3. Down's Syndrome (Trisomy 21)

Down's Syndrome (Trisomy 21)

This PatientPlus article is written for healthcare professionals so the language may be more technical than the condition leaflets. You may find the abbreviations list helpful.

Synonym: trisomy 21

It is a common genetic disorder characterised by learning difficulty, dysmorphic facial features and a host of structural abnormalities. It is named after John Langdon Haydon Down - an English physician, 1828-1896.

Incidence

One of the most common genetic disorders, affecting 1 in 650-1,000.[1]

  • The underlying genetic defect is trisomy 21 in 94% of cases.
  • Mosaicism (2.4%) and translocations (3.3%) also occur.
  • 75% of these translocations are de novo errors.

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Risk factors

  • Family history.
  • Age of mother:
    • 1:385 risk at 35 years
    • 1:106 at 40 years
    • 1:30 at 45 years

At birth there is a wide range of associated physical features. Not all babies have typical facies. Frequently the first feature noticed is hypotonia.

Neonatal features of Down's syndrome (most common features first)[2]

  • Brachycephaly.
  • Oblique palpebral fissures.
  • Gap between hallux and second toes.
  • Loose skin on nape of neck.
  • Hyperflexibility.
  • Ears set low, folded or stenotic meatus.
  • Protruding tongue (small narrow palate).
  • Flat nasal bridge.
  • Muscular hypotonia.
  • Epicanthic folds.
  • Ring of iris speckles - Brushfield's spots.
  • Short little finger.
  • In-curved little finger.
  • Short broad hands.
  • High arched palate.
  • Single palmar crease.
  • Congenital heart defects.
  • Transient myelodysplasia of the newborn*.
  • Duodenal atresia*.
*highly specific for Down's syndrome

Soon after birth all babies with Down's syndrome should be assessed for congenital heart disease by echocardiogram.[3] They should also have auditory evoked potential testing (before age 6 months) to check for hearing loss[4] and also be checked for congenital cataracts and glaucoma.[5]

Cardiological disorders

The most common cardiac abnormalities are:

  • Atrioventricular canal defects.
  • Ventricular septal defect.
  • Isolated secundum atrial septal defects.
  • Isolated persistent patent ductus arteriosus.
  • Fallot's tetralogy.

Adult patients, without known congenital heart disease, may develop mitral valve prolapse or aortic regurgitation. A second assessment in early adulthood may be appropriate.

Ear, nose and throat disorders

  • 90% of patients with Down's syndrome may have conductive, sensorineural, or mixed hearing loss.[1]
  • They are more susceptible to otitis media, sinusitis and pharyngitis.
  • Obstructive sleep apnoea may develop.

Ophthalmological disorders

Most commonly:

  • Cataracts
  • Refractive errors
  • Strabismus
  • Nystagmus
  • Congenital glaucoma
  • Keratoconus

Gastrointestinal disorders

  • Oesophageal atresia or tracheo-oesophageal fistula.
  • Duodenal atresia.
  • Pyloric stenosis.
  • Meckel's diverticulum.
  • Hirschsprung's disease.
  • Imperforate anus.
  • Gastro-oesophageal reflux.
  • Dental problems - delayed and unusual patterns of eruption, missing teeth.
  • Coeliac disease occurs frequently enough for screening to be recommended.

Orthopaedic disorders

  • Atlanto-axial instability*.
  • Hyperflexibility.
  • Scoliosis.
  • Hip dislocation after two years.
  • Patellar subluxation or dislocation.
  • Foot deformities.

*Few need treatment.[7] They present with neck pain, limited movements or symptoms/signs suggestive of cord compression. Less specific symptoms, eg bladder problems, gait abnormalities or clumsiness, may also indicate need for further investigation.

Endocrine disorders

These are common, particularly hypothyroidism. Annual TFTs are indicated.[8]

Neurological and psychiatric disorders

  • Learning difficulties (these range from severe, to those with 'low normal' IQ).
  • Behavioural problems.
  • Seizures occur in 5-10%.
  • In older patients an Alzheimer's-type picture develops in >60% of those over 60 years of age.

Haematological disorders

  • Patients have approximately 12 x greater risk of infections, eg pneumonia due to impaired cellular immunity.
  • They also have increased risk of acute myeloblastic leukaemia (AML), acute lymphoblastic leukaemia (ALL) and acute megakaryoblastic leukaemia (AMegL).
  • Polycythaemia and transient myeloproliferative disorder (a self-limiting type of leukaemia which regresses spontaneously by the age of two months) may occur in newborns.

This is covered in detail in Antenatal Screening for Down's syndrome.

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Further reading & references

  1. Down Syndrome, Online Mendelian Inheritance in Man (OMIM)
  2. Chen H, Genetics of Down Syndrome, Medscape, Feb 2011
  3. Basic medical surveillance essentials for people with Down's Syndrome: cardiac disease - congenital and acquired, Down's Syndrome Medical Interest Group (2007)
  4. Basic medical surveillance essentials for people with Down's Syndrome: hearing impairment, Down's Syndrome Medical Interest Group (2007)
  5. Basic medical surveillance essentials for people with Down's syndrome: ophthalmic problems, Down's Syndrome Medical Interest Group (2006)
  6. Weijerman ME, de Winter JP; Clinical practice. The care of children with Down syndrome. Eur J Pediatr. 2010 Dec;169(12):1445-52. Epub 2010 Jul 15.
  7. McKay SD, Al-Omari A, Tomlinson LA, et al; Review of Cervical Spine Anomalies in Genetic Syndromes. Spine (Phila Pa 1976). 2011 Oct 29.
  8. Basic medical surveillance for people with Down's Syndrome : thyroid disorder, Down's Syndrome Medical Interest Group (2001)
Original Author: Dr Hayley Willacy Current Version: Peer Reviewer: Dr Hannah Gronow
Last Checked: 14/12/2011 Document ID: 2071  Version: 22 © EMIS

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.