(Trisomy 21; 47,XX,+21 or 47,XY,+21)
Down syndrome refers to a common genetic disorder that results in birth defects, medical problems and some degree of mental retardation.
Down syndrome (DS) is the most frequent genetic cause of mild to moderate mental retardation. DS occurs in one out of 800 to 1,000 live births in all races and economic groups.
In all cases it is the extra chromosome 21 genetic material that causes problems.
- Chromosomes contain the genetic makeup of your body and they occur in pairs within the body. There are 23 pairs of chromosomes. Most individuals with Down syndrome, 95%, have three (rather than the normal 2) number 21 chromosomes, Trisomy 21. This results from an error in cell division in the egg or sperm.
- The remaining five percent have either Mosaic Trisomy 21 or Translocation Trisomy 21. Where mosaicism exists (1-2% of individuals) some cells have the extra chromosome, trisomy 21, and some have the normal count of two number 21 chromosomes. Where a translocation is found (3-4%) part of the chromosome number 21 has broken off and reattached itself onto another chromosome. In about one third of individuals the translocation is inherited from a parent.
A risk factor is something that increases your chance of getting a disease or condition.
- Genetics: If either parent is a carrier of a specific type of Down syndrome (Translocation Down Syndrome), there is an increased risk of giving birth to a child with this type of Down syndrome.
- Age: The chance of having a child with Down syndrome increases after a woman reaches age 35.
- Sex: More boys than girls are born with Down syndrome.
RELATIONSHIP OF DOWN SYNDROME INCIDENCE TO MOTHER'S AGE
|Mother's Age (Years)
||Incidence of Down Syndrome
||Less than 1 in 1,000
||1 in 900
||1 in 400
||1 in 300
||1 in 250
||1 in 180
||1 in 135
||1 in 105
||1 in 60
||1 in 35
||1 in 20
||1 in 16
||1 in 12
Reference : Hook, EG, Lindsjo A. Down syndrome in live births by single year maternal age interval in a Swedish study: comparison with results from a New York State study. Am J Hum Genet. 1978 Jan;30(1):19-27.
Researchers have established 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. Older mothers account for only about 9% of all live births each year but for 25% of Down syndrome births.
Infants born with Down syndrome 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 Down syndrome may be born with or develop:
Most of these health problems are treatable and the majority of people born with Down syndrome today have a life expectancy of approximately 55 years.
- 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
- At older ages there is an increased incidence of dementia
- 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
- An under-active thyroid (hypothyroidism)
- Slow growth
- Late to sit, walk, toilet train
- Speech problems
- Emotional problems
- Risk that others assume that a child is more retarded than he or she is
There are two types of procedures available to pregnant women: screening tests and diagnostic tests. Screening tests estimate the risk of the fetus having Down syndrome; diagnostic tests tell whether or not the fetus actually has the condition.
- Screening Tests:
- Triple Screen and the Alpha-fetoprotein Plus
- These tests measure quantities of various substances in the blood (alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol) and together with the woman's age, estimate her risk of having a child with Down syndrome.
- These screening tests have traditionally been offered between fifteen and twenty weeks of gestation, but new data suggests that optimal screening should begin as early as 11 weeks and include both ultrasound imaging and blood screening. For maximum test sensitivity, screening should be done at 11 weeks with a follow-up in the second trimester for those who screen negative.
- Recent data reports Down syndrome detection rates of up to 96% for combined first and second trimester screening.
- Five percent of women who undergo these tests will be given false-positive readings and as a consequence they may choose to have diagnostic testing done which could result in a miscarriage or other harm to a normal fetus.
- Diagnostic Tests:
- The procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling (CVS), amniocentesis and percutaneous umbilical blood sampling (PUBS).
- Each one of these procedures 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 percent accurate in the detection of Down syndrome.
- Amniocentesis is usually performed between 12 and 20 weeks of gestation, CVS between ten and 12 weeks and PUBS after 20 weeks.
Doctors can also usually identify a child born with Down syndrome at delivery. When features that suggest Down syndrome are present a blood test will be done for chromosome study and a definitive diagnosis.
There is no cure for Down syndrome. Some newborns may need surgery to repair serious medical problems like heart defects. Living at home and receiving special therapy helps children with Down syndrome 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.
Infants with Down syndrome may take longer to feed. A child with the condition will start talking, playing and performing other activities later than normal.
School programs designed to meet the child's special needs may help to increase learning opportunities. Children who have accomplished academic milestones may be mainstreamed into regular public school classes, with additional support as required.
Speech, physical and occupational therapy may improve the ability to speak, walk and perform activities of daily living.
Professional support helps a family cope with caring for a child with birth defects and mental retardation. Mental health professionals offer help with managing emotional problems.
There are no guidelines for preventing Down syndrome. If you have concerns about having a child with Down syndrome, consider getting genetic counseling prior to becoming pregnant.