Tuesday 2 November 2010

MRCP revision battle 40.6: Trisomies - Down's, Edward's and Patau's

A trisomy is a genetic abnormality in which there are 3 copies of a chromosome instead of the usual 2.  It is a form of aneuploidy (=when the number of chromosomes is not a multiple of 23).


This battle will look at the 3 commonest trisomies in order of their frequency, which helpful is the same as the order of their names alphabetically.



Down's Syndrome = Trisomy 21

Causes of Down's:
  • non-disjunction: 94%
  • Robertson translocation: 5%
  • mosaicism: 1%

Risk of Downs with age of mother:
  • 30 yrs: 1/1000
  • 35 yrs: 1/350
  • 40 yrs: 1/100
  • 45 yrs: 1/30


The characteristics of patients with Down's are varible but include:
  • microgenia (=small chin)
  • macroglossia
  • epicanthic folds of eyes
  • short neck
  • single palmar crease
  • flat nasal bridge
  • low IQ
  • short stature

Associations with Down's include:
  • duodenal atresia
  • Hirschsprungs disease
  • AV septal defects (40%)
  • ventricular septal defects (30%)
  • atrial septal defects (10%)
  • PDA/tetralogy of fallot
  • Brushfield spots (=white spots on iris)

Later complications include:
  • ALL
  • atlantoaxial instability
  • hypothyroidism


Edwards Syndrome = Trisomy 18


Characterised by:
  • prominent occiput
  • microcephaly
  • overlapping fingers
  • rockerbottom feet
  • cardiac defects

Rare to survive beyond infancy




Patau's Syndrome = Trisomy 13

Characterised by:
  • CNS, renal and cardiac abnormalities
  • low IQ
  • polydactyly
  • rockerbottom feet

Rare to survive beyond infancy.



To the final battle of the day - hereditary angioedema