Overview 

Intellectual disability, previously known as “mental retardation”, is a generalised neurodevelopmental disorder that is present at birth and manifests during childhood, typically before the age of 18. Intellectual disability is characterised by significant impairment in intellectual functioning and adaptive behaviour (everyday-functioning skills).¹  

Intellectual functioning refers to a person’s cognitive abilities that allow them to understand and interact with the world around them. It can include various cognitive processes such as learning, reasoning, problem-solving, memory, and understanding abstract concepts. Intellectual functioning is typically measured using the intelligence quotient (IQ) which is based on a total score from standardised tests.¹  

Adaptive behaviour involves a person’s ability to effectively interact and navigate the environment that is necessary for daily living and to function independently. These skills can include social skills (such as interpersonal skills, social responsibility, communication, and self-esteem), conceptual skills (such as the ability to understand time, fiances and, language), and practical skills (such as the ability to use tools or skills necessary to carry out daily living).¹  

Limitations in these domains can vary widely in severity from mild to profound. The level of impairment often determines the level of support an individual requires in daily life, such as occasional assistance with certain tasks to ongoing support across all levels of functioning.^1,2

Intellectual disabilities can often be associated with various related and co-occurring conditions, including mental health (e.g., depression), medical conditions (e.g., cerebral palsy), and neurodevelopmental disorders (e.g., autism spectrum disorders).¹  

This article will discuss intellectual disability, its causes, different levels of severity, assessment and diagnosis, and common co-occurring conditions of intellectual disability. 

Signs and symptoms 

Intellectual disability is often noticeable during infancy or early childhood, but in some cases, it may not become apparent until the child reaches school age. The signs and symptoms of intellectual disability can vary depending on the severity of the condition, its underlying cause, and individual factors. Furthermore, intellectual disability can be divided into different levels based on the severity of impairment, often determined by IQ scores. These levels can include mild, moderate, severe, and profound intellectual disability. Each level is associated with different degrees of cognitive impairment, adaptive functioning challenges, and support needs.²  

Some early signs of intellectual disability can include:³

• Delays in reaching developmental milestones (sitting, crawling, walking)
• Delays in speech and language development 
• Difficulties with learning and understanding new concepts
• Poor memory, reasoning, and problem-solving 
• Difficulties with everyday tasks (getting dressed, washing, and eating)
• Difficulties learning at school 

Mild and moderate intellectual disability 

People with mild intellectual disability will typically have IQ scores approximately between 50-70. Those with mild intellectual disability experience delays in various aspects of cognitive development, as well as in social and daily living skills. However, they can acquire practical life skills that can allow them to function independently with minimal support and assistance.³ 

People with moderate intellectual disability will have IQ scores ranging approximately from 35-49. Moderate intellectual disability is generally noticeable during the first five years of life.  Children with moderate intellectual disability may exhibit delays in speech and language development, motor skills, and social interaction, which may become more apparent as they grow and mature. Those with mild intellectual disability may require significant support in daily activities such as self-care, communication, and social interaction. While they may learn some practical skills with appropriate support they often require ongoing assistance in navigating daily life.³ 

Severe and profound intellectual disability 

Those with severe intellectual disability will exhibit IQ scores approximately between 20-34.   They experience profound delays in all areas of development and often have limited communication abilities, relying on non-verbal methods such as gestures or simple vocalisations. Individuals with severe intellectual disability need constant supervision and support for basic activities of daily living.³ 

People with profound intellectual disability have IQ scores below 20 and often have congenital syndromes. They have profound cognitive impairments may have little to no functional speech or communication abilities, and often have physical limitations. They require constant assistance and supervision for all aspects of daily life, including personal care, mobility, and safety. They often require specialised educational, therapeutic and medical intervention to support their needs and quality of life.  People with severe or profound intellectual disabilities are more likely to have medical conditions compared to those with mild or moderate intellectual disability.³   

Causes of intellectual disability

The exact cause of intellectual disability is not known in most cases but is thought to arise from a combination of genetic, environmental, and other factors. These factors may affect brain development and its functioning, leading to intellectual disability.

