10/20/2025
Down Syndrome
By Siegfried M. Pueschel, M.D., Ph.D., M.P.H.
What is Down Syndrome?
People with Down Syndrome are first and foremost human beings who have recognizable physical characteristics and limited intellectual endowment which are due to the presence of an extra chromosome 21.
The estimated incidence of Down Syndrome is between 1 and 1,000 to 1 in 1,100 live births. Each year approximately 3,000 to 5,000 children are born with this chromosome disorder. It is believed there are about 250,000 families in the United States who are affected by Down Syndrome.
How do children with Down Syndrome develop?
Children with Down Syndrome are usually smaller, and their physical and mental developments are slower, than youngsters who do not have Down Syndrome. The majority of children with Down syndrome function in the mild to moderate range of intellectual disabilities. However, some children are not intellectually disabled at all; they may function in the borderline to low average range; others may be severely intellectual disabled. There is a wide variation in mental abilities and developmental progress in children with Down Syndrome. Also, their motor development is slow; and instead of walking by 12 to 14 months as other children do, children with Down Syndrome usually learn to walk between 15 and 36 months. Language development is also markedly delayed
It is important to note that a caring and enriching home environment, early intervention, and integrated education efforts will have a positive influence on the child’s development
What are the physical features of a child with Down Syndrome?
Although individuals with Down Syndrome have distinct physical characteristics, generally they are more similar to the average person in the community than they are different. The physical features are important to the physician in making the clinical diagnosis, but no emphasis should be put on those characteristics otherwise. Not every child with Down Syndrome has all the characteristics; some may only have a few, and others may show most of the signs of Down Syndrome. Some of the physical features in children with Down Syndrome include flattening of the back of the head, slanting of the eyelids, small skin folds at the inner corner of the eyes, depressed nasal bridge, slightly smaller ears, small mouth, decreased muscle tone, loose ligaments, and small hands and feet. About 50 percent of all children have one line across the palm, and there is often a gap between the first and second toes. The physical features observed in children with Down Syndrome (and there are many more than described above) usually do not cause any disability in the child.
How many chromosome subtypes are observed in Down Syndrome?
There are three main types of chromosome abnormalities in Down syndrome:
1) The vast majority of children with Down Syndrome (approximately 95 percent) have an extra 21 chromosome. Instead of the normal of 46 chromosomes in each cell, the individual with Down Syndrome has 47 chromosomes. This condition is called Trisomy 21.
2) The second type is called translocation since the extra 21 chromosome is attached or translocated on to another chromosome, usually on chromosome 14, 21 or 22. If translocation is found in a child with Down syndrome, it is important to examine the parents’ chromosomes, since in at least one-third of the cases; a parent may be a carrier of the translocation. This form of chromosome error is found in 3 to 4 percent of the individuals with Down Syndrome.
3) Another chromosome problem, called mosaicism, is noted in about 1 percent of individuals with Down syndrome. In this case, some cells have 47 chromosomes and others have 46 chromosomes. Mosaicism is thought to be the result of an error in cell division soon after conception.
What is the cause of Down Syndrome?
Although many theories have been developed, it is not known what actually causes Down Syndrome. Some professionals believe that hormonal abnormalities, X-rays, viral infections, immunologic problems, or genetic predisposition may be the cause of the improper cell division resulting in Down Syndrome.
It has been known for some time that the risk of having a child with Down syndrome increases with advancing age of the mother; i.e., the older the mother, the greater the possibility that she may have a child with Down Syndrome. However, most babies with Down Syndrome (more than 85 percent) are born to mothers younger than 35 years. Some investigators reported that older fathers may also be at an increased risk of having a child with Down Syndrome.
It is well known that the extra chromosome in Trisomy 21 could either originate in the mother or the father. Most often, however, the extra chromosome is coming from the mother.
What kind of information can be provided through
genetic counseling?
Parents who have a child with Down Syndrome have an increased risk of having another child with Down Syndrome in future pregnancies. It is estimated that the risk of having another child with Down syndrome is about 1 in 100 in Trisomy 21 and mosaicism. If, however, the child has translocation Down Syndrome and if one of the parents is a translocation carrier, then the risk of recurrence increased markedly. The actual risk depends on the type of translocation and whether the translocation is carried by the father or the mother.
What health concerns are often observed in people with Down Syndrome?
The child with Down Syndrome is in need of the same kind of medical care as any other child. The pediatrician or family physician should provide general health maintenance, immunizations; attend to medical emergencies, and offer support and counseling to the family. There are, however, situations when children with Down Syndrome need special attention.
1. 60 to 80 percent of children with Down Syndrome have hearing deficits. Therefore, audiologic assessments at an early age and follow-up hearing tests are indicated. If there is a significant hearing loss, the child should be seen by an ear, nose and throat specialist.
2. 40 to 45 percent of children with Down Syndrome have congenital heart disease. Many of these children will have to undergo cardiac surgery and often will need long term care by a pediatric cardiologist.
3. Intestinal abnormalities also occur at a higher frequency in children with Down Syndrome. For example, a blockage of the food pipe (esophagus), small bowel (duodenum), and at the anus are not uncommon in infants with Down Syndrome. These may need to be surgically corrected at once in order to have a normal functioning intestinal tract.
4. Children with Down Syndrome often have more eye problems than other children who do not have this chromosome disorder. For example, 3 percent of infants with Down Syndrome have cataracts. They need to be removed surgically. Other eye problems such as cross-eye (strabismus), near-sightedness, far-sightedness and other eye conditions are frequently observed in children with Down Syndrome.
5. Another concern relates to nutritional aspects. Some children with Down Syndrome, in particular those with severe heart disease often fail to thrive in infancy. On the other hand, obesity is often noted during adolescence and early adulthood. These conditions can be prevented by providing appropriate nutritional counseling and anticipatory dietary guidance. Thyroid dysfunctions are more common in children with Down Syndrome than in normal children. Between 15 and 20 percent of children with Down Syndrome have hypothyroidism. It is important to identify individuals with Down Syndrome who have thyroid disorders since hypothyroidism may compromise normal central nervous system functioning.
6. Skeletal problems have also been noted at a higher frequency in children with Down Syndrome, including kneecap subluxation (incomplete or partial dislocation), hip dislocation, and atlantoaxial instability. The latter condition occurs when the first two neck bones are not well aligned because of the presence of loose ligaments. Approximately 15 percent of people with Down Syndrome have atlantoaxial instability. Most of these individuals, however, do not have any symptoms, and only 1 to 2 percent of the individuals with Down Syndrome have a serious neck problem that requires surgical intervention.
Other important medical aspects of Down Syndrome, including immunologic concerns, leukemia, Alzheimer disease, seizure disorders, sleep apnea, and skin disorders, may require the attention of specialists in their respective fields
