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or '''trisomy 21''' (usually '''Down's Syndrome''' in British English
1) is a ability and
Physical Growth as well as facial appearance. Down syndrome can be identified during pregnancy or at birth.
Individuals with Down syndrome can have a lower than average cognitive ability, often ranging from mild to moderate . A small number have severe to profound mental retardation. The
Incidence of Down syndrome is estimated at 1 per 800 to 1,000 births.
Many of the common physical features of Down syndrome also appear in people with a standard set of chromosomes. They include a
Single Transverse Palmar Crease (a single instead of a double crease across one or both palms), an almond shape to the eyes caused by an
Epicanthic Fold of the eyelid, shorter limbs, poor muscle tone, a larger than normal space between the big and second toes, and protruding tongue. Health concerns for individuals with Down syndrome include a higher risk for
Congenital Heart Defect s,
Gastroesophageal Reflux Disease , recurrent
Ear Infections ,
Obstructive Sleep Apnea , and
Thyroid dysfunctions.
Early Childhood Intervention , screening for common problems, medical treatment where indicated, a conducive family environment, and vocational training can improve the overall development of children with Down syndrome. Although some of the physical genetic limitations of Down syndrome cannot be overcome, education and proper care will improve quality of life.Roizen NJ, Patterson D.''Down's syndrome.'' Lancet. 2003
12 April ;361(9365):1281–9. Review. PMID 12699967
known as ''
Brushfield Spots '']]
Individuals with Down syndrome may have some or all of the following physical characteristics: oblique eye fissures with
Epicanthic Skin Fold s on the inner corner of the eyes,
Muscle Hypotonia (poor muscle tone), a flat nasal bridge, a
Single Palmar Fold (also known as a simian crease), a protruding tongue (due to small oral cavity, and an enlarged tongue near the tonsils), a short neck, white spots on the
Iris known as
Brushfield Spots ,
2 excessive flexibility in joints,
Congenital Heart Defect s, excessive space between
Large Toe and second toe, a single
Flexion furrow of the fifth finger, and a higher number of ulnar loop
Dermatoglyphs . Most individuals with Down syndrome have
Mental Retardation in the mild (
IQ 50–70) to moderate (IQ 35–50) range,
3 with scores of children having Mosaic Down syndrome (explained below) typically 10–30 points higher.
4 In addition, individuals with Down syndrome can have serious abnormalities affecting any body system.
See Also: Genetic origins of Down syndrome
for trisomy Down syndrome. Notice the three copies of chromosome 21]]
Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the mice with most of human chromosome 21 (in addition to the normal mouse chromosomes).
5 The extra chromosomal material can come about in several distinct ways. A typical human karyotype is designated as 46,XX or 46,XY, indicating 46 chromosomes with an XX arrangement typical of females and 46 chromosomes with an XY arrangement typical of males.For a description of human karyotype see
6
Trisomy 21 (47,XX,+21) is caused by a
Meiotic Nondisjunction event. With nondisjunction, a
Gamete (''i.e.'', a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the
Embryo now has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.
7
Trisomy 21 is caused prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21; or a Down syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the cause of 1–2% of the observed Down syndromes.
The extra chromosome 21 material that causes Down syndrome may be due to a
Robertsonian Translocation . In this case, the long arm of chromosome 21 is attached to another chromosome, often
Chromosome 14 (45,XX, t(14;21q)) or itself (called an
Isochromosome , 45,XX, t(21q;21q)). Normal
Disjunction s leading to gametes have a significant chance of creating a gamete with an extra chromosome 21. Translocation Down syndrome is often referred to as ''familial Down syndrome''. It is the cause of 2–3% of observed cases of Down syndrome. It does not show the maternal age effect, and is just as likely to have come from fathers as mothers.
Rarely, a region of chromosome 21 will undergo a duplication event. This will lead to extra copies of some, but not all, of the genes on chromosome 21 (46,XX, dup(21q)).Petersen MB, Tranebjaerg L, McCormick MK, Michelsen N, Mikkelsen M, Antonarakis SE. ''Clinical, cytogenetic, and molecular genetic characterization of two unrelated patients with different duplications of 21q.'' Am J Med Genet Suppl. 1990;7:104-9.
PMID 2149934 If the duplicated region has genes that are responsible for Down syndrome physical and mental characteristics, such individuals will show those characteristics. This cause is very rare and no rate estimates are available.
The incidence of Down syndrome is estimated at 1 per 800 to 1 per 1000 births.Based on estimates by National Institute of Child Health & Human Development
8
In 2006, the Center for Disease Control estimated the rate as 1 per 733 live births in the United States (5429 new cases per year).
9 Approximately 95% of these are
Trisomy 21. Down syndrome occurs in all ethnic groups and among all economic classes.
Maternal Age influences the chances of conceiving a baby with Down syndrome. At maternal age 20 to 24, the probability is 1/1490; at age 40 the probability is 1/60, and at age 49 the probability is 1/11. Although the probability increases with maternal age, 80% of children with Down syndrome are born to women under the age of 35,Estimate from
10 reflecting the overall fertility of that age group. Other than maternal age, no other risk factors are known. There does not appear to be a paternal age effect.
Pregnant women can be screened for various complications during pregnancy. Many standard prenatal screens can discover Down syndrome.
Genetic Counseling along with
Genetic Testing , such as
Amniocentesis ,
Chorionic Villus Sampling (CVS), or percutaneous umbilical blood sampling (PUBS) are usually offered to families who may have an increased chance of having a child with Down syndrome, or where normal prenatal exams indicate possible problems. Genetic screens are often performed on pregnant women older than 30 or 35.
