Introduction

Intellectual disability (ID) refers to a developmental condition causing limitations in an individual’s intelligence and adaptive behaviour. Usually diagnosed before the age of 18, common signs of ID include deficits in cognitive abilities (learning, reasoning, problem solving) and adaptive behaviour necessary for activities of daily living.

ID has both genetic and physiological basis, beginning from in-utero to producing long lasting effects throughout lifetime. This article focuses on understanding environmental factors on the development, manifestation, and symptomatology of ID – from maternal health & nutrition, to effects of education and stimulation. Environmental factors play a crucial role in shaping the physical and social environment that a child is reared in, and studying its effects can help develop targeted treatment intervention & prevention programs. This has great implications in shaping clinical, educational, and home environments to better suit and foster inclusivity and enhance learning and adjustment through integrated approaches.

Overview of Intellectual Disability 

The American Psychological Association defines intelligence as “the ability to derive information, learn from experience, adapt to the environment, understand, and correctly utilize thought and reason.” The “Intelligence Quotient” represents a quantitative measure of intelligence of an individual, typically centered around a mean value of 100.^`1

According to the DSM-5,² “Intellectual Disabilities” are neurodevelopmental disorders beginning in childhood, representing difficulties in cognitive and intellectual functioning, accompanied by deficits in occupational, social, and practical functioning.  Diagnosing ID requires comprehensive evaluations of neuropsychological, cognitive, speech, and occupational functioning to determine the severity of the condition. The assessment must only be carried out by trained personnel, such as psychometricians and clinicians. Another important consideration in assessing intelligence is incorporating age-based and cultural norms to assess capacity.³

Prevalent in about 1% of the world population, ID presents large variations in clinical symptomatology.⁴ While the causes of Intellectual disabilities are largely unknown, the growing literature classifies the main sources to be of the interplay of genetic and environmental factors.⁵

Genetic

Genetic abnormalities arise from specific chromosomal or gene mutations, defects in neurodevelopment and neurodegeneration, resulting in metabolic disorders,  behavioural abnormalities and cognitive decline. Some of the well-studied disorders include Down Syndrome &  Fragile-X Syndrome.

Environmental

Environmental contributors to ID include early exposure to certain toxins, radiation, alcohol/cigarettes, complications in birth such as hypoxia (lack of optimum level of oxygen within the brain), preterm birth, low birth weight, delivery complications, infections, postnatal trauma as well as quality of education. The upcoming sections delve deeper into each of these causes, discussing the nature of occurrence and subsequent side-effects on a child’s neurological, cognitive and developmental trajectories.

Assessing such cases in clinical settings requires drawing up detailed physical and medical histories, noting any hereditary patterns, delivery conditions and health during birth, potential for exposures to harmful substances, sensory and social reactions, parental living situation and behaviour in academic settings and towards caretakers.

Environmental Factors Contributing to Intellectual Disability 

Prenatal Factors

Maternal health and nutrition

One of the most widely recognized determinants of foetal development, crucial to optimum outcomes for maternal and foetal health is nutrition. Studies have demonstrated the impact of essential nutrient intake by the mother as necessary for the child’s in-utero organ development, skeletal growth, neural functioning and typical physiological states.

• Vitamin B-12: Vitamin B-12 is extremely essential for the functioning of the central nervous system, particularly in the early stages of brain development. B-12 deficiencies cause neural tube defects, resulting in life long disability and neurocognitive impairments. Coupled with birth and delivery conditions, Vitamin B-12 deficiencies affect a process known as myelination – the insulation of neurons by lipids called myelin – to ensure transmission and coordination of electrical signals⁵
• Iron: Iron is a critical mineral found in haemoglobin, assisting the red blood cells in transporting oxygen throughout the body. Maternal iron deficiency (anaemia) during pregnancy severely impairs oxygen delivery, causing behavioural and cognitive problems in the child. These manifest as poor attention, intelligence and learning deficiencies. Adequate levels of iron also help immune function, in reducing the child’s susceptibility to illnesses early in growth. Therefore, an optimum maternal diet is iron-rich, such as greens, fish, legumes and lean meats⁶
• Folic Acid: Folic acid is essential for cell division and DNA synthesis, which are crucial in neural tube development. Adequate intake prevents neural tube defects, such as spina bifida, anencephaly etc. In doing so, folic acid also supports optimum oxygen delivery throughout the cardiovascular and central nervous systems, preventing hypoxia and improving overall cognitive function

