Revision:Down syndrome

What is Down syndrome?

Down Syndrome is a cytogenetic disorder that causes human beings to have recognisable physical characteristics and limited intellectual endowment which is due to either an extra 21st chromosome or an specific extra portion of the 21st chromosome. This additional portion on chromosome, because of the genes it contains, causes and excessive amount of specific proteins to be formed in the cells. This disturbs normal growth in the body. The particular proteins involved and how they act are not yet known.

The estimated incidence of Down syndrome is between 1 in 1000 to 1 in 1100 live births. The incidence of Down syndrome is also influenced by factors such as the age of the mother. At age 20, the incidence of Down syndrome is 1 in 2000. While for a mother of the age of 49 the incidence is 1 in 10.

What are the physical features of a child with Down syndrome?

The physical features are important to the physician in making the clinical diagnosis, but no emphasis should be put on these 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 fifty 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 may chromosome subtypes are observed in Down syndrome?

There are three main types of chromosome abnormalities in Down syndrome:

  • The vast majority of children with Down syndrome (approximately 95 percent) have an extra 21 chromosome. Instead of the normal number of 46 chromosomes in each cell, the individual with Down syndrome has 47 chromosomes. This condition is called trisomy 21.
  • 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 - 4 percent of the individuals with Down syndrome.
  • 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 health concerns are often observed in people with Down syndrome?

  • Sixty 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.
  • Forty 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.
  • 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.
  • 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.
  • 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 per cent 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.
  • 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 -2 percent of individuals with Down syndrome have a serious neck problem that requires surgical intervention.
  • Other important medical aspects in 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.

Can Down syndrome be medically treated?

Although many medications and various therapies have been touted as treatment for people with Down syndrome, there is no effective medical treatment available at the present time. However, recent advances in molecular biology make it feasible now to examine the genetic basis for Down syndrome. Once we identify the genes on chromosome 21 (many already have been discovered) and once we find the mechanism of how these genes interfere with normal developmental sequences, and if one could counteract these specific actions, a rational approach to medical therapy could emerge.