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From Wikipedia, the free encyclopedia
Down's syndrome, Down's, trisomy 21
Illustration of the facial features of Down syndrome
Educational support, sheltered work environment[6][7]
Life expectancy 50 to 60 years (developed world)[8][9]
Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21.[2] It is usually associated with physical growth delays, mild to moderate intellectual disability, and characteristic facial features.[1] The average IQ of a young adult with Down syndrome is 50, equivalent to the mental ability of an eight- or nine-year-old child, but this can vary widely.[8]
The parents of the affected individual are usually genetically normal.[12] The probability increases from less than 0.1% in 20-year-old mothers to 3% in those of age 45.[3] The extra chromosome is believed to occur by chance, with no known behavioral activity or environmental factor that changes the probability.[13] Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing or after birth by direct observation and genetic testing.[5] Since the introduction of screening, Down syndrome pregnancies are often aborted.[14][15] Regular screening for health problems common in Down syndrome is recommended throughout the person's life.[8]
There is no cure for Down syndrome.[16] Education and proper care have been shown to improve quality of life.[6] Some children with Down syndrome are educated in typical school classes, while others require more specialized education.[7] Some individuals with Down syndrome graduate from high school, and a few attend post-secondary education.[17] In adulthood, about 20% in the United States do paid work in some capacity,[18] with many requiring a sheltered work environment.[7] Support in financial and legal matters is often needed.[9] Life expectancy is around 50 to 60 years in the developed world with proper health care.[8][9]
Down syndrome is one of the most common chromosome abnormalities in humans.[8] It occurs in about 1 in 1,000 babies born each year.[1] In 2015, Down syndrome was present in 5.4 million individuals globally and resulted in 27,000 deaths, down from 43,000 deaths in 1990.[10][11][19] It is named after British doctor John Langdon Down, who fully described the syndrome in 1866.[20] Some aspects of the condition were described earlier by French psychiatrist Jean-Étienne Dominique Esquirol in 1838 and French physician Édouard Séguin in 1844.[21] The genetic cause of Down syndrome was discovered in 1959.[20]
Those with Down syndrome nearly always have physical and intellectual disabilities.[22] As adults, their mental abilities are typically similar to those of an 8- or 9-year-old.[8] They also typically have poor immune function[12] and generally reach developmental milestones at a later age.[9] They have an increased risk of a number of other health problems, including congenital heart defect, epilepsy, leukemia, thyroid diseases, and mental disorders.[20]
Separation of first and second toes
People with Down syndrome may have some or all of these physical characteristics: a small chin, slanted eyes, poor muscle tone, a flat nasal bridge, a single crease of the palm, and a protruding tongue due to a small mouth and relatively large tongue.[27][28] These airway changes lead to obstructive sleep apnea in around half of those with Down syndrome.[20] Other common features include: a flat and wide face,[27] a short neck, excessive joint flexibility, extra space between big toe and second toe, abnormal patterns on the fingertips and short fingers.[24][27] Instability of the atlantoaxial joint occurs in about 20% and may lead to spinal cord injury in 1–2%.[8][9] Hip dislocations may occur without trauma in up to a third of people with Down syndrome.[20]
Growth in height is slower, resulting in adults who tend to have short stature—the average height for men is 154 cm (5 ft 1 in) and for women is 142 cm (4 ft 8 in).[30] Individuals with Down syndrome are at increased risk for obesity as they age.[20] Growth charts have been developed specifically for children with Down syndrome.[20]
This syndrome causes about a third of cases of intellectual disability.[12] Many developmental milestones are delayed with the ability to crawl typically occurring around 8 months rather than 5 months and the ability to walk independently typically occurring around 21 months rather than 14 months.[31]
Most individuals with Down syndrome have mild (IQ: 50–69) or moderate (IQ: 35–50) intellectual disability with some cases having severe (IQ: 20–35) difficulties.[1][32] Those with mosaic Down syndrome typically have IQ scores 10–30 points higher.[33] As they age, people with Down syndrome typically perform worse than their same-age peers.[32][34]
Commonly, individuals with Down syndrome have better language understanding than ability to speak.[20][32] Between 10 and 45% have either a stutter or rapid and irregular speech, making it difficult to understand them.[35] After reaching 30 years of age, some may lose their ability to speak.[8]
