NATHANIEL SCHERER: According to the World Health Organisation, 15% of the world’s population live with a disability. This equates to one billion people, of whom at least 150 million are under the age of 18. Of this population, 80% live in low- and middle-income countries. As we have learned, the term developmental disabilities covers a broad range of conditions. And as this term is used differently across different settings, countries, and cultures, it is difficult to accurately estimate the number of children with developmental disabilities across the world. Evidence from the United States indicates that one in six children under the age of 17 have a developmental disability.
When looking at children in the critical period of early childhood, global data suggests that 8% of children under the age of five have a developmental disability. This equates to roughly 53 million children worldwide. Although our best available data,
this figure is based on just six conditions: epilepsy, intellectual disability, vision loss, hearing loss, autism, and attention deficit hyperactivity disorder. Now, as we know, this is not a comprehensive overview of all of the conditions associated with developmental disability, and we can say that the prevalence of developmental disability in children under five years of age is likely much higher than this. It’s clear that we need more accurate data on the prevalence of developmental disability split across countries and regions. What this data does show, however, is that the magnitude of developmental disability is much higher in low- and middle-income countries, especially in sub-Saharan Africa, where the prevalence of developmental disability increased by 70% from 1990 to 2016.
So what are the causes of conditions such as cerebral palsy, autism, and hearing loss associated with developmental disability? Unfortunately, there is no easy answer. We’re talking about a broad range of conditions, and this means a broad range of causal factors that often interact with one another. In general, causes are split across two domains, environmental and genetic. Let’s start with environmental factors. These can have an adverse effect on a child before, during, and after birth. There are a variety of factors to consider, but for now, let’s think about the most common. The first is nutritional deficiencies of both a mother during pregnancy and a child in early development. Iodine deficiency, for example, is associated with congenital blindness, deafness, and intellectual impairments.
Another commonly known issue is that of folate deficiency, which can cause various neural tube defects, such as spina bifida. The second factor to consider is infection. Infections can damage a developing nervous system and cause a range of developmental disabilities. These infections can affect a child in utero, should a mother be infected, or in a child’s early years. Common examples include the Zika virus, which can cause microcephaly, meningitis, which may cause hearing loss, and measles, which may result in blindness. Another important point to consider is environmental toxins, which are well recognised to affect a developing nervous system.
A child is commonly exposed to such toxins through a mother’s health and behaviour patterns during pregnancy, the most common of which would be consuming alcohol or drugs. Children may also ingest toxins once born. Lead is a common example, and can be ingested through paint, batteries, and even gasoline. So these are the environmental factors we should be considering, but what about genetic factors? Congenital anomalies can occur as a result of genetic inheritance from one or both parents, or they may simply be spontaneous. These genetic conditions will commonly cause syndromes that typically manifest as a group of identifying characteristics. The most commonly recognised of these is Down syndrome, which is caused by an additional copy of chromosome 21.
However, there are many other syndromes caused by genetic conditions, such as Fragile X syndrome, Williams syndrome, and DiGeorge syndrome. Genetic syndromes and other genetic conditions can cause impairments across any domain, whether physical, intellectual, or sensory. For instance, children with Down syndrome typically experience intellectual impairment, but also difficulties with motor skills, as well as having a greater risk of heart defects and thyroid problems. When thinking further on the causes of a condition associated with developmental disability, it’s important to consider the interaction between genetic causes and environmental influences. Environmental factors may well increase the severity of impairments in children with genetic developmental disabilities, or they may produce comorbid conditions.
For example, a child born with phenylketonuria, a genetic condition, would develop intellectual disability and other conditions, but only if they eat foods containing the amino acid phenylalanine. If they avoid this amino acid, then the risk of these complications is much reduced. This is one way in which environmental and genetic causes together can contribute towards the development of impairments. Having discussed causes, let us now consider risk factors. While causes and risk factors sound like one and the same, they are, in fact, very different. Risk factors exacerbate the likelihood of a developmental disability, but they are not the underlying cause. For instance, a chromosomal abnormality causes Down syndrome, and increased maternal age is a risk factor for this chromosomal abnormality.
Many risk factors are as a result of social determinants. Evidence into the social determinants of health are clear, children and adults from a lower socioeconomic background are at an increased risk of poor health and disability. Difficulty accessing healthcare, for example, will increase the risk of infection, and increased poverty may increase the risk of malnutrition and nutritional deficiencies. In this way, the social determinants of health can increase the risk of many causes of developmental disability. Thinking on these factors, we can understand why the prevalence of developmental disabilities are higher in low- and middle-income countries.
It is not because they are genetically predisposed to having a developmental disability, but because the risk of environmental causes is heightened, as they may experience high levels of poverty and exclusion from healthcare and education. We will discuss this concept in further detail later in the course, when we think about the cycle of disability and poverty.