Down Syndrome: Causes, Symptoms, Types & Tests

Down syndrome is one of the most common genetic conditions worldwide, affecting approximately 1 in every 700 babies. Characterized by an extra copy of chromosome 21, it results in physical differences, intellectual disability, and a higher risk of certain medical conditions. Advances in medical care, education, and social inclusion have greatly improved the life expectancy and quality of life for people with Down syndrome. Today, many live healthy, fulfilling lives, actively participating in work, family, and community.

Down Syndrome

Down syndrome, medically known as trisomy 21, is a congenital genetic disorder caused by the presence of an extra (third) copy of chromosome 21. Typically, humans have 23 pairs of chromosomes, but individuals with Down syndrome have three copies of chromosome 21 instead of the usual two.This extra genetic material changes the way the brain and body develop, leading to a combination of physical traits, intellectual disability, and an increased risk of certain health issues. The severity and combination of these traits vary widely among individuals, meaning no two people with Down syndrome are exactly alike.

Causes of Down Syndrome

Down syndrome is caused by abnormal cell division involving chromosome 21, but why this happens isn’t fully understood. The condition is not caused by anything parents did before or during pregnancy.

There are three types of Down syndrome, each related to how the extra chromosome appears:

1. Trisomy 21 (about 95% of cases):

• Every cell in the body has three copies of chromosome 21.
• Occurs due to random events in cell division during the formation of the egg or sperm.

2. Translocation Down syndrome (about 3–4% of cases):

• Part of chromosome 21 attaches (translocates) to another chromosome before or at conception.
• Children still have the usual two copies of chromosome 21 plus extra genetic material from the translocated part.

3. Mosaic Down syndrome (about 1–2% of cases):

• Some cells have three copies of chromosome 21, while others have the usual two.
• This usually happens due to cell division errors after fertilization.
• Symptoms can be milder depending on the proportion of affected cells.

Symptoms of Down Syndrome

Symptoms and physical characteristics of Down syndrome can differ greatly but often include:

• Distinct facial features: Flattened facial profile, small nose, upward-slanting eyes with epicanthal folds, small ears, and a protruding tongue.
• Short stature: Slower growth and shorter adult height.
• Low muscle tone (hypotonia): Leading to floppiness in infancy.
• Single deep crease across the center of the palm (simian crease).
• Short neck and small hands and feet.
• Intellectual disability: Mild to moderate, affecting learning, language development, and reasoning.
• Developmental delays: Milestones like sitting, walking, and talking may occur later.

In addition, individuals with Down syndrome may have medical complications, including:

• Congenital heart defects (affecting about 40–50%).
• Gastrointestinal issues (e.g., duodenal atresia, Hirschsprung’s disease).
• Increased risk of hearing and vision problems.
• Higher susceptibility to infections and autoimmune conditions.
• Risk of thyroid disorders, sleep apnea, leukemia, and Alzheimer’s disease in adulthood.

Types of Down Syndrome

The three types of Down syndrome reflect how the extra chromosome 21 appears in cells:

1. Trisomy 21 (Standard Down syndrome):

• Every cell contains three copies of chromosome 21.
• This is by far the most common form.

2. Translocation Down syndrome:

• Extra chromosome 21 material is attached to another chromosome.
• Can be inherited from a parent who carries a balanced translocation, though most cases are still spontaneous.

3. Mosaic Down syndrome:

• Only some cells have the extra chromosome.
• Often leads to milder symptoms depending on the proportion and location of affected cells.

The type of Down syndrome does not necessarily predict severity, but mosaicism may sometimes present with milder developmental or physical differences.

Tests Used to Detect Down Syndrome

Screening and diagnostic tests for Down syndrome are typically offered during pregnancy and, if needed, after birth.

Prenatal screening tests:

These estimate the risk but do not confirm the condition.

1. First trimester combined screening:

• Blood test (checking hormones like hCG and PAPP-A).
• Nuchal translucency ultrasound (measuring fluid at the back of the baby’s neck).

2. Second trimester quadruple screening:

• Measures four substances in maternal blood: AFP, hCG, estriol, and inhibin A.

3. Non-invasive prenatal testing (NIPT):

• Analyses fetal DNA circulating in maternal blood.
• Highly accurate but still considered a screening test.

Diagnostic tests:

These confirm Down syndrome with almost 100% accuracy.

1. Chorionic villus sampling (CVS):

• Done between 10–13 weeks of pregnancy.
• Takes a sample of placental tissue.

2. Amniocentesis:

• Done usually after 15 weeks.
• Collects a sample of amniotic fluid.

Both carry a small risk of miscarriage, so are typically offered if screening tests show higher risk.

Postnatal diagnosis:

• Physical examination by a doctor.
• Confirmation through a karyotype test, which analyzes chromosomes from a blood sample.

Conclusion

Down syndrome is a genetic condition rooted in biology, not choice or fault. Each individual with Down syndrome is unique, with personal strengths, personalities, and challenges. With advances in medicine, therapies, inclusive education, and community awareness, people with Down syndrome can lead meaningful, healthy, and engaged lives. While there is currently no cure, early intervention programs, regular health monitoring, speech and occupational therapies, and supportive environments dramatically improve outcomes. Society benefits when everyone, regardless of genetics, is valued and included.

Frequently Asked Questions (FAQ’s)

1. What is Down syndrome?
Down syndrome is a genetic disorder caused by an extra copy of chromosome 21, leading to developmental and physical differences.

2. Can Down syndrome be prevented?
No, it occurs randomly in most cases; advanced maternal age slightly increases the risk.

3. How is Down syndrome diagnosed?
Through prenatal tests (like NIPT, CVS, or amniocentesis) or after birth with a chromosomal analysis.

4. Does everyone with Down syndrome have the same symptoms?
No, symptoms and severity vary widely among individuals.

5. Is Down syndrome inherited?
Most cases aren’t, but translocation Down syndrome can sometimes be passed from parent to child.

6. What is the life expectancy of people with Down syndrome?
With medical care, many live into their 50s, 60s, or beyond.

7. Can people with Down syndrome go to school and work?
Yes, many attend school, work, and live semi-independently or with support.

8. Does Down syndrome affect intelligence?
It usually causes mild to moderate intellectual disability, but learning abilities vary greatly.

9. Are there treatments for Down syndrome?
There’s no cure, but therapies, medical care, and education help manage symptoms and support development.

10. Why is it called Down syndrome?
It’s named after Dr. John Langdon Down, who first described the condition in the 19th century.