Down's syndrome

The Down's syndrome - A chromosomal abnormality that results in a variable degree of learning difficulty and a characteristic physical appearance in affected individuals.

Down’s syndrome child using sign language
There is a greater than normal risk of congenital deafness in Down’s babies. Typical features of the condition include upward sloping eyes that are covered at the corners, small facial features, and a large tongue that tends to stick out.

CAUSES
People with Down’s syndrome have an extra chromosome (47 instead of the normal 46). Affected individuals have three copies of chromosome number 21 instead of two; for this reason, the disorder is also called trisomy 21.

In most cases, it is the result of a sperm or egg being formed with an extra chromosome 21, due to the failure of the chromosome 21 pair to part and enter separate cells during meiosis. If one of these abnormal egg or sperm cells takes part in fertilization, the baby will also have the extra chromosome.

This type of abnormality is more likely if the mother is aged over 35. A less common cause is a chromosomal abnormality called a translocation, in which part of one parent’s own chromosome number 21 has joined with another chromosome. The parent is unaffected but has a high risk of having Down’s children.

SYMPTOMS
Typical physical features of a person with Down’s syndrome include small face and features; upward-sloping eyes with folds of skin that cover their inner corners; large tongue; flattened back to the head; short, broad hands, with a single horizontal crease on the palm (see simian crease); and short stature. Learning difficulties are very common, and range from mild to severe; however, affected people often have cheerful, friendly personalities.

People with Down’s syndrome have a greater than normal risk for certain disorders. One possible problem is a heart defect at birth (see heart disease, congenital), which affects up to two in five babies. Others include intestinal atresia (a narrowing in the intestines) and congenital deafness. Acute leukaemia is more common than in other children. Down’s syndrome children are also especially susceptible to ear infections. Affected adults over the age of 40 have a higher than normal risk of developing Alzheimer's disease.

DIAGNOSIS
Screening tests during early pregnancy, including a nuchal thickness scan (a form of ultrasound scanning to measure the skin at the back of the neck) indicate those fetuses likely to have the syndrome.

Amniocentesis is then offered; this test involves chromosome analysis of fetal cells. In some cases, Down’s syndrome is only recognized once the baby has been born. The diagnosis is confirmed by chromosome analysis of cells from the baby.

OUTLOOK
There is no cure for the condition, but many Down’s syndrome children can make the most of their capacities with appropriate environmental and educational stimulation and support. Some children learn skills such as reading and writing, and some adults may be able to work; however, most affected people cannot live independently and need long-term care from their families or in a residential home.