Prenatal Imaging Findings In Down Syndrome

What Is Down Syndrome?

Humans have 46 chromosomes (genetic material) arranged in 23 pairs. Down syndrome is a congenital genetic condition where children are born with an extra chromosome, where normally only two chromosomes are present. This extra genetic material results in physical and intellectual developmental delays and disabilities. Though these disabilities are lifelong, a Down syndrome kid can continue a healthy and satisfying life with strong social support.

What Are the Types of Down Syndrome?

Down syndrome is of three different types.

1. Trisomy 21: Most common, every cell has three copies of chromosome 21 instead of 2; abnormal cell division happens during the development of the sperm or egg.

2. Mosaicism: Rare form, only a few cells have an extra chromosome, occurs due to abnormal cell division after fertilization of an egg by the sperm.

3. Translocation: These patients have only 46 chromosomes, but a part of chromosome 21 is incorrectly attached to another chromosome.

Why Does Down Syndrome Happen?

The human body is made up of a certain set of chromosomes that contain genetic materials. These genetic materials are characteristic of the traits and functions carried out by every human being. Therefore, any slight change in this structural genetic makeup could significantly alter human characteristics.

Each cell in the body consists of 23 pairs of chromosomes, 46 in total. In Down syndrome, chromosome 21 has three copies instead of two. Hence Down syndrome is also known as Trisomy 21. Thereby, people with Down syndrome have 47 chromosomes in total. This extra genetic material results in altered physical and cognitive abilities.

There are certain risk factors associated with Down syndrome. They are as follows:

• Increased maternal age. Mothers over the age of 35 have a greater probability of having a child with Down syndrome.

• Few studies have shown that even a paternal age over 40 had twice the risk of a child with Down syndrome.

• People with a family history of Down syndrome.

• A Down syndrome mother has a 50 % chance of conceiving a child with Down syndrome.

What Are the Symptoms?

The symptoms of Down syndrome and their severity may vary between individuals. However, there are certain physical, intellectual, and behavioral characteristics which are as mentioned below:

Physical:

• Flat facial features with a flattened nasal bridge.

• The upward slant of the eye.

• Loose folds of the skin at the back of the neck.

• Short neck.

• Small ears.

• Large tongue.

• Poor facial muscle tone.

• Palm showing a single crease in the center.

• Short stature.

Intellectual:

• Speech and language development are delayed.

• The average IQ is 50.

• Delayed development of motor skills.

• Attention-deficit problems.

• Cognitive delays.

• Their learning ability is at a slow pace.

• Tantrums and impulsive nature.

Complications Associated with Down Syndrome:

• Congenital heart diseases such as ventricular septal defect and atrioventricular septal defect.

• Gastrointestinal problems such as Hirschsprung disease and celiac disease.

• Repeated dislocations.

• Loss of vision and hearing.

• Cataracts.

• More prone to respiratory tract infections, urinary tract infections, and skin infections.

• Hypothyroidism.

• Obstructive sleep apnea.

• Low fertility in males.

• Alzheimer’s disease.

• Autism.

How Can Down Syndrome Be Diagnosed?

Pregnant women are usually subjected to screening tests for any genetic or congenital abnormalities. There are screening tests available for Down syndrome also. If the screening tests are positive and women are associated with high-risk factors, doctors may advise doing certain confirmatory diagnostic tests. The screening tests do not confirm Down syndrome; they only indicate a high risk of the baby's Down syndrome. The diagnostic tests are, however, confirmatory. Diagnostic tests have a possibility of certain complications, including miscarriage. Hence diagnostic tests are not advised for all women. These prenatal tests, if positive, help the parents prepare and accept the fact of having a child with Down syndrome or can even help them consider medical termination. The diagnostic workup would be as mentioned below:

Screening Tests:

- Maternal Age

Maternal age has a strong correlation with Down syndrome. Increasing maternal age (over 35 years) is also incorporated in both trimesters while calculating the risk of having Down syndrome.

- First Trimester

1. Blood Test

Abnormal levels of two blood parameters that are pregnancy-associated plasma protein-A (PAPP-A) and the pregnancy hormone known as human chorionic gonadotropin (HCG) can be suggestive of a problem. This is also known as the double marker test, which is done roughly around 11 weeks of pregnancy.

2. Nuchal Translucency Test (NT Screening Test)

This specialized ultrasound screening test is done in the 11 to 13 weeks of gestation. This test measures the thickness of the nuchal folds, which are nothing but a clear space at the back of the neck of the developing fetus. In a fetus with any congenital or genetic abnormalities, this space tends to be accumulated with fluid resulting in an enlarged appearance. A measurement less than 3 millimeters (mm) is considered normal, and above 3 millimeters (mm) suggests any congenital disorder or genetic disorder, including Down syndrome. NT test is a highly specific marker for the risk of any congenital disabilities.

