Definition

Down syndrome (DS) is a common genetic disorder. It results in birth defects, medical problems, and some degree of mental retardation.

Causes

DS is the most frequent genetic cause of mild to moderate mental retardation. It occurs in one out of 800 to 1,000 live births. DS occurs in all races and economic groups.

Chromosomes contain the genetic makeup of your body. They occur in 23 pairs within the body. Three types of chromosomal changes that lead to DS include:

• An extra chromosome (3 instead of 2) develops in chromosome 21. This type is called Trisomy 21. This results from an error in cell division in the egg or sperm.
• In Mosaic Trisomy 21, some cells have the extra chromosome and some do not.
• In translocation trisomy, part of the chromosome number 21 has broken off. The broken piece reattaches onto another chromosome. In about one third of individuals, the translocation is inherited from a parent.

In all cases, it is the extra chromosome 21 genetic material that causes problems.

Risk Factors

Factors that increase the chance for DS include:

• Genetics: If either parent is a carrier of Translocation DS, there is an increased risk
• Age: The chance of having a child with DS increases after a woman reaches age 35
• Sex: More boys than girls are born with DS

Symptoms

Infants born with DS may have some or all of the following physical characteristics:

• Muscle hypotonia, low muscle tone
• Flat facial features, a somewhat depressed nasal bridge and a small nose
• Upward slanted eyes, small skin folds on the inner corner of the eyes
• Short neck
• Misshaped ears
• White spots on the colored part of the eye
• Single skin crease in the palm of the hand
• Excess flexibility in joints
• Heart defects
• Sight and hearing problems
• Large and protruding tongue
• Fifth finger has one flexion furrow instead of two
• Excessive space between large and second toe

The degree of medical problems and mental retardation varies. Talents, abilities, and pace of development differ. People with DS may be born with or develop:

• Vision problems
• Hearing loss
• Heart defects
• Increased incidence of acute leukemia
• Frequent ear infections and increased susceptibility to infection in general
• Gastrointestinal obstruction (imperforate anus and similar problems)
• Esophageal atresia or duodenal atresia
• One third of patients experience blocked airways during sleep
• Increased incidence of dementia in older ages
• Instability of the back bones at the top of the neck, can result in compression injury of the spinal cord
• Urinary system defects
• High blood pressure in the lungs
• Seizures
• An under-active thyroid (hypothyroidism)
• Slow growth
• Late to sit, walk, toilet train
• Speech problems
• Obesity
• Emotional problems
• Risk that others assume that a child is more retarded than he or she is

Diagnosis

Prenatal Diagnosis

There are two types of procedures available to pregnant women:

• Screening tests estimate the risk of the fetus having DS
• Diagnostic tests tell whether or not the fetus actually has the condition

Screening Tests: Triple Screen and Alpha-fetoprotein Plus

• These tests measure amounts of various things in the blood. Together with the woman's age, the doctor can estimate the risk of having a child with DS.
• New data suggests that screening should begin as early as 11 weeks. It also should include both ultrasound imaging and blood screening. There should be a follow-up in the second trimester for those who screen negative.
• Five percent of women who undergo these tests will be given false-positive readings. This means the test indicates DS even when it does not exist.

Diagnostic Tests

• The tests include chorionic villus sampling (CVS), amniocentesis, and percutaneous umbilical blood sampling (PUBS).
• Each one carries a small risk of miscarriage as tissue is extracted from the placenta or the umbilical cord to examine the fetus' chromosomes.
• The procedures are about 98% to 99% accurate in the detection of DS.
• Amniocentesis is usually performed between 12 and 20 weeks of gestation, CVS between 10 and 12 weeks, and PUBS after 20 weeks.

Amniocentesis

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Treatment

There is no cure for DS. Some newborns may need surgery to repair serious medical problems, like heart defects. Living at home and receiving special therapy helps children with DS achieve their full potential. Most people with the condition can actively participate in the community—at schools, jobs, and various leisure activities. Some live with family, some with friends, and some independently.

Prevention

There are no guidelines for preventing DS. If you have concerns about having a child with DS, consider getting genetic counseling prior to becoming pregnant.