Down Syndrome

By: John Rowbotham

The formal story began in 1866, when a physician named John Langdon Down published an essay in England in which he described a set of children with common features who were distinct from other children with mental retardation. Down was superintendent of an asylum for children with mental retardation in Surrey, England when he made the first distinction between children who were cretins (later to be found to have hypothyroidism) and what he referred to as "Mongoloids."

Down based this unfortunate name on his notion that these children looked like people from Mongolia, who were thought then to have an arrested development. This ethnic insult came under fire in the early 1960s from Asian genetic researchers, and the term was dropped from scientific use. Instead, the condition became called "Down's syndrome." In the 1970s, an American revision of scientific terms changed it simply to "Down syndrome," while it still is called "Down's" in hte UK and some places in Europe.

In the first part of the twentieth century, there was much speculation of the cause of Down syndrome. The first people to speculate that it might be due to chromosomal abnormalities were Waardenberg and Bleyer in the 1930s. But it wasn't until 1959 that Jerome Lejeune and Patricia Jacobs, working independently, first determined the cause to be trisomy (triplication) of the 21st chromosome. Cases of Down syndrome due to translocation and mosaicism (see definitions of these below) were described over the next three years.

Chromosomes are thread-like structures composed of DNA and other proteins. They are present in every cell of the body and carry the genetic information needed for that cell to develop. Genes, which are units of information, are "encoded" in the DNA. Human cells normally have 46 chromosomes which can be arranged in 23 pairs. Of these 23, 22 are alike in males and females; these are called the "autosomes." The 23rd pair are the sex chromosomes ('X' and 'Y'). Each member of a pair of chromosomes carries the same information, in that the same genes are in the same spots on the chromosome. However, variations of that gene ("alleles") may be present. (Example: the genetic information for eye color is a "gene;" the variations for blue, green, etc. are the "alleles.")

Human cells divide in two ways. The first is ordinary cell division ("mitosis"), by which the body grows. In this method, one cell becomes two cells which have the exact same number and type of chromosomes as the parent cell. The second method of cell division occurs in the ovaries and testicles ("meiosis") and consists of one cell splitting into two, with the resulting cells having half the number of chromosomes of the parent cell. So, normal eggs and sperm cells only have 23 chromosomes instead of 46.

This is what a normal set of chromosomes looks like. Note the 22 evenly paired chromosomes plus the sex chromosomes. The XX means that this person is a female. The test in which blood or skin samples are checked for the number and type of chromosomes is called a karyotype, and the results look like this picture.

Many errors can occur during cell division. In meiosis, the pairs of chromosomes are supposed to split up and go to different spots in the dividing cell; this event is called "disjunction." However, occasionally one pair doesn't divide, and the whole pair goes to one spot. This means that in the resulting cells, one will have 24 chromosomes and the other will have 22 chromosomes. This accident is called "nondisjunction." If a sperm or egg with an abnormal number of chromosomes merges with a normal mate, the resulting fertilized egg will have an abnormal number of chromosomes. In Down syndrome, 95% of all cases are caused by this event: one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. Hence the scientific name, trisomy 21. Recent research has shown that in these cases, approximately 90% of the abnormal cells are the eggs. The cause of the nondisjunction error isn't known, but there is definitely connection with maternal age. Research is currently aimed at trying to determine the cause and timing of the nondisjunction event.

Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation. In this case, two breaks occur in separate chromosomes, usually the 14th and 21st chromosomes. There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosome. So while the number of chromosomes remain normal, there is a triplication of the 21st chromosome material. Some of these children may only have triplication of part of the 21st chromosome instead of the whole chromosome, which is called a partial trisomy 21. Translocations resulting in trisomy 21 may be inherited, so it's important to check the chromosomes of the parents in these cases to see if either may be a "carrier."

The remainder of cases of trisomy 21 are due to mosaicism. These people have a mixture of cell lines, some of which have a normal set of chromosomes and others which have trisomy 21. In cellular mosaicism, the mixture is seen in different cells of the same type. In tissue mosaicism, one set of cells, such as all blood cells, may have normal chromosomes, and another type, such as all skin cells, may have trisomy 21.

The estimated incidence of Down syndrome is between 1 in 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 mental retardation. However, some children are not mentally retarded at all; they may function in the borderline to low average range; others may be severely mentally retarded. 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 to 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 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.

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.

1. 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.

2. 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.

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.

6. 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.

7. 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.

8. 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.