About Down syndrome
The term ‘syndrome’ is used to describe a collection of features that are often seen together. Dr. John Langdon Down first identified Down syndrome in 1866 when he noticed a similarity in appearance in some of his patients. These individuals possessed a broad, flat face, a thick tongue and a small nose and were intellectually impaired to a variable degree.
However, there are more differences than similarities between people with Down syndrome. Many babies with the condition will have one or all of the following features at birth: low muscle tone (a floppy baby), a face that appears flatter with eyes slanting upward, small ears and a wider neck than usual, a crease across the palm of the hand and a gap between the toes. Some may have heart problems. While intellectual disability is a feature of the syndrome, those with the condition will develop and learn throughout life, but at a slower pace than usual.
There is no cure for a child born with this condition but many symptoms can be treated and special early intervention programs are enabling these individuals to develop their potential. A child with Down syndrome can usually do most things that any young child can do, such as walking, talking, dressing and being toilet trained, although they may do these things later than other children.
About 1 in 660 babies are born with Down syndrome, and it is found in all population groups.
In each human cell, except for the egg and sperm cells, there are 46 chromosomes, made up of 23 pairs. The chromosomes are numbered according to their size. When egg and sperm cells are formed, the number of chromosomes they contain is halved to 23 with only one copy of each pair. A baby is formed when the sperm from the father fertilises the egg from the mother. The baby will now have 46 chromosomes, just like the parents, with one copy from each parent.
Sometimes, when the egg and sperm are forming, a mistake occurs so that the chromosome pairs do not separate in an ordered fashion. The result is an egg or sperm cell that has only 22 chromosomes while others have 24 chromosomes. If an egg or sperm carrying 24 chromosomes combines with an egg or sperm carrying the usual 23 chromosomes, the result will be an individual with cells in which there are 47 chromosomes instead of the usual 46. Thus, there will be three copies of a particular chromosome in the cells rather than two.
Down syndrome is caused by an extra copy of chromosome number 21. For this reason, it is also known as Trisomy 21. The presence of the extra chromosome causes the mental and physical characteristics of Down syndrome. In 95% of cases, the extra chromosome is present in all the cells of the baby. In about 1% of cases, some of the baby’s cells will contain the usual chromosome number of 46 and others will contain the extra chromosome 21. This situation is known as “mosaic” Trisomy 21.
In about 4% of cases, the extra copy of chromosome 21 is attached (translocated) to another chromosome. This is called “translocation” Trisomy 21. This type of Down syndrome can be inherited. So, where there is a previous child with Down syndrome or a close family history of Down syndrome, a discussion with a genetic counsellor can provide information about the risk of having an affected child and the availability of testing.
Who is at risk of having a child with Down syndrome
The extra chromosome 21 can come from either the egg or the sperm. However, the chance of having a baby with Down syndrome increases with a woman’s age. It is estimated that 1 in 3-4 fertilised eggs are chromosomally abnormal and this increases with the mother’s age. Therefore, most people have had a chromosomally abnormal conception at some time which may have miscarried or not even been recognised as a pregnancy because the miscarriage occurred so early.
Prenatal screening for Down syndrome
The newest, earliest and most accurate screening for Down syndrome during pregnancy is now offered at so+gi scan through the Non-Invasive Prenatal Testing (NIPT), a simple blood maternal blood test performed as early as 10 weeks gestation with >99% accuracy for detecting Down syndrome.
Additionally, so+gi scan continues to offer the combined first trimester screen(CFTS) based on a 12 week nuchal translucency measurement, observation of the nasal bone and a blood test called the First Trimester serum screen. This is traditionally performed between 12-14 weeks and has an accuracy of 95% detection rate for Down syndrome at so+gi scan.
It is important to understand that no screening is perfect and invasive testing would always be required to provide a definitive diagnosis.
Prenatal diagnosis of Down syndrome
Prenatal diagnostic tests that can be done during pregnancy include chorionic villous sampling (CVS) and amniocentesis. These tests involve removing some of the developing placenta (CVS) or amniotic fluid (amniocentesis) and looking at the chromosomes. There is an increased risk of miscarriage of 0.5 -1% associated with these tests.
Down syndrome Australia – http://www.downsyndrome.org.au/
Queensland chapter of Down syndrome Association – http://www.dsaq.org.au/
Down Syndrome Association – http://www.dsaq.org.au
Stillbirth & Neonatal Death Support Group – http://www.sands.org.au
Bonnie Babes – http://www.bbf.org.au/home1.asp