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Trisomy 18

Definition

Trisomy 18 is a genetic disorder in which a person has a third copy of material from chromosome 18, instead of the usual 2 copies. Rarely, the extra material may be attached to another chromosome (translocation). Most cases are not passed down through families. Instead, the problems that lead to this condition occur in either the sperm or the egg that forms the fetus.

Causes

Trisomy 18 occurs in 1 in 6000 live births. It is 3 times more common in girls than boys.

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

• Trisomy 18: the presence of an extra (third) chromosome 18 in all of the cells.

• Mosaic trisomy 18: the presence of an extra chromosome 18 in some of the cells.

• Partial trisomy 18: the presence of a part of an extra chromosome 18 in the cells.

Most cases of Trisomy 18 are not passed down through families (inherited). Instead, the events that lead to trisomy 18 occur in either the sperm or the egg that forms the fetus.

Symptoms

Symptoms may include:

• 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)

Exams 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 facial features and fingerprint patterns. X-rays may show a short breast bone.

Chromosome studies will show trisomy 18. The chromosome abnormality may be present in every cell or present in only a certain percentage of the cells (called mosaicism). Studies may also show part of the chromosome in some cells. Rarely, part of the chromosome 18 becomes attached to another chromosome. This is called 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

There are no specific treatments for trisomy 18. Which treatments are used depend on the person's individual condition.

Support Groups

More information and support for people with Trisomy 18 and their families can be found at:

• Support Organization for Trisomy 18, 13 and Related Disorders (SOFT): trisomy.org

• Trisomy 18 Foundation: www.trisomy18.org

• Hope for Trisomy 13 and 18: www.hopefortrisomy13and18.org

Outlook (Prognosis)

One half of infants with this condition do not survive beyond the first week of life. Nine out of ten children will die by 1 year of age. Some children have survived to the teenage years, but with serious medical and developmental problems.

Possible Complications

Complications depend on the specific defects and symptoms.

Complications may include:

• Breathing difficulty or lack of breathing (apnea)

• Deafness

• Feeding problems

• Heart failure

• Seizures

• Vision problems

When to Contact a Medical Professional

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

Prevention

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

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

References

Bacino CA, Lee B. Cytogenetics. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 98.

Madan-Khetarpal S, Arnold G. Genetic disorders and dysmorphic conditions. In: Zitelli BJ, McIntire SC, Nowalk AJ, eds. Zitelli and Davis' Atlas of Pediatric Physical Diagnosis. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 1.

National Center for Advancing Translational Sciences. Genetic and Rare Diseases Information Center website. Trisomy 18. rarediseases.info.nih.gov/diseases/6321/trisomy-18. Updated February 2023. Accessed October 17, 2023.

Review Date: 9/18/2023

Reviewed By: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.