Down's Syndrome

Down's syndrome is probably almost as old as history itself.

In fact, 7th and 8th century writings described people with Down's syndrome, and they are also depicted in many works of art which have withstood the ravages of time. Of course, the life expectancy of a child born with the Syndrome was relatively short, due to inferior medical knowledge of the day, and most of these children were taken and reared in convents where they became affectionately known as "Children of God".

It was in 1866 that Dr Langdon Down, an English physician, first described the Syndrome and remarked on the resemblance of the facial features to those of the Mongolian race, hence the unfortunate use of the words "Mongolian" or "Mongoloid". There are other features noticeable at birth, but the most common of which is poor muscle tone. Certain abnormalities in the heart or intestines may also be present and can be detected by doctors in the early days of life. The feature which most concerns parents at this early stage is intellectual disability.

What Causes the Syndrome?

Down's syndrome is an accident of nature - no one is to blame. Nothing that the mother eats or takes as medication, nor any physical activity nor emotional condition during her pregnancy, can result in the birth of a child with Down's syndrome. It happens because there is an imbalance of chromosomes in either the development of the egg or sperm, or in the first or second division of the fertilized egg when an extra chromosome appears. So there is an error in the correct number of chromosomes to be found in the cells of the new growing baby.

Each cell of the human body contains 23 pairs of chromosomes (making 46 chromosomes altogether), and each chromosome controls up to some thousand genes which govern growth and development. The child with Down's syndrome has 47 chromosomes instead of the usual 46. The smallest chromosome (21) is present in triplicate rather than as a pair. Although the genes of these number 21 chromosomes are normal, because there are three instead of two, the genes balance is upset and thrown out of harmony, and so alterations in development are produced. Therefore, it is not surprising that as the body grows, there are alterations in every cell and tissue of the body, including the brain cells. These characteristics we know as Down's syndrome.

Down's Syndrome Falls into Three Groups:

Trisomy 21
The majority (about 94%) of children with Down's syndrome are born with standard Trisomy 21, which is not hereditary. This is when either the egg or sperm carries the extra chromosomes prior to conception.

Mosaic
(About 1%). A person with mosaic Down's syndrome starts off with 23 pairs of chromosomes, and a mistake is made in the early division of cells (either the first or second division). So we have two cell lines - a mixture of normal and trisomatic.

Translocation Trisomy 21
(About 4%). In a small percentage of cases, there are not three identifiable separate chromosomes. The long arm of the extra 21 chromosomes attaches itself to another chromosome (usually number 14). So chromosomes are produced which do not fit into the size scale, and the extra chromosome 21 material will develop the characteristics of Down's syndrome. It is possible in about one in three of these cases that the parent can be a balanced translocation carrier, though not being Down's syndrome himself or herself. However, this brings about a hereditary factor (about 1% of Down's syndrome is hereditary). It is therefore advisable to have chromosome analysis performed to exclude the possibility of the translocation type of Down syndrome. If a baby has a translocation chromosome, parents should then have a chromoosomal study made to determine whether either of them is a balanced translocation carrier. Parents under 30 are more likely to be translocation carriers than those over 30.

Most couples who have given birth to a baby with Down syndrome have little risk of producing a second baby with the condition. However, GENETIC COUNSELLING is available in all states to parents who suspect they are likely to have another child with Down's syndrome. Today, through AMNIOCENTESIS (or ante-natal screening), it is possible to tell whether a pregnancy will result in a baby with Down's syndrome. This is a special test done when the lower part of the mother's abdominal wall and some fluid is withdrawn from the amniotic fluid space. This fluid, which contains casts of skin cells of the foetus, is then cultured for two to three weeks and the chromosomes are stained and counted. This shows whether a child has the normal number of chromosomes. Parents can then decide whether or not they wish the pregnancy to be terminated. There is a small element of risk attached to this procedure. If carried out by skilled people, the maximum risk of losing a foetus through miscarriage is under 1 in 1000. Many doctors recommend that this test be carried out on all mothers over 35, or those who have already had a child with a chromosomal disorder.

 

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