What is Down syndrome?
Down syndrome is a genetic condition caused by extra genetic material (genes) from the 21st chromosome. The extra genes cause certain characteristics that we know as Down syndrome. Individuals with Down syndrome also have all the other genes given to them by their parents. As a result, they have a combination of features typical of Down syndrome on top of the individual features from their parents. This includes some degree of mental retardation, or cognitive disability and other developmental delays. Some of the physical traits that are common but not always present are epicanthal folds over the eyes, flattened bridge of the nose, a single palmar crease and decreased muscle tone.
How did Down syndrome get its name?
Down syndrome gets its name from the British doctor, John Langdon Down, who first clinically identified the condition in 1866. The actual chromosomal nature of Down syndrome did not become documented until 1959 by Dr. Lejeune. However, Down's name remains associated with condition.
Also, though the term has nothing whatever to do with the physical direction "down", Ann Fortis who has Down syndrome has coined the term "Up Syndrome" to provide a different perspective on the condition.
What is the cause of Down syndrome?
We don't know what causes the extra 21st chromosome. We know that the extra 21st chromosome causes an extra dose of some proteins. Those proteins cause the typical features of Down syndrome. We don't know most of the proteins involved and how they cause Down syndrome. We do know there is an association between a mother's age and the chances of having a baby with Down syndrome. No causes have been identified and proven, however.
What are the types of Down syndrome?
About 95% of people with Down syndrome have Trisomy 21, ie. 3 instead of 2 number 21 chromosomes. We normally have 23 pairs of chromosomes, each made up of genes. During the formation of the egg (or the sperm) a woman's (or a man's) pair of chromosomes normally split so that only one chromosome is in each egg (or sperm). In Trisomy 21, or nondisjunction, the twenty-first chromosome pair does not split and a double-dose goes to the egg (or sperm). It is estimated that 95-97% of the extra chromosome is of maternal origin.
The second type of Down syndrome is known as translocation. It occurs in about 3-4% of people with Down Syndrome. In this type, an extra part of the twenty-first chromosome gets stuck onto another chromosome. In about half of these situations, one parent carries the extra twenty-first chromosome material in a "balanced", or hidden, form.
The third type of Down syndrome is called mosaicism. In mosaicism, the person with Down syndrome has an extra twenty-first chromosome in only some of the cells but not all of them. The other cells have the usual pair of twenty-first chromosomes. One to 2 percent of people with Down syndrome have this type. (Cooley & Graham, 1991)
What is the incidence of Down syndrome?
The incidence of Down syndrome has been variously reported as 1 in 800 live births to 1 in 1,100 live births. A recent estimate in the United States puts the incidence at about 1 in 1,000. There is no association between Down syndrome and any given culture, ethnic group, socioeconomic status or geographic region. (CDC, 1994) (Thompson, McInnes, & Willard, 1991)
What are a couple's chances of having a child with Down syndrome?
In the usual circumstance, the chances depend upon the age of the mother. The odds of having a child with Down syndrome at age 35 are approximately 1 in 350. Under age 25, the odds are about 1 in 1400 and at age 40 the odds are about 1 in 100. (Thompson, et al., 1991)
The chances of a parent of a child with Trisomy 21 having another child with Down syndrome is approximately 1 in 100. If the child has a translocation, the recurrence risk can be as high as 100% or as low as 2%. Parents of children with translocation type of Down syndrome should have chromosome analysis to detect a carrier state. (Pueschel, 1990)
Can people with Down syndrome have babies?
The chance of a woman with Down Syndrome having a baby with Down Syndrome is theoretically 50%. There is one documented instance of fertility in a male with Down syndrome; the mother also had Down syndrome. The pregnancy miscarried about half-way through.
When is prenatal diagnosis recommended?
Until recently, the answer was to offer amniocentesis to all pregnant women age 35 or older and to those women who had a previous child with Down syndrome. Now some doctors reccommend screening tests for all pregnancies.
What tests are used for prenatal diagnosis?
Tests are of two types- diagnostic and screening. Diagnostic tests sample fetal cells and give a definitive diagnosis. Screening tests are relatively simple tests that find most of the fetuses with Down syndrome, but they also find some without. Screening tests need to be confirmed by a diagnostic test.
Diagnostic tests are amniocentesis and chorionic villus sampling (CVS). With amniocentesis a needle is passed through the mother's belly into the womb to sample fetal cells in the amniotic fluid. These cells are then sent for chromosome analysis. Ultrasound is used to safely pass the needle. The test is usually done between 14 and 18 weeks of pregnancy. Although fairly safe there is a small risk of miscarriage. CVS samples cells from the chorionic villi, a structure in the womb that has fetal cells but is not the fetus. It is done between 9 and 12 weeks. It also has similar risks.
Screening tests include maternal alpha-fetoprotein and, more recently, the triple test. Ultrasound is not usually used as a screening test but has been investigated. It is done in conjunction with amniocentesis and chorionic villus sampling. The maternal alpha-fetoprotein test was initially devised to screen for neural tube defects, such as spina bifida. A low level of alpha-fetoprotein is correlated with Down syndrome and some other chromosome disorders. The test is estimated to detect approximately 35% of fetuses with Down syndrome after confirmation by amniocentesis.
The triple test measures a mother's blood levels of human chorionic gonadotropin (hCG), maternal serum alpha-fetoprotein (MSAFP) and unconjugated estriol. The three results are adjusted by a computer program to give an estimate of the risk of having a fetus with Down syndrome. Studies so far suggest a detection rate of 55-60% after confirmation by amniocentesis.(Haddow, Palomaki, Knight, Williams, Pulkkinen, Canick, et al., 1992) Because the accuracy of all of the serum tests depends on accurately knowing the number of weeks of pregnancy, a positive test should have an ultrasound done to confirm the due date of the pregnancy. If the adjusted test results are still positive, then either amniocentesis or chorionic villus sampling should be done.
Besides the medical issues involved, there are obvious ethical issues involved. The results of the testing are most likely used for one of two purposes. The couple may use the information to decide whether or not to have an abortion or the information may be used to prepare for the delivery. Although groups recommending routine use of the blood tests also recommend counseling, they do not specifically address the type of counseling. Counseling should expose the couple to all information and, in my opinion, should include contact either with families who have a child with Down Syndrome and/or with people with Down syndrome.
Are children with Down syndrome born only to older mothers?
No. Although the risk of Down syndrome increases with age, a child with Down syndrome can be born to a mother at any age. Although only 5-8% of pregnancies occur in women over the age of 35, they account for 20% of Down syndrome births. Therefore, 80% of children with Down syndrome are born to women who are less than 35 years of age.