Trisomy 18 is caused by an extra chromosome 18 and is usually associated with intellectual disability, small birth size, and various congenital anomalies, including severe microcephaly, heart defects, prominent occiput, low-set malformed ears, and a characteristic pinched facial appearance.
Trisomy 18 occurs in 1/6000 live births, but spontaneous abortions are common. More than 95% of affected children have complete trisomy 18. The extra chromosome is almost always maternally derived, and advanced maternal age increases risk. The female:male ratio is 3:1.
Symptoms and Signs
A history of feeble fetal activity, polyhydramnios, a small placenta, and a single umbilical artery often exist. Size at birth is markedly small for gestational age, with hypotonia and marked hypoplasia of skeletal muscle and subcutaneous fat. The cry is weak, and response to sound is decreased. The orbital ridges are hypoplastic, the palpebral fissures are short, and the mouth and jaw are small; all of these characteristics give the face a pinched appearance. Microcephaly, prominent occiput, low-set malformed ears, narrow pelvis, and a short sternum are common. A clenched fist with the index finger overlapping the 3rd and 4th fingers usually occurs. The distal crease on the 5th finger is often absent, and there is a low-arch dermal ridge pattern on the fingertips. Redundant skinfolds, especially over the back of the neck, are common. The fingernails are hypoplastic, and the big toe is shortened and frequently dorsiflexed. Clubfeet and rocker-bottom feet are common. Severe congenital heart disease is common, especially patent ductus arteriosus and ventricular septal defects. Anomalies of lungs, diaphragm, GI tract, abdominal wall, kidneys, and ureters are frequent. Boys may have undescended testes. Common muscular manifestations include hernias, separation of the rectus muscles of the abdominal wall, or both.
Diagnosis may be suspected prenatally on ultrasound (eg, with abnormalities of extremities and fetal growth restriction) or by amniocentesis or chorionic villi sampling or postnatally by appearance. Confirmation in all cases is by karyotyping.
No specific treatment is available. More than 50% of children die within the first week; < 10% are still alive at 1 yr. Children who survive have marked developmental delay and disability. Support for the family is critical.
Last full review/revision December 2012 by Nina N. Powell-Hamilton, MD
Content last modified January 2013