Overview

What is intellectual disability (ID)?

Intellectual disability intellectual(ID) is a neurodevelopmental condition which causes deficits in intellectual functioning (learning, having thoughts/ideas which go beyond considering physical concepts we may observe), problem-solving, reasoning and understanding complex ideas) and in adaptive behaviour/adaptive functioning (conceptual, social and practical skills).^1,2

Common intellectual disabilities include: 

• Fragile X syndrome
• Fetal alcohol syndrome
• Down syndrome
• Prader-Willi syndrome³

What are conceptual, social, and practical skills?

Conceptual/intellectual functioning skills include:

• Understanding and breaking down complex ideas/problems and finding solutions to them

Social skills include:

• Ability to communicate, listen, and understand 
• Solving problems 
• Recognise body language, facial expressions, and emotions
• Responsibility towards the environment, society, and themselves

Practical skills include:

• Being able to budget and spend money when needed
• Performing daily living tasks — cooking, eating and drinking, washing and cleaning (which includes personal hygiene)⁴

Importance of correct diagnosis and assessment of intellectual disability 

Intellectual disability is diagnosed by the DSM-5 and ICD-11 criteria.^1,6

Correct classification, diagnosis and assessment of intellectual disability allow patients to live a happy, healthy, and productive life by:

•  Guiding suitable treatment recommendations
• Preventing complications in the long-term (such as resulting from comorbid conditions) 
• Increasing public knowledge of intellectual disability therefore dispelling shame and stigma centred around the condition.
• Promoting further research on intellectual disability to decipher its causes and effective treatment.⁵

Criteria to meet in DSM-5 and ICD-11 to diagnose intellectual disability

According to DSM-5 (Diagnostic and Statistical Manual of Mental Disorders version 5) and ICD-11 (International Classification of Diseases version 11) guidelines, three criteria must be met to diagnose a patient with intellectual disability. These are:

1. Deficits in knowing, learning and understanding, solving problems, judgement, reasoning, and learning from previous experiences
2. Deficits in adaptive behaviour (which include daily tasks such as budgeting money, washing, cleaning, eating, drinking and maintaining one’s care) cause loss of independence and difficulty in managing tasks in daily life
3. Childhood-onset (before age 18) of the disability according to DSM-5, and onset in the developmental period according to ICD-11^1,6

Both DSM-5 and ICD-11 emphasise deficits in cognitive and adaptive functioning as criteria for diagnosis of intellectual disability.^1,6

Modifications from preceding versions of diagnostic manuals

DSM-4 has been modified to create DSM-5, the latest diagnostic manual for mental disorders. 

These modifications include:

• DSM-5 does not use specific Intelligence Quotient (IQ) scores (a score which measures human intelligence) as it did in DSM-4
• Whilst DSM-IV specified deficits in two or more skill areas (e.g. communication, problem-solving, leadership etc) the DSM-5 criteria point to a deficit in one or more superior skill domains (e.g. conceptual, social, practical), therefore, it is more inclusive than the DSM-4 criteria.
• DSM-5 focuses on the performance of daily tasks in life (adaptive behaviours) compared to DSM-4¹

ICD-10 has been modified to create ICD-11, the latest diagnostic manual for mental disorders.⁷

These modifications include:

• Introduction of new diagnoses 
• Improvement of diagnostic criteria of prevailing diagnoses
• Steps made in the direction of looking at the quality of having many different features/qualities for some diagnoses⁷

Both DSM-5 and ICD-11 redefined the term ‘mental retardation’ to intellectual disability.^7,8

Assessment of intellectual disability 

Intelligence ouotient (IQ) tests

Tests such as the Wechsler Scales of Intelligence and the Stanford Binet Intelligence Scales measure IQ scores, which are used to assess the intellectual functioning of patients.^4,9 An IQ score of 70 or below indicates an intellectual disability diagnosis.⁴ Based on the IQ score, the severity of the intellectual disability is discerned.⁴ 

Adaptive behaviour scales 

Vineland Adaptive Behaviour Scales assesses a patient’s adaptive functioning (such as exercising, maintaining personal hygiene, eating and drinking, communication and understanding skills)  by interviewing the patient or their carer(s).⁹

Adaptive Behaviour Assessment System uses rating forms featuring questions to rate and score a patient based on skill areas within conceptual, social, and practical domains. For example, social skills may be assessed, and on a four-point response scale, raters will indicate whether the individual can do this activity and if so, how often they do this activity when required.¹⁰

Psychologists, educators, and medical health professionals aid in the diagnosis and assessment of intellectual disability. They analyse and interpret results compiled from patients to deduce the most suitable course of treatment. 

