Down syndrome is a condition in which extra genetic material causes delays in the way a child develops, and often leads to learning difficulties. It affects 1 in every 800 babies born.
Down syndrome is a condition in which extra genetic material causes delays in the way a child develops, and often leads to learning difficulties. It affects 1 in every 800 babies born.
Some people have an increased chance of having a baby with Down syndrome, and they may consider whether to make use of the prenatal screening for Down syndrome which is available. There are several different prenatal screening procedures including a relatively simple, noninvasive screening test that examines a drop of the mother's blood to determine if there is an increased likelihood for Down syndrome. There is no cure for Down syndrome, nor is there any prevention for the chromosomal events that cause Down syndrome, but the additional medical needs of people with Down syndrome can be managed.
Incidence of Down syndrome
The incidence of Down syndrome is estimated at 1 per 800 births, about 95
percent of which are trisomy 21, making it the most common human aneuploidy — genetic disorder with the presence of an extra copy of genetic material on some chromosome. According to the National Down Syndrome Society, there are approximately 350,000 individuals with Down syndrome in US.
Maternal age influences the chance of conceiving a baby with the syndrome. At age 20 to 24, it is 1/1490, while at age 40 it is 1/106, and at age 49 is 1/11. Although the chance increases with maternal age, most children with Down syndrome (80 percent) are born to women under the age of 35. That represents a big diagnostic problem.
Most Down syndrome cases occur spontaneously. There is no known prevention, although some factors, such as increased maternal age, can increase the chance of occurrence. The genetic basis itself cannot be treated, and the variety of expression requires targeting treatment to each individual
What exactly is a Down syndrome?
Down syndrome is a disorder that includes a combination of birth defects. The most common of these are:
- Learning difficulties and a low IQ.
- Characteristic facial features. A child with Down syndrome will usually have eyes that slant upward and small ears that may fold over a little at the top. The child's mouth may be small, making the tongue appear large and nose may also be small, with a flattened nasal bridge. Some babies with Down syndrome have short necks and small hands with short fingers and, due to less muscle tone.
- Heart defects. Almost half of babies with Down syndrome have heart defects, of which some are minor and may be treated with medications, while others may require surgery. That is why all babies with Down syndrome should be examined by a pediatric cardiologist, a doctor who specializes in heart diseases of children, and have an echocardiogram in the first 2 months of life so that any heart defects can be treated.
- Increased infections. Children with Down syndrome tend to have many colds and ear infections, as well as bronchitis and pneumonia. They also are at increased risk of thyroid problems and leukemia
- Intestinal malformations - About 10 percent of babies with Down syndrome are born with that some intestinal malformations that require surgery soon after birth.
- Problems with vision and hearing. Common visual problems associated with Down syndrome include crossed eyes (amblyopia), near- or far-sightedness and cataracts. Most can be improved with glasses, surgery or other treatments.
- Other health problems…
Genetic abnormalities
Normally, each egg and sperm cell contains 23 chromosomes. The union of these creates 23 pairs, or 46 chromosomes in total. Sometimes, although the reasons are still unknown, an accident occurs when an egg or sperm cell is forming, causing it to have an extra chromosome number 21. This is Down syndrome! So, the bottom line is that Down syndrome is nothing more than one chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part (such as due to translocations).
Causes of Down syndrome
Several research studies done in the past have shown that Down syndrome has four root causes:
Non-disjunction event
Trisomy 21 is caused by a meiotic non-disjunction event. In this case the child has three copies of every gene on chromosome 21. This is the most common cause because about 96% of Down’s syndrome cases, with 88 percent coming from the maternal gamete and 8 percent coming from the paternal gamete, are caused by this non-disjunction!Translocation
The extra material is due to a change called translocation. The long part of 21 is attached to another chromosome. The parent with the translocation is missing information on the short arm of 21, but this does not have apparent effects. Through normal disjunction during meiosis, gametes are produced with extra copies of the long arm of chromosome 21. This isn't so common cause of Down syndrome because it is the cause of two to three percent of the observed cases.Normal arrangement and trisomy - combined
The individual is a mosaic of normal chromosomal arrangements and trisomy 21. This can occur in one of two ways:- A non-disjunction event during an early cell division leads to a fraction of the cells with trisomy 21; or
- A Down syndrome embryo undergoes non-disjunction and some of the cells in the embryo revert back to the normal chromosomal arrangement.
