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Down Syndrome

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 approximately 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 (non disjunction) 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 incidence 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 2,000 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

Individuals with Down Syndrome may not experience all of the symptoms. Some of the common symptoms are: decreased muscle tone at birth, asymmetrical or odd-shaped skull, round head with flat area at the back of the head, small skull, slanting eyes, small mouth with protruding tongue (see tongue problems), broad short hands, single crease on the palm, retarded growth and development, delayed mental and social skills (mental retardation)

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.

What are the physical Characteristics?

Children with Down Syndrome have a widely recognized characteristic appearance. Their head may be smaller than normal (microcephaly) and abnormally shaped. Other prominent characteristics are: a flattened nose, protruding tongue, upward slanting eyes, short hands and fingers, and a single crease in the palm.

Treatment

There is no specific treatment for Down syndrome. Special education and training is offered in most communities for mentally handicapped children. Specific heart defects may require surgical correction. The potential for visual problems, hearing loss, and increased susceptibility to infection will require screening and treatment at appropriate intervals.

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.

Prevention

There's no way to prevent Down syndrome. However, if you are at high risk of having a child with Down syndrome or you already have one child with Down syndrome, you may wish to consult a genetic counselor before becoming pregnant.

A genetic counselor can help you understand your chances of having a child with Down syndrome. He or she can also explain the prenatal tests you will be offered and help you figure out the pros and cons of testing for your particular situation.