Down Syndrome
When a couple finds out they are expecting a child, it is often a joyous occasion. The next nine months are filled with excitement, anticipation, wonder, and some justified apprehension. Everyone wonders what their new baby will look like and whether it will be a boy or a girl. Women look forward to their baby showers and men worry about the finances. The health of the baby is obviously a great concern, but we typically assume our baby will be healthy. Unfortunately, that is not the case with every birth. When new parents are told that their baby has a disease or disorder, the obvious reactions are shock, sadness, anger, and feelings of sadness and guilt. Those were my feelings when, only a few hours after my son was born, doctors told me they thought he had Down syndrome. Several days later, which seemed like an eternity, I was elated when a nurse called to tell me that my baby was healthy.
Down syndrome is a genetic abnormality that can affect any race and economic group. It was first discovered in 1866 when the characteristic features were described by Dr. John Langdon Down (1828-1896). Dr. Down (figure 1), who was an English doctor working in Surry, admittedly did not understand the cause of the condition that he'd described (Selikowitz 25). It wasn't until 1959, some 93 years after Dr. Down's original description, that the syndrome was fully proven to be associated with an extra chromosome. This discovery was made by Dr. Jerome Lejeune (figure 2) and his colleagues in Paris.
Each cell in the human body contains 46 chromosomes, or 23 pairs of chromosomes. The only exceptions to this are the egg and sperm cells; those each have only 23 chromosomes. The sperm fertilizes the egg and the chromosomes fuse together, therefore providing the newcell <>Figure 2
MercatorNet-New Media Foundation Ltd 2004 - 2007 <http://www.mercatornet.com/
articles/saintly_scientists_hate_the_disease_love_
the_diseased/
with 46 chromosomes. This newly formed cell,
called a zygote, undergoes mitosis millions of
times as it develops first into an embryo and then
into a fetus (Colbert et al 429). During this mitotic
phase, there is an error in cell division resulting in
the presence of a third number 21 chromosome. An excessive amount of certain proteins are formed because of the additional chromosome, causing the normal growth of the fetus to be disturbed. Fewer body cells are produced because they do not divide as quickly as they normally would, resulting in a smaller baby. There is also a disruption in the formation of different body parts, the brain in particular. Research indicates that 80% of all fetuses with Down syndrome will terminate in a miscarriage (Selikowitz 34).
It is not necessary for an individual to have an entire extra chromosome 21 to cause Down syndrome. Only a portion of the chromosome is necessary for the syndrome to occur. The amount of the chromosome present, and how it is structured, determines the severity of the physical features and intellectual disability of the person.
There are three forms of Down syndrome: trisomy 21, translocation, and mosaicism. Down syndrome is most commonly referred to as trisomy 21 (tri = three, somy = chromosome) because this form affects 95% of those who have it (Selikowitz 34). This is characterized by the individual having an extra whole chromosome 21. In trisomy 21, the division of cells from the egg and sperm is abnormal, causing the newly formed cells to have an extra chromosome 21. This is known as non-disjunction.
The mother's age is an extremely important factor in trisomy 21. Studies have found that approximately 80% of those affected received the extra chromosome from their mother (Selikowitz 37). All of the eggs that a female will produce in her entire lifetime are present in an immature form when she is born. These immature eggs linger in the ovaries until the cycle is reached for a particular egg to be released. With this being the case, the theory is that the older an egg is, the greater chance it has of causing an error. Sperm, on the other hand, do not begin to form until the male reaches puberty. New sperm is produced in a continuous 10-week cycle. Because they do not stay in one stage for very long, sperm are less likely to cause the particular error that results in this form of Down syndrome. According to Mark Selikowitz, the odds of bearing a child with trisomy 21 increase from 1 in 2000 when the mother is 20 years old to 1 in 900 when she is 30. The odds are further increased to 1 in 100 when the mother is 40 years old (36).
The translocation form of Down syndrome occurs in 4% of those affected (Selikowitz 37). This form of the syndrome is caused by the presence of an extra part of chromosome 21, rather than the entire whole chromosome. During the mitotic phase, a small portion of chromosome 21 breaks off and becomes stuck to another chromosome. Translocation is the process of one chromosome sticking to another, and can only occur with chromosomes 13, 14, 15, 22, or another 21, with 14 being the most common (Selikowitz 37). The reason only these chromosomes have the ability to become involved in translocation is because they all have genetically inactive tips. These tips can break off, making it easier for another chromosome to adhere to them. It is not believed that the age of the parents is a factor in translocation as it is in the trisomy form.
Figure 3 - Male Trisomy 21
Dr. Mark Hill. University of New South Wales. <http://embryology.med.unsw.edu.au/Defect/page21.htm>
The third and last form of Down syndrome is called mosaicism. This form is found in only 1% of individuals with the syndrome (Selikowitz 39). In mosaicism, an individual has some cells with the proper number of chromosomes and some cells with an additional number 21 chromosome. The facial appearance and mental impairment of an individual with the mosaicism form are likely to be much less severe than those
with the trisomy or translocation forms.
