Down syndrome

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Down syndrome,

congenital disorder characterized by mild to severe mental retardationmental retardation,
below average level of intellectual functioning, usually defined by an IQ of below 70 to 75, combined with limitations in the skills necessary for daily living.
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, slow physical development, and characteristic physical features. Down syndrome affects about 1 in every 730 live births and occurs in all populations equally. It was first described in 1866 by an English physician, J. Langdon Down. In 1959 a French physician, Jerome Lejeune, discovered that the syndrome was caused by an extra chromosome. It was later discovered that this extra chromosome appears as a third chromosome attached to the 21st of the 23 pairs of chromosomes normally present in the human genome. This third chromosome gives rise to the alternate name trisomy 21.

The extra genetic material is responsible for the physical characteristics of the syndrome: low muscle tone, flattish facial features, an upward slant to the eyes and epicanthal folds (which were the basis for the former name, mongolism), a single crease across the palm, hyperflexibility of the joints, and a displastic middle phalanx on the fifth finger. People with Down syndrome have an increased incidence of infection, childhood leukemia, congenital heart defects, and respiratory problems, but modern medical treatment has improved the life expectancy from 9 (in 1910) to 55 (in 1995).

Mental retardation varies widely, from minimal to severe. The great majority of those who have the disorder attend public schools and as adults can live independently or in group homes. After age 35 individuals with the syndrome develop the neurological changes of Alzheimer's diseaseAlzheimer's disease
, degenerative disease of nerve cells in the cerebral cortex that leads to atrophy of the brain and senile dementia and, ultimately, death. The disease is characterized by abnormal accumulation of plaques and by neurofibrillary tangles (malformed nerve
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, and many develop the dementia that accompanies them.

Eighty percent of children with Down syndrome are born to women under 35 years of age, but the incidence of Down syndrome births does increase with age. Approximately 5% of cases are transmitted by the sperm. Amniocentesisamniocentesis
, diagnostic procedure in which a sample of the amniotic fluid surrounding a fetus is removed from the uterus by means of a fine needle inserted through the abdomen of the pregnant woman (see pregnancy).
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 or chorionic villus samplingchorionic villus sampling
(CVS) or chorionic villus biopsy
(CVB) , diagnostic procedure in which a sample of chorionic villi from the developing placenta is removed from the uterus of a pregnant woman (see pregnancy) using a fine needle inserted through the abdomen or
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 can be used to detect the disorder in the fetus. Children born to women with Down syndrome have a 50% chance of having the disorder.

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Down syndrome

A developmental disability due to abnormal chromosome number or structure. It is characterized by physical and behavioral features and has been considered the most common form of genetic aberration. Incidence among the newborn is estimated at 3 in 1000, in the general population approximately 1 in 1000. The difference reflects the early mortality.

The most common type (trisomy 21) is due to a nondisjunction of chromosome 21 during the original cell division, resulting in an extra chromosome 21. These children have a total of 47 chromosomes instead of the usual 46. However, the extra material from chromosome 21 can also be attached to another chromosome through translocation; such children have Down syndrome but only 46 chromosomes. More rarely, the trisomy 21 breaks up, giving some cells with 47 chromosomes and some with 46 (mosaicism).

The characteristic physical features include almond-shaped eyes; a rounded, brachycephalic skull with flattened occipital region; a broad, flattened bridge of the nose; an enlarged fissured tongue; broad hands with stubby fingers; often a single “simian” palmar crease; hypotonic muscle development; thick, everted, and cracked lips; dry, rough skin; subnormal height; and infantile genitalia. Not all of these physical signs are present in every case, and some may be observed in individuals without Down syndrome. However, Down syndrome is diagnosed when most of the anomalies are present.

The degree of mental defect is not directly related to the number or gravity of the physical signs, but rather to a combination of those anomalies and the specific chromosomal defect. Few children with Down syndrome are classified today as severely retarded. Most are moderately to mildly retarded and are often educable and highly trainable. They tend to be curious, observant, skillful at mimicry, and usually, very affectionate. Aggression and hostility are rare; however, they are often stubborn and compulsive and are not easily frustrated. They are excellent candidates for vocational training.

Pathological research suggests nonspecific, generalized defective brain development. There is a tendency toward thyroid dysfunction and congenital heart defects. There may also be vision problems, but below-average dental caries. Medication has little effect on the physical condition or on the mental retardation. See Alzheimer's disease, Congenital anomalies

Although there are some reports of more than one child with Down syndrome in a single family, it is not a classical hereditary disease. Incidence is increased if the mother is under 16 or over 35 years old or the father is of advanced age. Furthermore, the Down syndrome child may result from a late or problem pregnancy or the last of numerous pregnancies. Thyroid deficiency, hypopituitarism, and pathology of the ovary have been observed in the mothers, and the probability of upset in their endocrine balance may increase with age. However, the basic etiology is still very much in doubt.

Prenatal identification of Down syndrome in the fetus is possible through amniocentesis. See Human genetics, Mental retardation

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
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