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Down syndrome

  • Down syndrome is a relatively common genetic disorder, which, in the absence of any prenatal detection programme, affects 1 in 700 live births.
  • It is most commonly caused by the presence of a third copy of chromosome 21 in all cells (‘trisomy 21’).
  • Individuals with Down syndrome display characteristic facial features and learning disabilities.
  • People with mosaicism for trisomy 21 (in whom only some of their cells have the extra chromosome 21, the rest having the usual two copies) present with less severe clinical effects.
  • Trisomies can be detected postnatally or prenatally, increasingly by employing DNA-based techniques, and confirmed by means of karyotyping (visualisation of the chromosome complement).
  • Individuals should be monitored for congenital and later health complications, and with the family offered counselling where helpful.

Clinical features

  • Individuals with Down syndrome have characteristic facial features, may have congenital anomalies, and as infants usually have hypotonia (poor muscle tone, floppiness).
  • Learning disabilities are present, and range from moderate to severe.
  • Forty per cent of individuals have congenital heart disease; 5% have duodenal atresia or Hirschsprung disease; 2% develop cataracts; 10%-30% have hypothyroidism; and 1% develop leukaemia.
  • Development of Alzheimer’s disease is common in people with Down syndrome beyond 40 years of age.


  • A clinical diagnosis should always be completed by chromosome testing (see 'Genetic testing', below).

Genetic basis

  • Down syndrome is caused by all cells having three copies of the genetic material of most or all of chromosome 21.
  • It is important to identify the underlying genetic cause of Down syndrome, as this will influence advice on recurrence risk.
  • In 95% of cases Down syndrome is caused by the situation of trisomy 21, where all cells contain an entire freestanding third copy of chromosome 21.
  • A translocation involving chromosome 21 (and that is associated with high recurrence risks and familial cases) accounts for 4% of cases, and should be considered when there is more than one affected person in a family.
  • Mosaicism for the trisomy (where only some cells have the extra chromosome 21, the rest having the usual two copies) accounts for 1% of cases, but has a milder clinical presentation.
  • The birth incidence of trisomy 21 (without prenatal testing) rises with increasing maternal age.

Clinical management

  • Infants and children with Down syndrome will normally be referred to the paediatrician at the local child development centre, for a planned programme of management, and involving paediatric cardiology and other specialties as appropriate.
  • Prevention of obesity and periodontal disease, as well as monitoring for coeliac disease, thyroid function, arthritis and other disorders should occur.
  • Counselling may be required for sexuality and sexual health, behavioural problems and psychological support.

Genetic testing

  • Increasingly, the diagnosis is made prenatally following antenatal screening and either chorionic villus sampling or amniocentesis.
  • The most valuable diagnostic tool is karyotyping, in which the full complement of an individual’s chromosomes can be visualised, showing the occurrence of an extra chromosome or a translocation.
  • Many prenatal testing programmes are now initially using DNA-based techniques, such as quantitative fluorescence PCR (QF-PCR), as an initial efficient way of detecting three copies of chromosome 21 material. 


This information is intended for educational use and was current in August 2013. For clinical management, it is recommended that local guidelines and protocols are used.