What is Down Syndrome?

Down Syndrome is a genetic condition involving the 21st chromosome. This results in developmental delay, some unique physical features and an increased chance for some health conditions.

If a person has three copies of chromosome 21 instead of two copies it causes Down Syndrome. This is why Down Syndrome is also referred to by the name Trisomy 21. Instead of a pair of chromosomes, as is found in all the other chromosomes, number 21 includes three chromosomes. It is important to understand that all chromosomes are normal. It is the fact that there is an extra chromosome that causes Down Syndrome. Every cell in a person with Down Syndrome will contain 47 chromosomes (with the exception of Mosaic Down Syndrome which is discussed below) instead of 46 chromosomes.

What causes Down Syndrome?

Down Syndrome results from a genetic imbalance in cells caused from an extra set of genes on chromosome 21.

There are many theories about how the extra chromosome causes the effects of Down Syndrome but little is currently known. Research, however, is continuing and a breakthrough may provide possible treatments to lessen the effects.

Either the father or the mother has to donate the extra chromosome that causes Down Syndrome. 80% of all babies born with Down Syndrome are born to mothers under the age of 35, although it is known that the chance increases with maternal age.

The same incidence applies to all races and economic levels. Recent research shows that slightly more males than females are born with Down Syndrome in comparison with the general population.

During cell division to create a germ cell (either sperm or egg), a cell containing 46 chromosomes divides into two germ cells each containing 23 chromosomes. Sometimes this division does not happen properly and one cell may contain 22 chromosomes and the other may contain 24 chromosomes. This can happen if the chromosomes do not properly separate and instead "stick together." This is called nondisjunction because the chromosomes have failed to disjoin or split-up. 75% of the time it is the egg cell which carries the additional chromosomal material, 25% of the time it is the sperm cell. If the cell containing 24 chromosomes combines with a cell containing 23 chromosomes, the new cell will contain 47 chromosomes instead of 46. If the trisomy is chromosome 21, the person will have Down Syndrome.

Different types of Down Syndrome:

There are three different types of Down Syndrome: Standard Trisomy 21, Translocation, and Mosaicism.

1. Standard Trisomy 21 is when the extra chromosome 21 comes from either the egg or sperm cell. Between 90% and 95% of all Down Syndrome is Standard Trisomy 21.
2. Translocation is caused when a piece of chromosome 21 is located on another chromosome such as chromosome 14. The person with Translocation Trisomy 21 will have 46 chromosomes but will have the genetic material of 47 chromosomes. The person with Translocation Trisomy 21 will exhibit all the same characteristics of a person with Standard Trisomy 21 since they have three copies of chromosome 21. Translocation occurs between 3% and 5% of cases of Down Syndrome.
3. Mosaicism is when a person has a mix of cells, some containing 46 chromosomes and some containing 47 chromosomes. This occurs either because:
   • The person received 46 chromosomes at fertilization but somewhere during early cell division the chromosome 21 cell pairs failed to split creating a cell with 47 chromosomes and a cell with 45 chromosomes. The cell with 45 chromosomes can not survive but the cell with 47 chromosomes will continue to divide. All cells that come from this cell will contain 47 chromosomes, or
     The person received 47 chromosomes at fertilization but later during cell division the extra chromosome is lost. Mosaicism occurs in 2% to 5% of cases of Down Syndrome. A person with Mosaic Down Syndrome may exhibit all, some, or none of the characteristics of Down Syndrome depending on the percent of cells carrying the extra chromosome and where these cells are located.

Is Down Syndrome Inherited?

The vast majority of cases of Down Syndrome are not inherited. Only in cases of Translocation Down Syndrome and then in only 1 of 3 cases of this type of Down Syndrome is the condition inherited. These inherited cases occur because one of the parents is a carrier. A carrier will have 45 chromosomes instead of 46 but they will have all the genetic material of a person with 46 chromosomes. Remember that in Translocation Down Syndrome the extra chromosome 21 material is located on a different chromosome. A carrier will have the extra material but will have only one chromosome 21. The carrier will not exhibit any of the symptoms of Down Syndrome because they have the correct amount of genetic material.

A carrier will have an increased chance of having a child with Down Syndrome. If the carrier is the mother, the chances are approximately one in five of having a child with Translocation Down Syndrome. If the carrier is the father the odds are reduced to between one in twenty to fifty. In cases where the carrier has no unattached chromosome 21, all the carriers children will have Down Syndrome. In all cases of Down Syndrome but especially in cases of Translocation Down Syndrome, it is important that the parents have genetic counseling to determine their risk.

What are chromosomes?

Our chromosomes contain the genetic code, which controls and instructs cell division, growth, and function. They are the structures inside the nucleus of living cells that contain hereditary information. A person normally has 46 chromosomes, 23 inherited from each parent. Every person has a unique genetic code (with the exception of identical twins). It is this uniqueness, which makes the physical appearance of each person different.

Our parents give our chromosomes to us. Each parent gives us 23 chromosomes that come from the egg and sperm cells. When they combine, they produce a cell with 46 chromosomes. This cell then divides, the result being that every cell contains identical genetic material.

Medical Considerations with Down Syndrome

Heart Defects - Around 40-45 percent of all babies born with Downs Syndrome are born with some form of a heart defect. IN the past, these defects usually led to death. However, today advances in cardiac surgery as made it possible to repair most defects.

