It’s probably safe to say that many of us have known and teased a child we used to describe as “Mongoloid.” This term is now politically incorrect (please, I am not talking politics for anybody). I just mean that nowadays, unlike in the past, we describe these persons by their correct medical designation, Down’s syndrome. And it is very encouraging that the attitude of the public toward persons with Down’s has taken an almost 180 degree turn.
WHAT IS DOWN’S SYNDROME – AN OVERVIEW
Down’s, or Down, syndrome is a genetic disorder resulting in varying degrees of physical and mental retardation. It causes problems in the development of the affected one ranging from mild to severe. The problem arises from the fact that the individual carries extra genetic material.
Genes are essentially what make up DNA, which determines every individual’s particular characteristics. They are contained on 23 pairs of chromosomes for a total of 46. Most normal human beings have this number. Most people with Down’s have an extra copy of the No. 21 chromosome, for a total of 47. Yes, we know them and number them, and unfortunately, more does not always mean better. As it turns out, Down’s is the most common disorder involving chromosomes, and is also a common cause of miscarriage.
WHAT ARE THE SIGNS AND SYMPTOMS?
A child with Down’s has an almost unmistakable facial appearance. All Down’s children do not have the same features, but some of the more common ones include:
• Flattened facial features
• A small nose
• Oddly shaped ears
• A protruding tongue
• Upward slanting eyes
Affected children may also have
• Relatively short fingers
• Poor muscle tone or flaccid muscles
• A single crease in the palm of the hand – a Simian crease
• Excessive flexibility
Children born with Down’s may be of average size, but they typically grow slowly and end up being smaller, certainly shorter, than children of similar age.
WHAT CAUSES DOWN’S SYNDROME?
Remember that each human cell normally contains 23 pairs of chromosomes. One chromosome from each pair comes from dad, the other from mom. That is why many of us have some ways or features of our mother and some of our father.
Now, follow me here. Each CELL has 23 pairs or 46 chromosomes. If each parent were to pass on their full complement to the child, the child would have 46 pairs or 92. Maybe this can happen in space aliens, but not humans. How is the child formed? From the fertilization of the woman’s egg with the man’s sperm, that is their coming together. So in order for the father not to carry all his 46 (23 pairs) chromosomes, and likewise the mother her 46 (23 pairs) to the child, a division by half, called meiosis, of the gametes (sperm and egg cells’ chromosome pair) must take place. So the sperm cell brings to the child one of the pairs of 23 and the egg cell brings the other of the pairs of 23 chromosomes. One from each parent makes a pair, 23 pairs. Follow?
Down’s syndrome is caused by one of three types of abnormal cell division involving the 21st chromosome. These include:
• Trisomy 21. Faulty chromosome division during formation of the egg or sperm results in an embryo (developing fetus) with three rather two chromosome 21’s. This is the most common cause, accounting for up to 92 percent of all Down’s cases.
• Translocation. During chromosome division, part of chromosome 21 separates and attaches to another chromosome. This accounts for 3 to 4 percent of all cases.
• Mosaicism. For the embryo to grow into a child, mitosis (division of the cells of the fertilized egg) must occur millions of times over. What happens in mosaicism is that in the early life of an embryo, abnormal cell division occurs, resulting in some cells with the normal 46 (23 pairs) chromosomes while others have 47 – a mosaic of normal and abnormal cells. This accounts for between 2 and 4 percent of Down’s cases.
Incidentally, no known behavioral or environmental factors are known to play a role in Down’s.
WHAT ARE THE RISK FACTORS?
A woman’s chances of giving birth to a child with Down’s increases with her age. At age 35, there is a one in 400 chance, at age 40, it is one in 105, and by age 49, there is a one in 12 chance of having a child with Down’s. You see, all the eggs a woman will ever have, she has at birth; she cannot make anymore. So, as the years go by, they age in her ovaries and therefore, there is a greater possibility for chromosomes to divide improperly at meiosis. But wait, let’s not blame it all on the woman though. Remember that improper divisions of the chromosomes can occur in the male too. So it means that a woman aged 25 can produce a Down’s child. I know a lot of women start panicking when they hit between 25 and 30 and haven’t conceived yet, thinking they must end up with a Down’s child. My mother had me at 40 and my last brother at 45. We’re all normal, as far as I know, I mean we haven’t got Down’s anyway.
Typically, a mother who has one child with Down’s has about a 1% chance of having another.
HOW TO SCREEN AND DIAGNOSE
There’s no way to screen for Down’s before pregnancy.
SCREENING TESTS DURING PREGNANCY.
We will often perform screening tests to determine if your fetus has an increased chance of Down’s at about the 16th week of pregnancy.
Blood tests such as the triple screen or the maternal serum alpha-fetoprotein (MSAFP) test identifies levels of certain biochemical markers, such as HCG, (Human chorionic gonadotropin, AFP (Alpha-fetoprotein), and estriol in the mother’s blood stream. A fetus with Down’s usually gives values that ate altered from those that occur in a normal pregnancy. It is not a certainty for Down’s but it identifies the possibility and prompts us to do more tests. Bear in mind that a normal screening test doesn’t guarantee Down’s isn’t present, only that the likelihood is lessened. In fact, only 60 percent of babies are identified by an abnormal blood screening test.
You may want to confirm a positive screening test by specific diagnostic tests, which include:
• Amniocentesis. A sample of amniotic fluid (the fluid bathing the baby in the womb) is withdrawn through a needle inserted into the womb, when the fetus is between 14 and 18 weeks old. The chromosomes of the fetus are analyzed in this fluid. There’s a risk of miscarriage from this procedure ranging from one in 200 to one in 400.
• Chrionic villus sampling (CVS) Cells taken from the mother’s placenta (the afterbirth) can be used to analyze the fetal chromosomes. This is performed between the 10th and 12th week of pregnancy. (Obviously this is done before even a screening test which is done at about 16 weeks. There is a 1% chance of miscarriage from this procedure.
These three tests are accurate to between 98 and 99 percent in diagnosing Down’s before birth.
DIAGNOSTICS TEST FOR NEWBORNS
After birth, the baby’s features may give it away if it has Down’s. If not, the doctor may do a karyotype, an analysis of the chromosomes. If there’s an extra chromosome 21, there is Down’s.
These include some type of heart defect, sometimes requiring surgery in early infancy, gastro intestinal blockage, thyroid problems, hearing loss, or poor vision.
• Leukemia. Down’s kids have a 10 to 15 times the risk to develop this compared to normal children.
• Infectious diseases. Because of an underdeveloped immune system, Down’s kids are more open to a number of infections.
• Impaired intellectual development. This is almost always certain but need not be severe. Mild cases occur.
• Dementia. Down’s sufferers have a high risk of dementia, often before age 40.
The lifespan of Down’s children has increased over the years, but is still quite low, average about 40.
They are being better integrated into society, are being cared for better through a variety of programs specifically designed to address their needs. Let’s continue to keep things up for Down’s kids.
See you next week.
Dr. Emanuel, based in the Commonwealth of Dominica, has been an educator of medical professionals, in training and the public, for over 20 years.