What is Down Syndrome?

In 1866, British physician John Langdon Down provided the first formal description of the syndrome. In great error, Dr. Down attributed the condition to a "reversion" to the "mongoloid race." He theorized that evolution had been reversed and there had been a sort of backslide from the superior Caucasian to the inferior Asian race. His original name for the disorder,"mongolism" is incorrect, racist and is to be avoided. In 1961 the name was changed to "Down syndrome".

Down syndrome is the most common and easily identifiable chromosomal condition that is associated with mental retardation. It is estimated to occur in roughly 1 out of every 800 births. It is caused by an abnormality in the persons chromosomes. For some reason, an accident in cell development results in 47 instead of the usual 46 chromosomes. This extra chromosome alters the course of physical and intellectual development. In the majority of cases, Down syndrome is diagnosed according to the results from a chromosome test that is given shortly after birth.

What are the Symptoms of Down Syndrome?

Babies born with Down syndrome often have medical complications such as heart defects and abdominal disorders. They also often suffer eyesight and hearing impairments from frequent ear infections, perhaps due to a less effective immune system. With early medical diagnosis and intervention, many of these complications can be treated. Also, not all children with Down syndrome exhibit all of these symptoms, and some children experience almost no complications.

Babies with Down syndrome are often identified shortly after birth because they may have several of the physical characteristics associated with the syndrome. However, some children show a few characteristics while others show many. Because of this, and because some of these features can also be seen in people without Down syndrome, genetic testing is necessary in order to confirm diagnosis. The most common physical features associated with Down syndrome include:

• Low muscle tone (babies often appear "floppy")
• An upward slant to the eyes.
• Small skin folds on the inner corner of the eyes
• Flat facial features, often with a small nose.
• Small ears, often abnormally shaped.
• Hyperflexibility, which is the ability to flex joints excessively.
• Extra space between the big toe and the second toe (big toe sticks out to the side).
• A Single deep crease across the center of the palm.
• Enlarged tongue that tends to stick out.

People with Down syndrome also typically have more hormonal problems than the average person. Roughly 10% of babies born with Down syndrome and up to 50% of adults with Down syndrome have thyroid disease. The most common thyroid condition is hypothyroidism, or a slowing of the thyroid gland. Luckily, hypothyroidism can be easily treated with medication.

Over half of the children born with Down syndrome also have visual problems such as cataracts, farsightedness or nearsightednes, or crossed eyes. Most of the time these problems can be treated with glasses or surgery when necessary. Hearing impairments are also common. For these reasons, children with Down syndrome should have regular hearing and vision examinations so any such problems can be corrected or compensated for before they interfere with normal language development.

About 10%-12% of babies born with Down syndrome also have abnormalities in the GI tract that require surgery for correction.

Roughly 25% of adults over the age of 35 with Down syndrome also show signs of dementia, often expressed as Alzheimer's disease. In the general population, dementia in the form of Alzheimer's disease is rare before age 50, and only 5%-10% of adults over age 65 develop symptoms.

Curious Connection To Cerebral Palsy...

An interesting aspect of Down syndrome is that fetuses and infants who have Down syndrome may be resistent to the development of pre-natal cerebral palsy, mainly thought to occur due to lack of oxygen. In several studies of children with Down syndrome, a much lower percentage of children had cerebral palsy than was expected. Of those children, most of them appear to have developed cerebral palsy after birth. This is curious since most often, cerebral palsy develops prior to or during birth. Doctors are trying to figure out what is protecting these children from developing cerebral palsy, in hopes of finding a way to decrease the occurance of cerebral palsy in non-Down syndrome children.

What Causes Down Syndrome?

