What is Down syndrome?

Down syndrome is a genetic disorder that is caused by abnormal cell division.
Down syndrome is a genetic disorder that is caused by abnormal cell division.

Down syndrome is a genetic disorder caused by abnormal cell division resulting in chromosomal abnormality. Abnormalities include an extra full or partial copy of chromosome 21. This genetic abnormality leads to developmental and physical changes that are characteristic of Down syndrome.

The severity of Down syndrome varies. Some cases may be mild, whereas others have severe mental disability and developmental retardation. They may also have systemic medical abnormalities, including heart and gastrointestinal (bowel) disorders. The lifespan of patients with Down syndrome has increased to more than 60 years, depending on the severity of health problems because of advancement in medical treatment options. Treatment requires a multidisciplinary approach involving doctors from several medical specialties, healthcare providers and special education teachers. Lifelong treatment is required to manage their medical and intellectual problems.

What causes Down syndrome?

Normal human cells contain 23 pairs of chromosomes. In each pair, one chromosome comes from the mother and the other from the father. Sometimes, there is abnormal cell division involving the chromosome that results in Down syndrome. This abnormal cell division results in an extra partial or full chromosome 21, which is responsible for all the features associated with Down syndrome. Down syndrome is usually not inherited. Abnormal cell division in the early stages of fetal development causes Down syndrome.

There are no known external or behavioral factors that cause Down syndrome.

There are three possible genetic variations that can cause Down syndrome

  • Trisomy 21Trisomy 21 is the most common and is seen in 95% of the cases. The person is born with three copies of chromosome 21. This abnormality occurs during cell division during the development of the sperm or egg.
  • Mosaic Down syndrome: This is rare. The person is born with only some abnormal cells with trisomy 21, whereas the rest are normal. Hence, the condition is called “mosaic” due to the mosaic pattern of normal and abnormal cells. This usually occurs due abnormal cell division after fertilization of the sperm and egg.
  • Translocation Down syndrome: This form of Down syndrome occurs when a portion of chromosome 21 gets attached to another chromosome (translocation). This can occur before or during conception. Children with this condition are born with the usual two copies of chromosome 21 but have additional genetic material from chromosome 21 attached to another chromosome.

Risk factors: Certain factors increase the risk of having a baby with Down syndrome. High-risk parents are usually advised to get genetic counselling

  • Advancing maternal age: A woman's risk of having a child with Down syndrome increases after 35 years of age because older eggs have a high risk of abnormal cells and chromosomal division. However, most children with Down syndrome are usually born to women under 35 years of age because more births are observed among younger women.
  • Being carriers of genetic translocation for Down syndrome: The child can inherit genetic translocation for Down syndrome from either parent.
  • Having one child with Down syndrome: Having one child with Down syndrome can increase the risk of having another child with Down syndrome.

What are the signs and symptoms of Down syndrome?

People with Down syndrome have characteristic facial features including 

  • Small head
  • Flattened face
  • Short neck
  • Small ears
  • Upward slanting eyelids
  • Protruding tongue
  • Broad, short hands with a single crease in the palm
  • Short fingers 
  • Small hands and feet
  • Short stature
  • Poor muscle tone
  • Excessive joint flexibility
  • Brushfield's spots (small white spots on the iris, the colored portion of the eyes)

Intellectual disabilities

Complications of Down syndrome

How is Down syndrome diagnosed?

Screening tests and diagnostic tests are usually recommended for all pregnant women, irrespective of age.

Screening tests during pregnancy: They indicate the risk of carrying a baby with Down syndrome. 

First trimester tests

  • Blood tests: Abnormal levels of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG) levels in the blood indicate a problem with the baby.
  • Nuchal translucency test: An ultrasound is done to measure a particular area of the back of the baby's neck. In the presence of abnormalities, there is increased fluid collection in the neck.

Second trimester tests: Quad screen is the measurement of four pregnancy-associated substances, namely HCG, alpha-fetoprotein, inhibin A and estriol in the blood.

Diagnostic tests during pregnancy: They can confirm the diagnosis of Down syndrome.

  • Chorionic villus sampling (CVS): Cells are taken from the placenta and analyzed between 10 and 13 weeks of pregnancy.
  • Amniocentesis: Amniotic fluid surrounds the fetus in the uterus. The fluid is aspirated from the mother's uterus using a needle. The cells in the amniotic fluid are analyzed for chromosomal abnormalities. This is usually performed after 15 weeks of pregnancy. 

Diagnosis in newborns: A preliminary diagnosis is based on the baby's appearance. The diagnosis can be confirmed by analyzing the chromosomes in the cells from a blood sample.

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Medically Reviewed on 9/29/2020
References
Medscape Medical Reference

CDC


National Down Syndrome Society


American Pregnancy Association


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