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At present there is no definitive cure for Down syndrome. However, an early detection and management of the affected individual by a multidisciplinary team can help reduce symptoms and improve the quality of life. Though the life expectancy of patients with Down syndrome has improved significantly due to advances in the medical field and improvised surgical techniques for correction of congenital disabilities, prevention is always a better approach than cure, says Dr Anurag Gupta, Consultant Molecular Pathologist and Head Cytogenetics, Ampath, Hyderabad.
In an exclusive interview with TheHealthSite, Dr Gupta, talked about some tests that can reduce the risk of having a child with Down syndrome and help going to be parents make informed decisions.
The expert explains: Down syndrome is a genetic disorder caused by the presence of an extra copy chromosome 21. It is the most common chromosomal abnormality affecting human beings with an incidence of about 1 in every 700 to 800 births. Clinical features of Down Syndrome were first described by John Langdon Down way back in 1866 however, gain of chromosome 21 as a cause was identified in 1959 by Dr. Jerome Lejeune. Chances of Down Syndrome increases with increasing maternal age. These patients present with varying degree of growth retardation, characteristic facial features, mild to moderate intellectual disability and abnormalities affecting heart, gastrointestinal tract and other organ systems including high risk of developing leukemia.
If you're pregnant or planning to conceive, Dr Gupta suggests a few tests that can help identify if your child is at risk of Down syndrome.
Screening tests are non-invasive tests that can help identify if a foetus is at risk of Down syndrome. These tests do not provide a definite diagnosis but can indicate if further diagnostic testing is needed. All abnormal screen tests should be confirmed by diagnostic tests before a making any actionable clinical decision. The most common screening tests for Down syndrome are:
First-Trimester Combined Screening: This test is also popularly known as "Double marker test" as it involves a blood test and an ultrasound. This is performed between 11-14 weeks of pregnancy. The blood test measures the levels of two hormones namely pregnancy-associated plasma protein A (PAPP-A) and free beta-human chorionic gonadotrophin (beta-hCG). An ultrasound is done to assess the thickness of the fluid-filled space at the back of the baby's neck, "the nuchal translucency". The thickness of the nuchal translucency on ultrasound scan and hormone levels along with the maternal age are taken into consideration to calculate a risk of Down syndrome. The report of this test describes the risk as "low", "intermediate" and "high".
Second-Trimester Quad Screen aka Quadruple marker test: This test is performed in the second trimester between 15-20 weeks of pregnancy and indicates risk of Down syndrome in the developing foetus. The test measures the levels of four hormones: alpha-fetoprotein (AFP) which originates from the developing foetus, -human chorionic gonadotropin (hCG) and inhibin-A of placental origin and unconjugated estriol from both the placental and foetal liver origin. The results of these blood tests are combined with maternal age and weight, ethnic background and gestational age of the foetus, to arrive at the risk of having a foetus with Down syndrome. When Inhibin A is not done, the test is called as a triple screen or triple marker test.
Non-Invasive Prenatal Testing (NIPT): Non-Invasive Prenatal Testing (NIPT) is based on analysis of cell free foetal DNA that is present in the maternal blood. It is the most sensitive and specific screening test for the common foetal aneuploidies including Down syndrome and can detect presence of other chromosomal abnormalities. It can be performed as early as 10 weeks of gestation. NIPT is usually recommended for women who are at high risk of having a baby with Down syndrome, such as women who are over 35 years old or who have had a previous child with Down syndrome. However, as per the latest recommendations of American college of Obstetrics and Gynaecology, ACOG, prenatal testing options should be discussed and offered to all pregnant patients regardless of maternal age or risk of chromosomal abnormality. Despite being very accurate, it has the potential for false-positive and false-negative results, therefore all abnormal results should be confirmed with one of the invasive but diagnostic tests.
Diagnostic tests provide a definite diagnosis of Down syndrome and are recommended if the screening tests indicate an increased risk of Down syndrome. These tests are invasive and are associated with relatively increased risk to the foetus as they involve obtaining tissues of foetal origin for genetic testing to look for extra copy of chromosome 21 which is diagnostic of Down Syndrome. Identification of extra copy of chromosome 21 can be done either by conventional karyotyping, a procedure that allows visualization of all the chromosomes of a cell simultaneously thereby enumerating them and characterising copy number for chromosome 21 or by Fluorescence in situ hybridization (FISH) that uses small sequences of DNA tagged with a fluorescent dye specifically targeting chromosome 21 or using Polymerase chain reaction (PCR) for chromosome 21. Though FISH/ PCR are relatively rapid compared to conventional karyotyping, analysis of the foetal genome using both karyotyping and FISH or PCR is recommended.
Diagnostic tests for Down syndrome
According to Dr Gupta, the two most common diagnostic tests for Down syndrome are:
Amniocentesis: Amniocentesis is usually performed between 15-20 weeks of pregnancy. This test involves the ultrasound guided insertion of a thin long needle through the abdomen and into the uterus to collect a sample of the amniotic fluid, a procedure called amniocentesis. The amniotic fluid is a fluid that is present all around the foetus in the amniotic sac. Besides serving as a reservoir of fluid and a source of nutrients, proteins, electrolytes, immunoglobulins, and vitamins for the developing foetus it also contains cells of foetal origin called amniocytes. These amniocytes are collected during amniocentesis and cultured to obtain and analyse baby's chromosomes. In experienced hands the risk of foetal loss after amniocentesis is around 0.11 per cent.
Chorionic Villus Sampling (CVS): CVS can be performed as early as 10-13 weeks of gestation. This test is also performed under ultrasound guidance. It involves the insertion of a needle through the abdomen or cervix to collect a sample of the chorionic villi (tiny finger-like projections on the placenta). The chorionic villi contain cells from the baby that are cultured in sterile media and are used to analyze the baby's chromosomes. Just like for amniotic fluid, the CVS sample is subjected to conventional karyotyping and FISH/ PCR to confirm or refute the diagnosis of Down syndrome. Being invasive, studies indicate a total foetal loss of 0.7 percent within 14 days using a transabdominal approach, 1.3 percent within 30 days, and 2 percent for loss anytime during pregnancy.
Dr Gupta says, "Screening tests carry absolutely no risk to foetal loss and can help identify risk of Down syndrome in an ongoing pregnancy, while diagnostic tests can provide a definite diagnosis. Non-Invasive Prenatal Testing (NIPT) is a relatively new test that is very safe with high sensitivity for risk of Down syndrome as well as other aneuploidies from a simple blood test."
All female during their ante-natal check-up should be offered screening tests for aneuploidy screening and it is important for couples to discuss the risks and benefits of these tests with their healthcare providers, he concludes.
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