DOWN SYNDROME, TRISOMY 21, TYPES, CAUSES AND PHYSICAL CHARACTERISTICS

Another name for the Down syndrome phenomenon is trisomy 21. This condition occurs when there is an extra copy of chromosome number 21 in the body system of an individual. Shildrick (2009) denotes that this condition causes a change in the body make of a child, and this leads to facial disfigurement. DePoy et al (2011) denotes that facial disfigurement occurs when the face of a child takes another form, which is against the normal. It changes the appearance of a person, and the condition has a direct influence on an individual's perception in the society. Social identity refers to the attitude, or perception that a group in the society, views another person or themselves. Social identity emanates from an individual self-conception. DePoy et al further denotes that this perception of self, results to an individual placing him or herself to a specific social group (2011).

In his publication in 1866 titled ‘Observation on an ethnic classification of idiots’, John Langdon Down an English physician at the London Hospital, was the first to describe the external appearance of the genetic condition that was later to bear his name (Down, 1866). Following his description, scientists have conducted researches aimed at identifying the presence of the condition in various populations, race and ethnic groups as well as its incidence at birth and prevalence in the population. Most of the widely circulated reports on the incidence and prevalence of Down syndrome in Nigeria and other countries in sub Saharan Africa are estimates extrapolated from statistics obtained in developed countries such as the United Kingdom, United States of America, Canada or Australia. The studies did not consider the influence of factors such as socio cultural, genetic, racial and environmental characteristics on the prevalence of the condition, thus affecting the reliability of the data.

The earliest reported study on the incidence of Down syndrome by Adeyokunnu, in Ibadan, South Western Nigeria, reported an incidence of 1 in 865 live births (Adeyokunu, 1982). Prior to this time, it was believed that Down syndrome is rare or non - existent among Africans (Tompkins, 1964). This is collaborated by the reports of other clinicians that found no case of Down syndrome in their clinical practice over several years in Nigeria (Jelliffe, 1954a, Tooth, 1950). Rather, few cases reported were among Jamaicans and were postulated to be derived genetically from non African sources (Jelliffe, 1954b). While there may be other reports from Nigeria, the case report by Tompkins in 1964 was the first to draw attention to the occurrence of the condition among Nigerian children (Tompkins, 1964).

After the report, it was now agreed that the condition is not as rare among Nigerians and Africans as was once believed. In spite of this however, there was still difficulty in achieving accurate data collection in Nigeria and other developing countries attributable to many factors. First, a large number of deliveries take place in non orthodox centres such as churches and traditional birth homes, most of which do not keep records. The government registered private maternity centres also have difficulty keeping accurate statistics (Oloyede, et.al, 2006). Secondly, within the community, cases of congenital malformations such as Down syndrome are not reported for record purpose because of the traditional belief that still associates them with witchcraft and witches. Consequently, true population based data are difficult to generate and most of the data reported are hospital based. In spite of this however, health planning are still based on these data with the assumption that they represent the actual situation.

Since the study by Adeyokunnu, there has been no other published report about the incidence in Nigeria till date. However, in South Africa, the Down Syndrome South Africa gave an incidence of about 1 in 500 live births in the country (DSSA). In same country, a study between January, 1974 and December, 1993, reported an overall prevalence rate of 1.49 per 1000, with a gradual decline to 1.3 per 1000 in the last 5 years of the study period, among the 3 study populations. The higher prevalence (1.88) among the white population compared with the prevalence in coloured (1.54) and blacks (1.29) could be attributed to the possibility that fewer blacks than whites undergo prenatal screened and diagnosis of the condition. This conclusion arose from the relative distribution of the number of terminations following prenatal diagnosis, being higher among whites (18.3%), intermediate in coloured (5.8%) and lowest in blacks (1.4%). The same study also confirms that the decline in the overall prevalence occur among the white population, while the blacks maintain their prevalence rate (Molteno, et, al., 1997). It should be noted that there is a fundamental difference in the statistical inferences from incidence and prevalence. Incidence is based on total birth in a year, while prevalence is based on actual population. While the difference is significant in developed countries because there is a better uptake of prenatal diagnosis services that influences the total birth incidence, same may not be true in Nigeria, where the uptake of such service is still poor. The incidence of Down syndrome could be higher in developing countries, with two factors as possible reasons. First the proportion of women that that conceive after 35 years is gradually rising in Nigeria compared to developed countries. Second, there is a higher mortality from complications of Down syndrome such as congenital heart defects in developing countries.

TYPES OF DOWN SYNDROME

There are three types of Down syndrome. People often can’t tell the difference between each type without looking at the chromosomes because the physical features and behaviors are similar (Shin M, Siffel C, Correa A, 2010).

• Trisomy 21: About 95% of people with Down syndrome have Trisomy 21. With this type of Down syndrome, each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2 copies.
• Translocation Down syndrome: This type accounts for a small percentage of people with Down syndrome (about 3%). This occurs when an extra part or a whole extra chromosome 21 is present, but it is attached or “trans-located” to a different chromosome rather than being a separate chromosome 21.
• Mosaic Down syndrome: This type affects about 2% of the people with Down syndrome. Mosaic means mixture or combination. For children with mosaic Down syndrome, some of their cells have 3 copies of chromosome 21, but other cells have the typical two copies of chromosome 21. Children with mosaic Down syndrome may have the same features as other children with Down syndrome. However, they may have fewer features of the condition due to the presence of some (or many) cells with a typical number of chromosomes.

CAUSES OF DOWN SYNDROME

Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells.  This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. 

The cause of the extra full or partial chromosome is still unknown. Maternal age is the only factor that has been linked to an increased chance of having a baby with Down syndrome resulting from nondisjunction or mosaicism.  However, due to higher birth rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age.

There is no definitive scientific research that indicates that Down syndrome is caused by environmental factors or the parents' activities before or during pregnancy.

The additional partial or full copy of the 21st chromosome which causes Down syndrome can originate from either the father or the mother. Approximately 5% of the cases have been traced to the father (National Down syndrome society, 2016).

PHYSICAL CHARACTERISTICS OF DOWN SYNDROME

According to Centers for Disease Control and Prevention (2016) they list the following as the physical characteristics of children with Down syndrome

• A flattened face, especially the bridge of the nose
• Almond-shaped eyes that slant up
• A short neck
• Small ears
• A tongue that tends to stick out of the mouth
• Tiny white spots on the iris (colored part) of the eye
• Small hands and feet
• A single line across the palm of the hand (palmar crease)
• Small pinky fingers that sometimes curve toward the thumb
• Poor muscle tone or loose joints
• Shorter in height as children and adults