There are several physical characteristics
that are displayed by an individual with Down
syndrome. It is very important to note that every
person with Down syndrome will not display all
of the characteristics (Selikowitz 27). It's equally
as important to note that, as in my son's case,
just because someone displays characteristics of Down syndrome, doesn't necessarily mean they have it. Since having the physical characteristics of Down syndrome does not prove one has it, a chromosome analysis, called a karyotype (figure 3), should be performed. This is a test that can be performed either before birth or after, using amniotic fluid or blood respectively. A cytogeneticist examines the sample under a microscope, identifies and matches the 23 pairs of chromosomes, along with the two sex chromosomes. The chromosomes are evaluated for size, shape, and the number of chromosomes that are present.
General physical characteristics of someone with Down syndrome include a short stature; hypotonia (weak muscles); a short, wide neck that may have excess fat and skin; a transverse
Figure 4
Emedicine from WebMD.
<http://www.emedicine.com/ped/
/images/637Down-SimianCrease.jpg>
Figure 5
U.S. National Library of Medicine. <http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/17169.jpg>
Figure 6
Wiki-pedia Foundations, Inc. <http://upload.wikimedia.org/wikipedia/
commons/9/9d/Brushfield.jpg
palmar crease, also called a simian line, on one or both of the hands
(figure 4); short, stocky arms and legs; and a wide space between the
first and second toes. Facial features include eyes that are slanted
slightly upwards and having an epicanthic fold (figure 5), with both
not being an attribute of the individual's race; a very low-set, wide
bridge; a tissue buildup on the iris, called Brushfield spots
(figure 6); and a slightly larger tongue combined with a slightly
smaller than average mouth (WebMD).
In addition to the external physical features displayed,
individuals with Down syndrome also tend to have internal
deformities putting them at a higher risk for gastroesophageal
reflux disease, chronic ear infections, obstructive sleep apnea,
and thyroid dysfunctions (Wikipedia). People with Down
syndrome have a small but still significant risk of acute leukemia.
In addition, approximately 50% of babies with Down syndrome
are born with a heart defect (WebMD). In adults with Down
syndrome, premature aging and dementia are common.
No matter which form of Down syndrome someone has, most individuals will suffer from some degree of mental retardation because they have fewer brain cells and a brain that is shaped differently. According to the March of Dimes website, it is impossible to predict the mental development of a child with Down syndrome based on physical features. Most individuals will fall within the mild- to-moderate range, with very few having severe mental retardation. Even with the milder mosaicism form, it is rare for individuals to be intellectually normal (Selikowitz 40).
The rate of development will vary from one person to another. Children with Down syndrome are said to have two “ages,” their physical age and their intellectual age. They tend to mature intellectually slower than normal children. Mark Selikowitz uses the analogy of cars in his book (41-42). “The normal child travels along the road of increasing skill-development at an average speed. The child with Down syndrome travels along the same road, picking up the same skills as he gets older. But because he travels along at a slower speed, he arrives at each stage of development at a later age than the normal child, and stays there for a longer period.”
In the 1920's, it was expected that someone with Down syndrome would not live to the age of 10. However, today they are living well into their 50's and beyond, living richer, fuller lives (Mayo Clinic). Although there is no cure for Down syndrome, early childhood intervention, education, supporting family members and proper medical care are essential to improving the quality of life for these individuals. With the help of family members and caretakers, many adults with Down syndrome are learning and developing the skills needed to live semi-independently, obtain and keep jobs, and become active contributors to their families and communities. Obviously, the severity of the person's health issues will always be a factor, but their life expectancy is continually rising due to early interventions and better health care (Mayo Clinic). More and more people with Down syndrome are getting married. While it's not unheard of for someone with the syndrome to marry a completely healthy person, it's more common for them to marry someone who is intellectually disabled because they tend to share the same outlook on life (Selikowitz 161).
When doctors told me they thought my son had Down syndrome, I was devastated. I couldn't understand what I had done to deserve having this happen to my child. At first I blamed myself and I blamed my husband. I was mad at the world and I hated everyone. But it didn't take me long to realize that no matter what his health, no matter what the blood test revealed, I loved him and I would do anything to help him. In the research I've done, I've learned that even though someone has a syndrome that is not curable, they can live a full life. Since my son was born, I've had the joy of meeting several people with Down syndrome. They are most often the happiest and most polite people you'll ever meet. They don't dwell on their syndrome and on the fact that they are different from other people. They laugh and joke and enjoy life. It's been a real awakening for me, because they've helped me put my life into perspective, and they've made me realize that it could be worse.
