Introduction
Human cells normally contain 23 pairs of chromosomes. Chromosomes are the parts of body cells that carry inherited information - the characteristics that are passed to you from your mother and father.
A chromosomal disorder means there is a change in the normal number of chromosomes. This can mean that physical and mental development is impaired and can result in learning difficulties and health problems.
Downs Syndrome is the most common chromosomal disorder and one of the most common causes of learning difficulties.
Named after the first person that described it, Dr J L Down, the condition affects one in every 1,000 people. Some are more severely affected than others.
Around 600 babies with Downs Syndrome are born in the UK every year. It occurs in all ethnic groups and affects slightly more boys than girls.
Symptoms
People with Downs Syndrome have lots of different characteristics.
There are thought to be up to 120 features of Downs Syndrome but many children have no more than six or seven of them.
Physical features:
Babies with Downs Syndrome usually weigh less than average at birth and are usually shorter than average as adults.
Children with Downs Syndrome often have a rounded face, with a flat profile (the face looks flat when you view it from the side). The back of the head is slightly flattened (this is called brachycephaly) and the eyes tend to slant upwards.
There are small folds of skin that run vertically between the inner corner of the eye and the bridge of the nose (epicanthic folds) and this can give the impression of crossed eyes (squint). There may be white or yellow speckling around the rim of the iris (coloured part of the eye). These are called Brushfield spots.
Children with Downs Syndrome often have straight, soft hair. As children they may have an extra fold of skin over the back of the neck and as adults, short broad necks.
People with Downs Syndrome often have smaller than average mouths with a bigger than average tongue that may stick out.
The hands may be broad with short fingers; the little finger may only have one joint instead of two and be slightly curved. The feet are often stubby with a wide space between the first and second toes.
Many people with Downs Syndrome have poor muscle tone (hypotonia). This muscular floppiness occurs in the limbs and neck and usually improves with age.
Children with Downs Syndrome learn new skills more slowly than other children and generally develop at a slower rate, meeting their developmental milestones such as walking or talking later. They may progress in stops and starts and may not fully catch up with other children their age. Often, a child with Downs Syndrome will not start to use language until his or her third year and may use some sign language before talking.
With treatment and support, the average life expectancy of someone with Downs Syndrome is about 60 years.
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Causes
We inherit genes from our parents and they determine our characteristics. Our genetic code is found in parts of our cells called chromosomes. Downs Syndrome is caused by an extra number 21 chromosome. It can come from either the mother or the father and there is no way to predict it.
The genes that the extra number 21 chromosome contains cause extra proteins to be made, and this disturbs normal growth. Cells do not divide as rapidly as normal and this leads to a smaller than average baby. As the baby develops, cells do not move around in the usual way, which means that the body parts do not form normally, especially the brain. This means that the baby will have fewer brain cells and a different brain formation.
The likelihood of having a baby with Downs Syndrome increases with the age of the mother, and is higher after the age of 35. However, more babies with Downs Syndrome are born to younger mothers, because overall more babies are born to women under 35.
There are three forms of Downs Syndrome:
Trisomy 21 94% of people with Downs Syndrome have an extra chromosome 21 in every cell of their body. The two number 21 chromosomes do not separate but stay together in one cell (this is called non-disjunction). This type of Downs Syndrome is often linked to the age of the mother.
Translocation In 4% of cases, Downs Syndrome is due to an extra part of chromosome 21 (rather than a whole extra chromosome). It occurs when small bits two chromosomes break off and stick together. Parental age is not a factor. In one third of these children a parent is found to be a carrier of Downs Syndrome and so has a higher than normal chance of having a child with the syndrome. But in two thirds the occurrence of Downs Syndrome is random.
Mosaicism In 2% of cases, there is an extra whole 21 chromosome in only some of the body cells and the rest of the cells are normal. These children are less noticeably affected and have less obvious physical characteristics. They may have little or no intellectual difficulties.
Diagnosis
Diagnosis of Downs Syndrome is made shortly after birth and is usually based on the appearance of the baby.
The diagnosis should be made as soon as possible, with both parents present. Parents should be encouraged to ask questions. At this stage, doctors will not be able to tell exactly how and to what extent the baby will be affected.
Diagnosis is confirmed by a chromosome test (blood test).
In cases of translocation (where Downs Syndrome is due to an extra part of chromosome 21) both parents should also have a chromosome test to see if either one is a carrier (carries the faulty gene, but does not have symptoms of the condition).
Treatment
There is no cure for Downs Syndrome but a range of treatment and support is available to improve the quality of life of people with the condition. Many people with Downs Syndrome benefit from specialised education, occupational therapy, physiotherapy, speech therapy and a dietician.
Tests for baby and beyond
If your baby is diagnosed with Downs syndrome they will have a physical examination at birth, another at six weeks old and then annually. Part of the newborn examination will include an echocardiogram, when the heart and blood vessels are examined. This is because children with Downs Syndrome are more likely to have heart defects.
It is important that your child has regular checks to monitor growth and to detect health problems early. Such problems may do more damage if they are untreated.
Children with Downs Syndrome should have regular vision and hearing tests. This is because they are at higher risk of long sightedness (hypermetropia) and short-sightedness (myopia), and glue ear which can result in hearing loss.
They should also have a thyroid test every year and a dental check at least every two years. This is particularly important for children with heart abnormalities because a tooth infection can cause bacteria to grow in the heart.