Genetic conditions 

There are several genetic conditions that are linked to intellectual disability. Some of the most common genetic syndromes can include Down’s syndrome, Fragile X syndrome, Rett syndrome, Prader-Willi syndrome, Angelman syndrome, Cri du chat syndrome, and phenylketonuria.⁴  

Prenatal factors 

Problems during pregnancy can impact foetal development and may cause intellectual disability. Some prenatal factors that can increase the risk of intellectual disability in the offspring can include maternal health (poorly controlled diabetes, hypertension, or thyroid disorders), maternal infections (such as rubella, cytomegalovirus, or syphilis), substance abuse (such as alcohol), and maternal malnutrition.⁵ 

Perinatal factors 

Problems during childbirth or immediately after birth can also cause intellectual disability.  These problems can include birth asphyxia (lack of oxygen to the brain), premature birth, low birth weight, perinatal infections (e.g., neonatal sepsis, meningitis, or encephalitis), neonatal hypoglycemia, and neonatal jaundice.⁵ 

Illness or injury 

Infections (e.g., meningitis, measles, or whooping cough), illness (e.g., childhood stroke), traumatic brain injury, and exposure to toxins during infancy or childhood can lead to intellectual disability.³ 

Assessment and diagnosis 

Assessing those with intellectual disability involves a healthcare professional examining the person’s cognitive abilities, adaptive skills, and developmental history which is based on a clinical interview of the individual and their caregivers. Healthcare professionals will assess intellectual functioning and adaptive behaviours by using cognitive assessments such as standardised tests and observations to evaluate their adaptive behaviour across various domains, including communication, self-care, and social skills. Assessment often involves collaboration with a multidisciplinary team, including psychologists, educators, speech therapists, and occupational therapists, to obtain a comprehensive understanding of the individual's strengths and needs.³

The diagnosis of intellectual disability is based on specific criteria outlined in diagnostic manuals such as the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) or the International Classification of Diseases (ICD-10). These criteria include having significant limitations in intellectual functioning (IQ scores less than 70) and adaptive functioning (impairments in conceptual, social, and practical skills). Intellectual and adaptive limitations must be present during the developmental period, before the age of 18.³

Common conditions that co-occur with intellectual disability 

Co-occurring conditions, also known as comorbidities or comorbid conditions, refer to when two or more medical, developmental, or psychological conditions occur alongside a main health condition in a person at the same time. These conditions can occur on their own or be related or interact with each other, such as the presence of symptoms, diagnostic criteria, and treatment approaches which can affect overall health, functioning and quality of life.³

Living with intellectual disabilities can be challenging and having co-occurring conditions can exacerbate its symptoms and presentation. 

Some common co-occurring conditions can include:

Behavioural disorders 

People with behavioural disorders such as attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are commonly observed in children with intellectual disabilities. The nature of these conditions may manifest as difficulties in impulse control, emotional regulation, aggression, self-injurious behaviour, and social interaction.⁶ 

Mental health conditions 

People with intellectual disability can frequently experience mental health conditions such as mood disorders (e.g., depression and bipolar disorder), anxiety disorders, and psychotic disorders (e.g., schizophrenia). Diagnosing a mental health condition in people with intellectual disabilities may be difficult due to communication difficulties and overlapping of behavioural symptoms.⁷ 

Medical conditions 

Medical conditions such as epilepsy, sensory impairments (e.g., vision or hearing loss), cerebral palsy, and congenital heart defects, were seen in people with intellectual disabilities.⁸ 

Genetic syndromes 

Certain genetic syndromes such as Down syndrome, Fragile X syndrome, Prader-Willi syndrome, Williams syndrome, and Angelman syndrome have been linked to intellectual disabilities.⁸

Communication disorders 

Various communication disorders such as speech sound disorders (e.g., articulation and phonological disorders), language disorders (e.g., expressive or receptive language disorders), and social communication disorders (e.g., pragmatic language impairment) can be experienced in those with intellectual disabilities.⁹

Learning disabilities 

Certain learning disabilities such as dyslexia (difficulties reading, writing and spelling), dyscalculia (difficulty with maths), and dysgraphia (difficulty with writing) can also occur in people with intellectual disabilities.¹⁰  

Summary 

Intellectual disability is characterised by limitations in intellectual functioning and adaptive behaviour which can present challenges that vary widely in severity, ranging from mild to profound levels. Intellectual disability is thought to arise from a combination of genetic and environmental factors, and it can be associated with various co-occurring conditions including physical and mental health conditions, and communication disorders.