What Facts Should I Know about Trisomy 18?
What is the medical definition of trisomy 18?
- Trisomy 18 (Edwards syndrome) results from the presence of an extra copy of chromosome 18.
- Trisomy 18 leads to severe intellectual and physical defects.
What happens when you have trisomy 18?
- Common features of trisomy 18 include a low birth weight; a small jaw and mouth; an abnormally shaped, small head; overlapping fingers and clenched fists; and defects in the heart and other organs.
- Trisomy 18 occurs on average in 1 out of every 5,500 births.
Is trisomy 18 always fatal?
- Most fetuses with trisomy 18 die in the prenatal period (prior to delivery).
- The prognosis of trisomy 18 is poor. Half of those affected do not live beyond the first week of life and a low percentage of infants with this condition live beyond the first year.
- Girls are affected more commonly than boys.
What is Trisomy 18?
Trisomy 18, also known as Edwards Syndrome, is a severe genetic disorder associated with intellectual disability and abnormalities in many organs. Trisomy 18 was first described in 1960 and results from an additional copy of chromosome 18 (normal cells carry two copies of each chromosome; the term, "trisomy," refers to the presence of three of a given chromosome). Only about 10% of affected infants survive past one year of age, and half of affected infants do not live beyond the first week of life.
What Causes Trisomy 18?
In most cases of trisomy 18, all the cells in the body have an extra copy of chromosome 18. Around 5% of affected people have the extra chromosome 18 in some, but not all, of the body's cells. This phenomenon is known as mosaic trisomy 18. Mosaic trisomy 18 may be very severe or hardly noticeable, depending upon the number of cells that have the extra chromosome.
In rare cases, there is no extra chromosome present; rather, a portion of the long arm of chromosome 18 becomes attached to another chromosome during the formation of egg and sperm cells or very early in development of the embryo. In this case, the individual has two copies of chromosome 18 plus the additional material from chromosome 18 that is attached to another chromosome. This is known as translocation, and the extra genetic material causes the developmental abnormalities in the same way as the presence of an entire extra chromosome. The signs and symptoms of this form of trisomy 18 are dependent upon the amount of chromosomal material that was translocated to another chromosome.
Trisomy 18 is not an inherited condition. It occurs as the result of random events during egg and sperm formation. The type of error that occurs is known as nondisjunction, and this leads to an egg or sperm cell with an abnormal number of chromosomes.
It is not known precisely why the extra genetic material causes the abnormalities specific to trisomy 18. As with Down syndrome (trisomy 21), experts believe that the presence of the extra chromosomal material interferes with the expression and interaction of various genes, resulting in impaired development and function.
How Common Is Trisomy 18?
Trisomy 18 occurs, on average, in 1 out of every 5,500 babies. Girls are affected far more commonly than boys. Any woman can have a child with trisomy 18, but the risk increases with increasing maternal age.
What Are the Sings and Symptoms of Trisomy 18?
Common birth defects in infants with trisomy 18 include
- a low birth weight; a small jaw and mouth;
- an abnormally shaped, small head;
- overlapping fingers and clenched fists;
- “rocker-bottom” feet; low-set ears; and
- defects in the heart and other organs.
Surviving infants may display poor feeding, respiratory problems, delayed growth, and other life-threatening complications. Mental impairment is also characteristic of this syndrome.
How Is Trisomy 18 Diagnosed?
In newborns, chromosome analysis can be carried out to determine the exact cause of birth defects. Prenatal testing for trisomy 18 is possible.
Screening tests such as measurement of alpha fetoprotein (AFP) levels (sometimes referred to as a triple screen) is one screening test. The triple test screens the mother's blood for AFP, hCG (human chorionic gonadotropin, the so-called "pregnancy hormone"), and estriol (a type of estrogen). The test is performed between the 15th and 17th week of pregnancy. A positive result on a screening test does not mean that the baby will have trisomy 18 or any chromosomal abnormality. In fact, only about 11% of those women who receive a positive result for trisomy 18 in this test will actually have an affected fetus.
Ultrasound is another commonly used screening test. Like the AFP test, a simple ultrasound cannot be used to establish the diagnosis of trisomy 18. A more detailed ultrasound may be performed to look for the characteristic signs of trisomy 18, but it cannot confirm alone that trisomy 18 is present.
Analysis of fetal chromosomal material obtained during amniocentesis or chorionic villus sampling is necessary to prove that the additional copy of chromosome 18 is present. An amniocentesis is usually performed at 15-18 weeks of pregnancy and is the most commonly used test for the prenatal diagnosis of trisomy 18. During this procedure, a thin needle is inserted through the abdominal wall and a small sample of amniotic fluid is taken.
Chorionic villus sampling is another type of test that allows the examination of fetal genetic material. It is performed earlier in pregnancy (at 10-12 weeks after the last menstrual period) and therefore carries the advantage of allowing for an earlier diagnosis. This procedure involves the collection a chorionic villus cell sample from the placenta either through the insertion of a needle in the abdominal wall or through a catheter in the vagina.
What Is the Prognosis and Life Expectancy for Trisomy 18?
Due to the presence of several life-threatening medical problems, many infants with trisomy 18 die within their first month. Five percent to 10 percent of children with this condition live past their first year.