Down’s Syndrome is caused by three copies of chromosome 21. It is also called trisomy 21. It gives characteristic dysmorphic features and is associated with a number of associated conditions. The extent to which the person is affected and the associated conditions they have vary between individuals.

Epidemiology

• 1/1000 births
• 85% survive 1 year
• Life expectancy - 60 years

Pathophysiology

• Non-dysjunction (95% of cases)
  • results in some cells having extra genetic material (trisomy 21)
  • 90% of cases it is as a result of the egg - more likely as maternal age increases
• Robertsonian translocation
  • additional chromosome 21 attached to another chromosome, when they split it results in an error, an additional chromosome 21 attaching to another chromosome
  • can happen between different chromosomes as well - 13,14,15,22
  • some of the translocations are not viable with life
• Mosaicism
  • occurs post conception - a non-dysjunction in mitosis
  • not all cell lines will have trisomy 21 (as this occurs later on)
  • results in a milder phenotype - may be less severely affected

Clinical features

• Hypotonia (reduced muscle tone)
• Brachycephaly (small head with a flat back)
• Short neck - extra skin folds at the back
• Short stature
• Brushfield spots in the iris
• Flattened face and nose
• small high arching palate with a larger tongue/smaller mouth