What is Down syndrome?
Down syndrome is a chromosomal disorder characterized by the presence of an extra #21 chromosome. Instead of having 46 chromosomes in each of his/her cells, a person with Down syndrome has 47. The extra chromosome may be derived from either the egg or the sperm. It is believed that during cell division of a sperm or egg, the two #21 chromosomes do not separate properly (nondisjunction). Thus, one cell will have an extra #21 chromosome which later, if united with a normal germ cell which has among others one #21 chromosome, may lead to trisomy 21. The first cell at conception then has 47 chromosomes instead of “normal set” of 46 chromosomes. When this cell with three #21 chromosomes continues to multiply, and the pregnancy is carried to term, a baby with Down syndrome will be born. This form of chromosome error, trisomy 21, is found in about 95% of persons with Down syndrome.
The 5 percent of babies with Down syndrome who do not have trisomy 21 may have Translocation, Mosaicism or partial trisomy. In translocation Down syndrome, there are also three copies of the #21 chromosome. However, one of the #21 chromosomes is attached or translocated on another chromosome, usually a #14, #21, or #22 chromosome. Three to four percent of children with Down syndrome have translocation. About one third to one half of translocations are inherited from one of the parents. When this happens, the carrier parent has the normal amount of genetic material; however one of the #21 chromosomes is attached to another chromosome. As a result this individual’s total chromosome count is 45 instead of 46. The carrier parent is clinically unaffected because there is no loss or excess of genetic material. Doctors refer to a parent like this as a “balanced carrier.“
Another form of Down syndrome is known as mosaicism, which affects about 1 percent of all people with Down syndrome. In mosaicism, a faulty cell division most often occurs in one of the earliest cell divisions after conception. This is in contrast to other types of Down syndrome when the error in cell division occurs before fertilization. As in trisomy 21, something causes one of the cells to divide incorrectly. But when this occurs in one of the early cell divisions, only some of the cells of the growing embryo have the extra #21 chromosome and the other cells have the normal number of chromosomes. Depending on the ratio of normal cells to trisomic cells, the child may have fewer Down syndrome features as well as higher cognitive abilities.
Although details of the mechanism of nondisjunction are for the most part unknown, the presence of three copies of the #21 chromosome causes a genetic imbalance that alters the typical course of growth and development of the fetus and child with Down syndrome.
What causes Down syndrome?
Down syndrome occurs in approximately one out of every 800 to 1000 live births; it is one of the most often observed chromosome anomalies. Scientists have investigated the causes of Down syndrome for the past century. So far its exact cause has eluded discovery. Although many factors have been considered to be possible causes, the age of the mother is the most often discussed factor related to the likelihood of having a baby with Down syndrome. It has been known for some time that the risk of having a child with Down syndrome increases with the advancing age of the mother, i.e., the older the mother, the greater chance that she may give birth to a child with Down syndrome. The risk at age 20 for instance is about 1 in 1600, the risk at age 35 is 1 in 365 and the risk at 40 is 1 in 100. However, because there are more younger women having babies, 75-80% of all children with Down syndrome are born to women under 35 years of age.
If I already have a child with Down syndrome, is the risk higher that I will have another child with Down syndrome?
If your baby has trisomy 21 the risk for having another child with Down syndrome is approximately 1 in 100 or 1% in addition to the mother’s age related risk. For families who have a baby with translocation Down syndrome, the risk of recurrence is also about one in one hundred unless the condition is inherited from one of the parents. The risk of recurrence depends on the type of translocation and the sex of the carrier parent.
Are any prenatal tests available to detect Down syndrome?
Yes. There are several types of testing available.
Screening Tests: Screening tests are used to look for potential problems and to identify those who are at high risk of having a baby with a genetic disorder.
The triple screen and the alpha-fetoprotein plus, and more recently, the quad test measure the amounts of certain hormones and proteins in the blood including alpha-fetoprotein, human chorionic gonadotropin, unconjugated estriol and inhibin. The results of these tests together with the woman’s age, will provide an estimate of her risk of having a child with Down syndrome. These tests are usually performed between the fourteenth and sixteenth week of gestation. Approximately 60-80% of fetuses with Down syndrome can be identified prenatally by considering the mother’s age and employing these screening tests.
In addition ultrasound examinations are almost always performed. During an ultrasound examination the physician looks for “markers”, such as a thickening of the skin at the back of the neck (nuchal fold), bright spots on the kidneys or heart, short arms or legs, reduced head size, congenital heart disease, and gastrointestinal problems. If any of these “markers” are observed, diagnostic testing is generally recommended.
