A condition involving mental retardation due to the presence of an extra chromosome 21. Also called trisomy 21.

Detailed Definition

Individuals with this condition tend to have similar facial features to each other (i.e. skin folds in the inside corners of their eyes, upslanting eyes, somewhat flatter appearing face, smaller mouth, protruding tongue). Physical complications seen in this condition include the possibility of a congenital heart defect, digestive problems, respiratory and ear infections, decreased functioning of the thyroid gland, lower muscle tone, and vision problems.

Individuals with Down syndrome should be monitored on a regular basis because of the higher risk for these types of problems. Many of the physical complications seen in this condition can be treated or corrected surgically. There is a great deal of variability in how individuals with Down syndrome are affected physically and most people do not have all the physical problems possible in the condition; rather, they have a combination of findings.

The one feature that is consistently seen in all individuals with Down syndrome is mental retardation. The majority of individuals with Down syndrome have mental retardation in the mild to moderate range. This means that these individuals can and will learn new skills although at a slower rate than someone without mental retardation. They do learn to walk, talk, and possibly read and write. Some may live semi-independently in adulthood.

Causes/Types

Down syndrome is a genetic condition caused by an extra 21 chromosome. Chromosomes are rod-like structures that are located inside all of our cells. Chromosomes carry genes which are the instructions for how our bodies grow and develop. Healthy individuals who do not have Down syndrome have 46 chromosomes, or 23 pairs in each of their cells. One chromosome from each pair is inherited from the mother; the other chromosome is inherited from the father. Sperm and egg cells are the exception to the rule and contain half the number of chromosomes- 23 (one from each pair).

Sometimes, errors in the cell division process can cause a sperm or egg cell to contain an extra chromosome- 24, rather than the usual 23 chromosomes. This error always occurs by chance and there is nothing a parent can do to cause or prevent this from happening. If, for example, an egg cell with an extra chromosome (24 chromosomes) is fertilized by a sperm cell with the usual number (23 chromosomes), the baby will have 47 chromosomes in all of its cells rather than the expected 46. In Down syndrome, it is the extra 21 chromosome that causes the characteristic features seen in individuals with this condition.  

The majority (about 95%) of individuals with Down syndrome have three copies of chromosome 21. This type of Down syndrome, also known as Trisomy 21, is NOT inherited and happens by chance alone.

Another type of Down syndrome is called mosaic Down syndrome. In this situation, an individual has two different cell types- some containing 46 chromosomes, others containing 47. The cells with 47 chromosomes have an extra 21 chromosome which can cause varying degrees of Down syndrome depending on how many cells in the person's body have the extra chromosome 21. Mosaic Down syndrome is also not inherited and occurs by chance alone. It is a rare occurrence and is seen in approximately 1-2% of individuals with features of Down syndrome.

About 3-4% of individuals with Down syndrome have the usual number of chromosomes in their cells but have an extra piece of chromosome 21 attached to, or translocated to, another chromosome. The features in this type of Down syndrome are the same as those seen in Trisomy 21 and mosaic Down syndrome. However, Down syndrome resulting from a translocation can be inherited and other family members could also be at risk to have children with the condition.  

Diagnosis

Down syndrome can often be suspected shortly after birth due to the characteristic facial features present in babies with the condition. To confirm the suspected diagnosis, a blood sample is obtained and the baby's chromosomes are studied to determine if there is an extra chromosome. This testing is extremely accurate and can provide a definitive answer as to whether or not a baby has Down syndrome or other chromosome condition. If an individual has Down syndrome, chromosome analysis is also useful in determining the type or cause of the Down syndrome. If the type of Down syndrome is known, then an accurate risk of recurrence (the chance of having another child with Down syndrome) can be given. 

Prenatal Diagnosis

Down syndrome can be detected through diagnostic tests such as chorionic villus sampling (CVS) and amniocentesis. Diagnostic tests provide a definitive answer as to whether or not a fetus has Down syndrome. CVS is performed during the first trimester of pregnancy between 10-12 weeks. Amniocentesis is performed in the second trimester routinely between 15-18 weeks. Results of testing usually take about 2 weeks.

An alternative to diagnostic testing is the XAFP screening test. This test provides an individual risk for the baby to have Down syndrome but it does not provide a definitive answer. If the risk is increased, follow-up diagnostic testing (amniocentesis) would be offered if the couple desired a definitive answer. Each of these prenatal tests have different benefits, limitations, and risks and a genetic counselor can discuss the information with you.

If results of testing show that the baby has Down syndrome, the options available to the couple would be to continue or end the pregnancy, depending on their personal choice. Some couples also choose to continue the pregnancy and then place the baby for adoption at birth.

Although Down syndrome can result from an extra 21 chromosome found in either the sperm or the egg, the chances to have a child with this condition increases with maternal age. Any woman who will be 35 or older at the time of delivery or anyone with a family history of Down syndrome should contact a genetic counselor to determine their individual risk and to discuss the testing options available during pregnancy.

Treatment/Services

At this time, there is no known cure for the condition and no known way to remove the extra genetic material from each cell. Physical complications can be treated medically or surgically when necessary. Special education programs are available even from infancy to determine areas of strengths and weaknesses in the child and to maximize their full potential.

Other Information

If the type of Down syndrome is Trisomy 21 or mosaic Down syndrome, the chance to have another child with the same condition is no higher than 1%. However, if a translocation is the cause of Down syndrome, the risk could be higher than 1% if one of the parents is a carrier for the translocation.