27th October 2023

Intellectual Disability (ID)

Classification

The evolution of classification for what is now commonly known as ID has developed significantly since it was first introduced into diagnostic manuals. Historical, underspecified terminology included terms such as simpleton and feeble-minded, have since progressed as greater efforts towards systemisation, including early IQ testing, yielded the historical classifications of moron (IQ 51-70), imbecile (IQ 26-50) and idiot (IQ 0-25).

These terms have developed further due to the ‘euphemistic treadmill’ – where a word becomes pejorative because it refers to something that attracts stigma, eventually leading to its replacement. In due course, this replacement might come to attract the same stigma, necessitating a further change in terminology. Change is often driven by activists working on behalf of the marginalised group, e.g. Rosa’s Law passed in 2010, which mandated the change of the term ‘mental retardation’ to ‘intellectual disability’ in the United States.

Advancements in descriptive psychopathology and testing have also helped clarify the diagnostic process, which now includes the consideration of:

• Standardised IQ and neuropsychological tests of domains such as working memory, processing speed, verbal comprehension and perceptual reasoning.
• Increasing specification of intellectual functioning and adaptive skills across different domains (see below).
• The inclusion of presentation and domain specific development across the lifespan.
• The inclusion of culture-, sex- and/or gender-related features.

The domains used by contemporary classification systems include:

• Conceptual – Communication, knowledge based tasks, e.g. problem solving, reading, writing, calculating.
• Social – Managing relationships, following rules and laws, avoiding victimisation.
• Practical – Skills for self care, work and recreation, finances, transport, health and safety, technology.

Contemporary diagnoses are made on the basis of a battery of standardised tests of intellectual and adaptive functioning (where possible) with scores compared to the mean, as these methods are more useful than IQ scores in isolation for understanding intellectual ability. These tests are performed in conjunction with clinical judgement of behaviours across the three domains highlighted.

Presentation

The following is a table depicting a breakdown of this in comparison to previous diagnostic categories and assessments:

Another relevant inclusion from the DSM-V is ‘Global developmental delay’, which can be diagnosed when individuals fail to meet expected developmental milestones across multiple domains of intellectual functioning and are unable to undergo systematic assessment, e.g. because they are too young (<5yrs). Individuals must be reassessed once capable of undergoing formal testing.

Specific aetiologies

As with many other neurodevelopmental disorders, the specific aetiology in the majority of cases remains unknown. However, there are a number of specific causes of ID to be aware of:

• Chromosomal and genetic conditions – Down syndrome, fragile X syndrome, phenylketonuria.
• Pregnancy and parturition related – Foetal alcohol spectrum disorder, maternal rubella or HIV infection, hypoxia during delivery.
• Problems in childhood – Infection (e.g. measles, meningitis), malnutrition (e.g. iodine deficiency), poisoning (e.g. lead or mercury), traumatic brain injury, asphyxia, stroke.
• Foetal alcohol spectrum disorder (FASD) – Describes the clinical features that can present in children from birth following maternal alcohol consumption during pregnancy, including facial anomalies (see picture); growth deficiency (weight/height <10th percentile); microcephaly; cognitive and behavioural impairments (e.g. problems with executive function, memory, attention, learning, language, visuospatial and motor processing etc.) Any alcohol intake at any time during pregnancy can lead to these features, especially above five units per day, but the greater the intake, the greater the risk. The prevalence is thought to be on par with ASD, perhaps unsurprising given that >40% of UK women consume some alcohol during pregnancy (Popova el al., 2023).

Image Source: Wikimedia Commons (2021). hoto of baby with FAS, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Photo_of_baby_with_FAS.jpg (Accessed on: 18 October 2023).

References

• Mentalhelp. History of stigmatizing names for intellectual disabilities continued. Available online at: https://www.mentalhelp.net/intellectual-disabilities/history-of-stigmatizing-names-for-intellectual-disabilities-continued/ [Accessed March 2023]
• American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders (4th ed.).
• American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.).
• Lee, K., Cascella, M., & Marwaha, R. (2019). Intellectual disability.
• Popova, S., Charness, M. E., Burd, L., Crawford, A., Hoyme, H. E., Mukherjee, R. A., … & Elliott, E. J. (2023). Fetal alcohol spectrum disorders. Nature Reviews Disease Primers, 9(1), 11.
• Stavrakantonaki, M., & Johnson, T. P. (2018). Effects of Rosa’s Law on Intellectual-Disability Reporting. Public Opinion Quarterly, 82(3), 593-604.
• World Health Organization. (1993). The ICD-10 classification of mental and behavioural disorders: Diagnostic criteria for research (7th ed.).
• World Health Organisation. (2019). Neurodevelopmental disorders. International statistical classification of diseases and related health problems (11th ed.).