Diagnostic Testing: Diagnostic testing tells whether or not the baby has the condition. However, there is no diagnostic test that is 100% reliable. Amniocentesis and chorionic villus sampling are the two diagnostic tests most often used to determine whether the fetus has Down syndrome.
Amniocentesis is typically performed around the 16th week of pregnancy. Before the procedure, an ultrasound examination is done which shows the location of the placenta, the amniotic cavity, and the fetus. During the procedure, a needle is inserted into the amniotic cavity through the mother’s abdomen. A small amount of amniotic fluid is obtained and analyzed. The amniotic fluid contains cells from the fetus, which are cultured and then examined to determine whether or not the fetus has Down syndrome. It generally takes 12 to 20 days to obtain results. Amniocentesis has about a two percent rate of miscarriage as well as other side effects, such as infection, bleeding, cramping and needle puncture of the fetus.
Chorionic villus sampling (CVS) is done in early pregnancy, usually between nine and twelve weeks of gestation. Following an ultrasound examination, a thin tube is inserted through the vagina and a small piece of placental tissue is obtained. Because the cells from the chorionic villi are fetal tissue, they can be cultured and then examined for chromosome abnormalities in one week to ten days. In terms of side effects, CVS is slightly more likely than amniocentesis to be followed by miscarriage or other complications such as infections, bleeding and leaking of amniotic fluid.
What are the physical characteristics of Down syndrome?
The following characteristics are most commonly associated with Down syndrome, but can also be found in the general population. There is some variety of physical features among babies with Down syndrome; not every baby possesses all of the characteristics. Moreover, there is no correlation between the number of characteristic features a baby has and the child’s cognitive ability.
Babies with Down syndrome usually have low muscle tone, called hypotonia. This means that their muscles appear relaxed and feel “floppy”.
The baby’s face may be broader and his/her nasal bridge may be flatter than usual. Often children with Down syndrome have a small nose. The child’s eyelids may appear to slant upward and may also have small folds at the inner corners (epicanthal folds). The baby’s mouth may be small and the roof of the mouth may be narrow. The baby’s ears are often small and the upper part of the ear may fold over.
Babies with Down syndrome usually have small heads. The difference in size, however, is not usually noticeable. The back of the head may be flatter and the neck may appear shorter. In newborns, there may be loose folds of skin on the back of the neck, but these tend to disappear later with advancing age.
The child’s hands may be smaller, and his/her fingers may be shorter than other children’s. In about 50% of children with Down syndrome only one palmar crease is observed, and the 5th finger may be curved slightly inward. Usually, the feet of babies with Down syndrome appear fairly normal, but there may be a wide gap between the first and second toe.
Because children with Down syndrome have an extra #21 chromosome, they may have features that resemble other babies with Down syndrome in some way. However, they will also resemble their parents, brothers and sisters.
What are the developmental aspects?
The baby with Down syndrome will grow and develop like other babies. He/she will do all the things a typical child does, only somewhat later. Compared with typical children, individuals with Down syndrome are usually smaller and their development is somewhat slower. For example, instead of walking at 12 to 14 months, a child with Down syndrome may learn to walk between 18 and 36 months. Most children with Down syndrome will display a delay in their speech and language development. It should be noted that there is a wide variation in the mental, behavioral and developmental progress in children with Down syndrome. A caring and enriching home environment, early intervention and improved special education services have a positive influence on the child’s development.
Do all children with Down syndrome have intellectual disabilities?
No. A few children with Down syndrome do not have intellectual disabilities; they may function in the borderline or low average range. The vast majority of children with Down syndrome function in the mild to moderate range of cognitive impairments. Children with Down syndrome often attend regular schools in regular education classes with differing levels of support. Most graduate from high school and some are enrolled in post secondary educational programs including colleges and vocational programs.
What about adults with Down syndrome? Where do they Live?
Today, adults with Down syndrome have choices about where they live and with whom they live. Some live in apartments, condominiums, or houses with roommates and/or with support services and some adults choose to remain in homes with their parents or siblings.
Do adults with Down syndrome work?
Adults with Down syndrome are working at a variety of jobs ranging from regular competitive jobs in the community, to supported employment or sheltered employment. People with Down syndrome are clerical workers, computer operators, assistant coaches, photographers, teacher assistants, etc. People with Down syndrome are becoming more and more contributing and tax paying members of their communities.
What is the life expectancy of people with Down syndrome?
Improved medical care has primarily prolonged the life span of people with Down syndrome. It is not unusual for people with Down syndrome to live into their 50’s, 60’s and even 70’s.
References: Pueschel, Siegfried M., M.D., Ph.D., J.D., M.P.H. A Parent’s Guide to Down Syndrome: Toward A Brighter Future, 2001.