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Down syndrome
(trisomy 21) is a chromosomal disorder resulting in mental retardation
and physical abnormalities.
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. Older mothers, especially those older than 35, contribute an extra chromosome more often than do younger mothers. As a result, they more often bear children with Down syndrome (see Genetic Disorders Detection: Chromosomal Abnormalities). However, the extra chromosome may come from the father.
Symptoms
In Down syndrome, physical and mental development is delayed. Infants with Down syndrome tend to be quiet, passive, and have somewhat limp muscles. 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 hearing. Thus, their language skills typically develop slowly. Early intervention with educational and other services improves the functioning of young children with Down syndrome.
Children with Down syndrome tend to have a small head, a face that is broad and flat with slanting eyes and a short nose. The tongue is large. The ears are small and set low in the head. The hands are short and broad, with a single crease across the palm. The fingers are short; the fifth finger, which often has two instead of three sections, curves inward. A space is visible between the first and second toes.
Children with Down syndrome often have heart defects. Many people with Down syndrome develop thyroid disease. They are prone to hearing problems because of recurring ear infections and the associated accumulation of inner ear fluid (serous otitis). They are also prone to vision problems because of problems in their corneas and lenses. Many people with Down syndrome develop symptoms of Alzheimer-like dementia in their 30s, such as memory loss, further lowering of intellect, and personality changes.
Diagnosis
The diagnosis of Down syndrome can often be made before birth (see Genetic Disorders Detection: Prenatal Diagnostic Testing). An infant with Down syndrome has a physical appearance that suggests the diagnosis. A doctor confirms the diagnosis by testing the infant's chromosomes for trisomy 21 or other disorders of the 21st chromosome. After the diagnosis is made, doctors use tests, such as ultrasound and blood tests, along with examinations by specialists, to detect abnormalities associated with Down syndrome. Treating abnormalities that are detected can often prevent them from impairing health.
Prognosis
Most children with Down syndrome survive to adulthood. Life expectancy for a child with Down syndrome with mild or moderate retardation is 55 years and with profound mental retardation is 45 years. Many have progressively worsening mental functioning. Heart abnormalities are often treatable with drugs or surgery. Heart disease and leukemia account for most deaths among children with Down syndrome.
Last full review/revision February 2003
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