| المجلد 4 , العدد 4 , ذو الحجة 1427 - كانون الثاني (يناير) 2007 |
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| What is Down Syndrome? |
| ماهي متلازمة داون ؟ |
| د. محمد كامل كبه وار |
| Kebbewar M.K. |
| الملخص Abstract |
يتصف المصابون بمتلازمة داون بخصائص مادية قابلة للتعرف بها كأول متلازمة يؤدي وجود تثلث في الصبفي 21 فيها إلى مقدرات ذهنية محدودة.
يتراوح وقوع المتلازمة بين 1 في 1000 إلى 1 في 1200 من المواليد الأحياء ففي كل عام يولد حوالي 3000 إلى 5000 طفل مصاب بهذا الاضطراب الصبغي. ويعتقد أن هنالك 250000 أسرة تقريباً في الولايات المتحدة متأثرة بمتلازمة داون.
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People with Down syndrome are first and foremost human beings who have recognizable physical characteristics and limited intellectual endowments which are due to the presence of an extra chromosome 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.
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| 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.
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| 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 chara-cteristics; 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.
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| 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.
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| 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.
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| What kind of information can be provided through genetic counseling? |
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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 one 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 increases markedly. The actual risk depends on the type of translocation and whether the translocation is carried by the father or the mother.
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| 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. 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.
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| Can Down syndrome be medically treated? |
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Although many medications and various therapies have been touted as treatment for people with Down syndrome, there is no effective medical treatment available now. 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.
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| What education services and vocational opportunities are available for people with Down syndrome? |
Today early intervention programs, pre-school nurseries, and integrated special education strategies have demonstrated that youngsters with Down syndrome can participate in many learning experiences, which can positively influence their overall functioning.
Research has shown that early intervention, environmental enrichment, and assistance to the families will result in progress that is usually not achieved by those infants who have not had such educational and stimulating experiences.
Children with Down syndrome, like all children, can benefit from sensory stimulation, specific exercises involving gross and fine motor activities, and instruction in cognitive development. Also, preschool nurseries play an important role in the young child's life since exploring the environment beyond the home enables the child to participate in a broader world.
Later, the school can give the child a foundation for life through the development of academic skills and physical as well as social abilities. Experiences provided in school assist the child in obtaining a feeling of self-respect and enjoyment. School should provide an opportunity for the child to engage in sharing relationships with others and help to prepare the child to become a productive citizen. Contrary to some views, all children can learn, and they will benefit from placement in a normalized setting with support as needed.
During adolescence, youngsters with Down syndrome should be exposed to prevocational training in order to learn good work habits and to engage in proper relationships with co-workers. Appropriate vocational counseling and job training will result in meaningful employment, and this, in turn, should lead to a feeling of self-worth and of making a contribution to society.
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| What attitude should society have? |
It is important that society develop attitudes that will permit people with Down syndrome to participate in community life and to be accepted. They should be offered a status that observes their rights and privileges as citizens, and in a real sense preserves their human dignity.
When accorded their rights and treated with dignity, people with Down syndrome will, in turn, provide society with a most valuable humanizing influence.
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| Parental screening for Down's Syndrome |
Down's Syndrome (Trisomy 21) is the most common autosomal chromosome aberration. During the 1980's, it was recognized by several groups that, in the 2nd trimester, the mean maternal serum concentrations of alpha-fetoprotein (AFP) and unconjugated estriol (UE3) are significant lower in trisomy 21 pregnancies, as compared to normal pregnancies, whereas the human chorionic gonadotropin (hCG) concentrations are significantly higher.
The simultaneous determination of AFP, UE3 and hCG in maternal serum in order to perform a screening for Down's Syndrome is called "triple test". The risk for having a Down's Syndrome pregnancy in the 2nd trimester can be calculated using the algorithm as described by wald and the respective assay-specific parameters.
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| References |
1-Rogers PT. and Coleman M.
Medical Care in Down Syndrome.
Marcel Dekker, NY, 14-16, 1992.
2-Pangalos C et al.
Understanding the mechanism(s) of mosaic trisomy 21, by using DNA polymorphism analysis.
Am. J. Hum. Genet. 54: 473-481, 1994.
3-Fishler K. and Koch R.
Mental Development in Down Syndrome Mosaicism.
Am. J. Mental Retardation, 96(3): 345-351, 1991.
4-Luke A; Rozien NJ; Sutton M. and Schoeller DA.
Energy Expenditure in children with Down Syndrome: Correcting Metabolic Rate for Movement.
Journal of Pediatrics, 125, 1994.
5-Cooley WC. and Graham JM.
Down Syndrome - an update and review for the primary paediatrician.
Clin Paed, 30 (4): 233-253, 1991.
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| المجلد 4 , العدد 4 , ذو الحجة 1427 - كانون الثاني (يناير) 2007 |
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