Down's syndrome

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Down's syndome is disorder caused by having an extra [[Chromosome|chromosome]] on chromosome 21 (referred to as [[Trisomy 21|trisomy 21]]), giving a total of 47 chromosomes instead of the normal 46 in humans.<span style="font-size: 13px; line-height: 1.5em">This extra chromosome copy disrupts the normal course of&nbsp;[[Brain|brain]] and body development, thus affecting both physical development and intellectual ability.&nbsp;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 </span>[[Alzheimer's|Alzheimer's]]<span style="font-size: 13px; line-height: 1.5em">, a [[Neurodegeneration|neurodegenerative]] disease that causes </span>[[Dementia|dementia]]<span style="font-size: 13px; line-height: 1.5em">: a gradual loss of memory, motor skills, and judgement.<ref name="Red">Genetics Home Reference. Down syndrome. Available at: http://ghr.nlm.nih.gov/condition/down-syndrome (Last accessed 01/12/15)</ref>&nbsp;</span>
+
Down's syndrome is a disorder caused by having an extra [[Chromosome|chromosome]] on chromosome 21 (referred to as [[Trisomy 21|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|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|Alzheimer's]], a [[Neurodegeneration|neurodegenerative]] disease that causes [[Dementia|dementia]]: a gradual loss of memory, motor skills, and judgement<ref name="Red">Genetics Home Reference. Down syndrome. Available at: http://ghr.nlm.nih.gov/condition/down-syndrome (Last accessed 01/12/15)</ref>.
  
== '''Types''' ==
+
== Types  ==
  
 
There are three main types of Down's syndrome.  
 
There are three main types of Down's syndrome.  
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=== Trisomy 21  ===
 
=== Trisomy 21  ===
  
An error during [[Gamete|gamete]] formation results in a failure of the chromosomal pair on chromosome 21 to separately properly, which results in an [[Embryo|embryo]] with three copies of chromosome 21. This is known as [[Non disjunction|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.&nbsp;
+
An error during [[Gamete|gamete]] formation results in a failure of the chromosomal pair on chromosome 21 to separately properly, which results in an [[Embryo|embryo]] with three copies of chromosome 21. This is known as [[Non disjunction|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 trisomy 21  ===
  
Mosaic refers to mixed or a combination. Most cells in the body have 47 chromosomes, whereas some of the cells have 46 chromsomes. 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.  
+
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  ===
 
=== Translocation trisomy 21  ===
  
Part of chromosome 21 is in the cell, which becomes attached to another chromosome during gametic cells formation or during early [[Foetus|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.<ref>NDSS. (n.d.). What is Down syndrome? Available from: http://www.ndss.org/Down-Syndrome/What-Is-Down-Syndrome/ (Last accessed 01/12/15)</ref><ref>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)</ref>  
+
Part of chromosome 21 is in the cell, which becomes attached to another chromosome during gametic cells formation or during early [[Foetus|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<ref>NDSS. (n.d.). What is Down syndrome? Available from: http://www.ndss.org/Down-Syndrome/What-Is-Down-Syndrome/ (Last accessed 01/12/15)</ref><ref>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)</ref>.
  
== '''Causes'''<br> ==
+
== 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 [[Javascript:void(0);/*1448929934861*/|environmental]] factors or parental activities before or during preganancy are the causes.&nbsp;
+
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''' ==
+
== Symptoms  ==
  
 
=== Physical  ===
 
=== Physical  ===
  
*Flattened face and nose<br>
+
*Flattened face and nose  
 
*Small hands and feet  
 
*Small hands and feet  
 
*Upward slanting eyes with a skin fold from upper eyelid that covers inner corner of the eye  
 
*Upward slanting eyes with a skin fold from upper eyelid that covers inner corner of the eye  
*Brushfield spots: white spots on coloured part of the eye&nbsp;
+
*Brushfield spots: white spots on coloured part of the eye  
 
*Abnormally shaped ears  
 
*Abnormally shaped ears  
 
*Poor [[Muscle|muscle]] tone (hypotonia)  
 
*Poor [[Muscle|muscle]] tone (hypotonia)  
*[[Heart disease|Heart disease]]
+
*[[Heart disease|Heart disease]]  
*Coeliac disease
+
*[[Coeliac Disease|Coeliac disease]]
*Hearing problems
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*Hearing problems  
*[[Hypothyroidism|Hypothyroidism]]&nbsp;
+
*[[Hypothyroidism|Hypothyroidism]]
  
 
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.  
 