Down syndrome is most commonly caused by an error in cell division where an extra chromosome was present at conception, either from the father's sperm or the mother's egg, each of which normally contributes 23 chromosomes. It's important to note that the error occurs when the sperm or the egg splits it's 46 chromosomes in half. Usually this leaves each with 23 chromosomes to contribute to the fetus. Sometimes the sperm cell or egg cell keeps an extra copy of chormosome number 21, and if this cell becomes fertilized, the extra chromosome is passed on to the child. Nothing the father or mother can do will have an effect on whether this will happen or not. Instead of 46 chromosomes, a person with Down syndrome has 47. The additional chromosome is added to the 21st pair. For this reason Down syndrome is medically referred to as Trisomy 21 (Trisomy meaning three chromosomes, 21 refering to the pair which is effected). Trisomy 21 accounts for 95% of the cases of Down syndrome. Translocation and mosaicism are two other chromosome errors which lead to the 47th chromosome and account for the other 5%.

Translocation Down syndrome is similar to regular Down syndrome in that the person has extra chromosome #21 material. The difference is that the extra #21 material is attached to another chromosome, and not separate. When a child has translocation Down syndrome, it means that one of the two parents actually has 45 chromosomes in each cell of his/her body (not the normal 46). That parent is known as a "balanced translocation carrier". In a balanced translocation carrier, the person doesn't have any additional genetic material - they simply have a smaller than normal chromosome #21 with a piece broken off, a normal second chromosome #21, a chromosome #14 with the broken piece of #21 attached, and a normal chromosome #14. So the person appears entirely normal with no related health problems. When that balanced carrier reproduces, there are three possible outcomes.

1. The carrier passes on his/her normal #21 chromosome and normal #14 chromosome, resulting in a healthy baby which is not a carrier.
2. The broken #21 (with a bit of material missing) and affected #14 (with that bit of material from #21) may be passed on. This results in a healthy baby who is also a balanced carrier.
3. The normal #21 and the affected #14 (with that bit of material from #21) are passed on. Since there is now an extra bit of chromosome #21 attached to chromosome #14, along with two normal #21's, the baby has too much genetic material from chromosome #21. The translocation becomes "unbalanced" and the baby will have Down syndrome.

While normally Down syndrome can occur randomly when two normal adults concieve, translocation Down syndrome is the only form of Down syndrome that can sometimes be inherited from a parent. If the carrier is the father, the chances are somewhere around 1 in 35 that the child will have Down syndrome. If the carrier is the mother, the chances are roughy 1 in 5. Also, If the carrier has that extra bit of chromosomal material attached to his/her other #21 chromosome, then the child will always have Down syndrome.

1 in 500 people are believed to carry a chromosome translocation. In many cases these are balanced translocations and the person has no idea that they are a carrier. Couples who have multiple miscarriages are often tested for chromosome translocation as this can be a cause.

The third form of Down syndrome, and the most rare, is known as Mosaicism Down syndrome. This occurs when a person has a mix of cells, some containing a normal set of 46 chromosomes and some containing 47 chromosomes. This can happen for a number of reasons.

1. The fetus received 46 chromosomes at fertilization but somewhere during early cell division chromosome #21 failed to split creating a cell with 47 chromosomes and a cell with 45 chromosomes. The cell with 45 chromosomes does not survive but the cell with 47 chromosomes will continue to divide. All future cells that come from this cell will also contain 47 chromosomes.
2. The person received 47 chromosomes at fertilization but later during cell division the extra chromosome is somehow lost. The original cells containing 47 chromosomes will continue to divide, alongside the now normal cells containing 46 chromosomes.

Mosaicism occurs in 2% to 3% of cases of Down syndrome. Since a person with Mosaic Down syndrome has normal cells as well as cells with an extra chromosome, they may exhibit all, some, or none of the characteristics of Down syndrome. This depends mostly on percent of cells carrying the extra chromosome and where in the body these cells are located.

Down syndrome is not related to race, nationality or economic status.

Because of higher fertility rates, 80% of all babies born with Down syndrome are born to women under the age of 35. The average age of the woman giving birth to a child with Down syndrome is 26. However, the chance of bearing a child with Down syndrome does increase with the age of the parents, from 1/1450 births between the ages of 20 and 30 to 1/32 births at age 45.