Barriers to diagnosis and assessment of intellectual disability

Difficulties in diagnosis of intellectual disability in culturally and linguistically diverse patients

For patients who speak limited/no English and/or come from a different culture, it may be challenging to diagnose intellectual disability. This is due to:

• Shame and stigma around discussing and seeking treatment for intellectual disability
• Stigma and suspicion around approaching psychologists, educators, and medical health professionals to diagnose and assess a disability
• Lack of interpreters who have experience working with individuals with intellectual disability and lack of medical health professionals who may speak the language spoken by the individual¹¹

Differential diagnosis and comorbid conditions

Commonly, other comorbid conditions may meet the criteria to diagnose intellectual disability therefore a differential diagnosis is required to decipher the most accurate diagnosis.¹²

Differences between ID and learning disabilities, autism, and psychiatric conditions 

Learning disabilities and difficulties such as dyslexia, dyspraxia, dyscalculia and dysgraphia, autism, ADHD, and other psychiatric conditions may present with features of intellectual disability. A patient of any intellectual ability may be diagnosed with learning disabilities or difficulties, autism and psychiatric conditions, compared to patients who meet the criteria for diagnosis of intellectual disability, who have deficits in intellectual and adaptive functioning.^13,14

Impact of age in assessment of intellectual disability

The barriers posed by age to the assessment of intellectual disability include:

• In children, intellectual disability may be assessed in school as children with ID learn and develop at a slow speed compared to children of the same age without intellectual disability. To assess intellectual disability in adults, the same assessment does not apply, which can be a barrier for those who may have it.
• Standardised tests for measuring intellectual and adaptive behaviours are difficult to use in extremely young children as results can be unreliable or invalid.
• Older children with comorbid medical and mental health disorders may be difficult to assess.^1,15

These impacts may be reduced by reassessing children who are diagnosed with intellectual disability in childhood when they reach teenage years and adulthood respectively to identify any required support from medical health professionals, educators, and psychologists for individuals and any changing needs. 

Future studies

New neuroimaging and genetic testing techniques

• Non-invasive neuroimaging techniques include different variations of fMRI, EEG, DTI (a type of MRI), and TES which help to understand brain activity, structure and function in more detail to understand intellectual disability and combine these findings with existing genetic and behavioural data on intellectual disability. 
• Faster DNA sequencing of the entire human genome to diagnose rare genetic diseases/find genetic variants which predispose an individual to intellectual disability^16,17

Summary

Developmental delays in intellectual, social and practical domains characterise intellectual disability (ID). Diagnosis involves meeting criteria set by current DSM-5 and ICD-11 guidelines; which are deficits in patient intellectual and adaptive functioning, and a childhood onset of ID before age 18. Assessment of ID involves IQ and adaptive behaviour scales to measure scores of intelligence and ability to carry out activities of daily living respectively in each test. 

Barriers to diagnosis of intellectual disability include patient age, comorbid conditions similar to ID, difficulty in differentiating ID from autism, psychiatric conditions and learning disabilities, and linguistic and cultural diversity.

FAQs

How do you diagnose intellectual disability?

Intellectual disability is diagnosed upon meeting three compulsory criteria in the DSM-5 and ICD-11, which include deficits in intellectual and adaptive functioning, and a childhood onset of intellectual disability before age 18.

What assessments are used for intellectual disability?

Assessments for intellectual disability include IQ tests used to measure intellectual functioning/intelligence and adaptive behaviour scales used to measure adaptive functioning.

What is the diagnosis and assessment of a disability?

Diagnosis involves the detection of a disability by signs and symptoms and the assessment involves tests of the ability to perform skills and daily living tasks and medical history checks, as well as explaining relevant local and community services/support mediums for patients with a particular disability.