Trisomy 18 is a genetic disorder in which a person has a third copy of material from chromosome 18, instead of the usual two copies.A.D.A.M.

Causes, incidence, and risk factors

Trisomy 18 is a somewhat common syndrome. It is three times more common in girls than boys.

The syndrome occurs when there is extra material from chromosome 18. The extra material affects normal development.

Symptoms

• Clenched hands
• Crossed legs
• Feet with a rounded bottom (rocker-bottom feet)
• Low birth weight
• Low-set ears
• Mental delay
• Poorly developed fingernails
• Small head (microcephaly)
• Small jaw (micrognathia)
• Undescended testicle
• Unusual shaped chest (pectus carinatum)

Signs and tests

An exam during pregnancy may show an unusually large uterus and extra amniotic fluid. There may be an unusually small placenta when the baby is born.

A physical exam of the infant may show unusual fingerprint patterns. X-rays may show a short breast bone. Chromosome studies will show trisomy 18, partial trisomy, or translocation.

Other signs include:

• Hole, split, or cleft in the iris of the eye (coloboma)
• Separation between the left and right side of the abdominal muscle (diastasis recti)
• Umbilical hernia or inguinal hernia

There are often signs of congenital heart disease, such as:

• Atrial septal defect (ASD)
• Patent ductus arteriosus (PDA)
• Ventricular septal defect (VSD)

Tests may also show kidney problems, including:

• Horseshoe kidney
• Hydronephrosis
• Polycystic kidney

Treatment

Treatment of children with trisomy 18 is planned on a case-by-case basis. Which treatments are used depend on the patient's individual condition.

Support Groups

• Support Organization for Trisomy 18, 13 and Related Disorders (SOFT) -- www.trisomy.org
• Trisomy 18 Foundation -- www.trisomy18.org
• Hope For Trisomy 13 and 18 -- www.hopefortrisomy13and18.org

Expectations (prognosis)

Half of infants with this condition do not survive beyond the first week of life. Some children have survived to the teenage years, but with serious medical and developmental problems.

Complications

Complications depend on the specific defects and symptoms.

Calling your health care provider

Genetic counseling can help families understand the condition, the risks of inheriting it, and how to care for the patient.

Prevention

Tests can be done during pregnancy to find out if the child has this syndrome.

Genetic testing is recommended for parents who have a child with this syndrome and who want to have more children.

Review Date: 7/8/2012.

Reviewed by: Chad Haldeman-Englert, MD,FACMG, Wake Forest School of Medicine, Department of Pediatrics, Section on Medical Genetics, Winston-Salem, NC. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.