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<br> ===
+
=== Intellectual and behavioural ===
  
*Dementia  
+
*[[Dementia|Dementia]]
 
*Attention deficiency behavioural disorder  
 
*Attention deficiency behavioural disorder  
 
*Delayed development of speech and language  
 
*Delayed development of speech and language  
*Short attention span&nbsp;<ref>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)</ref>&nbsp;&nbsp;&nbsp;
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*Short attention span<ref>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)</ref>.
  
== '''Screening and diagnosis''' ==
+
=== Complications ===
  
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.&nbsp;<br>
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Individuals with Down Syndrome may be at a higher risk of:
  
'''Screening''' <br>
+
*Respiratory problems
 +
*Hearing difficulties
 +
*Childhood leukaemia
 +
*Epilepsy
 +
*Thyroid disorders
  
[[Blood|Blood]] (serum screening) tests - measures amount of substances in maternal blood, and along with the mother's age, doctors use this information to estimate the probability of the baby developing Down's syndrome.&nbsp;
+
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.  
  
Ultrasound - often used in conjuction with blood test, this creates a picture of the baby. Doctors observe any fluid behind the baby's neck as this can indicate genetic defects.<ref>Driscoll, D. A., &amp;amp;amp;amp;amp; Gross, S. (2009). Prenatal screening for aneuploidy. New England Journal of Medicine, 360, 2556–2562.</ref>&nbsp; &nbsp;
+
== Screening and diagnosis  ==
  
'''Prenatal 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.
  
Chorionic villus sampling - obtain a small sample of material from the [[Placenta|placenta]] (chorionic villus) and test for abnormalities.&nbsp;
+
=== Screening  ===
  
<span style="font-size: 13px; line-height: 19px">[[Amniocentisis|Amniocentisis]] - examines amniotic fluid (fluid sac surrounding the baby), and test it for protein levels.</span><br>
+
[[Blood|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.  
  
<span style="font-size: 13px; line-height: 19px">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</span><span style="font-size: 13px; line-height: 1.5em">&nbsp;and amniocentisis, however, this can only be carried out in the later stages of pregnancy.<ref>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</ref></span>
+
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<ref>Driscoll, D. A., and Gross, S. (2009). Prenatal screening for aneuploidy. New England Journal of Medicine, 360, 2556–2562.</ref>.
  
'''Diagnosis at birth'''
+
=== Prenatal diagnosis  ===
  
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.&nbsp;
+
Chorionic villus sampling - obtain a small sample of material from the [[Placenta|placenta]] (chorionic villus) and test for abnormalities.  
  
[[Karyotype|Karyotope]] analysis - doctors obtain a blood sample from the newborn and examine karyotypes to detect any chromosomal abnormalities.<ref>MedlinePlus. Karyotyping. Available from: https://www.nlm.nih.gov/medlineplus/ency/article/003935.htm (Last accessed 01/12/15)</ref><br>
+
[[Amniocentisis|Aminocentesis]] - examines amniotic fluid (the fluid sac surrounding the baby), and test it for protein levels.  
  
== '''References'''<br> ==
+
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<ref>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</ref>.
  
<references /><br>
+
=== 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.
 +
 
 +
[[Karyotype|Karyotope]] analysis - doctors obtain a blood sample from the newborn and examine karyotypes to detect any [[Chromosomal abnormalities|chromosomal abnormalities]]<ref>MedlinePlus. Karyotyping. Available from: https://www.nlm.nih.gov/medlineplus/ency/article/003935.htm (Last accessed 01/12/15)</ref>.
 +
 
 +
== References  ==
 +
 
 +
<references />

Latest revision as of 06:47, 5 December 2018

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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