Down syndrome


Down syndrome (or Down's syndrome) was named after John Langdon Down (1828-1896), who was the first physician to identify it, in 1866.

Down syndrome occurs in about 1 out of every 700-900 live births worldwide (depending on contraception and termination attitudes in different communities), to mothers of all social, economic and racial groups, and is the most common genetic cause of developmental disability.

In the UK around one in every 1000 babies will be born with Down syndrome and there are 60,000 people in the UK living with the condition.

Although the chance of a baby having Down syndrome is higher for older mothers, more babies with Down syndrome are born to younger women. This is because women under 35 make up the majority of the child-bearing population.

What is Down syndrome?

Down syndrome is a chromosomal disorder caused by an error in cell division and the most common form produces an additional third chromosome 21, or "trisomy 21" (it is also known in some countries as Trisomy 21). There are three forms of Down's syndrome, with 94% being affected with Trisomy 21.

The other two are:-

** Translocation, which is caused when a piece of chromosome 21 attaches itself to another chromosome in a cell. It affects around 4% of people with Down's syndrome. The symptoms of translocation Down's syndrome are similar to those of Trisomy 21, but in rare cases it may be possible for a parent to pass translocation Down's syndrome on to their child.

Mosaicism is the least common type, affecting around 2% of people with Down's syndrome. Only some of the cells have an extra copy of chromosome 21. People with mosaic Down's syndrome may experience less delay with some aspects of their development.

** Source: NHS

Due to advances in medical technology, as many as 80% of people with Down syndrome can now expect to live to around 40-60 years of age, whereas in 1910 their life expectancy was about 9.

Is it the mother or father that is the 'carrier' of Down syndrome?

The additional genetic material which causes Down syndrome can originate from either the mother or the father, with approximately 5% of cases having been traced to the latter.

What is the probability of having more than one child with Down syndrome?

The risk of a term pregnancy with Down syndrome increases with advancing maternal age (e.g. 1 in 940 at age 30, and 1 in 35 at age 45).

* Where a previous pregnancy has been affected with Down syndrome or an open neural tube defect was reported, the result will be classified as 'screen-positive' regardless of the level of the screening markers so that further testing can be discussed with the woman.

A risk is calculated which takes account of a woman’s previous pregnancy with Down syndrome. The woman's age at the time of her previous pregnancy with Down syndrome affects the recurrence risk and this is taken into account in the risk calculation.

* Source: Barts & The London School of Medicine & Dentistry.

What is the link between maternal age and Down syndrome?

Since Down syndrome is a chromosomal disorder caused by an error in cell division, the likelihood of such an error occuring increases with maternal age. This means that an older mother is more likely to give birth to a child with Down sydrome than her younger counterpart. However, overall, fewer older mothers have babies than younger ones.

According to the National Association for Down syndrome, "80% of babies born with Down syndrome are born to women younger than 35. The average maternal age is 28 years old." The likelihood of a woman under 30 years of age giving birth to a child with Down syndrome is less than 1:1000, but increases the older the woman gets (see chart below), with an incidence of about 1:85 at 40 years of age.

An annual occurence of Down syndrome of about 9% is seen in women 35 years of age or older, but about 25% of babies with Down syndrome are born to women in this age group. 20-25% of babies conceived with Down syndrome survive past birth.

 


Calculation of the risk of Down syndrome.


(Note: This chart should not be used to assess an individual's risk of having a baby with Down syndrome, it is purely a general guide).

Age:
Risk:
20
1:1450
25
1:1350
30
1:940
35
1:350
36
1:270
37
1:200
38
1:150
39
1:110
40
1:85
41
1:70
42
1:55
43
1:45
44
1:40
45
1:35
46
1:30
47
1:30
48
1:30
49
1:25
Source: Barts & The London School of Medicine & Dentistry

 

Screening for Down syndrome

Click here for details of the various tests available during pregnancy


What are the problems that a child born with Down syndrome might face?

Associated with Down syndrome is an increased risk of hearing and vision defects, heart abnormalities (about 50% are born with congenital heart defects, for which the majority can be surgically corrected), infection, leukaemia, thyroid disorders, and of developing Alzheimer-type dementia in later life. There can also be delayed motor skills - sitting, crawling and walking in infancy, and delayed cognitive skills such as speech and language acquisition and in short-term memory abilities.

