Micrognathia (Small Lower Jaw)

Micrognathia, or a small lower jaw, can appear in three clinical forms: as an isolated finding, micrognathia that is associated with a certain genetic disorder (combined with additional findings), or micrognathia as part of Pierre Robin sequence.

What are the symptoms?

When inherited micrognathia is an isolated finding, the problem is usually purely aesthetic. However, certain functional difficulties may occur which can require a patient to undergo corrective orthodontic surgery for lengthening the lower jaw and for bite correction.

When associated with Pierre Robin sequence, the smaller lower jaw causes a backward displacement of the tongue that disrupts the closure of the palate. This causes many patients to be born with a cleft palate. These cases include severe micrognathia and are characterized by a high level of amniotic fluid due to obstruction of the esophagus. 

15% of patients also have intellectual disabilities, most likely caused by breathing difficulties that occur immediately after birth. In addition, 15% of patients have heart defects that can be detected through echocardiography.

Additional symptoms of Pierre Robin sequence include ear infections that may cause hearing impairments, breathing difficulties, feeding difficulties, finger deformities, and more.

Cases of micrognathia associated with a genetic disorder may include various symptoms. Usually, other symptoms will raise more concern than the smaller jaw. For instance, trisomy 13 can cause polydactyly, heart defects, kidney defects, brain defects, and more. Another example is trisomy 18, in which 90% of patients do not survive past the first year of life.


How is micrognathia inherited?

When micrognathia is an isolated finding, the inheritance pattern is autosomal dominant. When this finding is part of a set of symptoms caused by a certain genetic disease or disorder, the inheritance type varies and depends on the disorder or disease.

In Pierre Robin sequence, most cases are caused by de-novo mutations. Only in rare cases is this condition inherited in an autosomal recessive or x-linked pattern.


Can this defect be detected in an ultrasound scan?

Micrognathia can be diagnosed through a fetal ultrasound scan. When detected, a fetal echocardiography is also required, along with comprehensive testing including amniocentesis, and in some cases FISH testing to detect deletions in chromosome 22 (Velo-cardio-facial syndrome).

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