Down Syndrome

Named after John Langdon Down, Down Syndrome the syndrome, Down syndrome is the most frequent genetic cause of mild to moderate mental retardDown Syndrome 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 that rDown Syndrome presence of an additional third chromosome 21 or "trisomy 21."

The Chromosomal Basis of Down Syndrome

To understand why Down syndrome occurs, the structurDown Syndrome function of the human chromosome Down Syndromee human body is made of cells; all cells contain chromosomes, structures that transmit genetic information. Most cells of the human body contain 23 pairs of chromosomes, half of which are inherited from each parent. Only the human reproductive cells, the sperm cells in males and the ovum in females, have 23 individual chromosomes, not pairs. Scientists identify these chromosome pairs as the XX pair, present in females, and the XY pair, present in males, and number them 1 through 22.

When the reproductive cells, the sperm and ovum, combine at fertilization, the fertilized egg that results contains 23 chromosome pairs. A fertilized egg that will develop into a female contains chromosome pairs 1 through 22, and the XX pair. A fertilized egg that will develop into a male contains chromosome pairs 1 through 22, and the XY pair. When the fertilized egg contains extra material from chromosome number 21, this results in Down syndrome.

Genetic Variations That may Cause Down Syndrome

Three genetic variations can cause Down syndrome. In Down Syndrome cases, approximately 92% of the time, Down syndrome is causDown Syndrome the presence of an extra chromosome 21 in all cells of Down Syndromel. In such cases, the extra chromosome originates in the development of either the egg or the sperm. Consequently, when the egg and sperm unite to form the fertilized egg, three--rather than two--chromosomes 21 are present. As the embryo develops, the extra chromosome is repeated in every cell. 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 simiDown Syndrome trisomy 21, but, in this instance, the extra chromosome 21 is present in some, but not all, cells of the individual. For example, the fertilized egg may have the right number of chromosomes, but, due to an error in chromosome division early in embryonic development, some cells acquire an extra chromosome 21. Thus, an individual with Down syndrome due to mosaic trisomy 21 will typically have 46 chromosomes in some Down Syndrome but will have 47 chromosomes (including an extra chromosome 21) in others. In this situation, the range of the physical problems may vary, depending on the proportion of cells that carry the additional chromosome 21.

Most of the time, the occurrence of Down syndrome isDown Syndrome to a random event that occurred during formation of the reproDown Syndrome cells, the ovum or sperm. As far as we know, Down syndrome is not attributable to any behavioral activity of the parents or envDown Syndrome factors. The probability that another child with Down syndrome will be born in a subsequent pregnancy is about 1 percent, regardlesDown Syndrome age.

For parents of a child with Down syndrome due to translocation trisomy 21, there may be an increased likelihooDown Syndromedrome in future pregnancies. This is because one of the two parents may beDown Syndromelanced carrier of the translocation. The translocation occurs when a piece of chromosome 21 becomes attached to another chromosome, often number 14, during cell division. If the resulting sperm or ovum receives a chromosome 14 (or another chromosome), with a piece of chromosome 21 attached and retains the chromosome 21 that lost a section due to translocation, then the reproductive cells contain the normal or balanced amount of chromosome 21. While there will be no Down syndrome associated characteristics exhibited, the individual who develops frDown Syndromelized egg will be a carrier of Down syndrome. Genetic counseling can be sought to find the origin of the translocDown Syndrome

However, it is important to realize that not all parents of individuals with translocation trisomy 21 are themselves balanced carriers. In such situations, there is no increased risk for Down syndrome in future pregnancies.

Researchers have extensively Down Syndrome studied the defects in chromosome 21 that cause Down syndrome. In 88% of cases, the extra copy of chromosome 21 is derived from theDown Syndrome mother. In 8% of the cases, the father provided the extra copy of chromosome 21. In the remaining 2% of the cases, Down syndrome is due to mitotic errors, an error in cell division which occurs afteDown Syndrome fertilization when the sperm and ovum are joined.

DOWN SYNDROME AND MATERNAL AGE

Researchers have established Down Syndrome that the likelihood that a reproductive cell will contain an extra copy of chromosome 21 increases dramatically as a woman ages. Therefore, an older mother is more likely than a younger mother to have a baby with Down syndrome. However, of the total population, older mothers have fewer Down Syndromeabies with Down syndrome are born to younger women because more younger women than older womenDown Syndrome Only about nine percent 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.

The incidence of Down Syndrome syndrome rises with increasing maternal age. Many specialists recommend Down Syndromeme pregnant at age 35 or older undergo prenatal testing for Down syndrome. The likelihood that a woman under 30 who becomes pregnant will have Down Syndromeown syndrome is less than 1 in 1,000, but the chance of having a baby Down Syndromeses to 1 in 400 for women who become pregnant at age 35. The liDown Syndromeown syndrome continues to increase as a woman ages, so that by age 42, Down Syndromeat a pregnant woman will have a baby with Down syndrome, and by age 49, the chance is 1 in 12. But using maternal age Down Syndromet over 75% of pregnancies that will result in Down syndrome.