They typically do fairly well with social skills.[20] Behavior problems are not generally as great an issue as in other syndromes associated with intellectual disability.[32] In children with Down syndrome, mental illness occurs in nearly 30% with autism occurring in 5–10%.[9] People with Down syndrome experience a wide range of emotions.[36] While people with Down syndrome are generally happy,[37] symptoms of depression and anxiety may develop in early adulthood.[8]
Children and adults with Down syndrome are at increased risk of epileptic seizures, which occur in 5–10% of children and up to 50% of adults.[8] This includes an increased risk of a specific type of seizure called infantile spasms.[20] Many (15%) who live 40 years or longer develop Alzheimer’s disease.[38] In those who reach 60 years of age, 50–70% have the disease.[8]
Hearing and vision disorders occur in more than half of people with Down syndrome.[20] Vision problems occur in 38 to 80%.[1] Between 20 and 50% have strabismus, in which the two eyes do not move together.[1] Cataracts (cloudiness of the lens of the eye) occur in 15%,[9] and may be present at birth.[1] Keratoconus (a thin, cone-shaped cornea)[8] and glaucoma (increased eye pressure) are also more common,[1] as are refractive errors requiring glasses or contacts.[8] Brushfield spots (small white or grayish/brown spots on the outer part of the iris) are present in 38 to 85% of individuals.[1]
Hearing problems are found in 50–90% of children with Down syndrome.[39] This is often the result of otitis media with effusion which occurs in 50–70%[9] and chronic ear infections which occur in 40 to 60%.[40] Ear infections often begin in the first year of life and are partly due to poor eustachian tube function.[41][42] Excessive ear wax can also cause hearing loss due to obstruction of the outer ear canal.[8] Even a mild degree of hearing loss can have negative consequences for speech, language understanding, and academics.[1][42] Additionally, it is important to rule out hearing loss as a factor in social and cognitive deterioration.[43] Age-related hearing loss of the sensorineural type occurs at a much earlier age and affects 10–70% of people with Down syndrome.[8]
The rate of congenital heart disease in newborns with Down syndrome is around 40%.[24] Of those with heart disease, about 80% have an atrioventricular septal defect or ventricular septal defect with the former being more common.[8] Mitral valve problems become common as people age, even in those without heart problems at birth.[8] Other problems that may occur include tetralogy of Fallot and patent ductus arteriosus.[41] People with Down syndrome have a lower risk of hardening of the arteries.[8]
Although the overall risk of cancer in Down syndrome is not changed,[44] the risk of testicular cancer and certain blood cancers, including acute lymphoblastic leukemia (ALL) and acute megakaryoblastic leukemia (AMKL) is increased while the risk of other non-blood cancers is decreased.[8] People with Down syndrome are believed to have an increased risk of developing cancers derived from germ cells whether these cancers are blood- or non-blood-related.[45]
Leukemia is 10 to 15 times more common in children with Down syndrome.[20] In particular, acute lymphoblastic leukemia is 20 times more common and the megakaryoblastic form of acute myeloid leukemia (acute megakaryoblastic leukemia), is 500 times more common.[46] Acute megakaryoblastic leukemia (AMKL) is a leukemia of megakaryoblasts, the precursors cells to megakaryocytes which form blood platelets.[46] Acute lymphoblastic leukemia in Down syndrome accounts for 1–3% of all childhood cases of ALL. It occurs most often in those older than nine years or having a white blood cell count greater than 50,000 per microliter and is rare in those younger than one year old. ALL in Down syndrome tends to have poorer outcomes than other cases of ALL in people without Down syndrome.[47][48]
In Down syndrome, AMKL is typically preceded by transient myeloproliferative disease (TMD), a disorder of blood cell production in which non-cancerous megakaryoblasts with a mutation in the GATA1 gene rapidly divide during the later period of pregnancy.[46][49] The condition affects 3–10% of babies with Down.[46] While it often spontaneously resolves within three months of birth, it can cause serious blood, liver, or other complications.[50] In about 10% of cases, TMD progresses to AMKL during the three months to five years following its resolution.[46][50][51]
People with Down syndrome have a lower risk of all major solid cancers, including those of lung, breast, and cervix, with the lowest relative rates occurring in those aged 50 years or older.[45] This low risk is thought due to an increase in the expression of tumor suppressor genes present on chromosome 21.[52][45] One exception is testicular germ cell cancer which occurs at a higher rate in Down syndrome.[45]
