An Introduction to Down Syndrome – The Most Common Chromosomal Disorder in the World

chromosomal disorders
What is Down Syndrome, and What Causes It?

Down syndrome is a chromosomal abnormality that leads to a wide range of developmental delays and physical disabilities in a person. It is one of the most common chromosomal disorders, with an estimated incidence of 1 in 1000 live births worldwide. Every year in India, between 23,000 and 29,000 children are born with Down syndrome, which is the highest in the world.

In most cases, Down Syndrome is caused by an error in cell division, that results in an extra copy of chromosome 21. Having this extra copy changes the brain’s and body’s normal development. To date, there is no evidence of parents’ behaviour or environmental factors increasing or decreasing the chances of having a child with down syndrome.

Types of Down Syndrome

There are three types of chromosomal abnormalities seen in Down syndrome – Trisomy 21, Translocation and Mosaicism.

Trisomy 21, the most common type of Down syndrome, occurs when there are three number 21 chromosomes, instead of the usual two. As the embryo develops, the extra chromosome is replicated in every cell of the body. This additional genetic material alters the course of development and causes the characteristics associated with the syndrome. Trisomy 21 accounts for 95% of cases.

Translocation occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome in a cell. The total chromosomes in the cells remain 46, but the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome. Translocation accounts for 2-4% of all cases of Down syndrome.

Mosaicism is the least common type of Down syndrome, accounting for only about 1% of all cases. In these cases, some but not all cells have an extra copy of chromosome 21. The symptoms of mosaicism Down syndrome tend to be milder and less wide ranging compared with the other types.

What are the Risk Factors for Down Syndrome?

Maternal age is the most significant factor in increasing the risk for having a child with Down syndrome. A woman who is 30-34 years of age has a risk of 1 in 800, and the likelihood continues to increase as she ages. However, due to higher birth rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age.

Parents who already have a child with Down syndrome, or who are carriers of the genetic translocation of Down syndrome are also at a higher risk of having a child with Down syndrome. Both men and women can pass the abnormal chromosomes for Down syndrome on to their children.

What are the Health Conditions Associated with Down Syndrome?

Everyone born with Down’s syndrome will have a degree of learning disability, but the level of disability will be different for each individual. They have delayed language development and slow motor development. They also have varying degrees of mental and social development delays, which means that the child could have impulsive behaviour, poor judgment, short attention spans and slow learning capabilities.

An infant with Down syndrome can be born at normal size but will develop more slowly than a child without the condition. At birth, babies with Down syndrome usually have certain characteristic signs, including flat facial features, poor muscle tone, eyes that slant upwards and a short neck.

Medical complications often accompany Down syndrome. The may include congenital heart defects, hearing loss, poor vision, hypothyroidism, sleep apnea and obesity. They may also struggle with respiratory infections, urinary tract infections, and skin infections.

Prognosis & Treatment

There is no cure for Down syndrome, but there is a wide variety of support and educational programs that can help both, people with the condition and their families. People with Down syndrome can achieve optimal quality of life through parental care and support, behavioural training, speech therapy, physical therapy and community based support systems like special schools.

How Is Down Syndrome Diagnosed?

Non-invasive prenatal screening is currently the safest option to identify the risk of having a child with Down syndrome. A simple blood test can help screen for the genetic condition from as early as the 9th week of pregnancy. MedGenome Claria provides a safe, non-invasive and highly accurate screening test for expectant parents.

A diagnostic test such as Amniocentesis can be taken to conclusively identify if a baby has Down syndrome. However, unlike NIPT, diagnostic tests such as Amniocentesis carry various risks.

A genetic counselling session can give you clarity and reassurance at a time when you need it the most. Speak to one of our genetic experts to get answers to all your questions, absolutely free of cost.

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