Down syndrome (also called
Trisomy 21 ) is a genetic disorder that occurs in approximately
1 of 800 live births. It is the leading cause of cognitive impairment.
- Down syndrome is associated with mild to moderate learning disabilities, developmental delays, characteristic facial features, and low muscle tone in early infancy.
- Many individuals with Down syndrome also have heart defects, leukemia, early-onset Alzheimer's disease, gastro-intestinal problems, and other health issues. The symptoms of Down syndrome range from mild to severe.
- Life expectancy for individuals with Down syndrome has dramatically increased over the past few decades as medical care and social inclusion have improved. A person with Down syndrome in good health will on average live to age 55 or beyond.
About Down's syndrome
Down's syndrome is caused by an extra chromosome. Chromosomes are structures that contain genes - these contain the instructions for life and are inherited from parents. Normally, our cells contain 46 chromosomes: 23 inherited from each parent. In Down's syndrome a mistake is made during cell division, this is most likely to occur when the sperm or egg is being formed causing 24 chromosomes to be present rather than the usual 23. After the egg is fertilised by the sperm the cells have 47 chromosomes rather than the normal 46 chromosomes.
Types of Down's syndrome
- Trisomy 21 - this is when all the cells have an extra chromosome 21. This happens in most people with Down's syndrome.
- Translocation - this is when an extra fragment of chromosome 21 attaches to another chromosome. This happens in about one in 25 people with Down's syndrome.
- Mosaicism - this is when only some cells have an extra chromosome 21 while others don't. This happens in about one in 50 people with Down's syndrome.
Symptoms of Down's syndrome
The extra chromosome 21 causes characteristic physical features in people with Down's syndrome. These usually include some, but not always all, of the following.
Physical Features
- Low-set eyes that slope upwards, with vertical skin folds (epicanthic folds) between the upper eyelids and the inner corner of the eye.
- A small mouth, which means the tongue may seem big and may stick out.
- A flattening at the back of the head.
- A flattened nose bridge.
- Broad hands with a single crease.
- Floppiness due to loose muscle tone.
- Small, low-set ears.
- A low birth weight and short stature.
Intellectual Disability
Most people with Down syndrome have some degree of
mental retardation, or intellectual disability, usually in the mild to moderate range. This is usually characterized by delay in development, language, and memory. Down syndrome is among the most common causes of mental retardation.
Note: Many of these physical features can be found in the general population; having some of these characteristics doesn't necessarily mean that a person has Down's syndrome.
Complications of Down's syndrome
People with Down's syndrome are more likely to have the following.
- Heart problems.
- Eye problems, such as short- or long-sightedness or cataracts (cloudy patches in the lens of the eye).
- Hearing problems, ranging from mild to complete deafness.
- Thyroid problems, including low or more rarely, high levels of the thyroid hormones.
- Poor immunity and so are prone to chest infections, coughs and colds.
- Problems with the digestive system, such as persistent diarrhoea or constipation; babies may have feeding problems and may not gain weight normally.
- Dementia at an earlier age (it occurs 20 to 30 years earlier than in the rest of the population).
It's important for people with Down's syndrome to have regular health checks so that these conditions can be diagnosed and treated at an early stage.
Development
All people with Down's syndrome have some level of learning disability but the severity can differ between individuals. Children usually learn to walk, talk, read and write, but more slowly than other children of their age. People with Down's syndrome learn to do things throughout their lives at different rates.
Causes of Down's syndrome
Down's syndrome is caused by an extra chromosome. This happens as a result of a problem in cell division but it's not known what causes that to happen. However, the chance of having a baby with Down's syndrome increases with the mother's age.
Mother's age at conception Risk of Down syndrome:- 25 years 1 in 1,250
- 30 years 1 in 1,000
- 35 years 1 in 400
- 40 years 1 in 100
- 45 years 1 in 30
However, most babies with Down's syndrome are born to women under 35, since these women account for the majority of the childbearing population.
The chance of you having a baby with Down's syndrome has nothing to do with where you live, your social class or your race. You can't do anything before or during pregnancy to change the chance of your baby having Down's syndrome.
Diagnosis of Down's syndrome
Babies with Down's syndrome are usually diagnosed in the first few days after birth. Doctors and midwives are trained to identify the physical characteristics associated with the condition. Some babies have almost no physical signs while others have all of them. A chromosome test is then used to confirm the diagnosis. The doctor will take a blood sample from the baby. This is sent to a laboratory for tests.
There are also screening tests for Down's syndrome that you can have while you're pregnant. Screening takes place during either the first trimester (three months) or second trimester (six months) by either ultrasound or through a blood test, or a combination of both. Screening tests don't give a definite answer, but can tell you if your baby has an increased risk of having Down's syndrome.