Amniocentesis and CVS are considered invasive procedures, in that they involve inserting instruments into the uterus, and therefore carry a small risk of causing fetal injury or miscarriage. There are several common non-invasive screens that can indicate a fetus with Down syndrome. These are normally performed in the late first trimester or early second trimester. Due to the nature of screens, each has a significant chance of a
False Positive , suggesting a fetus with Down syndrome when, in fact, the fetus does not have this genetic abnormality. Screen positives must be verified before a Down syndrome diagnosis is made. Common screening procedures for Down syndrome are given in Table 1.
]]
Even with the best non-invasive screens, the detection rate is 90%–95% and the rate of false positive is 2%–5%.
False Positive s can be caused by undetected multiple fetuses (very rare with the ultrasound tests), incorrect date of pregnancy, or normal variation in the proteins.
Confirmation of screen positive is normally accomplished with
Amniocentesis . This is an invasive procedure and involves taking
Amniotic Fluid from the mother and identifying fetal cells. The lab work can take several weeks but will detect over 99.8% of all numerical chromosomal problems with a very low false positive rate.
12
Due to the low incidence of Down syndrome, a vast majority of early screen positives are false.Assume the false positive rate is 2% (at the low end), the incidence of Down syndrome is 1/500 (on the high side) with 95% detection, and there is no
Ascertainment Bias . Out of 100,000 screens, 200 will have Down syndrome, and the screen will detect 190 of them. From the 99,800 normal pregnancies, 1996 will be given a positive result. So, among the 2,186 positive test results, 91% will be false positives and 9% will be true positives. Since false positives typically prompt an amniocentesis to confirm the result, and the amniocentesis carries a small risk of inducing
Miscarriage , there is a slight risk of miscarrying a healthy fetus. (The added miscarriage risk from an amniocentesis is traditionally quoted as 0.5%, but recent studies suggest that it may be considerably smaller (0.06% with a 95% CI of 0 to 0.5%).
13 PMID 17077226)
A 2002 literature review of elective abortion rates found that 91–93% of pregnancies with a diagnosis of Down syndrome were terminated.
14 PMID 10521836 This is similar to 90% results found by
15 PMID 10951466 Physicians and ethicists are concerned about the ethical ramifications,
16 with some commentators calling it "
Eugenics by abortion".
17 Many members of the
Disability Rights movement "believe that public support for prenatal diagnosis and abortion based on disability contravenes the movement's basic philosophy and goals."
18 PMID 12587137
Cognitive Development in children with Down syndrome is quite variable. It is not currently possible at birth to predict the capabilities of any individual reliably, nor are the number or appearance of physical features predictive of future ability. The identification of the best methods of teaching each particular child ideally begins soon after birth through early intervention programs.
19 Also
20 Since children with Down syndrome have a wide range of abilities, success at school can vary greatly, which stresses the importance of evaluating children individually. The cognitive problems that are found among children with Down syndrome can also be found among typical children. Therefore, parents can use general programs that are offered through the schools or other means.
Language skills show a difference between understanding speech and expressing speech. It is not uncommon for children with Down Syndrome to have a speech delay. Although it is common for them to need speech therapy to help with expressive language.
21 Also,
22 Fine Motor Skill s are delayed
23 and often lag behind
Gross Motor Skill s and can interfere with cognitive development. Gross Motor Skills can be affected anywhere from minor to major. Some children will walk at around 2 while others around 4, it all depends. A physical therapist or APE will help a child with this.
24
In education,
Mainstreaming of children with Down syndrome is becoming less controversial in many countries. For example, there is a presumption of mainstream in many parts of the UK. Mainstreaming is the process whereby students of differing abilities are placed in classes with their chronological peers. Children with Down syndrome may not age emotionally/socially and intellectually at the same rates as children without Down syndrome, so over time the intellectual and emotional gap between children with and without Down syndrome may widen. Complex thinking as required in sciences but also in history, the arts, and other subjects can often be beyond the abilities of some, or achieved much later than in other children. Therefore, children with Down syndrome may benefit from mainstreaming provided that some adjustments are made to the curriculum.
25 Also, see
26 Finally, see a survey by NDSS on inclusion,
27
Some European countries such as
Germany and
Denmark advise a two-teacher system, whereby the second teacher takes over a group of children with disabilities within the class. A popular alternative is cooperation between
Special School s and mainstream schools. In cooperation, the core subjects are taught in separate classes, which neither slows down the typical students nor neglects the students with disabilities. Social activities, outings, and many sports and arts activities are performed together, as are all breaks and meals.There are many such programs. One is described by Action Alliance for Children, Also, see
See Also: Health aspects of Down syndrome
The medical consequences of the extra genetic material in Down syndrome are highly variable and may affect the function of any organ system or bodily process. The health aspects of Down syndrome encompass anticipating and preventing effects of the condition, recognizing complications of the disorder, managing individual symptoms, and assisting the individual and his/her family in coping and thriving with any related disability or illnesses.
28
The most common manifestations of Down syndrome are the characteristic facial features, cognitive impairment, s that contribute to cancer risk, or some other as-yet unspecified factor. Down syndrome can result from several different genetic mechanisms. This results in a wide variability in individual symptoms due to complex gene and environment interactions. Prior to birth, it is not possible to predict the symptoms that an individual with Down syndrome will develop. Some problems are present at birth, such as certain heart malformations. Others become apparent over time, such as epilepsy.
