A chromosome disorder affecting mental development and physical appearance
- Present at birth
- Due to an extra chromosome
- Gender and lifestyle are not significant factors
Down’s syndrome, also called trisomy 21, is the most common chromosomal abnormality. Approximately 1 in 600 babies born each year in the UK is affected. The condition usually occurs because a baby has three copies of chromosome 21 instead of the normal two copies, giving a total of 47 chromosomes instead of the usual 46. In most cases, the extra chromosome is inherited from the mother due to an abnormality in the egg; less commonly, it is inherited from the father due to a sperm abnormality. Children who have Down’s syndrome have a characteristic physical appearance and generalized learning disabilities.
The risk of a baby having Down’s syndrome becomes greater with increasing age of the mother. In women who conceive at the age of 20, there is a risk of 1 in 1,500 that the baby will have the condition. For women aged 40, the risk of Down’s syndrome rises to 1 in 100. A woman who has already had an affected child has a slightly increased risk of having other children with the condition.
What are the symptoms?
Many of the physical characteristics of Down’s syndrome are present at birth and may include:
Round face with full cheeks.
Eyes that slant up at the outer corners with folds of skin covering the inner corners of the eyes.
Flattening of the back of the head.
Excess skin on the back of the neck.
Short, broad hands that have a single transverse crease on the palm.
Children with Down’s syndrome have abnormalities that vary in severity. They are slow in learning to walk and talk, have learning disabilities ranging from mild to severe, and are typically short in stature. However, these children are usually cheerful and affectionate.
Are there complications?
More than 1 in 3 children with Down’s syndrome have an abnormality of the heart (see Congenital heart disease), usually a hole in the wall (septum) that divides the main chambers of the left and right sides of the heart. Sometimes, abnormalities can occur in the digestive tract, causing the tract to become narrowed or blocked. Affected children are also more likely to have ear infections and an accumulation of fluid in the ear, which may lead to impaired hearing (see Chronic secretory otitis media). Eye problems, such as cataracts are also common. The neck joints may become unstable, which can prevent children with Down’s syndrome from taking part in sports. Affected children also have an increased risk of developing cancer of the white blood cells (see Acute leukaemia) and a higher than average risk of having an underactive thyroid gland (see Hypothyroidism). In later life, people with Down’s syndrome are at increased risk of developing Alzheimer’s disease, a progressive disorder of the brain that results in a gradual decline in mental abilities.
How is it diagnosed?
An increased risk of Down’s syndrome can be determined by tests early in pregnancy (see Antenatal genetic tests). These tests include blood tests and ultrasound scanning. Ultrasound can detect thickening at the back of the neck of the fetus (called nuchal translucency), which indicates an increased risk of Down’s syndrome. A definitive diagnosis can only be made by examining the chromosomes from fetal cells obtained at amniocentesis or CVS.
Sometimes, Down’s syndrome may not be diagnosed until after the baby is born. The condition is usually recognized by the distinctive appearance of the face. The diagnosis is confirmed by taking a sample of the baby’s blood and performing a blood test to look for an extra copy of chromosome 21. Further tests, such as ultrasound scanning of the heart (see Echocardiography) or a contrast X-ray of the intestines, called a barium enema, may then be carried out to look for complications.
What is the treatment?
In the past, many children with Down’s syndrome were given institutional care and lacked the necessary stimulation and support to maximize their mental development. Today, improved facilities mean that it is possible to make the most of their mental capabilities and to treat any physical problems as they arise. In the majority of cases, children are able to remain in their home environment.
Children who have Down’s syndrome and their families usually receive medical care and long-term support from a team of professionals. Specialized care may include physiotherapy and speech therapy. Special educational programmes may be adopted to help affected children to develop intellectually as much as possible. In some cases, surgery is necessary to correct abnormalities of the heart or intestine. Parents of affected children may find it helpful to contact a support group.
What is the prognosis?
Most children with Down’s syndrome lead happy and fulfilling lives. Although intellectual ability varies widely, affected children are now able to achieve much more because of the improvement in teaching methods and availability of special educational programmes. Some children learn to read and write and may acquire sufficient skills to enable them to work later in life. However, most are unable to live independently and need long-term supervision, provided either by their parents or in a special residential home. Although many now survive into late middle age or beyond, people with Down’s syndrome are at increased risk of dying in childhood, often as a result of congenital heart disease. The average life expectancy of a person with Down’s syndrome is about 60–65 years, but more than 40 per cent of those over 50 develop Alzheimer’s disease.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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