Trisomy 21 syndrome: A common chromosome disorder, often called Down syndrome, due to an extra chromosome number 21 (trisomy 21). The chromosome abnormality affects both the physical and intellectual development of the individual.
Trisomy 21 (Down) syndrome is associated with a 45% risk for heart malformations, a lesser risk of duodenal atresia (part of the small intestines is not developed), and a minor but still significant risk of acute leukemia.
Trisomy 21 is due to an extra copy of chromosome number 21. Instead of having the normal 2 copies of chromosome number 21, the person with Down syndrome has 3 copies of chromosome number 21. Confirmation of such a condition requires a chromosome study (analysis under the microscope of the chromosomes). A chromosome study is also valuable to rule in or out a translocation (a type of rearrangement) of chromosome 21 that can be heritable in which case it can give rise to more cases of Down syndrome in the family. The evaluation of the Down syndrome baby and the family by a medical geneticist is often useful.
In Down syndrome there are certain characteristic features in the appearance which may individually be quite subtle but together permit a clinical diagnosis of Down syndrome to be made at birth. These signs of Down syndrome include slight flattening of the face, minimal squaring off of the top of the ear, a low bridge of the nose (lower than the usually flat nasal bridge of the normal newborn), an epicanthic fold (a fold of skin over top of the inner corner of the eye, which can also be seen less frequently in normal babies), a ring of tiny harmless white spots around the iris, and a little narrowing of the palate. There are many, many more minor malformations in Down syndrome.
Down syndrome is also associated with a number of major malformations. For example, nearly half of Down syndrome children are born with a heart defect, most often a hole between the two sides of the heart. For another example, Hirschprung's disease (congenital aganglionic megacolon) which can cause intestinal obstruction occurs more frequently in children with Down syndrome than in other children.
The intellectual handicaps in Down syndrome are often the most important problem. These handicaps may not be evident in early infancy. However, they tend to become increasingly noticeable later in infancy and during childhood as developmental delay. In adults with Down syndrome, the intellectual handicap is manifest as mental retardation.
Very few adults with Down syndrome can lead independent lives because of their mental retardation. It was once thought that nearly all adults with Down syndrome developed Alzheimer's disease (dementia) so that on top of their mental handicap most people with Down syndrome were slated for premature senility. However, it now appears that a much lower proportion, perhaps 20% to 25%, of Down syndrome adults develop dementia. The majority of adults with Down syndrome may thus be spared this fate.
The name Down syndrome refers to a 19th century English physician Langdon Down. Ironically, he was not the first person to describe the condition, he added little to our knowledge of it and, in great error, he attributed the condition to a "reversion" to the mongoloid race. The disorder was also once called mongolism, a term now considered obsolete.