Children with Downs Syndrome should have the usual childhood immunisations (see Immunisation topic) and may be offered the flu jab as well.
Leading a fulfilling life
Children with Downs Syndrome can benefit from specialised educational programmes and therapy as well as physiotherapy, occupational therapy and speech therapy. Some may thrive better in a specialised school and some in a mainstream school. Encourage your child and let them do things for themselves. With appropriate education, therapy, and social support, the majority of people with Down syndrome can lead fulfilling and productive lives. They can live independently and have personal relationships.
Some children may need constant care and support from family or carers. For advice, guidance and support see your midwives, social worker, counsellor or health visitor. You can also attend special health and development clinics. Parents of children with Downs Syndrome may like to join a parents group so they can talk to other parents, share experiences and get support.
Complications
Certain conditions are more common in people with Downs Syndrome. Most of these are usually mild and easy to treat. The more severe conditions only affect about 1 in 100 children with Down syndrome.
Heart problems : 40-50% of children with Downs Syndrome have heart problems such as a heart defect, murmur or hole in the heart. Many of these children will need surgery.
Upper respiratory tract infections: these are infections that affect the ear nose and throat and include coughs, colds, middle ear infections and chest infections. Your child should always be examined by your GP to see if the infection is bacterial or viral.
Dry skin, obesity, diabetes.
Lower immune system: this means people with Down's Syndrome are prone to catching infections.
Eye problems: these can include a squint, nystagmus (where the eyes move to and fro rapidly because the brain does not control the movement of muscles around the eyeball), cataracts (clouding of the eye lens) and keratoconus (when the cornea becomes more conical in shape).
Teeth problems: children with Downs Syndrome keep their milk teeth longer and get their adult teeth later. Parents should take extra care to avoid sugary foods and drinks, and consider giving extra fluoride to protect teeth.
Hypothyroidism (under-active thyroid gland): Congenital hypothyroidism (you are born with an under-active thyroid gland) this affects about 1 in 150 children with Downs Syndrome and may go unnoticed at first. Acquired (you develop an under-active thyroid gland) hypothyroidism this affects 1 in 5 adults and 1 in 10 children. Hypothyroidism needs to be detected and treated (with hormones) as soon as possible, because it can result in serious and permanent intellectual problems.
Intestinal problems: 10-15% of children with Downs Syndrome have duodenal atresia, when narrowing of the small intestine causes vomiting because food cannot travel into the intestine from the stomach.
Hirschsprung disease: the lack of nerves in the rectum can cause constipation in babies and infants, but this can be treated.· Leukaemia is more common later in life (1 in 100).
Klinefelter syndrome: 1 in 100 boys with Downs Syndrome have an extra X chromosome in addition to the extra number 21 chromosome. They will have smaller than average testicles, will not go through the normal changes during adolescence and will be infertile. This syndrome can be treated with male hormones.
Prevention
Screening
All pregnant women, whatever their age, should be offered screening before 20 weeks. These tests are all optional.
The triple test is performed after 14 weeks of pregnancy but before 20 weeks. The best time for the most accurate results is between 16-18 weeks. It is a blood test that tests for the hormone Human Chorionic Gonadotrophin (hCG), the protein alpha-feto (AFP ) and the protein unconjugated oestriol (uE3 ).
Low levels of AFP, low levels of uE3 or high levels of hCG in the blood may mean a higher than average risk of having a baby with Downs Syndrome.
The results of the blood test are combined with the mothers age to estimate the risk of Downs Syndrome. The results will be either screen negative (risk is less than 1 in 250) or screen positive (risk is more than 1 in 250). A screen positive test result does not mean that there is an abnormality. It only means that there is an increased risk and further tests may be needed to be more accurate.
Results are not 100% accurate. At the moment, less than 5 in 100 people have a false-positive result (at least 60 out of 100 cases of Downs Syndrome are detected). By April 2007, this false-positive rate should be less than 3 in 100 (more than 75 out of 100 cases of Downs Syndrome will be detected).
The quadruple test is similar to the triple test but also tests inhibin A. This is only used if you are at a very high risk of having a baby with Downs Syndrome.
The nuchal translucency scan (taken between 10 and 13 weeks of pregnancy) is only available in certain hospitals. It involves measuring the width of the babys spinal cord.
Diagnostic tests
An amniocentesis test involves taking a sample of the amniotic fluid that surrounds the unborn baby. It is carried out with local anaesthetic using a thin needle to remove the amniotic fluid. This contains some of the babys cells and can be used to try and diagnose Downs Syndrome. Results take 3-4 weeks. There is a 1 in 200 chance of miscarriage with this test.
A chorionic villous sampling (CVS) test involves taking a sample of tissue from the placenta to measure certain markers in the blood. It is carried out under local anaesthetic using a thin needle passed into the wall of the abdomen or by inserting a small tube through the vagina and the cervix (neck of the womb) to remove some of the chorionic villi (placental tissues). Results take 10-14 days. There is a 1 in 100 chance of miscarriage with this test.
Decisions
Although results are not 100% accurate, it can be a worrying and difficult time if your diagnostic tests indicate a high risk of having a baby with Downs syndrome. You will be encouraged to have some counselling and to discuss the results with a doctor, nurse or midwife and your partner. There are a number of options available to parents who are expecting a baby with Downs Syndrome.
Research into a cure
Research is being carried out on chromosome 21 to better understand the development of people with Downs Syndrome and why there is a incidence of certain medical complications in people with the condition.
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