Down syndrome (DS) is the most frequent of all chromosomal abnormalities and arises at the time of conception. It is not a disease but a purely random occurrence that occurs in 1 of 1000 births in South Africa, traversing all nationalities and backgrounds.
The term Down syndrome is taken from the name of the English physician, Dr. John Langdon Down. He was the first person to describe the condition in 1866. It was only in 1959 that the actual chromosomal abnormality associated with the syndrome was discovered by Dr. Jerome Lejuene. He found that an individual with Down syndrome had extra genetic material, usually an extra chromosome. Instead of having 46 chromosomes in each cell (23 pairs) individuals with Down syndrome commonly have 47 chromosomes with the extra chromosome associated with the 21st pair.
The term Trisomy 21 is therefor used to describe this configuration of three number 21 chromosomes. About 95 percent of individuals with Down syndrome have Trisomy 21. There are three types of Down syndrome: trisomy 21 (nondisjunction) accounts for 95% of cases, translocation accounts for about 4%, and mosaicism accounts for about 1%.
WHAT ARE THE CHARACTERISTICS OF INDIVIDUALS WITH DOWN SYNDROME?
Individuals with Down syndrome may vary significantly in terms of physical and psychological characteristics. The list of possible characteristics however should not obscure two important facts: clearly individuals with Down syndrome are first and foremost people who have similar needs, desires, and rights as others; and, the effects of intensive interventions with young children with Down syndrome are only now being evaluated, hence making many of the historical descriptions of Down syndrome no longer accurate.
Some of the physical characteristics observed in individuals with Down syndrome include the following: the back of the head is often flattened, the eyelids may be slightly slanted, small skin folds at the inner corners of the eyes may be present, the nasal bridge is slightly depressed, and the nose and ears are usually somewhat smaller. In the new born babies there is often an excess of skin at the back of the neck. The hands and feet are small and the fingerprints are often different from A-typical children.
Individuals with Down syndrome have loose ligaments and their muscle strength and tone are usually reduced. If the ligaments between the first two neck bones are loose, there may be a condition referred to as Atlanto-Axial Instability. About one-third of children with Down syndrome have congenital heart disease. Other congenital defects such as blockage in the bowels and cataracts, although rare, may also be present. Hearing deficits, visual problems, and thyroid dysfunction are often observed in persons with Down syndrome.
HOW OFTEN DOES DOWN SYNDROME OCCUR/RECUR?
Down syndrome is the most common chromosomal condition and clinical cause of intellectual disability in the world. On average 1: 1000 births is a baby with Down syndrome. Down syndrome occurs in people of all races and economic levels.
Age however does play a role in an individual’s chance to have a baby with Down syndrome.
1: 1,500-2,000 births when the mother’s age is below 25;
1:400 births when the mother’s age is over 35;
1:40 when the mother’s age is over 45.
A mother with a child with Down syndrome has a one-in-25 chance of recurrence. Siblings of Down syndrome have the same risk of having a child with Down syndrome as does the general population unless the disorder is of genetic origin.
CAN DOWN SYNDROME BE DETECTED BEFORE BIRTH?
Down syndrome can be identified during the prenatal and perinatal periods. There are two types of tests that can be performed before your baby is born: screening tests and diagnostic tests. Prenatal screenings estimate the chance of the foetus having Down syndrome. These tests do not tell you for sure whether your foetus has Down syndrome; they only provide a probability.
Diagnostic tests, on the other hand, can provide a definitive diagnosis with almost 100 percent accuracy. There is an extensive menu of prenatal screening tests now available for pregnant women. Most of these screening tests involve a blood test and an ultrasound (sonogram). Together with a woman’s age, these are used to estimate her chance of having a child with Down syndrome. Typically offered in the first and second trimesters, maternal serum screening tests are only about 80 percent reliable. It is important to note that none of these prenatal tests will be able to definitively diagnose Down syndrome. Instead, mothers should expect results such as, “You have a 1 in 240 chance of having a child with Down syndrome” or “You have a 1 in 872 chance of having a child with Down syndrome.”
These blood tests are often performed in conjunction with a detailed sonogram (ultrasound) to check for physical features that are associated with Down syndrome in the fetus. Recently, researchers have developed a maternal serum/ultrasound/age combination that can yield a much higher accuracy rate at an earlier stage in the pregnancy. Even with the ultrasound, however, the screening will not definitively diagnose Down syndrome.
In recent years, non-invasive prenatal screening tests have been made available. These tests involve blood being taken from the expectant mother as early as 10 weeks of gestation and rely on the detection of cell-free DNA that circulates between the fetus and the expectant mother. These blood tests can detect up to 98.6% of foetuses with trisomy 21. A “positive” result on the test means that there is a 98.6% chance that the foetus has trisomy 21; a “negative” result on the test means that there is a 99.8% chance that the foetus does not have trisomy 21.
While the initial clinical trials were conducted just on cases with trisomy 21, one form of Down syndrome, a “positive” result cannot distinguish between trisomy 21, translocation Down syndrome, and high-percentage mosaic Down syndrome. Put another way, a “positive” result will pick up all forms of Down syndrome, except mosaic Down syndrome that is 33% or lower. All mothers who receive a “positive” result on this test are encouraged to confirm the diagnosis with one of the diagnostic procedures described below.
Prenatal screening tests are now routinely offered to women of all ages. If the chance of having a child with Down syndrome is high from prenatal screening, doctors will often advise a mother to undergo diagnostic testing.
The diagnostic procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling (CVS) and amniocentesis. These procedures, which carry up to a 1% risk of causing a spontaneous termination (miscarriage), are nearly 100% accurate in diagnosing Down syndrome. Amniocentesis is usually performed in the second trimester between 15 and 22 weeks of gestation, and CVS in the first trimester between 9 and 14 weeks. Amniocentesis and CVS are also able to distinguish between these genetic types of Down syndrome: trisomy 21, translocation Down syndrome and mosaic Down syndrome.