Down Syndrome

Introduction

The term "Mongolism," is incorrect and racist and it was once used when describing a specific group of individuals. However, with technological advancements in the scientific world, more information is being collected about the condition that is prevalent in 1/700 children born in the world. These "Mongoloids" are now known as people and are individuals that possess a certain condition known as Down syndrome.

Known Causes

Down syndrome was incorrectly characterized over 150 years ago; however, this is where the story began. According to Korenberg, "scientists and physicians were comparing the epicanthic fold, or skin fold of the upper eye lid, of Down syndrome individuals to the tarsal epicanthal fold seen in oriental populations" (3). This mischaracterization took place in 1866 and it was not until 1959 that scientists determined that Down syndrome was caused by trisomy 21. Down syndrome results from an abnormal amount of otherwise perfectly normal genetic material. Chromosome 21 is the smallest of all the human chromosomes and it only comprises approximately 2 percent of the human genome. However, a bulky number of cloned DNA segments are assigned to this portion of the genome, which makes it difficult to comprehend the mechanism whereby specific genes on chromosome 21 contribute to the pathology of Down syndrome.

Before the arrival of human molecular genetic techniques, scientific knowledge of the organization of chromosome 21 DNA was detained to a cytogenetic level of organization. According to Neve, "The long arm of chromosome 21 has been subdivided into three bands, on the basis of their width and intensity of staining; only the distal segment, the band 21q22, is involved in the pathogenesis of Down syndrome" (2). Therefore, it is now known that although Down syndrome is identified as trisomy for chromosome 21, it can be narrowed down to chromosome 21q22.

Motor Development Characteristics

Korenberg says that, "Down syndrome is a major cause of mental retardation and congenital heart disease... in addition Down syndrome is associated with a characteristic set of facial and other physical features, congenital gut disease, an increased risk of leukemia, defects of the immune system, endocrine abnormalities, and an Alzheimer-like dementia" (1). As can be seen, a child that is diagnosed with Down syndrome is at risk for many other developmental disorders. The severity of these disorders depends on the form of Down syndrome the child has. There are three different classifications of Down syndrome. The first is Trisomy 21, which is the most common form where all cells are affected because it occurs from the beginning of cell division. Another form is translocation, which is where one of the three chromosomes is attached to another chromosome. All cells are also affected in translocation because it occurs from the beginning of cell division. The last form is mosaicism, which is the least severe form because it takes place later in cell division so not all cells are affected.

The probability of being born with Down syndrome is 1/700 births. According to Hassold, "the association between increasing maternal age and trisomy is one of the most important etiological factors in human genetic disease" (5). The numbers discussed in lecture further strengthens this point. A woman between the ages of 15-29 has a 1/1500 incidence of Down syndrome at delivery whereas, a woman above the age of 45 has a 1/50 incidence of Down syndrome at delivery. However, Hassold explains that, "despite the obvious clinical importance of the maternal age effect, we still know little about its basis" (5). Many tests and models have been proposed to account for the effect but it still remains unknown.

The intelligence quotient (IQ) range varies for children with Down syndrome but a majority fall in the moderate range. There are three classifications of intellectual disabilities that are based off of IQ. An IQ between 50-70 characterizes mild classification, an IQ between 36-49 characterizes moderate, and severe is characterized by an IQ below 36. The consistency of developmental progression is low within the Down syndrome population because there is large variability in the population. They typical onset of walking for a normal developing child is 10 to 12 months whereas for children with Down syndrome this milestone is typically achieved at 24 months. There are multiple theories out there that help explain the differences in motor development patterns. The first is the plateau concept, which explains that a child with Down syndrome hits more plateaus during development and these plateaus last longer compared to normally developing children. Children with Down syndrome hit more frequent plateaus because at their level of motor development they run into more proficiency barriers. The plateaus last longer because inappropriate skill behaviors are reinforced through continuous use, thus it takes longer to change and correct the movement errors. In 1986, S.F Henderson proposed that children with Down syndrome have a different motor pattern of development rather than a delayed pattern. His theory states that children with Down syndrome have, "continuous neurological deterioration, where neurons begin to deteriorate much earlier than the normal developing population." Furthermore, Jon F. Miller performed a study on the individual differences in vocabulary acquisition in children with Down syndrome compared to normal developing children. After the study was performed he concluded that, "...work to date has documented consistent differences in the rate of vocabulary learning in children with Down syndrome relative to their mental age matched peers. These deficits increase with advancing age indicating that the learning rate of the Down syndrome group is significantly slower than that of the normal developing group" (102). Bottom line, the motor development progression for children with Down syndrome is at a declined rate when compared to normal developing children. Moreover, the rate of development decreases more as a child with Down syndrome increases with age.

Children with Down syndrome also have differences in brain development in comparison to normal developing children. Problems in brain development are not present at birth; however, after 6 months of age they occur over time. A major difference in the brain of a child with Down syndrome is the size of the cerebellum. The cerebellum is 2/3 the size of a typical brain.

Motor Behavior

Children with Down syndrome exhibit many different motor behavior characteristics. They are typically shorter with regards to height because their physical