Down Syndrome Diagnosis Methods

Down syndrome, also known as a chromosomal disorder, is one of the most common causes of intellectual disability. This disorder affects an individual's development throughout their life and can affect babies of mothers of all ages; however, the likelihood is higher in babies of older mothers. The most common cause of the syndrome is the presence of three copies of the 21st chromosome instead of the usual two. Therefore, it is also referred to as trisomy 21. The diagnosis of Down syndrome can be made during pregnancy or after birth. This process is sensitive and requires information for parents. Therefore, understanding the available testing methods is crucial for making informed decisions. The diagnostic methods are divided into two main groups: screening tests and diagnostic tests. Screening tests only indicate risk or probability, while definitive diagnoses are made with diagnostic tests.

Down Syndrome Screening Tests: Determining Probability

Screening tests are non-invasive tests conducted to determine the likelihood of the baby having Down syndrome. While these tests indicate the presence of a risk, they do not provide a definitive diagnosis. A positive screening test usually necessitates further diagnostic testing.

• Tests Conducted in the First Trimester: (First Trimester Screening Test): This test is performed between the 11th and 14th weeks of pregnancy. It measures the levels of two proteins (free β-hCG and PAPP-A) in the mother's blood. Additionally, the fetus's nuchal translucency (NT) is determined via ultrasound. By combining these three data points, a risk percentage for Down syndrome and other chromosomal abnormalities is calculated. A high-risk result provides a justification for referring the mother to definitive diagnostic tests.

• Tests Conducted in the Second Trimester: (Triple and Quadruple Screening Tests): These tests are performed between the 16th and 18th weeks of pregnancy.

• Triple Screening Test: This test calculates the likelihood of Down syndrome in the baby based on the levels of three different proteins (AFP, hCG, and uE3) in the mother's blood.

• Quadruple Screening Test: This test expands on the triple test by including a fourth protein called inhibin A from the mother's blood. The accuracy rate of the quadruple test is slightly higher than that of the triple test.

• Cell-Free DNA Tests (NIPT): This test, which has gained popularity in recent years, determines the likelihood of Down syndrome by examining free DNA fragments from the baby in the mother's blood. The accuracy rate of this test is much higher than that of other screening tests (over 99%). Its non-invasive nature, meaning it poses no risk to the mother or baby, is its greatest advantage.

Diagnostic Tests for Definitive Diagnosis: Risky but Necessary Procedures

Diagnostic tests provide definitive results, unlike non-invasive screening tests. These tests carry a small risk of miscarriage as they directly examine fetal cells. Therefore, they are generally recommended for mothers who receive high-risk results from screening tests or have risk factors such as advanced maternal age.

• Chorionic Villus Sampling (CVS): This procedure is performed between the 9th and 14th weeks of pregnancy. A thin needle is inserted through the abdomen or cervix to obtain a sample of the placental tissue (chorionic villus) that nourishes the baby. Since the genetic structure of this tissue is the same as that of the baby, it provides definitive results.

• Amniocentesis: This procedure involves taking a sample of the amniotic fluid in which the baby floats. The cells shed in this fluid are examined to determine the chromosomal structure. It may seem frightening as it can lead to miscarriage; however, when performed by an experienced doctor, this risk is quite low (usually less than 0.5%). It is generally performed between the 15th and 20th weeks of pregnancy, but it can also be done at different times depending on the purpose. Performing it before the 15th week of pregnancy increases the risk of miscarriage.

• Cordocentesis: This diagnostic test is performed between the 18th and 22nd weeks of pregnancy. It involves taking blood from the umbilical cord of the baby. This test is usually used when the results of other tests are uncertain or for the detection and treatment of blood disorders such as fetal anemia.

Postnatal Diagnosis of Down Syndrome: Clinical Observation and Definitive Analysis

The diagnosis of Down syndrome can also be made after birth. A baby with Down syndrome can be easily recognized due to its typical physical features. These physical characteristics help in making a preliminary diagnosis; however, a genetic test is essential for a definitive diagnosis.

• Clinical Observation: In the first hours after birth, a doctor may suspect Down syndrome by observing the baby's physical characteristics. These typical features include:

• The head is smaller than normal and the back is flat.

• The face is flat, especially the bridge of the nose is depressed.

• The eyes are slanted upwards and widely spaced.

• The ears are smaller than normal and have less developed lobes.

• The mouth is small, which may cause the tongue to protrude.

• Excess skin at the back of the neck and a short neck structure may be observed.

• The hands are short and wide. There may be a simian line, which is a single line that runs across the palm, found in 70-80% of babies with Down syndrome and is rare in the normal population.

• Due to weak muscle and joint structure, their bodies are generally more relaxed (hypotonia).

• Definitive Diagnosis (Karyotype Analysis): Clinical observation alone is not sufficient. For a definitive diagnosis, a blood sample taken from the baby is analyzed through karyotype analysis. In this analysis, the baby's chromosomes are examined under a microscope, and the presence of three copies of the 21st chromosome is confirmed. This test finalizes the diagnosis of Down syndrome and allows the family to plan their future steps.