May 29, 2024
Genetic defects

Down Syndrome or Trisomy 21: Chromosomal disorder

  • October 23, 2015
  • 3 min read
Down Syndrome or Trisomy 21: Chromosomal disorder

Trisomy 21 or  Down Syndrome was first described by John Langdon Hayden Down a Physician ( 1828-1896 AD) in The London Hospital in 1866 AD.In 1959, Lejeune and Jacobs et al independently determined -trisomy 21 as cause of Down Syndrome.

March 21st every Year is Marked  as ” Down Syndrome Day” – 3rd month 21st


  1. Trisomy 21: 47 XX or XY +21 . Usually caused by an extra copy of Chromosome 21
  2. Translocational type- 46 XX, der ( 14;21)(q10;q10),+21
  3. Mosaic type- 46 XX/ 47 XX ,+21

Most common of the chromosomal disorders ( incidence 1 in 700 births).Risk increases with maternal age. Significant increase in risk with Maternal age above 40 years. 1:2000 in  under 25 years, 1: 300 at 35 years and 1:100 at 40 years. Co-relation with maternal age suggests that meiotic  non-disjunction of Chromosome 21 occurs in Ovum.

Male:Female ratio is 1.15:1 not much difference.


  1. Meiotic non-dysjunction (95%)
  2. Robertsonian translocation (4%)
  3. Mosaicism due to miotic non-dysjunction during the embryogenesis. (1%) : Mosaics are people having 2 or more populations of cells within individuals.  They have variable features and milder.

Clinical features-

Severe mental retardation- most common cause of genetic mental retardation.  IQ is generally low to very low.

Facial  features :

  • Mongoloid facies-flat face,low-bridged nose, epicanthal folds, mongoloid slant of eyes
  • brachycephaly commonly , microcephaly, delayed closure of  fontanels, a patent metopic suture
  • Absent frontal and sphenoid sinuses, and hypoplasia of the maxillary sinuses may be seen.
  • low-set ears ,dysplastic ears-small ear with overfolded helix
  • hypertelorism –wide set eyes
  • high arched palate,open mouth with protruded and furrowed tongue ( macroglossia)
Click to view full size


  • Brushfield spots-speckled appearance of the iris
  • Cataract and squint sometimes
  • Refractive error and nystagmus


  • Simian hand- short broad hands with short stubby fingers
  • Simian Crease- Single palmar flexion crease.
  • Clinodactyly -( Hypoplasia of the middle phalanx of 5th finger producing incurving little finger)
  • Dermatoglyphics-  more than 8 ulnar loops on fingers and distal axial triradii becomes obtuse.


  • Sandal gap- Increased gap between 1st and 2nd toe
  • Sub-hallucial pad of fat
  • Single deep longitudinal plantar crease

Cardiovascular System-Congenital heart defects- Endocardial cushion defect leads to formation of an atrioventricular canal ( a common connection between all 4 chambers), VSD, ASD, PDA

GI system – Duodenal atresia ( ‘ double bubble ‘ sign), Hirchsprung disease, tracheo-esophageal fistula, Imperforate anus.

Other System-

  • Muscular hypotonia
  • Broad short neck, Short height, Growth retardation
  • Atlantoaxial subluxation with spinal cord compression <1 : A delay in recognizing atlantoaxial and atlanto-occipital instability may result in irreversible spinal-cord damage and add disability like visual and auditory loss.
  • Infertility
  • Personality- Cheerful, fond of Music.
  • Premature Aging.
  • Umbilcal hernia and diastasis rectii

Increase susceptibility to-

  • Increased risk ( 15-20 X) of Acute Lymphoblastic Lymphoma- Trisomy 21, GATA 1 mutation and unknown genetic alternation.
  • Alzheimer disease ( by age 40 virtually all will develop Alzheimer Disease)
  • Coeliac disease may be associated .
  • Increased susceptibility to infection (pneumonia, otitis media, sinusitis, pharyngitis, periodontal disease)
  • Autoimmune hypothyroidism and Hashimoto disease
  • Hyperuricemia, Diabetes Mellitus,

Prenatal Diagnosis-

  1. In-utero:  Triple test using Maternal serum levels of alpha-fetoprotein, hCG, and unconjugated  estriol.
  2. Ultrasonography- increased nuchal fold >4 mm, failure to visualize fetal nasal bone at age 11-13 weeks POG,femur and the humerus tend to be shortened.
  3. Amniocentesis
  4. Chorionic Villi sampling
  5. Percutaneous umbilical blood sampling (PUBS)



Down syndrome decreases prenatal viability and increases prenatal and postnatal morbidity as well. About 75%  with trisomy 21 die in fetal life.~ 85% of infants survive to age 1 year, and 50% can be expected to live longer than age 50 years. Down Syndrome is compatible to life . With good medical care , current median age at death is 47 years.

10 Questions about Down syndrome : FAQ


  • Robbins and Cotran Pathological Basis of Disease
  • Langman’s Medical embryology.
  • USG measurement in down Syndrome
  • Kundu Bedside Medicine
  • Kaplan’s Lecture notes
  • Medscape-Emedicine
About Author



  • Reading horrifying articles like this is why so many mothers choose to abort when told they are pregnant with a baby with Down Syndrome. Such a shame! As the mother of an AMAZING son with Trisomy 21, you did not even begin to depict the true picture of what a person with Down syndrome is like.

  • Your writing aided me with my seminary assignment in time. I think I will give my assignment next week.

Comments are closed.