If the screening tests show that your baby has an increased risk of having Down's syndrome; you will be offered further diagnostic tests, such as chorionic villus sampling (following a first-trimester screening test) or amniocentesis (following a second-trimester screening test). These tests involve some risk to mother and baby so are usually only offered to women if earlier screening tests suggest the baby is likely to have Down's syndrome. For more information about these tests, see related topics.
When Down syndrome is diagnosed after birth, it is usually suspected based on the physical characteristics of the infant. The definitive diagnosis, though, is made by chromosomal karyotype ("mapping") analysis.
There are several
screening tests done in pregnancy that may help diagnose Down syndrome:
- Alpha fetoprotein (AFP) screening is a simple blood test done between 15 and 20 weeks' gestation in all pregnancies.
It is not a specific test (meaning that it will not give an exact diagnosis), but, if abnormal, will generally lead to further testing. The blood test is used along with gestational age and maternal age to indicate if a higher risk of Down syndrome, neural tube defect, or other genetic abnormalities are present.
- High-resolution ultrasound of the fetal neck during the second trimester of pregnancy may also help to diagnose Down syndrome. Generally, more testing is done if the ultrasound is suspicious.
- Amniocentesis or chorionic villus sampling are more invasive tests that carry some risk but can give a definitive diagnosis of Down syndrome by chromosomal analysis.
Depending on a woman's risk of Down syndrome, she may see a geneticist or discuss having a diagnostic procedure done during her pregnancy (prenatal diagnosis).
Living with Down's syndrome
People with Down's syndrome have special medical and social needs, but they can live full lives, take part in further education, have jobs and relationships, and live independently.
Medical and social support
A team of professionals will help support people with Down's syndrome, and their families. This team may include your GP, a pediatrician, midwife, health visitor, occupational and speech therapists and a physiotherapist for example.
Specialist doctors monitor all babies with Down's syndrome for health problems, and children with the condition have regular growth, hearing and sight, and thyroid checks. It's also important for adults with Down's syndrome to have regular sight, hearing and thyroid function tests.
Occupational therapists and dietitians can help with issues such as nutrition and educational support. Most children with Down's syndrome go to mainstream schools, but there are schools for children with special needs.
Down's syndrome Q&A
See our answers to common questions about
Down's syndrome, including:
Sources:
- Antenatal care: Routine care for the healthy pregnant woman. National Institute for Health and Clinical Excellence (NICE), March 2008.www.nice.org.uk
- Why is Down's syndrome referred to as a genetic condition? Down's Syndrome Association. www.downs-syndrome.org.uk, accessed 15 December 2009
- Learning about intellectual disabilities and health: Down's syndrome. St Georges University of London. www.intellectualdisability.info, accessed 15 December 2009
- Basic medical surveillance essentials for people with Down's syndrome. Cardiac disease: Congenital and acquired. UK Down's Syndrome Medical Interest Group, 2007. www.dsmig.org.uk
- Down's syndrome: Coeliac disease/gluten sensitivity. UK Down's Syndrome Medical Interest Group. www.dsmig.org.uk, accessed 15 December 2009
- Health FAQs. Down's Syndrome Association. www.downs-syndrome.org.uk, accessed 15 December 2009
- Testing for Down's syndrome in pregnancy. NHS Antenatal and Newborn Screening Programmes. www.fetalanomaly.screening.nhs.uk, accessed 15 December 2009
- Pre-conception - advice and management. Clinical Knowledge Summaries. www.cks.nhs.uk, accessed 15 December 2009
- Information for parents: Down syndrome. Department for Children SAF. http://publications.everychildmatters.gov.uk, accessed 15 December 2009
- Choosing the right school for your child: A guide for parents and carers. Mencap. www.mencap.org.uk, accessed 15 December 2009
- Down's syndrome: A guide for new parents. Down's Syndrome Association. www.downs-syndrome.org.uk, accessed 15 December 2009
- Pradhan M, Dalal A, Khan F, et al. Fertility in men with down syndrome: A case report. Fertil Steril 2006; 86(6):1765
- Van Cleve SN, Cannon S, Cohen W. Part 2: Clinical practice guidelines for adolescents and young adults with down syndrome: 12 to 21 years. J Pediatr Health Care 2006; 20(3):198-205
- National strategy for speech, language and communication: 2005-2010. Down's Syndrome Association. www.downs-syndrome.org.uk, accessed 15 December 2009
- Behind Closed Doors: Down Syndrome http://www.npr.org/templates/story/story.php?storyId=10167662
- Health Supervision for Children With Down Syndrome http://aappolicy.aappublications.org/cgi/reprint/pediatrics;107/2/442.pdf
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