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Down Syndrome (Trisomy 21; Trisomy G)
(Trisomy 21; Trisomy G)
Down syndrome is a chromosomal disorder resulting in intellectual disability and physical abnormalities.
Most cases of Down syndrome are caused by an extra copy of chromosome 21.
Children with Down syndrome have delayed physical and mental development, specific head and facial features, and are short.
The diagnosis is suggested by the child’s physical appearance and is confirmed by finding an extra copy of chromosome 21 in a blood sample.
Most children with Down syndrome survive to adulthood.
An extra chromosome, making three of a kind, is called trisomy. The most common trisomy in a newborn is trisomy 21 (three copies of chromosome 21). Trisomy 21 causes about 95% of the cases of Down syndrome. Thus, most people with Down syndrome have 47 chromosomes. The remaining 5% of people with Down syndrome have 46 chromosomes, but the extra chromosome 21 is incorrectly joined with another chromosome (called translocation), creating an abnormal, but not extra, chromosome. The extra chromosome may come from the father. However, older mothers, especially those older than 35, more commonly contribute the extra chromosome. Yet, because most births occur to younger women, just 20% of infants with Down syndrome are born to mothers older than 35. Women who have Down syndrome have a 50% chance of having a child with Down syndrome. However, many affected fetuses abort spontaneously. Men with Down syndrome are usually infertile.
Down syndrome affects many parts of the body. Not all complications are present in each person.
Some Complications of Down Syndrome
In Down syndrome, physical and mental development is typically delayed. Infants tend to be placid and passive and do not cry as often as expected. Many infants are born with heart and gastrointestinal defects, and they have somewhat limp muscles. They tend to have a small head and a face that is broad and flat with slanting eyes and a short nose. However, some newborns appear normal at birth and then develop characteristic facial features during infancy. The tongue is sometimes large. The larger tongue plus low facial muscle tone frequently causes children to hold their mouth open. There may be extra skin around the back of the neck. The ears are small, rounded, and set low in the head. The hands are often short and broad, with a single crease across the palm. The fingers are short, and the fifth finger, which often has two instead of three sections, curves inward. The space between the first and second toes may also be widened. Children with Down syndrome are short and are at increased risk of becoming obese.
The intelligence quotient (IQ) among children with Down syndrome varies but averages about 50, compared with normal children, whose average IQ is 100. Children with Down syndrome have better visual motor skills (such as drawing) than skills that require listening. Thus, their language skills typically develop slowly. Behavior suggestive of attention-deficit disorder (sometimes with hyperactivity) is often seen in childhood. Children with Down syndrome are at greater risk of autistic behavior, especially those with severe intellectual disability. Depression is also common among adults and may also occur in children. Early intervention with educational and other services improves the functioning of young children with Down syndrome.
Children with Down syndrome often have heart defects. They have hearing loss and are prone to recurring ear infections. They are also prone to vision problems because of problems in their corneas and lenses. The joints in the neck may be unstable, which can lead to weakness or paralysis. Many people with Down syndrome develop thyroid disease (such as hypothyroidism) and diabetes. They are also at a higher risk of developing infections and leukemia.
Down syndrome may be suspected before birth based on findings detected during an ultrasound of the fetus or based on abnormal levels of certain proteins found in the mother’s blood in the first 15 to 16 weeks of pregnancy. Doctors can sometimes find deoxyribonucleic acid (DNA) from the fetus in the mother's blood and use this DNA to diagnose Down syndrome. Screening for Down syndrome before 20 weeks of pregnancy is recommended for all women regardless of age.
An infant with Down syndrome has a physical appearance that suggests the diagnosis. A doctor confirms the diagnosis by testing the infant’s blood for trisomy 21 or other disorders of the 21st chromosomes. After the diagnosis is made, doctors use tests, such as ultrasonography of the heart and blood tests, along with examinations by specialists, to detect abnormalities associated with Down syndrome. Treating such abnormalities can often prevent them from impairing health. Thus, these children should have regular screenings for thyroid disease, vision problems, and hearing problems. Height, weight, and head circumference measurements are plotted at each well-child visit using a growth chart created specifically for children with Down syndrome. Children are also evaluated for obstructive sleep apnea (see Sleep Apnea). Children who have neck or nerve pain, weakness, or other neurologic symptoms should have x-rays of the bony joints of the neck to check for instability. Children and adults who want to participate in the Special Olympics or other sporting events also may need to have x-rays of the bony joints of the neck.
The aging process seems to be accelerated, but most children with Down syndrome survive to adulthood. The average age at death is 49. Recently, however, some people have been living into their 70s. Symptoms of Alzheimer-like dementia, such as memory loss, further lowering of intellect, and personality changes, may develop at an early age. Heart abnormalities are often treatable with drugs or surgery. Heart disease and leukemia account for most deaths among children with Down syndrome.
Recent findings indicate that blacks with Down syndrome have a substantially shorter life span than whites. This finding may be the result of poor access to medical, educational, and other support services.
There is no cure for Down syndrome. However, some specific symptoms and problems caused by the syndrome can be treated. Doctors can surgically repair some heart and gastrointestinal defects. Doctors give thyroid hormone replacement therapy to people with hypothyroidism. Care for people with Down syndrome should also include genetic counseling for the family, social support, and educational programming appropriate for the level of intellectual functioning (see Intellectual Disability).
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