Problems of the thyroid gland occur in 20–50% of individuals with Down syndrome.[8][20] Low thyroid is the most common form, occurring in almost half of all individuals.[8] Thyroid problems can be due to a poorly or nonfunctioning thyroid at birth (known as congenital hypothyroidism) which occurs in 1%[9] or can develop later due to an attack on the thyroid by the immune system resulting in Graves' disease or autoimmune hypothyroidism.[53] Type 1 diabetes mellitus is also more common.[8]
Constipation occurs in nearly half of people with Down syndrome and may result in changes in behavior.[20] One potential cause is Hirschsprung's disease, occurring in 2–15%, which is due to a lack of nerve cells controlling the colon.[54] Other frequent congenital problems include duodenal atresia, pyloric stenosis, Meckel diverticulum, and imperforate anus.[41] Celiac disease affects about 7–20%[8][20] and gastroesophageal reflux disease is also more common.[41]
People with Down syndrome tend to be more susceptible to gingivitis as well as early, severe periodontal disease, necrotising ulcerative gingivitis, and early tooth loss, especially in the lower front teeth.[55][56] While plaque and poor oral hygiene are contributing factors, the severity of these periodontal diseases cannot be explained solely by external factors.[56] Research suggests that the severity is likely a result of a weakened immune system.[56][57] The weakened immune system also contributes to increased incidence of yeast infections in the mouth (from Candida albicans).[57]
People with Down syndrome also tend to have a more alkaline saliva resulting in a greater resistance to tooth decay, despite decreased quantities of saliva,[58] less effective oral hygiene habits, and higher plaque indexes.[55][57][58][59]
Higher rates of tooth wear and bruxism are also common.[57] Other common oral manifestations of Down syndrome include enlarged hypotonic tongue, crusted and hypotonic lips, mouth breathing, narrow palate with crowded teeth, class III malocclusion with an underdeveloped maxilla and posterior crossbite, delayed exfoliation of baby teeth and delayed eruption of adult teeth, shorter roots on teeth, and often missing and malformed (usually smaller) teeth.[55][57][58][59] Less common manifestations include cleft lip and palate and enamel hypocalcification (20% prevalence).[59]
Taurodontism, an elongation of the pulp chamber, has a high prevalence in people with DS.[60][61]
Males with Down syndrome usually do not father children, while females have lower rates of fertility relative to those who are unaffected.[62] Fertility is estimated to be present in 30–50% of females.[63] Menopause usually occurs at an earlier age.[8] The poor fertility in males is thought to be due to problems with sperm development; however, it may also be related to not being sexually active.[62] As of 2006, three instances of males with Down syndrome fathering children and 26 cases of females having children have been reported.[62] Without assisted reproductive technologies, around half of the children of someone with Down syndrome will also have the syndrome.[62][64]
Down syndrome is caused by having three copies of the genes on chromosome 21, rather than the usual two.[2][65] The parents of the affected individual are typically genetically normal.[12] Those who have one child with Down syndrome have about a 1% risk of having a second child with the syndrome, if both parents are found to have normal karyotypes.[63]
The extra chromosome content can arise through several different ways. The most common cause (about 92–95% of cases) is a complete extra copy of chromosome 21, resulting in trisomy 21.[64][66] In 1.0 to 2.5% of cases, some of the cells in the body are normal and others have trisomy 21, known as mosaic Down syndrome.[63][67] The other common mechanisms that can give rise to Down syndrome include: a Robertsonian translocation, isochromosome, or ring chromosome. These contain additional material from chromosome 21 and occur in about 2.5% of cases.[20][63] An isochromosome results when the two long arms of a chromosome separate together rather than the long and short arm separating together during egg or sperm development.[64]
Trisomy 21 (also known by the karyotype 47,XX,+21 for females and 47,XY,+21 for males)[68] is caused by a failure of the 21st chromosome to separate during egg or sperm development (nondisjunction).[64] As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes. When combined with a normal cell from the other parent, the baby has 47 chromosomes, with three copies of chromosome 21.[2][64] About 88% of cases of trisomy 21 result from nonseparation of the chromosomes in the mother, 8% from nonseparation in the father, and 3% after the egg and sperm have merged.[69]