The majority of those with Down syndrome have IQs that fall in the mild to moderate range of retardation, but some are so mildly affected that they can lead independent and relatively normal lives. However, it is important to remember that the range of abilities and thus, disabilites, interests and achievements varies greatly, just as it would for any individual and not just those with Down syndrome.

Individuals with Down syndrome can often lead fulfilling and productive lives and this is due to the range of health care, education and support that is now available to them.

Is there a cure for Down syndrome?

There is no 'cure' for Down syndrome, but researchers are working all the time to develop effective interventions and treatment strategies.

What is Trisomy 18 and Trisomy 13?

Edwards' syndrome, also known as Trisomy 18, (named after Dr John Edwards), is another abnormality caused by an extra chromosome - number 18. It is less common than Down syndrome - occuring in about 1 in 2500 pregnancies and 1 in every 6,000 live births. Three times as many girls as boys are affected.

Children born with Trisomy 18 suffer severe mental retardation, heart defects and other health problems. They usually die early on: 30% by 1 month of age, 50% by 2 months of age and 90% die by 12 months of age. Advanced maternal age is a risk factor for Trisomy 18.

Patau syndrome, also known as Trisomy 13, (named after Dr Klaus Patau, who identified the chromosomes responsible for it in 1960) is caused by an extra chromosome - chromosome number 13.

Trisomy 13 occurs in about 1 out of every 10,000 newborns, with boys and girls affected in equal numbers. Trisomy 13 is the third most common autosomal anomaly after Trisomy 21 (Down Syndrome) and Trisomy 18 (Edwards' Syndrome).

There are other Trisomy conditions in addition to the above, but these are not as common.

Disorders

Tests are designed principally to screen for Down syndrome and cystic fibrosis. However, in addition the Down syndrome screening tests can also detect other disorders. These are Edwards' syndrome and, if testing is after 15 weeks gestation, Neural Tube Defects (NTDs) and Abdominal Wall Defects (AWDs). The table below shows that most of the additional disorders are either rare, incompatible with survival, or surgically correctable:


Disorder:
Rate at birth:
Prognosis:
Down syndrome (Trisomy 21)
1 in 700-900
Untreatable
Edwards' Syndrome (Trisomy 18)
1 in 6,000

Median life expectancy is 4 days. However, about 5-10% survive beyond one year, so the mean age at death is 48 days.

Newborns have a 40% chance of surviving to age 1 month. Infants have a 5% chance of surviving to age 1 year. Children have a 1% chance of surviving to age 10 years.
[Source: patient.co.uk]

Patau Syndrome (Trisomy 13)
1 in 10,000

Life expectancy very limited. Median survival is 2.5 days. About 50% live longer than 1 week. 5-10% of infants live longer than 1 year.
[Source: patient.co.uk]

Spina Bifida
1 in 1000

Children with spina bifida can lead active lives. Prognosis, activity, and participation depend on the number and severity of abnormalities and associated personal and environmental factors.

Most children with the disorder have normal intelligence and can walk, often with assistive devices. If learning problems develop, appropriate educational interventions are helpful.

[Source: National Institute of Neurological Disorders and Stroke (NINDS)]

Anencephaly
Less than 1 in 10,000

The prognosis for babies born with anencephaly is extremely poor. If the infant is not stillborn, then he/she will usually die within a few hours or days after birth.

[Source: National Institute of Neurological Disorders and Stroke (NINDS)]

Exomphalos
1 in 3,000 live births
Operable
Gastroschisis
1-5 per 10,000 live births
Operable
Cystic fibrosis
70,000 worldwide
Lifetime treatment

† 2013 - Less than 1 in 10,000 live births in some places. Due to dietary folic acid supplementation by mothers-to-be before and during pregnancy can reduce the incidence of such birth defects. [Source: University of Utah].

There are some other disorders that are not specifically screened for, but are detected as a result of tests. Rarer chromosomal anomalies (e.g. Turner's syndrome, triploidy), rather than Down syndrome or Edwards syndrome, may be found following invasive prenatal diagnosis. Structural abnormalities (e.g. heart defects) may be found when measuring nuchal transparency.

Source: Understanding Intellectual Disability & Health