What are the other Names for this Condition? (Also known as/Synonyms)

• Down's Syndrome
• T21 (Trisomy 21)
• Trisomy 21

What is Down Syndrome? (Definition/Background Information)

• Down Syndrome is the trisomy of chromosome 21. The term ‘trisomy’ indicates that there are three chromosomes instead of a typical pair of chromosomes. The trisomy of chromosome 21 means that there is an extra chromosome 21 
• All chromosomes are numbered in pairs. Usually, 46 chromosomes are present; but individuals with Down Syndrome have 47 chromosomes
• Chromosomes are structures made of deoxyribonucleic acid (DNA). DNA is responsible for the makeup and functioning of every human being

There are 3 common types of chromosomal abnormality that result in Down Syndrome:

• Trisomy 21: The extra copy of chromosome 21 is typically from the mother, often due to errors in the egg cell. Over 95% of all Down Syndrome cases, are due to this  occurrence
• Translocation: This occurs when a piece of chromosome 21 attaches itself to another chromosome, usually chromosome 14. Here, the individual has two copies of chromosome 21 and an additional fragment attached to chromosome 14
• Mosaic: This means that the individual has 3 copies of chromosome 21 in some, but not every cell of the body

Who gets Down Syndrome? (Age and Sex Distribution)

• Down Syndrome is the most common chromosomal abnormality among live births (about 1 in 691 live births, as per CDC). Both male and female sexes are equally affected
• No definitive racial or ethnic predominance is noted

What are the Risk Factors for Down Syndrome? (Predisposing Factors)

The risk factors associated with Down Syndrome include:

• Age of the mother: Older women (an increased maternal age) are shown to have a higher risk of giving birth to a child with Down Syndrome
• Previous pregnancies resulting in a child with Down Syndrome
• Mother with Down Syndrome
• If either parent carries a translocation or an additional copy of chromosome 21 themselves, the risk of inheriting the extra chromosomal material increases

It is important to note that having a risk factor does not mean that one will have a child with Down Syndrome. A risk factor increases one's chances of having a child with a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not have a child with Down syndrome. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Down Syndrome? (Etiology)

An extra chromosome at the 21st chromosome pair, resulting in a triplet of chromosomes, rather than the typical pair, causes Down Syndrome. The extra chromosomal genes alter typical genetic and cellular processes.

Different forms of Down Syndrome are characterized by how this extra chromosome comes about.

• Trisomy 21: Three copies of chromosome 21 are observed in all of the individual’s cells. This is due to an abnormal cell division at, or prior to, conception
• Translocation Down Syndrome: This form occurs when a part of chromosome 21 becomes attached (translocated) to another chromosome, prior to or during conception. The occurrence of this form is rare (4-5% of all cases)
• Mosaic Down syndrome: Only some cells containing an extra chromosome characterize this very rare form of Down Syndrome (1-2% of all cases). This mosaic of cells is caused by abnormal cell division after fertilization

What are the Signs and Symptoms of Down Syndrome?

The signs and symptoms of Down Syndrome may include:

• Flat facial profile
• Upward slanting eyes
• Short neck, small mouth, and protruding tongue
• Low muscle tone
• Hyper-extensive joints
• Short stature
• Small ears (low set)
• Single palmar crease
• Large space between the first two toes
• Premature aging
• Brushfield spots (white spots on the peripheral portion of the iris)
• Intellectual disability

How is Down Syndrome Diagnosed?

Down Syndrome can be diagnosed prenatally. Prenatal screenings look for certain markers that indicate Down Syndrome, either through ultrasound or in the mother’s blood. The diagnosis can be confirmed by a chromosomal analysis (karyotype).

• Nuchal translucency is a parameter that is evaluated by ultrasound. This measures the thickness of the folds of the neck, which may be increased in fetuses with Down Syndrome
• Amniocentesis may be used to extract a small amount of amniotic fluid for analysis
• Chorionic villus sampling takes chorionic villus cell samples from the placenta, in order to find any abnormalities

Nuchal translucency is a screening test, whereas amniocentesis and chorionic villus sampling are diagnostic tests.

In Down Syndrome, the following pattern of blood results is used for screening:

• Alpha-fetoprotein (AFP) - decreased
• Unconjugated estriol (uE3) - decreased
• Human chorionic gonadotropin (hCG) - increased
• Inhibin A - increased

If it is not detected during pregnancy, then a physician may suspect Down Syndrome, based on the infant’s appearance. The presence of the following characteristics will most likely lead to testing for Down Syndrome:

• Physical characteristics, such as short neck, small mouth, protruding tongue, upward slanting eyes
• Problems with vision, hearing, respiratory infection
• Abnormal eating
• Absence of stool, due to imperforate anus defect
• Vomiting, due to GI tract blockage

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Down Syndrome?

Following are the possible complications of Down Syndrome:

• About 50% of the individuals have congenital heart defects
• Problems with vision and hearing
• Sleep apnea
• Blood disorders, including some forms of leukemia
• Immune system abnormalities, putting the individuals at higher risk of infection and autoimmune disorders
• Gastrointestinal disorders, which may cause bowel obstruction, or problems with nutrient absorption
• Skeletal problems
• Obesity, with associated complications
• Infertility
• Adults with Down Syndrome stand a greater risk of developing early-onset Alzheimer’s disease and dementia

How is Down Syndrome Treated?

Down Syndrome has no cure as it is a genetic condition. The treatment measures to manage the symptoms may include:

• Corrective surgery may be required for some disorders, like heart defects or GI abnormalities
• Early education programs and family counseling have been very helpful, for families having a child with Down Syndrome

How can Down Syndrome be Prevented?

Currently there are no specific methods or guidelines to prevent Down Syndrome.

• Genetic testing of the expectant parents (and related family members) and prenatal screening or diagnosis (testing of the fetus during pregnancy) helps  give parents an understanding of the chance that their fetus has Down Syndrome
• If there is a family history of the condition, then genetic counseling will help assess risks before planning for a child
• Active research is currently being performed to explore the possibilities for treatment and prevention of inherited and acquired genetic disorders
• Women who wish to conceive at an advanced age, should be aware of increased likelihood of having a child with Down Syndrome
• The main focus should be on providing educational programs for the children and adults (and to their families) with Down Syndrome

What is the Prognosis of Down Syndrome? (Outcomes/Resolutions)

• Family support and educational programs are a must for those with Down Syndrome
• Life expectancy for an individual with Down Syndrome is 60 years. Many individuals are active in their communities; they have a full-time job and lead fulfilling lives
• Individuals with Down Syndrome are more likely to develop other medical conditions associated with having an extra copy of chromosome 21

Additional and Relevant Useful Information for Down Syndrome:

There are voluntary organizations and support groups for Down Syndrome that provides counsel, help, and understanding, to the affected individuals and their families.

Acknowledgment: We sincerely thank National Down Syndrome Society for their valuable input and feedback, which has helped enrich the contents of this article.