Down Syndrome (Trisomy 21)

What are trisomies?

The term "trisomy" is used to describe the presence of three chromosomes, rather than the usual pair of chromosomes. For example, if a baby is born with three #21 chromosomes, rather than the usual pair, the baby would be said to have "trisomy 21." Trisomy 21 is also known as Down syndrome. Other examples of trisomy include trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means there are three copies of the #18 chromosome (or of the #13 chromosome) present in each cell of the body, rather than the usual pair.

What is Down syndrome?

Down syndrome is a genetic disorder that includes a combination of birth defects, including some degree of mental retardation, characteristic facial features and, often, heart defects, visual and hearing impairment, and other health problems. The severity of all of these problems varies greatly among affected individuals. Down syndrome is one of the most common genetic birth defects, affecting approximately one in about 800 babies. In this country, there are more than 400,000 individuals with Down syndrome, according to the National Down Syndrome Society. Life expectancy among adults with Down syndrome is about 60 years, though average lifespan varies.

The name "Down syndrome" comes from the physician, Dr. Langdon Down, who first described the collection of physical symptoms in 1866. It was not until 1959 that the cause of Down syndrome (the presence of an extra #21 chromosome) was identified.

What causes Down syndrome?

Normally in reproduction, the egg cell of the mother and the sperm cell of the father start out with the usual number of 46 chromosomes. The egg and sperm cells undergo cell division where the 46 chromosomes are divided in half and the egg and the sperm cells end up with 23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes, the baby ends up with a complete set of 46 chromosomes, half from the father and half from the mother.

Sometimes, an error occurs when the 46 chromosomes are being divided in half and an egg or sperm cell keeps both copies of the #21 chromosome instead of just one copy. If this egg or sperm is fertilized, the baby ends up with three copies of the #21 chromosome and this is called "trisomy 21" or Down syndrome. The features of Down syndrome result from having an extra copy of chromosome 21 in every cell in the body.

Ninety-five percent of Down syndrome results from trisomy 21. Occasionally, the extra chromosome 21 or a portion of it is attached to another chromosome in the egg or sperm; this may result in what is called "translocation" Down syndrome (3 to 4 percent of cases). This is the only form of Down syndrome that may be inherited from a parent. Some parents have a rearrangement called a balanced translocation, where the #21 chromosome is attached to another chromosome, but it does not affect their own health. Rarely, a form of Down syndrome called "mosaic" Down syndrome may occur when an error in cell division occurs after fertilization (1 percent to 2 percent of cases). These individuals have some cells with an extra chromosome 21 and others with the normal number.

What does a child with Down syndrome look like?

A child with Down syndrome may have eyes that slant upward and small ears that may fold over slightly at the top. Their mouth may be small, making the tongue appear large. Their nose also may be small, with a flattened nasal bridge. Some babies with Down syndrome have short necks and small hands with short fingers. Rather than having three "creases" in the palm of the hand, a child with Down syndrome usually has one single crease that goes straight across the palm, and a second crease that curves down by the thumb. The child or adult with Down syndrome is often short and has unusual looseness of the joints. Most children with Down syndrome will have some, but not all, of these features.

What types of problems do children with Down syndrome typically have?

  • About 40 percent to 50 percent of babies with Down syndrome have heart defects. Some defects are minor and may be treated with medications, while others may require surgery. All babies with Down syndrome should be examined by a pediatric cardiologist, a physician who specializes in heart diseases of children, and have an echocardiogram (a procedure that evaluates the structure and function of the heart by using sound waves recorded on an electronic sensor that produce a moving picture of the heart and heart valves) in the first two months of life, so that any heart defects can be treated.

  • About 12 percent of babies with Down syndrome are born with intestinal malformations that require surgery.

  • Children with Down syndrome are at increased risk for visual or hearing impairment. Common visual problems include crossed eyes, near- or farsightedness, and cataracts. Most visual problems can be improved with glasses, surgery, or other treatments. A pediatric ophthalmologist (a physician who specializes in comprehensive eye care and provides examinations, diagnosis, and treatment for a variety of eye disorders) should be consulted within the first year of life.

  • Children with Down syndrome may have hearing loss due to fluid in the middle ear, a nerve defect, or both. All children with Down syndrome should have regular vision and hearing examinations so any problems can be treated before they hinder development of language and other skills.

  • Children with Down syndrome are at increased risk of thyroid problems and leukemia. They also tend to have many colds, as well as bronchitis and pneumonia. Children with Down syndrome should receive regular medical care including childhood immunizations. The National Down Syndrome Congress publishes a "Preventive Medicine Checklist" which outlines which checkups and medical tests are recommended at various ages.

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