Researchers outside the NICHD are also developing a new method of diagnosis, called preimplantation diagnosis or blastomere analysis before implantation (BABI), which allows clinicians to detect chromosome imbalances before an embryo is implanted during in vitro fertilization. This technique would primarily be used in couples who are at risk of passing on X-linked disorders, couples who have suffered repeated terminations of pregnancy, subfertile couples, or those at risk for single gene disorders. This technique, which allows the clinician to provide a genetic diagnosis prior to implantation, has been successful so far for cystic fibrosis, Tay Sachs disease, and Lesch-Nyhan syndrome. BABI allows a couple to begin their pregnancy knowing that the fetus is unaffected with the genetic disease of concern. For couples at high risk, this procedure provides an alternative to prenatal testing in the first or second trimester.

A Diagnosis of Down Syndrome

A newborn baby with Down syndrDown Syndrome features the attending physician will most likelyDown Syndrome 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 evDown Syndrome 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 visualization of the chromosomes to determine if extra material from chromosome 21 is present.

When parents are told that their newborn baby has Down syndrome, it is not unusual for them to have feelings of sadness Down Syndromeparents report that at the time their child is first diagnosed with Down syndrome and during the weeks that follow, they feel overwhelmed by feelings Down Syndrome of loss and anxiety. While caring for a child with Down syndrome frequently requires more time and energy, parents of newborn childreDown Syndromeyndrome should seek the advice of a knowledgeable pediatrician and/or the maDown Syndrome syndrome support groups and organizations available.

Down Syndrome initial diagnosis of Down syndrome has no way of knowing the intellectual or physical capabilities thisDown Syndrome or any other child, may have. Children and adults with Down syndrome have a wide range of abilities. A person with Down syndrome may be vDown Syndrome or they may present unusual and demanding medicDown Syndrome problems at virtually every stage of life. However, every person with Down syndrome is a unique individual, and not all people with Down syndrome Down Syndromehe medical disorders discussed below.

During the first dDown Syndrome and months of life, some disorders may be immediately diagnosed. Congenital hypothyroidism, characterized 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. A routine blood test for hypothyroidism that is performed on newboDown Syndrome detect this condition if present.

Several other well-known medical conditions, including hearing loss, congenital heart disease, and vision disorders, are more prevalent among those with Down syndrome.

Recent studies indicate that 66 to 89% of children Down Syndrome Down syndrome have a hearing loss of greater than 15 to 20 decibels in at least oDown Syndromeo the fact that the external ear and the bones of the middle and inner ear may develop differently in children with Down syndrome. Many hearing problems can be corrected. But, because of the high pDown Syndrome hearing loss in children with Down syndrome, an objective measure of hearing should be taken to establish Down Syndrome addition to hearing disorders, visual problems also may be present early in life. Cataracts occur in approximately 3% of children with Down syndrome, but can be surgically removed.

Approximately half of the children with Down syndrome have congenital heart disease and associated early onset of pulmonary hypertension, or high blood pressure in the lungs. Echocardiography may be indicated to identify any congenital heart disease. If the defects have been identified before the onset of pulmonary hypertension, surgery has provided favorable results.

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. There is an unusually high incidence of infantile spasms or seizures in children less than one year of age, some of which are precipitated by neonatal complications and infections and cardiovascular disease. However, these seizures can be treated with anti-epileptic drugs.

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

Newborns

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.

Infants and Preschool Children

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, orthopedic 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 leukemia, a potentially fatal disease. These children should receive an appropriate cancer therapy, such as chemotherapy. 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, usually the t-cell and antibody-mediated immunity functions that fight off infections. 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.

Parents of other children with Down syndrome are often valuable sources of information and support. Parents should keep in mind that children with Down syndrome have a wide range of abilities and talents, and each child develops at his or her own particular pace. It may take children with Down syndrome longer than other children to reach develop mental milestones, but many of these milestones will eventually be met. Parents should make a concerted effort not to compare the developmental progress of a child with Down syndrome to the progress of other siblings or even to other children with Down syndrome.

Early Intervention and Education

The term "early intervention" refers to an array of specialized programs and related resources that are made available by health care professionals to the child with Down syndrome. These health care professionals may include special educators, speech therapists, occupational therapists, and social workers. It is recommended that stimulation and encouragement be provided to children with Down syndrome.

The evaluation of early intervention programs for children with Down syndrome is difficult, due to the wide variety of experimental designs used in interventions, the limited existing measures available that chart the progress of disabled infants, and the tremendous variability in the developmental progress among children with Down syndrome