Edwards Syndrome, commonly known as Trisomy 18, is a rare genetic disorder caused by the presence of a third copy of all or part of chromosome 18 which causes abnormal development in many of the baby’s organs. About 1 in every 5,000 babies is born with Trisomy 18, and most are female.
There are three types of Trisomy 18:
- Full Trisomy 18. The extra chromosome is in every cell in the baby’s body. This is the most common type of Trisomy 18.
- Mosaic Trisomy 18. The extra chromosome 18 is only in some of the baby’s cells. Approximately 5% of individuals born with Trisomy 18 will carry this type. This form is also rare.
- Partial Trisomy 18. Affected individuals have two copies of chromosome 18, plus the extra material from chromosome 18 (part of the long (q) arm) attached to another chromosome. This type of Trisomy 18 is very rare.
Women of all ages can have a child with Trisomy 18, however the chance increases as a woman gets older. A first trimester screening that includes a blood test and ultrasound offers early information about a baby’s risk of having Trisomy 18. A second trimester blood test called a quad screen can also aid in early detection. More precise methods take cells from the amniotic fluid (amniocentesis) or placenta (chorionic villus sampling) and analyze the chromosomes to confirm diagnosis.
Individuals with Trisomy 18 often suffer from intrauterine growth retardation and subsequently, low birth weight. Some features of Trisomy 18 include having a small, abnormally shaped head, small jaw and mouth, cleft lip and palate, clenched hands and fists with overlapping fingers, rocker bottom feet, groin hernia, birth defect with intestinal organs outside of the body, kidney defects, heart defects, low-set ears, single line palms and drooping of the upper eyelid. There is no cure or treatment for Trisomy 18 and many babies don’t survive past the second or third trimester of pregnancy.
About half of babies who are carried full-term are stillborn. Boys with Trisomy 18 are more likely to be stillborn than girls. Babies who survive birth usually die within their first month of life. Only five to 10 percent of children with Trisomy 18 live past their first year of life but with severe intellectual disabilities.
Most cases of Full and Mosaic Trisomy 18 are not inherited, but occur as random events during the formation of eggs and sperm. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 18 in each of the body’s cells. For Mosaic Trisomy 18 specifically, some of the body’s cells have the usual two copies of chromosome 18, and other cells have three copies of this chromosome.
Partial Trisomy 18 can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 18 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 18. Although they do not have signs of Trisomy 18, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
In the United States, a rare disease is defined as a condition affecting fewer than 200,000 people. There are as many as 7,000 rare diseases. The total number of Americans living with a rare disease is estimated at between 25-30 million.
- May involve chronic illness, disability, and often premature death
- Often have no treatment or not very effective treatment
- Are frequently not diagnosed correctly
- Are often very complex
- Are often caused by changes in genes
Source: U.S. National Library of Medicine. Medline Plus. http://www.medlineplus.gov