Down's Syndrome

Down's syndrome is a chromosomal condition, with an estimated incidence of about 1 in every 800 live births. This figure is an overall general population risk but it is known that the incidence rises sharply with the mother's age. Down's syndrome affects both boys and girls.

Three different changes in the chromosomes can result in babies developing Down's syndrome:

What is a chromosome?

1. Trisomy 21. This is seen in approximately 95 percent of individuals with Down's syndrome and means that the person has an extra copy of a chromosome 21 in each cell, i.e. 3 instead of the usual 2.

2. Translocations are found in 3 - 4 percent of the individuals with Down's syndrome. A translocation occurs when one chromosome gets stuck to another one. Translocations that lead to Down's syndrome involve an extra copy of chromosome 21 (or part of chromosome 21) attached to another chromosome. In these cases the extra material may have been inherited from either parent or may have arisen when the baby was conceived. Where a translocation has caused Down's syndrome, there may be a risk of recurrence in a subsequent pregnancy.

3. Mosaicism is seen in about 1 percent of people with Down's syndrome. In this case, some cells have 47 chromosomes (with an extra 21) and others have the usual 46. This is thought to be the result of an error in cell division soon after conception. Because not all their cells have the trisomy, people with Down's syndrome mosaicism may be less affected by the condition.

	Characteristics of Down's Syndrome

In general, people who have the same chromosomal variation have the same characteristics. However, it is important to realise that not all of the features will be seen in all people with Down's syndrome, and that every person with Down's syndrome is a unique individual.

The most common consequence of this condition is learning disability. This is very variable, and no test prior to the birth will be able to ascertain the degree of intellectual limitation. Some children with Down's syndrome have passed GCSE exams; others have little speech or comprehension. In every case, appropriate teaching and support can enable the child to maximise their potential.

As well as learning disability, there are a range of other health issues which may affect people with Down's syndrome. Approximately half of people with the condition require spectacles, and many have some degree of hearing loss. Around 40 percent of children are born with a congenital heart abnormality, most commonly a hole in the heart (atrioventricular septal defect). Depending on how serious the heart condition is, surgery may be required. Over- and underactive thyroid problems, diabetes and problems with the intestinal and digestive tract are also seen in people with Down's syndrome. Most children with Down's syndrome experience respiratory tract problems, due to the small nasal bridge and the shorter lung capacity. There is some evidence to suggest that people with Down's syndrome have an increased probability of developing Alzheimer's disease. However, with improved medical treatments, people with Down's syndrome are living longer and with less health problems.

People with Down's syndrome have a characteristic facial appearance. For example, their eyes appear to slant upwards and outwards. The head is usually flatter at the back and people with Down's syndrome often have a low hairline. The face often appears flatter with a flat nasal bridge and the tongue is often larger than usual. The ears often appear small and low set and there is often a distinctive fold of loose skin on the back of the neck. This increased thickness on the back of the neck may be detected in pregnancy (nuchal translucency) and can contribute to a diagnosis of Down's syndrome. People with Down's syndrome often have broad hands with short fingers. Other common features associated with Down's syndrome include floppiness or poor muscle tone (hypotonia), and loose or bendy joints. For this reason, children are often slower to achieve their physical milestones such as sitting up and walking. They may also have difficulty feeding.

The life experience of people with Down's syndrome varies widely. Some children attend special schools, while others attend mainstream schools. Some adults live independently and find paid work. Others require support from family or paid helpers, and may attend day centres or other projects for people with learning difficulties. People with Down's syndrome have a reputation for being warm and friendly individuals, although some children and adults may have behavioural difficulties.

Find out more about Down's syndrome from the Down's Syndrome Association