In most cases, Down syndrome is caused by an extra chromosome 21. It is the most common single cause of human birth defects, with an occurence in 1 out of every 660 births.

Children with Down syndrome have a widely recognized characteristic appearance. The head may be smaller than normal (microcephaly) and abnormally shaped. Prominent facial features include a flattened nose, protruding tongue, and upward slanting eyes. The inner corner of the eyes may have a rounded fold of skin (epicanthal fold) rather than coming to a point. The hands are short and broad with short fingers and often have a single crease in the palm (simian crease). Retardation of normal growth and development is typical and most affected children never reach average adult height.

Congenital heart defects are frequently present in Down syndrome children. Early mortality is often a result of cardiac abnormalities. Gastrointestinal abnormalities such as esophageal atresia (obstruction of the esophagus) and duodenal atresia (obstruction of the duodenum) are also relatively common. Obstruction of the gastrointestinal tract may require major surgery shortly after birth. Children with Down syndrome also have a higher than average incidence of acute lymphocytic leukemia (ALL).

• Decreased muscle tone at birth
• Separated sutures (joints between the bones of the skull)
• Asymmetrical or odd-shaped skull
  • Round head with flat area at the back of the head (occiput)
  • Small skull (microcephaly)
• Upward slanting eyes, unusual for ethnic group
• Small mouth with protruding tongue (see tongue problems)
• Broad short hands
• Single crease on the palm
• Retarded growth and development
• Delayed mental and social skills (mental retardation)
• Iris lesion (an abnormality of the colored part of the eye called Brushfield spots)

A heart murmur may be revealed by listening to the chest with a stethoscope. Characteristic abnormalities are revealed by a physical examination. These include a flattened facial profile, small ears, separation of the abdominal muscles, joint hyperflexibility, awkward gait, extra skin on back of neck at birth, and an abnormal bone in the middle of the 5th finger.

Early and massive vomiting may indicate obstruction of the esophagus or duodenum and less often lower segments of the gastrointestinal tract. This is sometimes discovered by inability at birth to pass a tube from the nose into the stomach or duodenum as well as by special x-rays.

Tests include:

• Chromosome studies (demonstrates three copies of 21st chromosome in 94% of cases)
• Chest x-ray (to determine presence of cardiac abnormalities)
• Echocardiogram (to determine nature of cardiac abnormality)
• ECG
• Gastrointestinal x-ray (to show obstruction if symptoms are suggestive)

• Vision problems
• Hearing loss
• Cardiac abnormalities
• Increased incidence of acute leukemia
• Frequent ear infections and increased susceptibility to infection in general
• Gastrointestinal obstruction (imperforate anus, and similar problems)
• Esophageal atresia or duodenal atresia
• Blocked airways during sleep (one third of patients)
• Increased incidence of dementia in older patients
• Instability of the back bones at the top of the neck, compression injury of the spinal cord

There is a risk that uninformed people may assume a Down syndrome child is more retarded than he or she is.

A geneticist should be consulted to help determine the diagnosis and interpret rare chromosomal translocation cases of Down syndrome.

The health care provider should be consulted to evaluate the child for the need for special education and training. The need for follow-up of physical problems varies.

Genetic counseling is recommended in all families with Down syndrome. Down syndrome can be detected in a fetus in the first few months of pregnancy by examination of the chromosomes obtained by amniocentesis or chorionic villus sampling. The parents of a child with Down syndrome are at increased risk for having another child with Down syndrome. Mothers who become pregnant after age 40 are also at increased risk for having a child with Down syndrome.