Conditions
Down's Syndrome is a chromosomal disorder affecting roughly 1 in every 1000 live births. Children born with this syndrome have limited intellectual ability.
Summary
- Down's syndrome is a chromosomal disorder affecting roughly 1 in a thousand live births.
- 30% of these babies are born to women over the age of 35.
- Children with Down's syndrome have limited intellectual ability and recognisable physical characteristics.
- They often have hearing problems, heart problems and thyroid problems and average life expectancy is about 35 years depending on whether there is a life threatening cardiac defect.
- Down's syndrome can be detected in a foetus by means of a number of tests, such as amniocentesis.
Description
This condition, named after John Langdon Down, the first physician to identify the syndrome, is also known as mongolism or trisomy 21. The term 'mongolism' refers to the fact that someone with Down's syndrome is often Eastern in appearance.
Down's syndrome is a chromosomal disorder affecting roughly 1 in every 1000 live births. Children born with this syndrome have limited intellectual ability and specific recognisable physical characteristics.
Almost 95 percent of these children are born with one extra chromosome in every cell, making 47 instead of the usual 46.
Almost 30 percent of children born with this syndrome, are to mothers over the age of 35, despite the fact that older women bear a very small percentage of all children.
Children who have Down's syndrome are easily recognisable in that they have broad and flat faces, flattened nose bridges and slanting eyes. Their tongues are also thicker than those of other children, their ears are small and low set, their hands short and broad, with a single crease across the palm. Not every child born with Down's syndrome has all these characteristics.
Almost 40-50 percent of children born with Down's syndrome have heart problems and the average life expectancy is 35 years, if they survive the first five critical years.
In the older child/adult the risk for development of Alzheimer's disease is increased.
Physical and mental development is delayed in children with Down's syndrome. Because of decreased muscle tone, children with Down's syndrome can take months longer to walk, and also to start speaking. Their IQ is on average in the range of 50, where other children have an average of 100. Some children are much more severely affected than others, who manage to cope in normal schools, sometimes up to high grades. Children with Down's syndrome are generally very loving individuals.
Causes
Improper cell division lies at the heart of the cause of Down's syndrome. Many theories abound about what exactly causes this, but so far no definite answers have been forthcoming. Suggestions and theories have included X-rays, viral infections immunologic problems, hormonal abnormalities and genetic predisposition.
It has long been known that advancing age of the mother is a risk factor for Down's syndrome. Almost 30 percent of children born with this syndrome, are to mothers over the age of 35, despite the fact that older women bear a very small percentage of all children. But many Down's syndrome children are also born to younger mothers, so other causes, besides age, obviously also play a part.
Almost 95 percent of these children are born with one extra chromosome in every cell, making 47 instead of the usual 46. But in 3 percent of the children with this syndrome, the extra chromosome 21 is translocated on to another chromosome, usually on chromosome 14, 21 or 22. The third possibility, a chromosome problem called mosaicism, describes a condition in which some cells have 46 and some 47 chromosomes. This is the case in approximately 2 percent of children born with this syndrome.
Diagnosis
Because most Down's Syndrome babies are born to younger mothers, screening tests have been developed to identify the high risk person. Screening tests are not diagnostic, but only pick up those mothers with an increased risk. However, none of these screening tests are 100% accurate.
The best screening test (nuchal translucency together with first trimester biochemical screening) can pick up only about 80% of at high risk mothers. 20% will therefore be missed. The Alpha feto protein test is not done on amniotic fluid, but on maternal blood (for screening test) and should be done between 15-20 weeks pregnancy (best 17 weeks). It is usually combined with two other biochemical tests for greater accuracy and better detection rate.
Interpretation of these results depend on gestation and maternal age and a risk for Down's syndrome is given. A 5% false positive rate is obtained with this so-called triple test. A positive screening test, therefore, is followed up by a diagnostic test like amniocentesis.
At birth Down's syndrome can be recognised, but chromosome analysis is done to confirm the chromosomal type, for the sake of counselling for the recurrence risk of the condition.
Signs and symptoms
Both physical and mental development is slow in Down's syndrome children.
Down's syndrome babies are generally very placid and seldom cry, but they do require constant care.
Their muscle tone is weak, so motor development is slow. They can take up to 18 months longer to walk than other children, and speech acquisition can also be a slow process, with a limited vocabulary by age 6.The average IQ of a Down's syndrome child is roughly half of the average of normal children - 50 as opposed to 100. But this is by no means true in all cases.
Children who have Down's syndrome are easily recognisable in that they have broad and flat faces, flattened nose bridges and slanting eyes. Their tongues are also thicker than usual, their ears are small and low set, their hands short and broad, with a single crease across the palm. The fifth finger also often has only two sections instead of three. Not every child born with Down's syndrome has all these characteristics.
Treatment
Down's syndrome is an irreversible chromosomal disorder, so there is no treatment for the syndrome as such.
In South Africa there are few facilities available for the full-time care of a Down's syndrome child. These children therefore need to be cared for at home.
When treating a Down's syndrome child, doctors need to be on the lookout for conditions more often found in these children, such as heart defects, leukaemia and thyroidal problems.
It must also be noted that 60 - 80 percent of children with Down's syndrome have hearing deficits. Intestinal problems, eye problems, obesity and skeletal problems are also frequently found.
With the recent advances in molecular biology, it is not impossible that genetic intervention could in future reverse the specific actions of abnormal developmental sequences.
Prevention
A screening test such as nuchal translucency can be done. Nuchal translucency is an area of increased translucency on an ultrasound picture. A positive nuchal translucency is followed by chromosomal analysis either by amniocentesis or chorionic villus biopsy. At 22 weeks gestation one would normally do a congenital abnormality check, which would include cardiac evaluation.
(Reviewed by Dr G. de Jong)