Down Syndrome (cont.)

It is now well known that the extra genes on the additional chromosome 21 are the cause of Down syndrome. Scientists are trying to determine which of the genes on chromosome 21 cause different characteristics of the disorder when present in three copies. Some genes may be more active and others less active due to the extra copy, and some of the genes may have a stronger influence on the characteristics of Down syndrome than others. Currently, about 400 genes on chromosome 21 have been identified, but the function of most remains unknown.

Until recently, scientists hypothesized that the most important genes involved in Down syndrome were located in an area on chromosome 21 called the Down syndrome critical region. However, scientists have found that genes outside this region are also important in Down syndrome.

Despite the variability in Down syndrome, individuals with Down syndrome have a widely recognized characteristic appearance. Typical facial features include a flattened nose, small mouth, protruding tongue, small ears, and upward slanting eyes. The inner corner of the eyes may have a rounded fold of skin (epicanthal fold). The hands are short and broad with short fingers, and may have a single palmar crease. White spots on the colored part of the eye called Brushfield spots may be present. Babies with Down syndrome often have decreased muscle tone at birth. Normal growth and development is usually delayed and often individuals with Down syndrome don't reach the average height or developmental milestones of unaffected individuals.

Down syndrome is the leading cause for impaired cognition. Cognitive development is usually delayed and learning difficulties persist throughout life. Scientists are trying to determine what causes this dysfunction. The average brain volume of a person with Down syndrome is small and certain brain structures such as the hippocampus and the cerebellum do not function properly. The hippocampus in particular, is important for learning and memory. Through human studies and mouse models of Down syndrome, scientists are trying to find out which genes on the extra chromosome 21 affect cognition in Down syndrome.

• Up to half of people with Down syndrome are born with a heart defect. The atrioventricular septal defect is the most common heart defect found in newborns with Down syndrome. Other heart defects include ventricular septal defect, atrial septal defect, tetralogy of Fallot, and patent ductus arteriosus. Some newborns with these types of heart defects will require surgery shortly after birth.
• Gastrointestinal abnormalities also occur quite frequently in Down syndrome. Esophageal atresia, tracheoesophageal fistula, duodenal atresia or stenosis, Hirschsprung's disease, and imperforate anus are some of the more common conditions. Approximately some people with Down syndrome develop celiac disease. Surgery may be necessary for some of these gastrointestinal conditions.
• Children with Down syndrome are also at an increased risk of developing acute lymphoblastic leukemia, myeloid leukemia, and testicular cancer; however, the risk of developing most solid tumors is reduced in individuals with Down syndrome.
• Other medical conditions include, infantile spasms, frequent ear infections (otitis media), hearing loss, visual impairment, sleep apnea, underactive thyroid (hypothyroidism), cervical spine-instability, constipation, obesity, seizures, dementia, and early-onset Alzheimer's disease.
• Coexisting psychiatric and behavior disorders occur in a significant percentage of individuals with Down syndrome. These include attention deficit hyperactivity disorder (ADHD), autism spectrum disorders, stereotypical movement disorders, obsessive compulsive disorder (OCD), and depression.

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