muscles, and cranial nerves, with severity ranging from mild asymmetry to complete absence of structures.
Diagnosis is based on characteristic unilateral facial features including mandibular hypoplasia, maxillary deficiency, microtia or aural atresia, and soft tissue underdevelopment. The Pruzansky-Kaban classification system helps grade mandibular involvement from mild hypoplasia to complete absence of the ramus and condyle, guiding treatment planning and surgical intervention timing.