Atrioventricular Canal Defect (AV canal) - The most common type of heart defect associated with Downs Syndrome is the AV canal defect. Also sometimes referred to as an endocardial cushion defect. An AV canal is a large hole in the center of the heart. Essentially what this means is that the walls between the two upper chambers and the two lower chambers, as well as the valves may be deformed. The large hole in the center of the heart allows the red blood (blood containing oxygen) to mix blue blood (low oxygen) and return to the lungs. This extra effort in turn can cause the heart to enlarge, in addition the body receives less oxygen since it receives red blood that is mixed with the low oxygen blue blood.  Children that suffer from an AV canal defect, tend to grow very slow and remain small. Also because of the high volume of blood that is pumped through the lungs high blood pressure may also occur resulting in damage to the lungs and blood vessels. Surgery can normally repair an AV canal defect to help restore normal blood circulation.

Ventricular Septal Defect (VSD) - Another common heart defect associated with Downs Syndrome is the VSD. The VSD is a large opening between the ventricles in which, as with the AV canal defect oxygenated and deoxygenated blood mix. This results in similar problems as those mentioned for the AV canal.

Atrial Septal Defect (ASD) - Not as common as AC and VSD but similar in nature, blue and red blood mix causing an inefficiency with the heart. This happens when there is a large defect between the atria. Most if not all heart defects CAN be corrected by surgery, and as of late the success rates for most surgeries are in the 95-99% range. Many of these conditions are rare and may be difficult to identify immediately because they do no cause problems all of the time.

GI Tract Problems - Generally speaking some signs that there may be a GI tract problem would include feeding problems like vomiting, coughing with feeding, a distended belly (swollen belly) frequent diarrhea or constipation or a failure to thrive (appropriate weight gain). Remember to use a chart designed specifically for Down Syndrome.

Aganglionic Megacolon (Hirshprung's Disease) - This is the absence of nerve ending in the colon (large intestine). Another way of saying it is that a child can not have consistent bowel movements on their own.

Duodenal Altresia - This is the most common of all GI tract anomalies. It is a narrowing or a complete blockage of the small intestines.

Imperforate Anus - An absence of an anal opening.

Pyloric Stenosis - An obstruction of the outlet of the stomach or in other words a joining of the intestines.

Tracheo-Esophageal Fistula  - An abnormal opening between the Trachea or wind pipe and the Esophagus or food pipe. All of the GI tract anomalies just listed can be corrected with surgery and early diagnosis is vital.

What factors can affect Development?

There is a wide range of development in children with Down Syndrome. Many factors influence this range, including early intervention programs; other genes inherited from the family, effects of fetal distress or a difficult birth, health problems resulting in low oxygen or prolonged hospitalization for health reasons.

Down Syndrome Characteristics

Children born with Down Syndrome usually exhibit definite features that help doctors diagnose Downs Syndrome. Typically these features involve the neck, face, hands, and feet. After a complete inspection of these areas doctors will usually order some chromosome testing to confirm their findings. The following characteristics are most commonly associated with Down Syndrome. Please remember that there is large variety and not everyone with Downs Syndrome will possess all of these features. There is also no connection between the number of features a person has and their cognitive abilities.

• Hypotonia or Low Muscle Tone - Down Syndrome can have "hypotonia" or low muscle tone. What this means is that their muscles usually appear relaxed and under developed. Low muscle tone can affect movement, strength and development. Low muscle tone will affect the development of skills like rolling over, crawling, standing and walking. Hypotonia can also have an effect with feeding. Holding a bottle is sometimes not that easy, and the acceptance of solid foods is sometimes more difficult due to the low muscle tone around the mouth. Hypotonia can be cured, although the child's muscle tone will generally be lower than others. Early intervention is vital.
• Facial Features
  • Eyes - Eyes may appear to slant upwards, (Slanting Palpebral Fissures). The eyes may also have small folds of skin at the inner corners, (Epicanthal Folds). The coloured part of the eye (the iris) may also have tiny light spots (Brushfeild spots) these spots are most commonly found in babies with blue eyes and do not affect the baby's sight. Eyesight should be checked frequently, as vision problems tend to be more common in children with Down Syndrome.
  • Nose - The face may be a little broader and the bridge of the nose may be somewhat flatter than usual. Children with Down Syndrome often have smaller noses and therefore the nasal passages will be somewhat smaller, and prone to becoming congested more quickly.
  • Mouth - It was believed for a long time that babies with Downs Syndrome had big tongues. This is incorrect; in fact their tongues are of normal size. Although their mouth may be a bit smaller, and the roof of the mouth may be shallow. This in combination with low muscle tone may affect the ability to keep their tongue inside their mouth.
  • Teeth - Teeth may come in late, and in a sporadic order. The teeth may also be small, oddly shaped, and sometimes out of place. Sometimes these problems fix themselves when permanent teeth come in.
  • Ears - The ears may appear set a little lower on the head, and the tops may fold over slightly. The ear canals like the nasal canals tend to be a little smaller, and prone to infection. Due to the smallness they tend to become blocked more easily. Schedule hearing exams frequently, and practice special care when cleaning them.
  • Hands and Feet - Hands may be smaller and her fingers shorter and the fifth finger may curve inwards slightly. Also only be one crease across the palm of each hand, called a transverse palmar or simian crease. The feet usually appear normal, but there may be a gap between the first and second toes, and quite often there is a deep crease on the sole of the feet at this gap.
  • Skin - Skin can fair, and very sensitive to irritation.
  • Hair - Hair is usually very soft, fine and sometime sparse.
  • Chest - Chest may be somewhat funnel shaped, or pigeon boned. These differences in shape results do not result in any additional medical problems.