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Chromosome 18 includes 18p-, 18q-, Ring 18, Tetrasomy 18p, and Trisomy 18.
18p can cause heart defects and holoprosencephaly, a brain malformation, as well as thyroid problems, growth hormone deficiency, and impaired vision, and neurological issues, such as seizures or a movement disorder.

The symptoms of 18q, including Distal 18q- and Proximal 18q-, vary greatly and include short stature, intellectual disability, poor muscle tone (hypotonia), and malformations of the hands and feet.

The problems associated with Ring 18 depend on which parts of chromosome 18 are missing. Most people with Ring 18 have features that overlap with 18p- and Distal 18q-, including holoprosencephaly or a heart defect. Babies with Ring 18 may have jaundice, a cleft palate, or breathing or feeding difficulties.

Tetrasomy 18p can cause heart defects, spina bifida, hernias, foot abnormalities, seizures, kidney problems, hearing and vision difficulties, and breathing or feeding difficulties in newborns.

Trisomy 18 can cause complex medical problems, including defects of the heart, brain, spinal cord, and other internal organs. Cleft lips and palates are also more common in babies with Trisomy 18. They may have hand and foot anomalies. Babies with Trisomy 18 may have difficulty feeding properly and regulating their breathing and temperature.