A genetic disorder in which the aﬀected person usually carries an extra chromosome – 47 instead of the usual 46. The extra chromosome occurs in the no. 21 group, hence the disorder is described as trisomy 21. The condition was named after Dr J L H Down, the London doctor who ﬁrst described it in 1866. The incidence is around one in 600 births. The disorder is characterised by a particular physical appearance and learning diﬃculties, with the aﬀected individuals having an INTELLIGENCE QUOTIENT (IQ) ranging from 30 to 80 (normal is 100). Most people with the syndrome have eyes that slope up at the outer corners with skin folds that cover the inner ones. The face and features are smaller than normal, while the tongue is larger; the back of the head is ﬂattened and the hands are usually short and broad. The facial features led to the syndrome being described as ‘mongolism’, a term that is no longer used.
Children with Down’s syndrome are usually friendly and ﬁt in well with the family. Despite their learning disabilities, some learn to read and, if they have appropriate educational and environmental stimulation, can make the most of their abilities.
A heart defect is present in around 25 per cent of the children at birth, and deafness and acute LEUKAEMIA occur more frequently than in unaﬀected youngsters. Those with the syndrome are particularly prone to developing ear infections. ATHEROSCLEROSIS often develops early in adults and ALZHEIMER’S DISEASE tends to occur as early as 40 years of age. A friendly home environment helps them to enjoy life, but a few individuals with the syndrome may eventually require institutional care. Improved social and medical care means that many now live until their 60s.
Routine screening tests early in pregnancy, starting with blood analysis but going on if necessary to AMNIOCENTESIS and chorionic villus sampling (see PRENATAL SCREENING OR DIAGNOSIS), can identify fetuses likely to develop the disorder. If a sample of fetal cells conﬁrms the chromosome defect (triple marker test – see PREGNANCY AND LABOUR), the parents may consider termination of the pregnancy. In the UK, screening is normally oﬀered to women over 35 because of their increased risk. When younger parents have a child with Down’s syndrome, the chances of a subsequent child with the disorder are relatively high as it is probable that both parents carry a chromosome abnormality insuﬃcient to cause ill-health until combined. So they may wish to discuss with their medical advisers the question of further pregnancies.
Parents who have a child with Down’s syndrome will understandably feel a combination of strong emotions, including anger and guilt, and constructive counselling can be valuable. Among societies oﬀering advice and support is the Down’s Syndrome Association.