Diagnosis of Down Syndrome
In most cases, Down syndrome is diagnosed at birth. Diagnosis involves a physical examination and diagnostic tests (e.g., blood tests). Signs of Down syndrome that are related to the baby's appearance often are apparent at birth. If a newborn exhibits signs of the condition (e.g., flat face, eyes that slant upward unrelated to ethnicity, ears that fold at the top), additional testing is performed to confirm the diagnosis.
Diagnostic tests for Down syndrome include a test called the chromosome karyotype test, which uses cells from a blood sample. By photographing the newborn's blood cells, physicians can group and organize the chromosomes to accurately determine whether an extra chromosome 21 is present.
Prenatal Tests for Down Syndrome
A number of prenatal tests are available to help pregnant women learn more about their risk for having a baby with Down syndrome. During pregnancy, screening tests provide a general idea of the risk and diagnostic tests can determine with 98–99 percent accuracy whether the unborn baby has the condition. Because they carry a small risk for miscarriage, diagnostic tests are conducted only when preliminary screening tests show cause for concern.
The maternal serum screening test is a blood test that generally is offered to women, usually between the 15th and 20th week of pregnancy. In this test, physicians look for abnormal levels of pregnancy-associated plasma protein (PAPP-A) and the hormones estriol and human chorionic gonadotropin (HCG). If any of these substances are higher than normal, there may be an increased risk that the unborn baby has Down syndrome. However, this test is only 60 percent accurate.
The maternal serum screening test may be performed with an ultrasound (sonogram) screening, which uses high frequency sound waves to create images that allow physicians to look for "markers" related to Down syndrome. Many doctors recommend both screening tests as a two-step process, taking the mother's age into account.
One type of ultrasound screening, the nuchal translucency screening test analyzes the area behind the unborn baby's neck. Babies with Down syndrome or with other chromosomal disorders tend to have more fluid in this area, making the space appear larger on ultrasound. Overall, this test is about 80 percent accurate.
A quad marker screen test is also available. This maternal blood test can be used to check for a number of birth defects, genetic conditions, and chromosomal disorders, including Down syndrome and spina bifida.
Three types of prenatal diagnostic tests are available. These tests are more conclusive than prenatal screening tests.
Amniocentesis is performed between 15 and 22 weeks of pregnancy. In this test, physicians use a needle to take a sample of the amniotic fluid around the fetus. The sample is then checked for chromosomal abnormalities. The risk for miscarriage caused by amniocentesis is 1 in 200.
Chorionic villus sampling (CVS) can be performed between 9 and 14 weeks. In this procedure, a sample of the placenta is removed and used to analyze the fetus’s chromosomes. The risk for miscarriage from CVS is 1 in 100 (about 1 percent).
Percutaneous umbilical blood sampling (PUBS) is conducted after 18 weeks of pregnancy. In this test, doctors remove a blood sample from a vein in the umbilical cord and use the sample to analyze the unborn baby's chromosomes. This test carries a higher risk for miscarriage (about 2%) than amniocentesis or CVS, so it is only performed when it is thought to be absolutely necessary.
Differential Diagnosis of Down Syndrome
Before confirming a diagnosis of Down syndrome, the following conditions are ruled out:
- Trisomy 18. This condition, which also is called Edward syndrome, is caused by an extra copy of chromosome 18. Babies with Trisomy 18 usually have severe mental retardation and heart problems. They also may have problems of the stomach, digestive tract, reproductive organs, and urological tract. Trisomy 18 often is fatal in infancy or early childhood.
- Multiple X chromosomes. Males normally have one X chromosome and one Y chromosome. (Females have two X chromosomes, but this condition does not occur in females.) In some cases, male infants are born with one or more additional X chromosomes in each cell, increasing the number of chromosomes beyond the normal total of 46. Multiple X chromosomes can cause decreased IQ, a distinct facial appearance and speech, skeletal, and coordination problems in males.
- Zellweger syndrome and other peroxisomal problems. Peroxisomes are the part of a cell's structure that helps the body get rid of toxins. Some children with peroxisomal disorders do not have enough of these elements and some do not have any at all. In the case of Zellweger syndrome, peroxisomes are either deficient or missing in the liver, kidneys, and brain. Children with this condition have vision problems, an enlarged liver, and too much iron and copper in their blood. There is no cure for Zellweger syndrome and the condition usually is fatal within 6 months of birth.