28. The most common chromosomal abnormalities.

Page created on February 28, 2022. Not updated since.

Down syndrome (trisomy 21)

Down syndrome is the most common chromosomal abnormality, caused by an extra chromosome 21. It affects approx. 1/650 live born infants. Its incidence increases with maternal age. Most live longer than 50 years.

It’s characterised by:

• Characteristic facies
  • Flat nasal bridge
  • Upslanted palpebral fissures
  • Epicanthic folds
• Other physical anomalies
  • Hypotonia
  • Single palmar crease
  • Wide gap between first and second toe
• Organ defects
  • Congenital heart defects
  • Duodenal atresia
  • Hirschsprung disease
• Later complications
  • Delayed motor milestones
  • Intellectual disability
  • Short stature
  • Otitis media leading to hearing impairment
  • Visual impairment
  • Increased risk of leukaemia, obstructive sleep apnoea, epilepsy, Alzheimer

Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13)

Edwards syndrome and Patau syndrome are less common than Down syndrome, and most affected babies die in infancy. Both are associated with cardiac and renal malformations, as well as typical physical anomalies.

Turner syndrome (45XO)

Turner syndrome is characterised by 45XO, with the other X chromosome lacking or having structural defects. It affects 1/2500 live-born females. It’s characterised by:

• Short stature
• Webbed neck
• Lymphoedema of the hands and feet
• Infertility (due to ovarian dysgenesis)
• Normal intellectual function (in most cases)
• Increased risk for congenital heart defects, hypothyroidism, renal anomalies

Growth hormone therapy prevents short stature, and initiation of oestrogen replacement therapy at the time of puberty allow for normal pubertal development.

Klinefelter syndrome (47XXY)

Klinefelter syndrome is characterised by the presence of two X chromosomes and one Y. It affects 1 – 2/1000 live-born males. It’s characterised by:

• Infertility
• Hypogonadism
• Gynecomastia
• Tall stature

Prader-Willi syndrome

Prader-Willi syndrome is characterised by microdeletion of the paternal copy at the long arm of chromosome 15 (15q11-13). It’s characterised by:

• The 4 H’s:
  • Hypotonia
  • Hyperphagia -> obesity
  • Hypogonadism
  • “Hypomentia” (intellectual disability)
• Characteristic facies

Angelman syndrome

Angelman syndrome is characterised by microdeletion of the maternal copy at the long arm of chromosome 15 (15q11-13). It’s characterised by:

• Characteristic facies
• Ataxia
• Epilepsy
• Intellectual disability
• Characteristic happiness, frequent laughing