Down syndrome is a chromosomal condition related to chromosome 21. It affects 1 in 800 to 1 in 1000 live born infants.
People who have Down syndrome have learning difficulties, mental retardation, a characteristic facial appearance, and poor muscle tone (hypotonia) in infancy.
Individuals with Down syndrome also have an increased risk for having heart defects, digestive problems such as gastroesophageal reflux or celiac disease, and hearing loss. Some people who have Down syndrome have low activity of the thyroid gland (hypothyroidism) - an organ in the lower neck that produces hormones.
Down syndrome can be diagnosed in infancy based on the characteristic clinical findings. When Down syndrome is suspected in a person, a genetic test called a chromosome analysis is performed on a blood or skin sample to look for an extra chromosome 21 (trisomy 21). Trisomy 21 means that each cell in the body has three copies of chromosome 21 instead of the usual two copies.
Having an extra number 21 chromosome interrupts the normal course of development, causing the characteristic clinical features of Down syndrome. Some people who have Down syndrome have an extra number 21 chromosome in only some of their body's cells. This type of Down syndrome is called mosaic Down syndrome.
A small number of individuals have Down syndrome because part of chromosome 21 becomes attached (translocated) to another chromosome before or at the time of conception. These individuals have two copies of chromosome 21, and additional material from chromosome 21 that is attached to another chromosome. The chromosomes of parents of a child with Down Syndrome caused by a translocation are studied to see whether the translocation was inherited.
Treatment for Down syndrome is based on the person's physical problems and intellectual challenges. Many babies who have Down syndrome do not have good muscle tone, which makes it harder for them to roll over and walk. Physical therapy can help with these problems.
About 40 - 60 percent of babies born with Down syndrome have a heart defect. Therefore, all newborns with Down syndrome have their heart checked with an electrocardiogram and an echocardiogram. When there is a heart defect present in an infant with Down syndrome, the infant is referred to a pediatric cardiologist for medical management or to a pediatric cardiac surgeon for early surgical repair.
Some infants with Down syndrome have difficulties with swallowing or they may have blockages in their bowels. Surgery can be performed to correct these problems. Once corrected, they usually cause no further health issues.
Children with Down syndrome may have frequent colds and sinus and ear infections. These are treated early and aggressively to prevent hearing loss and chronic infections.
Low thyroid levels are more common in infants who have Down syndrome. It is recommended that thyroid level testing be performed at least yearly.
Some infants with Down syndrome have eye problems such as cataracts (cloudy lenses) or crossed eyes (strabismus). Surgery can help with these problems.
Sucking problems related to low muscle tone or heart problems may make breast feeding difficult initially. Occupational therapists, speech therapists, breast feeding consultants and support groups usually have specific resources for the mothers of infants with Down syndrome.
Intelligence in individuals with Down syndrome ranges from low normal to very slow to learn. At birth it is not possible to tell the level of intelligence a baby with Down syndrome will have. All areas of development including motor skills, language, intellectual abilities, and social and adaptive skills are followed closely in children with Down syndrome. Early referral, beginning at birth, to an early intervention program will help enhance development. Preschool programs for children with Down syndrome include physical, occupational, speech and educational therapies.
Many adults with Down syndrome have jobs and live independently.
More detailed information about treatment for individuals who have Down syndrome can be found at Information for Kids - Down Syndrome [kidshealth.org].
Most cases of Down syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 21. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 21 in each of the body's cells.
Mosaic Down syndrome is also not inherited. It occurs as a random error during cell division early in fetal development. As a result, some of the body's cells have the usual two copies of chromosome 21, and other cells have three copies of the chromosome. Translocation Down syndrome can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 21. Although they do not have signs of Down syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
Currently, NHGRI is not conducting studies on Down syndrome.
Last Updated: June 29, 2017