Friday, May 25, 2012

Friday's Facts 052512: Interesting Tidbits

Down syndrome was named for John Langdon Down who, in 1866, described the syndrome in a paper entitled 'Observations on the Ethnic Classification of Idiots'. He classified Down syndrome as being the 'Mongolian type of idiot'.

John Langdon Down's terminology for Down syndrome is now said to have misleading connotations, as well as being an embarrassing term. The World Health Organisation officially dropped the term mongolism in 1965. However, it does still appear is some medical texts.

In 1959, Jérôme Lejeune discovered that an extra 21st chromosome causes Down syndrome.

There are many different variations of Down syndrome with Trisomy 21 accounting for around 95% of all recorded cases.

Of all recorded cases of Down syndrome, only around 4% are the result of translocation (when a segment of the 21st chromosome breaks off and attaches itself to another chromosome), which can take on several different forms. Before translocation occurred, the baby would have had the normal number of chromosomes.

Mosaic Down syndrome accounts for around 1 - 2% of all recorded cases. In this form, there is a chance that the individual will have less prominent physical features of Down syndrome.  In Trisomy 21, which is the most common form of Down syndrome, all of the cells within the body are affected by the additional chromosome. In mosaicism, there is a mixture of cells affected by trisomy 21 and cells with the normal number of chromosome. The name comes from the fact that, in mosaicism, the cells form a mosaic pattern within the body.

Although there are health problems which are associated with having Down syndrome, there are also health benefits. Having Down syndrome gives a reduced risk of developing many common malignancies. This does not, however, include leukaemia, which individuals are 15 - 30 times for likely to develop , and testicular cancer. It is unclear as to why the incidents of fatal cancers in reduced amongst individuals who have Down syndrome although it could be related to the tumour suppressor genes on the 21st chromosome. Another explanation could be that individuals with Down are not as exposed to the environmental factors which increase the risk of some cancers.

It is not possible to prevent the genetic abnormality which causes Down syndrome. However, it has become easier to identify babies who are at risk during pregnancy. Test which can indicate a risk of Down include, a triple screen which is offered at 15 - 20 weeks of detection, a quad screen which is also offered at 15 -20 weeks and a nuchal translucency test (measures the width of the spinal cord at the back of the baby's neck) which is offered between 10 - 13.5 weeks.  The test requires an ultrasound, which measures the amount of fluid at the back of the baby's neck; this tends to be higher in babies who have Down syndrome. Although it detects around 62% of all cases of Down syndrome, there is a small chance of a false positive being given. Therefore, an amniocentesis is needed to confirm a genetic abnormality. However, due to it being an invasive procedure, which involves taking amniotic fluid from the amniotic sac, it does come with a risk of miscarriage. An amniocentesis may also be used to diagnosis trisomy 18 (Edwards syndrome) and spina bifida.

Down syndrome affects people from all cultures and ethnic groups and, in many cases, is sporadic. This means that neither of the parents are affected by the disorder and that the chances of any other pregnancies resulting in a Down syndrome baby are slim. However, there are factors which can increase the risk.

Although the risk of an infant having Down syndrome does increase with age, most babies with the syndrome are born to younger parents. This is due to most people choosing to have their children at a young age. At the age of 20, a woman's chance of having an child with Down is about 1 in 1700. This increases to 1 - 900 at age 30, 1 in 100 at age 40 and 1 -35 at age 45. The age of the father may also have an effect of the likelihood of conceiving a child who has Down syndrome.

Individuals who have Down syndrome have a life expectancy of 50 - 60+ years of age, although this can be lessened if the individual has significant health problems. This has increased from 15 years in the 1950s. As the life expectancy of individuals with Down syndrome has increased, a link has emerged between it and another disorder. Individuals with Down syndrome have a 50% chance of developing dementia. 

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