- Second Trimester

1. Triple Test

A triple marker test is usually done at the 15 to 18th week of pregnancy. The serum markers used are Alpha-fetoprotein (AFP), unconjugated estriol, and human chorionic gonadotropin (hCG). To diagnose trisomy 21, the serum levels of AFP and unconjugated estradiol are lower than normal (25 %), and maternal HCG is twice higher than the normal level. However, the sensitivity of the triple marker test is lower with advanced maternal age.

2. Quadruple Test

A quadruple marker test measures alpha-fetoprotein (AFP) serum levels, HCG, Estriol, and Inhibin A. It is done between 15 to 18 weeks of pregnancy. A positive screening indicates high risk and the need for a confirmatory diagnostic test.

3. Ultrasound

Ultrasound is considered a vital tool in diagnosing any congenital or chromosomal defects. A second-trimester ultrasound is 60-91% positive in diagnosing Down syndrome. It is usually done in the 18 to 22 weeks of pregnancy. A Doppler done along with an ultrasound can better diagnose any associated cardiac anomalies. The below findings in ultrasonography may be associated with Down syndrome. These findings are also known as “soft markers.”

• Increased nuchal translucency.

• Absence or short nasal bone (characteristic of Down syndrome).

• Restricted growth.

• Cardiac anomalies such as septal defects, ventricular anomalies, and atrioventricular anomalies.

• Short femur and humerus.

• Duodenal atresia (the first part of the small intestine is poorly developed ).

• The echogenic bowel (ultrasound appearance of the fetal intestine) appears relatively brighter than normal.

• Cardiac echogenic chordae tendineae (bright spots on the fibrous tissues attaching the heart valves to the heart muscles.

• Choroid plexus cysts (fluid accumulation in a part of the brain).

• Large iliac angle and short iliac length.

• Renal pyelectasis (increased fluid in kidneys).

• Umbilical cord with two vessels.

4. Prenatal Cell-Free DNA Screening (cfDNA)

This test uses DNA extracts from both mother and fetus to evaluate chromosomal abnormalities. It is indicated only in high-risk cases. It is not a diagnostic test but also expensive; hence it is not routinely done.

Diagnostic Tests

Diagnostic tests are confirmatory. They are usually suggested to only patients with high risk and those with positive screening tests. These tests also have a chance of miscarriage and other associated complications.

1. Amniocentesis:

This test is usually conducted in the 15th week. First, a sample of amniotic fluid is extracted by inserting a long thin needle through the abdomen into the uterus. This fluid detects a chromosomal abnormality in the fetus as the fluid will contain sloughed skin cells and other extracts containing the chromosomes. The results will take approximately two weeks.

2. Chorionic Villus Sampling (CVS):

This test is done in the 9th to 14th week of pregnancy, when cells from the placenta will be analyzed for fetal chromosomes. The procedure is similar to amniocentesis, where it is done by inserting a needle, or it can be done through the cervix, similar to a Pap smear. The results are obtained in a couple of weeks.

3. Cordocentesis:

Cordocentesis is also known as percutaneous umbilical blood sampling (PUBS). It is done after the 18th week of gestation when blood from the umbilical cord is tested for any chromosomal defect. Unfortunately, it has the highest risk for miscarriage; hence it will be performed only when other tests are uncertain, and parents feel the need to diagnose.

4. FISH Technique:

Fluorescent in situ hybridization (FISH testing) is specialized diagnostic testing done on the tissue samples obtained via amniocentesis or CVS. The advantage of this procedure is that no cell culturing is required; hence reports are available rather quickly. However, the disadvantage is that it does not offer any information on the chromosomal structure, thus not distinguishing the types.

What Are the Treatments Available?

There is no cure as such for Down syndrome. However, the child will need adequate support to build their intellectual, cognitive, and developmental traits. Any congenital anomalies, such as cardiac, gastrointestinal, or neurological, may require a surgical approach. In addition, an interdisciplinary treatment approach may be required in some cases. A speech therapist, occupational therapist, physiotherapist, and behavioral therapist may all be needed to work concurrently for the betterment of the child.

Conclusion:

Down syndrome is a genetic disorder resulting in certain physical and intellectual abnormalities. It is a condition and not a disease. Though Down syndrome cannot be prevented, parents can consider the risk factors. Appropriate prenatal diagnosis can help the family cope with the possibility of having a child with Down syndrome or even consider medical termination. Proper support, health care, and education can help a child with Down syndrome lead a happy and satisfying life like any normal kid.