Down syndrome
Down syndrome: Excerpt from Handbook of Diseases
The first disorder attributed to a chromosomal aberration, Down syndrome (trisomy 21) characteristically produces mental retardation, dysmorphic facial features, and other distinctive physical abnormalities. It’s commonly associated with heart defects (in about 60% of patients) and other congenital disorders.
Life expectancy for patients with Down syndrome has increased significantly because of improved treatment for related complications (heart defects, respiratory and other infections, acute leukemia). Nevertheless, up to 44% of such patients who have associated congenital heart defects die before age 1.
Causes
Down syndrome usually results from trisomy 21, a spontaneous chromosomal abnormality in which chromosome 21 has three copies instead of the normal two because of faulty meiosis (nondisjunction) of the ovum or, sometimes, the sperm. This results in a karyotype of 47 chromosomes instead of the normal 46. In about 4% of patients, Down syndrome results from an unbalanced translocation (chromosomal rearrangement) in which the long arm of chromosome 21 breaks and attaches to another chromosome. The disorder may also be due to chromosomal mosaicism, a mixture of two cell types — one with a normal number of chromosomes (46) and some with 47 (an extra chromosome 21).
Down syndrome occurs in 1 in 650 to 700 live births, but the prevalence increases with advanced parental age, especially when the mother is age 34 or older at delivery or the father is older than age 42. At age 20, a woman has about 1 chance in 400 of having a child with Down syndrome; by age 49, she has 1 chance in 12. If a woman has had one child with Down syndrome, the risk of recurrence is 1% to 2%. This risk varies according to the type of translocation the parents carry, and genetic counseling should be discussed.
Signs and symptoms
The physical signs of Down syndrome (especially hypotonia) as well as some dysmorphic facial features and heart defects may be apparent at birth. The degree of mental retardation may not become apparent until the infant grows older. People with Down syndrome typically have craniofacial anomalies, such as upslanting palpebral fissures, almond-shaped eyes with epicanthic folds; a flat face; a protruding tongue; a small mouth and chin; a single transverse palmar crease (simian crease); small white spots (Brushfield spots) on the iris; strabismus; a small skull; a flat bridge across the nose; slow dental development, with abnormal or absent teeth; small ears; a short neck; and cataracts.
Other physical effects may include dry, sensitive skin with decreased elasticity; umbilical hernia; short stature; short extremities, with broad, flat, squarish hands and feet; clinodactyly (finger that curves inward); a wide space between the first and second toe; and abnormal fingerprints and footprints. Hypotonic limb muscles impair reflex development, posture, coordination, and balance.
Congenital heart disease (septal defects or pulmonary or aortic stenosis), duodenal atresia, megacolon, and pelvic bone abnormalities are common. The incidence of leukemia and thyroid disorders may be increased. Frequent upper respiratory infections can be a serious problem. Genitalia may be poorly developed and puberty delayed. Females may menstruate and be fertile. Males are infertile with low serum testosterone levels; many have undescended testes.
Down syndrome patients may have an IQ between 30 and 70; however, social performance is usually beyond that expected for mental age. The level of intellectual function depends greatly on the environment and the amount of early stimulation received in addition to the IQ.
Diagnosis
Physical findings at birth, especially hypotonia, may suggest this diagnosis, but no physical feature is diagnostic in itself.
A karyotype showing the specific chromosomal abnormality provides a definitive diagnosis. Amniocentesis allows prenatal diagnosis and is recommended for pregnant women older than age 34, even if the family history is negative. Amniocentesis is also recommended for any pregnant woman if she or the father carries a translocated chromosome.
Treatment
Down syndrome has no known cure. Surgery to correct heart defects and other related congenital abnormalities and antibiotic therapy for recurrent infections have improved life expectancy considerably. Plastic surgery is occasionally done to correct the characteristic facial traits, especially the protruding tongue. Benefits beyond improved appearance may include improved speech, reduced susceptibility to dental caries, and fewer orthodontic problems later. Most Down syndrome patients are now cared for at home and attend special education classes. As adults, some may work in a sheltered workshop or live in a group home facility.
Special considerations
Support for the parents of a child with Down syndrome is vital. By following the guidelines listed below, you can help them meet their child’s physical and emotional needs.
❑ Establish a trusting relationship with the parents, and encourage communication during the difficult period after diagnosis. Recognize signs of grieving.
❑ Teach parents the importance of a balanced diet for the child. Stress the need for patience while feeding the child, who may have difficulty sucking and may be less demanding and seem less eager to eat than normal babies.
❑ Encourage the parents to hold and nurture their child.
Clinical tip Emphasize the importance of adequate exercise and maximal environmental stimulation; refer the parents for infant stimulation classes, which may begin in the early months of life.
❑ Help the parents set realistic goals for their child. By the time he’s 1 year old, the child’s development may begin to lag behind that of other children. (See Setting goals for children with Down syndrome.)
❑ Refer the parents and older siblings for genetic counseling to help them evaluate future reproductive risks. Discuss options for prenatal testing.
❑ Encourage the parents to remember the siblings’ emotional needs.
❑ Refer the parents to national or local Down syndrome organizations and support groups.
Pictures
Book Source Details
- Book Title: Handbook of Diseases
- Author(s): Springhouse
- Year of Publication: 2003
- Copyright Details: Handbook of Diseases, Copyright © 2003 Lippincott Williams & Wilkins.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Handbook of Diseases
Authors: Springhouse
Publisher: Lippincott Williams & Wilkins
Copyright: 2003
ISBN: 1-58255-266-5
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