Overview
A healthy individual is born with 46 chromosomes, present in 23 pairs. The 22 pairs are common in all, known as autosomes and the 23rd pair is the sex chromosome (XX or XY). Chromosomes carry genes that pass certain characteristics from the parents to their children. Abnormalities occur when there is an extra chromosome.
What is Down Syndrome?
People with Down syndrome (also referred to as Trisomy 21) have an extra chromosome, three copies of chromosome 21 instead of two. A child born with Down syndrome has characteristic features such as mongoloid face, weakness of muscles, small low set ears, short and broad hands and protruding tongue. Heart defects and abnormal thyroid levels are common and present at birth. Incidence of Down syndrome accounts for 1 of every 800 live births.
Types
There are three genetic mechanisms for Down syndrome:
- The first and most common is non-disjunction, faulty cell division where there is an entire extra chromosome 21 in all the cells. This is responsible for 95% of all cases of Down syndrome.
- The second is mosaic Down syndrome, where the cells having an extra copy of chromosome 21(47, XX, +21/46, XX) are mixed with normal cells. An individual with this form of Down syndrome is frequently milder in their presentation, depending on the proportion of the normal cells.
- Translocation is the third mechanism which is known to occur in only 3%-4% of people with Down syndrome. Translocation occurs when part of the chromosome 21 breaks off during cell division and attaches to another chromosome. While the total number of chromosome in the cell remains 46, the presence of an extra part of chromosome 21 causes the feature of Down syndrome. This can be due to faulty cell division or one of the parents is a carrier of a balanced translocation.
Combine First Trimester Screening
Every woman is at the risk of having a baby with Down syndrome. For instance, Down syndrome occurs 1 in every 1200 births to the women aged 25 years. However, the rate increases to 1 in 350 for women aged 35 years. Hence first-trimester screening is recommended at the best hospital for pregnancy in Bangalore to all pregnant women. The screening test is known to identify 4 out of 5 Down syndrome pregnancies. The first stage of screening comprises a biochemical screening and Nuchal Translucency (NT) scan.
The biochemical screening measures the levels of two hormones β-HCG and PAPP-A released during pregnancy. Ultrasound scan measures the fluid behind the baby’s neck called NT. An overall detection rate of 92-94% for trisomy 21 is achieved by using first‐trimester screening. Depending on the results of these tests, some women (about 1 in 10) falling in the high-risk group are offered the diagnostic test.
Non-invasive Prenatal Screening (Nips)
NIPS is a new screening test that has an advanced detection rate of common chromosomal aneuploidies. The screening estimates the chance of whether an unborn baby has Down syndrome. These test analyses small fragments of cell-free DNA (cf-DNA) that are circulating in pregnant women’s blood. This testing is more accurate than conventional screening methods, detecting more than 99% of pregnancies where the baby has Down syndrome.
Prenatal Diagnosis
Diagnostic tests are invasive and carried out to confirm if the baby will have Down syndrome. Pre-natal samples are taken as chorionic villi or amniotic fluid (under ultrasound guidance) and examined for any chromosomal abnormalities. There is 1 in 300 risks of miscarriage after the invasive procedure. Consult with the genetics doctor in Bangalore for the diagnosis.
Recurrence Risk
In general, it is estimated that there is 1 in 100 chance of recurrence of trisomy 21 in the next pregnancy. However, the risk is higher if one of the parents is a carrier of a balanced translocation.
Management
It is very important to understand that there is no way to remove the extra chromosome 21 that a child is born with. However, there are numerous management strategies that can minimize its associated complications. When a child is detected with Down syndrome at an early age, it is usually seen that they lead a better and more independent life. A number of neurodevelopmental therapies are available for Down syndrome kids in their early ages and throughout life to help and promote the greatest possible development and independence.
The Down syndrome children are good at visual learning techniques and sign-language & pictures can be used as different forms of communication. Most Down syndrome kids have an inborn talent for creativity. Parents are advised to identify these unique qualities in their child and motivate them to make a successful career out of it.
Head of Department & Consultant, Medical Genetics
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