Hemifacial microsomia

The underlying cause of hemifacial microsomia remains unknown. However, most researchers agree that something occurs in the early stages of development, such as a disturbance of the blood supply to the first and second branchial arches in the first 6 to 8 weeks of pregnancy.^[1][^[2] Tissue layers in the six pairs of pharyngeal arches give rise to the muscles, arteries, nerves, and cartilage of the face and neck. Specifically, the first and second pharyngeal arches develop into the lower jaw, the nerves and muscles used for chewing and facial expression, the external ear, and the bones of the middle ear. Interference with the normal development of these structures can result in the abnormalities seen in hemifacial microsomia. It is not known why certain disruptions to development affect the first and second pharyngeal arches in particular. Researchers suggest that these structures may develop together in such a way that they respond as a unit to these disruptions.^[2]

There appear to be several factors that can disrupt the normal development of the first and second pharyngeal arches and lead to hemifacial microsomia. Some individuals with this condition have chromosomal abnormalities such as deletions or duplications of genetic material; these individuals often have additional developmental problems or malformations. Occasionally, hemifacial microsomia occurs in multiple members of a family in a pattern that suggests inheritance of a causative gene mutation, but the gene or genes involved are unknown. In other families, individuals seem to inherit a predisposition to the disorder. The risk of hemifacial microsomia can also be increased by environmental factors, such as certain drugs taken by the mother during pregnancy. In most affected individuals, the cause of the disorder is unknown.^[2]

Hemifacial microsomia most often occurs in a single individual in a family and is not inherited.^[2][3] If the condition is caused by a chromosomal abnormality, it may be inherited from one affected parent or it may result from a new abnormality in the chromosome and occur in people with no history of the disorder in their family.^[2] In a very small number of cases, hemifacial microsomia is inherited in an autosomal dominant pattern, which means one copy of an altered gene in each cell is sufficient to cause the disorder.^[2][3] In rare cases, the condition is inherited in an autosomal recessive pattern, which means both copies of a gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition. The gene or genes involved in hemifacial microsomia are unknown.^[2]

In some affected families, people seem to inherit an increased risk of developing hemifacial microsomia, not the condition itself. In these cases, some combination of genetic changes and environmental factors may be involved.^[2]