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Trisomy 18 -ˈā(t)-ˈtēn

(Edwards Syndrome; Trisomy E)

By Nina N. Powell-Hamilton, MD, Clinical Assistant Professor of Pediatrics ;Medical Geneticist , Sidney Kimmel Medical College at Thomas Jefferson University;Nemours/Alfred I. duPont Hospital for Children

Trisomy 18 is a chromosomal disorder caused by an extra chromosome 18 that results in intellectual disability and physical abnormalities.

  • Trisomy 18 caused by an extra copy of chromosome 18.

  • Infants are small and have many physical abnormalities and problems with internal organs.

  • Several tests can be done before or after birth to confirm the diagnosis.

  • There is no treatment available for trisomy 18.

Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function.

An extra chromosome, making three of a kind, is called trisomy (see also Overview of Chromosome and Gene Disorders). Children who have trisomy 18 have a third chromosome 18. Trisomy 18 occurs in about 1 out of 6,000 live births. However, many affected fetuses spontaneously abort. The extra chromosome almost always comes from the mother. Mothers who are over age 35 are at increased risk of having a child with trisomy 18. Girls are affected more often than boys.

Symptoms

In the womb, affected fetuses are not typically very active, and there is often excess amniotic fluid and a small placenta.

Physical abnormalities

At birth, newborns are often very small because their muscles and body fat are underdeveloped. Newborns are typically limp and have a weak cry. The mouth and jaw may be small, which gives the newborn's face a pinched appearance. Other visible deformities are common, including a small head, low-set and malformed ears, a narrow pelvis, and a short breastbone (sternum). Physical abnormalities may be obvious at birth. However, some newborns have abnormalities that are not as severe.

The hands are clenched in fists, and the index fingers overlap the middle and ring fingers. The fingernails are underdeveloped. Skinfolds, especially over the back of the neck, are common. The big toes are shortened and frequently bend upward. Clubfeet and rocker-bottom feet are common.

Common Types of Clubfoot

Internal abnormalities

Internal organs also have defects. Severe abnormalities may be present in the heart, lungs, digestive tract, and kidneys. Boys may have undescended testes. Newborns may also have hernias, muscles that have separated from the abdominal wall, or both.

Diagnosis

  • Before birth, ultrasonography of the fetus or blood tests of the mother

  • Chorionic villus sampling, amniocentesis, or both

  • After birth, the appearance of the infant and blood tests of the infant

Before birth, trisomy 18 may be suspected based on findings detected during an ultrasound of the fetus. Doctors can sometimes find deoxyribonucleic acid (DNA) from the fetus in the mother's blood and use this DNA to detect an increased risk of trisomy 18. If doctors suspect trisomy 18 based on these tests, they often confirm the diagnosis using chorionic villus sampling, amniocentesis, or both (see Testing for chromosome and gene abnormalities).

After birth, the infant's physical appearance may suggest the diagnosis of trisomy 18. To confirm the diagnosis of trisomy 18, the infant's chromosomes are analyzed using a blood test.

Prognosis and Treatment

  • Support for the family

There is no specific treatment available for trisomy 18. More than 50% of children die within the first week, and less than 10% are still alive at 1 year of age. Children who survive have severe developmental delay and disability. Family members should seek support.

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