The extra chromosome 21 material may also occur due to a Robertsonian translocation in 2–4% of cases.[63][70] In this situation, the long arm of chromosome 21 is attached to another chromosome, often chromosome 14.[71] In a male affected with Down syndrome, it results in a karyotype of 46XY,t(14q21q).[71][72] This may be a new mutation or previously present in one of the parents.[73] The parent with such a translocation is usually normal physically and mentally;[71] however, during production of egg or sperm cells, a higher chance of creating reproductive cells with extra chromosome 21 material exists.[70] This results in a 15% chance of having a child with Down syndrome when the mother is affected and a less than 5% probability if the father is affected.[73] The probability of this type of Down syndrome is not related to the mother's age.[71] Some children without Down syndrome may inherit the translocation and have a higher probability of having children of their own with Down syndrome.[71] In this case it is sometimes known as familial Down syndrome.[74]
The extra genetic material present in Down syndrome results in overexpression of a portion of the 310 genes located on chromosome 21.[65] This overexpression has been estimated at around 50%, due to the third copy of the chromosome present.[63] Some research has suggested the Down syndrome critical region is located at bands 21q22.1–q22.3,[75] with this area including genes for the amyloid precursor protein, superoxide dismutase, and likely the ETS2 proto oncogene.[76] Other research, however, has not confirmed these findings.[65] microRNAs are also proposed to be involved.[77]
The dementia that occurs in Down syndrome is due to an excess of amyloid beta peptide produced in the brain and is similar to Alzheimer's disease, which also involves amyloid beta build-up.[78] Amyloid beta is processed from amyloid precursor protein, the gene for which is located on chromosome 21.[78] Senile plaques and neurofibrillary tangles are present in nearly all by 35 years of age, though dementia may not be present.[12] Those with Down syndrome also lack a normal number of lymphocytes and produce less antibodies which contributes to their increased risk of infection.[20]
Down syndrome is associated with an increased risk of many chronic diseases that are typically associated with older age such as Alzheimer's disease. The accelerated aging suggest that trisomy 21 increases the biological age of tissues, but molecular evidence for this hypothesis is sparse. According to a biomarker of tissue age known as epigenetic clock, trisomy 21 increases the age of blood and brain tissue (on average by 6.6 years).[79]
When screening tests predict a high risk of Down syndrome, a more invasive diagnostic test (amniocentesis or chorionic villus sampling) is needed to confirm the diagnosis.[80] The false-positive rate with screening is about 2–5% (see section Screening below).[81] Amniocentesis and chorionic villus sampling are more reliable tests, but they increase the risk of miscarriage between 0.5 and 1%.[82] The risk of limb problems may be increased in the offspring if chorionic villus sampling is performed before 10 weeks.[82] The risk from the procedure is greater the earlier it is performed, thus amniocentesis is not recommended before 15 weeks gestational age and chorionic villus sampling before 10 weeks gestational age.[82]
About 92% of pregnancies in Europe with a diagnosis of Down syndrome are terminated.[15] As a result, there is almost no one with Down's in Iceland and Denmark, where screening is commonplace.[83] In the United States, the termination rate after diagnosis is around 75%,[83] but varies from 61% to 93% depending on the population surveyed.[14] Rates are lower among women who are younger and have decreased over time.[14] When asked if they would have a termination if their fetus tested positive, 23–33% said yes, when high-risk pregnant women were asked, 46–86% said yes, and when women who screened positive are asked, 89–97% say yes.[84]
The diagnosis can often be suspected based on the child's physical appearance at birth.[9] An analysis of the child's chromosomes is needed to confirm the diagnosis, and to determine if a translocation is present, as this may help determine the risk of the child's parents having further children with Down syndrome.[9] Parents generally wish to know the possible diagnosis once it is suspected and do not wish pity.[20]
Guidelines recommend screening for Down syndrome to be offered to all pregnant women, regardless of age.[80][85] A number of tests are used, with varying levels of accuracy. They are typically used in combination to increase the detection rate.[20] None can be definitive, thus if screening is positive, either amniocentesis or chorionic villus sampling is required to confirm the diagnosis.[80] Screening in both the first and second trimesters is better than just screening in the first trimester.
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