This is the cause of one to two of the observed Down syndrome cases.
Duplication
Rarely, a region of the 21st chromosome will undergo a duplication event. This will lead to extra copies of some, but not all, of the genes on chromosome 21.Prenatal screening and diagnosis
A new study supports screening for Down syndrome in earlier weeks of pregnancy than what has traditionally been recommended. These are very good, easy to do and effective prenatal diagnostic tools that can help in detecting abnormalities characteristic to Down syndrome!
Maternal serum screening
The proven fact is that this maternal serum screening allows the detection of trisomy 21 pregnancies in women in younger than 35 years who give birth to about 70 percent of infants with Down syndrome.The serum markers most widely used are:
- Alfa-fetoprotein
- Unconjugated estriol
- Human chorionic Gonadotropin
That’s why - this combination is known as "triple test"! The test is usually performed at 15 to 18 weeks of pregnancy. The chances of trisomy 21 is calculated on the basis of each of the serum marker results and the patient’s age.
Results - The triple test can detect 60 percent of trisomy 21 pregnancies. Young mothers also must understand that a normal result reduces the likelihood of trisomy 21 but does not exclude it.
There are several factors that have influence on test. The factors are:
- Gestational age
- Maternal weight
- Maternal race
- Maternal insulin-dependant diabetes
- Multiple fetal pregnancy
- Family history of Down syndrome
Test results also can help a woman manage her pregnancy more effectively.
Ultrasound assessment
It is proven that ultrasound test improves the performance of the triple test. This test uses sound waves to take a picture of the fetus. Ultrasound can help determine the gestational age of the fetus and show if a woman is carrying twins.
A Simple Ultrasonographic test can detect many abnormalities associated with fetal Down syndrome. Some of the common findings are:
- Increased nuchal fold thickness >5 mm
- Intrauterine growth restriction
- Flat facies
- Small ears
- Choroids plexus cysts
- Congenital heart defects
- Renal pelvis dilation
- Shortened humerus and femur
- Increased iliac wing angle
- Incurving of the fifth finger
- Increased space between first and second toes
- Two-vessel umbilical cord
If the ultrasound does not provide an explanation for an abnormal test result, additional diagnostic testing is recommended.
Chorionic villus sampling
Women who receive abnormal results on other screening test may be offered a prenatal test called chorionic villus sampling, which is done between 10 and 12 weeks of pregnancy. Chorionic villus sampling is one very good and effective prenatal test used to identify various genetic problems, including Down syndrome. Using a thin tube guided through the mother’s cervix or a needle inserted into your uterus, doctor takes a sample of chorionic villi from the placenta.
What's so good about this diagnostic tool? Chorionic villus sampling can provide genetic information about your baby earlier in your pregnancy than can other diagnostic tests, such as amniocentesis. The test might be offered if your baby has an increased risk of a specific chromosomal or genetic disorder. On the other hand - chorionic villus sampling cannot detect neural tube defects, such as spina bifida.
Amniocentesis
Amniocentesis is a prenatal test used to identify various genetic problems. It is done by using a thin needle which is inserted into the uterus by doctor! Then, the doctor withdraws a sample of the amniotic fluid that surrounds and protects the baby. This fluid contains fetal cells and various chemicals produced by the baby. With genetic amniocentesis, the chromosomes and genes in these cells or the chemicals in the amniotic fluid can be tested for certain abnormalities, such as Down syndrome and spina bifida. What's also good? With maturity amniocentesis, the amniotic fluid could be also tested to determine whether a baby's lungs are mature enough for birth.
Indications for an amniocentesis include:
- A chromosomal abnormality or neural tube defect in a previous pregnancy
- Abnormal results from a prenatal screening test, such as first trimester screening or the quad marker screen
- A family history of central nervous system defects, Down syndrome or other genetic disorders
One problem with amniocentesis is that it carries various risks, including:
- Miscarriage.
- Rupture of the amniotic sac
- Cramping
- Light vaginal bleeding.
- Rh sensitization
- Needle injury
- Neonatal respiratory distress syndrome
- Neonatal pneumonia
- Limb reduction defects
- Bleeding
These risks are small, and the pros and cons should be discussed with the doctor before having the test.