Down's Syndrome

Down's Syndrome

Introduction

Down syndrome is a chromosomal abnormality that affects both mental and physical development. It is also known as trisomy 21 syndrome. One of the most common chromosomal abnormalities, Down syndrome is usually the result of an extra copy of chromosome 21; affected people therefore have 47 chromosomes in all of their body cells, rather than 46 (Girod, 2001). It may also result when part of chromosome 21 breaks off and attaches to another chromosome, a process called translocation, so that cells have the normal number of chromosomes but chromosome 21 is abnormally sized. Very rarely, Down syndrome may be the result of mosaicism, in which some body cells have 47 chromosomes and some have 46 (Brill, 2007). Exactly how these abnormalities produce the characteristic mental and physical features of Down is not known (Dyke, 1995).

Symptoms of Down's Syndrome

In most cases there is no identifiable reason for the chromosomal abnormality, although maternal age is a risk factor—after the early 30s, the risk of having a child with Down's Syndrome increases significantly. Paternal age can also be a risk factor, if the father is over 50 (Skallerup, 2008). Parents who already have a child with Down or who have abnormalities of their own chromosome 21 have a higher risk of having a baby with Down syndrome (Pueschel, 2006).

There is considerable variation in the severity of symptoms, but typically they include slow motor and language development and learning difficulties. Physical symptoms may include a small face with upward-sloping eyes; a flattened back of the head; a short neck; a large tongue; small hands with a single horizontal crease on the palm; and short stature (Margulies, 2007). There is also increased risk of various disorders, such as heart disease (often associated with congenital heart problems), hearing problems, under activity of the thyroid gland, narrowing of the intestines, leukemia, and respiratory-tract and ear infections. Adults are at increased risk of eye problems such as cataracts (Skallerup, 2008). In older people there is a heightened risk of Alzheimer's disease (Tocci, 2000). People with Down syndrome have lower than normal life expectancy, but some survive into old age.

Diagnosis and Classification

DS is classified as a chromosomal disorder. Although there are three types of the syndrome, trisomy 21 is the most prevalent (i.e., 94 percent of all cases). Trisomy 21 occurs following faulty chromosomal distribution in the formation of the sperm or egg prior to fertilization (Selikowitz, 1997). Specifically, non-dysjunction during meiosis results in either the sperm or the egg having an extra 21st chromosome. As a consequence, the fertilized egg contains a complement of 47, as opposed to 46, chromosomes (Dyke, 1995). In a less frequent number of cases (about 2 percent), non-dysjunction occurs after fertilization during one of the initial cell divisions. When this takes place the individual has a mixture of cells containing 46 and 47 chromosomes. People with this 'mosaic' form of Down Syndrome maybe less intellectually disadvantaged than persons with trisomy 21 (Brill, 2007). Although it is unknown why non- dysjunction of the ...