Down's syndrome

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Down's syndrome is a disorder caused by having an extra chromosome on chromosome 21 (referred to as trisomy 21), giving a total of 47 chromosomes instead of the normal 46 in humans. This extra chromosome copy disrupts the normal course of brain and body development, thus affecting both physical development and intellectual ability. Although individuals with Down's syndrome behave and look similar, their intellectual capability varies from mild to moderately low. As sufferers age (often about the age of 50), they may often experience a progressive decline in cognitive function, which can result in an increased risk of Alzheimer's, a neurodegenerative disease that causes dementia: a gradual loss of memory, motor skills, and judgement[1].

Contents

Types

There are three main types of Down's syndrome.

Trisomy 21

An error during gamete formation results in a failure of the chromosomal pair on chromosome 21 to separately properly, which results in an embryo with three copies of chromosome 21. This is known as non-disjunction. Most cases of Down's syndrome are a result of a random chromosomal defect during the formation of reproductive cells, especially egg cells. As the embryo develops, the extra chromosome is replicated in every cell of the body. This is the most common form and accounts for about 95% of cases.

Mosaic trisomy 21

Mosaic refers to mixed or a combination. Most cells in the body have 47 chromosomes, whereas some of the cells have 46 chromosomes. Like trisomy 21, this isn't inherited; instead, it is caused by random events that occur during cell division in early foetal development. This is the least common form and accounts for about 1% of cases.

Translocation trisomy 21

Part of chromosome 21 is in the cell, which becomes attached to another chromosome during gametic cells formation or during early foetal development. This is transferred to other cells as it divides. In some cases, parents unaffected by Down's syndrome may have translocation trisomy 21, which can be inherited by their offspring[2][3].

Causes

To date, the cause of the extra chromosome is still unknown, Increasing maternal age (especially pregnant women aged 35 or over) is a contributory factor towards a higher risk of a baby having Down's syndrome with trisomy 21 or mosaicism. There is no conclusive scientific research to suggest that environmental factors or parental activities before or during pregnancy are the causes.

Symptoms

Physical

Young children are more susceptible to developmental delay. This may be a consequence of poor muscle tone, which impacts the ability to stand, balance, and sit normally, so they may reach these milestones slower than other children.

Intellectual and behavioural

Complications

Individuals with Down Syndrome may be at a higher risk of:

Note that individuals with Down Syndrome may have a decreased risk of diabetic retinopathy (hardening of the arteries), and also a reduced risk of cancers.

Screening and diagnosis

Before a clinical diagnosis is made, prenatal screening tests are carried out to predict the probability of the foetus having Down's syndrome. Conversely, diagnostic tests are more accurate in determining whether or not the foetus will have Down's syndrome.

Screening

Blood (serum screening) tests - measures amount of substances in the maternal blood, and along with the mother's age, doctors use this information to estimate the probability of the baby developing Down's syndrome.

Ultrasound - often used in conjunction with a blood test, this creates an image of the baby. Doctors observe any fluid behind the baby's neck as this can indicate genetic defects[5].

Prenatal diagnosis

Chorionic villus sampling - obtain a small sample of material from the placenta (chorionic villus) and test for abnormalities.

Aminocentesis - examines amniotic fluid (the fluid sac surrounding the baby), and test it for protein levels.

Percutaneous umbilical blood sampling - examines blood from the umbilical cord, which is then tested for the extra chromosome. This is the most accurate method and can be used to confirm CVS and amniocentesis, however, this can only be carried out in the later stages of pregnancy[6].

Diagnosis at birth

At birth, doctors observe whether or not the newborn has any of the typical characteristics such as: weak muscle tone, upward slanting eyes and flattened facial profiles. However, these traits may also be present in those without Down's syndrome. Therefore it is important to carry out further analysis.

Karyotope analysis - doctors obtain a blood sample from the newborn and examine karyotypes to detect any chromosomal abnormalities[7].

References

  1. Genetics Home Reference. Down syndrome. Available at: http://ghr.nlm.nih.gov/condition/down-syndrome (Last accessed 01/12/15)
  2. NDSS. (n.d.). What is Down syndrome? Available from: http://www.ndss.org/Down-Syndrome/What-Is-Down-Syndrome/ (Last accessed 01/12/15)
  3. Genetic and Rare Diseases Information Center (GARD). (2012). Down syndrome. Available from: http://rarediseases.info.nih.gov/GARD/QnASelected.aspx?diseaseID=10247 (Last accessed: 01/12/15)
  4. Down syndrome. (2012). In A.D.A.M. Medical Encyclopedia. Available from: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001992 (Last accessed: 01/12/15)
  5. Driscoll, D. A., and Gross, S. (2009). Prenatal screening for aneuploidy. New England Journal of Medicine, 360, 2556–2562.
  6. American Association for Clinical Chemistry. (2012). Second trimester: Cordocentesis (percutaneous umbilical blood cord sampling, PUBS). Available from: http://labtestsonline.org/understanding/wellness/pregnancy/second-cordo
  7. MedlinePlus. Karyotyping. Available from: https://www.nlm.nih.gov/medlineplus/ency/article/003935.htm (Last accessed 01/12/15)
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