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Down Syndrome

Down syndrome or trisomy 21 is the most frequent genetic cause of mild to moderate mental retardation and associated medical problems and occurs in one out of 800 live births, in all races and economic groups. Down syndrome is a chromosomal disorder caused by an error in cell division, when a fertilised ovum , which will develop into the foetus , contains extra material from chromosome number 21 (cf. Genetic Definitions for further detail on this process). Three genetic variations can cause Down syndrome:

  • In most cases, approximately 92% of the time, Down syndrome is caused by the presence of an extra chromosome 21 in all cells of the individual. In such cases, the extra chromosome originates in the development of either the egg or the sperm. This condition, in which three copies of chromosome 21 are present in all cells of the individual, is called trisomy 21 .
  • In approximately 2-4% of cases, Down syndrome is due to mosaic trisomy 21 . This situation is similar to simple trisomy 21, but, in this instance, the extra chromosome 21 is present in some, but not all, cells of the individual. This may be due to an error in chromosome division early in embryonic development. In this situation, the range of the physical problems may vary, depending on the proportion of cells that carry the additional chromosome 21.
  • Approximately 3-4% of individuals with Down syndrome have cells containing 46 chromosomes, but still have the features associated with Down syndrome. In such cases, material from one chromosome 21 gets stuck or translocated onto another chromosome, either prior to or at conception . In such situations, the individual with Down syndrome is said to have translocation trisomy 21 . For parents of a child with Down syndrome due to translocation trisomy 21, there may be an increased likelihood of Down syndrome in future pregnancies. This is because one of the two parents may be a balanced carrier of the translocation.

Most of the time, the occurrence of Down syndrome is due to a random event that occurred during formation of the reproductive cells, the ovum or sperm. As far as we know, Down syndrome is not attributable to any behavioural activity of the parents or environmental factors.

Researchers have established that the likelihood that a reproductive cell will contain an extra copy of chromosome 21 increases dramatically as a woman ages. Only about 9 % of total pregnancies occur in women 35 years or older each year, but about 25% of babies with Down syndrome are born to women in this age group. Many specialists recommend that women who become pregnant at age 35 or older undergo prenatal testing for Down syndrome (see below).

The probability that another child with Down syndrome will be born in a subsequent pregnancy is about 1 percent, regardless of maternal age.

Click here for a table outlining the relationship of maternal age to the incidence of Down Syndrome.

Prenatal screening

Prenatal screening for Down syndrome is available. There is a relatively simple, non-invasive screening test that examines a drop of the mother's blood to determine if there is an increased likelihood for Down syndrome. This blood test measures the levels of three markers for Down syndrome: serum alpha feto-protein (MSAFP), chorionic gonadotropin (hCG), and unconjugated estriol (uE3). While these measurements are not a definitive test for Down syndrome, a lower MSAFP value, a lower uE3 level, and an elevated hCG level, on average, suggests an increased likelihood of a Down syndrome foetus, and additional diagnostic testing may be desired. These tests include amniocentesis , chorionic villus sampling (CVS), and percutaneous umbilical blood sampling (PUBS). However, before undergoing any of these diagnostic tests, patients and their families should seek detailed genetic counselling to discuss their family history in relationship to the risks and benefits of performing these diagnostic procedures.

Symptoms and Signs

A newborn baby with Down syndrome often has physical features the attending physician will most likely recognise in the delivery room. These may include a flat facial profile, an upward slant to the eye, a short neck, abnormally shaped ears, white spots on the iris of the eye (called Brushfield spots), and a single, deep transverse crease on the palm of the hand. However, a child with Down syndrome may not possess all of these features; some of these features can also be found in the general population.

To confirm the diagnosis, the doctor will request a blood test called a chromosomal karyotype . This involves "growing" the cells from the baby's blood for about two weeks, followed by a microscopic visualisation of the chromosomes to determine if extra material from chromosome 21 is present.

Associated Medical Disorders

Medical disorders such as congenital hypothyroidism , characterised by a reduced basal metabolism, an enlargement of the thyroid gland, and disturbances in the autonomic nervous system, occurs slightly more frequently in babies with Down syndrome. Several other well-known medical conditions, including hearing loss (estimated at 66 to 89%), congenital heart disease (approximately half ), and vision disorders, are also more prevalent among those with Down syndrome. Seizure disorders , though less prevalent than some of the other associated medical conditions, still affect between 5 and 13% of individuals with Down syndrome, a 10-fold greater incidence than in the general population.

The incidence and severity of these associated medical ailments will vary in babies with Down syndrome.

Newborns and Infants

Babies with Down syndrome often have hypotonia , or poor muscle tone. Because they have a reduced muscle tone and a protruding tongue, feeding babies with Down syndrome usually takes longer. Mothers breast-feeding infants with Down syndrome should seek advice from an expert on breast feeding to make sure the baby is getting sufficient nutrition.

Hypotonia may affect the muscles of the digestive system, in which case constipation may be a problem. Atlantoaxial instability , a malformation of the upper part of the spine located under the base of the skull, is present in some individuals with Down syndrome. This condition can cause spinal cord compression if it is not treated properly.

Medical care for infants with Down syndrome should include the same well-baby care that other children receive during the first years of life, as well as attention to some problems that are more common in children with Down syndrome. If heart, digestive, orthopaedic or other medical conditions were identified during the neonatal period, these problems should continue to be monitored.

During the early years of life, children with Down syndrome are 10-15 times more likely than other children to develop leukaemia , a potentially fatal disease. Infants with Down syndrome are also more susceptible to transient myelodysplasia , or the defective development of the spinal cord.

Compared to the general population, individuals with Down syndrome have a 12-fold higher mortality rate from infectious diseases, if these infections are left untreated and unmonitored. These infections are due to abnormalities in their immune systems. Children with Down syndrome are also more likely to develop chronic respiratory infections , middle ear infections , and recurrent tonsillitis. In addition, there is a 62-fold higher incidence of pneumonia in children with Down syndrome than in the general population.

Children with Down syndrome may be developmentally delayed. A child with Down syndrome is often slow to turn over, sit, stand, and respond. This may be related to the child's poor muscle tone. Development of speech and language abilities may take longer than expected and may not occur as fully as parents would like. However, children with Down syndrome do develop the communication skills they need.

For further, more detailed information on this topic, please refer to the reference source for this page.

The information in this page is presented in summarised form and has been taken from the following source(s):
1. United States National Institutes of Health, National Institute of Child Health and Human Development: http://www.nichd.nih.gov


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  http://www.hon.ch/Dossier/MotherChild/birth_disorders/down_syndrome.html Last modified: Jun 24 2002