Down Syndrome

A hereditary mental disorder present at birth resulting from an abnormality in the number of chromosomes; also known Trisomy 21.

Down syndrome was named after John Langdon Haydon Down, a British physician and advocate of education for the mentally retarded, who first described it in 1866. In 1959, the French pediatrician Jerome Lejeune discovered that the disorder is caused by a chromosomal abnormality. Ninety-five percent of individuals with Down syndrome have Trisomy 21, an extra chromosome in the 21st pair (altogether, they have 47 chromosomes instead of the normal 46); four percent have translocation, a chromosomal abnormality; and one percent have mosaicism. Down syndrome is characterized primarily by varying degrees of mental and motor retardation.

Genetic map of a person with Down syndrom. Note three copies of chromosome 21. (Phototake/NYC. Reproduced by permission.)

Most people with the disorder are retarded. Individuals with Down syndrome have I.Q.s ranging from 20 to more than 90, with the mean being 49. They are also prone to possible heart defects, poor vision and hearing, cataracts, and a low resistance to respiratory infections. Until the discovery of antibiotics, most Down syndrome children died of pneumonia before reaching adulthood. People with Down syndrome are 20 times more likely than the general population to develop leukemia and a neurological condition similar to Alzheimer's disease.

Individuals with Down syndrome have a distinctive physical appearance characterized by almond-shaped eyes (on which the condition's former alternate name— mongolism—was based); a short, stocky build; a flat nose and large, protruding tongue (which makes normal speech difficult); a small skull flattened in the back; a short neck with extra skin; and small hands with short fingers. Other features include a fold of skin on the inner side of the eye; speckling at the edge of the iris; and a small amount of facial and body hair. Muscle tone is often poor, and newborns are prone to hypotonia, or "floppiness." People with Down syndrome are widely recognized to have docile temperaments; and are generally cheerful, cooperative, affectionate, and relaxed, although there are no scientific studies to confirm this. Their motor, speech, and sexual development is delayed, and their cognitive development may not peak until the age of 30 or 40. In infancy, speech development is delayed by about seven months.

Recent research has led to the conclusion that Down children are capable of expressing complex feelings, of developing richer personalities, and of mastering higher degrees of learning using adaptive strategies (such as computer-aided learning to teach reading and writing). One developmental program that began with Down children as young as 30 months old and stressed positive parent-child communication eventually enabled the children to read at a second-grade level. The theory is that early stimulation helps to develop connections in the brain that might otherwise not have developed.

Although most people with Down syndrome were institutionalized until the 1970s, those with only moderate retardation are capable of achieving some degree of self-sufficiency. Today, with changed social attitudes and expanded educational opportunities, many lead productive, fulfilling lives. In less than a century, the expected lifespan of a person with Down syndrome has increased from nine years of age in 1910 to 19 or 20 after the discovery of antibiotics to age 55 today, due to recent advancements in clinical treatment.

More than 350,000 people in the United States have Down syndrome; one baby is born with Down syndrome for every 800 to 1,000 births in the U.S. Down syndrome babies are found in every ethnic group and socioeconomic class. About 5,000 babies are born yearly in the United States with Down syndrome. Twenty to 25 percent of children conceived with Down syndrome survive beyond birth. Women over age 35 have a one in 400 chance of conceiving a child with Down syndrome. For women age 40, the incidence becomes one in 110. For mothers age 45, the incidence increases to one child in every 35. Under age 30, women give birth to Down syndrome babies at a rate of one for every 1,500 babies born. However, women under age 35 actually bear 80 percent of Down infants, and recent studies suggest that the father's age may play a role as well. Prenatal detection of Down syndrome is possible through amniocentesis and chorionic villus sampling and is recommended for pregnant women over the age of 35.