Exposure to toxins (e.g., alcohol, drugs, pollutants)

Parental substance abuse produces a multitude of side-effects on a child’s heath, affecting behavioural, cognitive, psychological, and neurological development of the child.⁷ A study in 2020 noted the intake of alcohol, tobacco and drugs corresponding to higher rates of preterm birth and developmental delays. Specifically, children presented with lower IQ, and were more likely to present signs of emotional instability and aggression. These behavioural abnormalities also manifested in poor academic performance, conduct disorders, as well as learning disabilities.Specifically, a disorder known as Fetal Alcohol Syndrome (FAS), characterized by maternal alcohol intake during pregnancy, poses a rising concern today. Children affected by FAS experience intellectual disability, whether mild or severe. Understanding the aetiology of FAS, patterns of consumption, susceptibility and the overarching impact of developmental and psychosocial growth of a child is essential for efforts in prevention, education, and providing instrumental support.⁸

Infections during pregnancy

One of the most significant risk factors for ID in children is maternal infections. Infections that trigger the mother’s immune responses can trigger inflammation and release substances called the pro-inflammatory cytokines, crossing the placenta and affecting infant neurodevelopment. Moreover, as these infections are associated with birth complications, they impact intellectual functioning and adaptive abilities of growing children. Some of the most common infections known to produce such teratogenic effects are rubella, Toxoplasma Gondii, etc affecting brain health and vision.  

Each of the infections also have differential effects depending on the time of pregnancy. For instance, when a mother contracts rubella in the first trimester, it leads to a condition called the congenital rubella syndrome, causing abnormalities in vision, heart and brain health, and deafness. The widespread movement of MMR Vaccines (Measles, Mumps & Rubella) demonstrates the effectiveness of early intervention health initiatives and programs in preventing or slowing the progress of such conditions.⁹

Perinatal Factors

During foetal development, the occurrence of insufficient oxygen levels within the brain (hypoxia) creates a profound impact on the infant’s growth and maturation. Without sufficient oxygen for a short period of time, the placenta may compensate by facilitating redistribution of blood flow to more critical regions such as the brain and heart. However, if the effects extend over longer durations, developmental delays manifest and effects sustain longer, causing structural abnormalities, as well as neurological deficits in foetuses. Moreover, hypoxia alters adaptive responses of the brain, through changes in gene expression and metabolic pathways, which lead to drastic regressions in developmental trajectories and increases susceptibility to health challenges.¹⁰

Postnatal Factors

Exposure to toxins

Exposure to lead especially during the third trimester of pregnancy is associated with reduced intellectual development in children. Various studies highlight that the exposure during week 28 had lasting effects on a child’s IQ up until the age of 10. This was not exclusive to high concentrations, which suggests that there is no safe threshold for lead exposure. Primarily, lead exposure affects brain development in the following ways:¹¹

• Neurogenesis – the process of production of neurons, disrupted by lead exposure
• Neuronal synaptogenesis – formation of synapses, affected by lead, impacting brain connectivity and reducing efficiency of neuronal networks.
• Oxidative stress – causes damage to cellular components by depleting anti-oxidants in the brain, resulting in cell death
• Disruption of neurotransmitter activity – altering levels and functioning of dopamine, glutamate etc impacts learning, memory and behavior

Impact of Stress 

Rise in studying the effects of psychological stresses on foetal outcomes has presented interesting insights. Prenatal Maternal Stress (PNMS) has been shown to contribute to foetal development, resulting in deficits in cognitive and motor functioning, altered behavioural responses as well as lowered IQ Scores. An interesting study outlined how stress affecting a particular time of the gestation period, in this case, mid-gestation exposure, has the most long-lasting developmental effects on the child. This outlines the need for specific interventions at crucial time periods, to prevent rise of neurological issues as pregnancy progresses.¹²

Summary

• Intellectual disability (ID is a neurodevelopmental condition resulting in deficits in cognition, intelligence and adaptive behaviour
• Attributable to both genetic and environmental factors, the effects of ID significantly impact a child’s psychosocial development, neurological status and academic and social performance and adjustment
• Some of the common environmental factors include exposure to toxins, prenatal maternal mental health, substance abuse, nutrition deficiencies and maternal infections
• Understanding the prevalence, presentation and causes of ID is crucial to develop effective health and educational interventional programs
• Research in addressing genetic and environmental causes, clinical outcomes, academic support and parental involvement and education form a holistic approach to promote inclusive and encouraging environments to improve well-being