12 Intellectual disability12.4 Clinical
12.4.2 Psychiatric presentation, diagnosis and diagnostic overshadowing
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Written by Resident Doctor Committee · Published
Defining intellectual disability
Intellectual disability (learning disability in UK services) requires both intellectual functioning at least two standard deviations below the population mean and deficits in adaptive functioning.
On a test with mean 100 and standard deviation 15, this cut-off is an intelligence quotient (IQ) of about 70.
Borderline intellectual functioning (IQ about 70 to 84) falls short of intellectual disability because adaptive functioning is generally preserved.
ICD-11 severity
ICD-11 calls the condition a disorder of intellectual development and grades it using both intellectual functioning and adaptive behaviour. Mild and moderate grades correspond approximately to 2 to 3 and 3 to 4 standard deviations below the mean, respectively, on appropriately normed assessments.
Grade | Adaptive functioning |
|---|---|
Mild | Largely independent adult living and work, with only intermittent practical help |
Moderate | Considerable, consistent support; academic skills limited to familiar words and simple coin handling |
Severe | Small vocabulary of words and signs; basic self-care gained only by intensive teaching and daily supervision |
Profound | Largely no words or signs; concrete skills only; near-total dependence for self-care |
Severe and profound are separated by adaptive behaviour alone, because intelligence tests cannot measure below their floor.
The provisional category is used when the child is younger than four, or when sensory, physical or behavioural problems prevent valid assessment. No severity level is assigned.
Related classification
Developmental learning disorder in ICD-11 is subtyped by the academic skill impaired (reading, written expression, mathematics, other specified). Dyslexia and dyscalculia fall inside it, and other neurodevelopmental diagnoses are separate categories.
Coexisting developmental, psychiatric and medical disorders are recorded separately; ICD-11 does not use a multiaxial system.
Autism spectrum disorder without a disorder of intellectual development can involve marked social-communication difficulties despite preserved general cognitive ability; functional language is qualified separately.
How mental illness presents
When someone cannot describe their inner experience, mental illness shows as change in sleep, appetite, skills, self-care, activity or interaction, and no single change points to one diagnosis. Judge the change against the person's own baseline.
Diagnostic overshadowing
Diagnostic overshadowing is the error of attributing new symptoms or behaviour change to the intellectual disability itself, so that a coexisting psychiatric or physical condition is missed.
Error | What is missed | Example |
|---|---|---|
Overshadowing | A new treatable problem | New withdrawal and weight loss dismissed as the disability |
Mirror error | A longstanding, non-pathological pattern | Self-talk or fantasy labelled psychosis |
Familiarity does not make a harmful or distressing longstanding pattern harmless.
Ruling out other causes
Physical causes such as constipation, dental or ear pain and reflux can present as agitation or withdrawal.
Epilepsy can cause altered awareness, postictal confusion or fatigue, and antiseizure medicines can change mood and behaviour, especially after a change in treatment.
Also review hearing, vision, pain, anticholinergic burden, akathisia, unmet communication needs, environmental stress and safeguarding.
Presentation of specific disorders
Adapted criteria such as the Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities (DC-LD) support current criteria. They do not replace a full assessment.
Psychosis
Schizophrenia presents more simply, and thought disorder and complex systematised persecutory delusions are less evident.
Earlier onset, prominent negative symptoms, epilepsy and behavioural presentations in severe disability are genuine features.
Recognition is delayed mainly because the person struggles to communicate subjective experiences such as hallucinations and delusions. A thin verbal account does not rule psychosis out.
Depression
Depression can show as withdrawal, irritability, changed sleep or appetite, lost skills or self-injury, without any report of low mood.
Self-injurious behaviour
Self-injury first appears around 2 to 3 years. It is most frequent and severe from later childhood through adolescence (roughly 5 to 20 years), and it eases in adulthood.
Post-traumatic stress disorder and abuse
Abuse by a known carer or acquaintance, rather than assault by a stranger, is the commonest identified cause of post-traumatic stress disorder (PTSD) in intellectual disability.
PTSD presents mainly as behavioural change from baseline: aggression (what most often brings it to attention), self-injury, regression and sleep disturbance.
PTSD is under-recognised because limited verbal ability restricts self-report, so recognition depends on change that informants observe.
Sexual abuse of people with learning disability is usually committed by someone the victim knows, and perpetrators are almost all male.
None of these signs proves that abuse took place, so the safeguarding assessment must be safe and non-leading.
Dementia
Dementia means decline from previous functioning, not a low absolute score. Down syndrome carries a higher and earlier risk of Alzheimer's disease, so new loss of skills or orientation in middle age warrants assessment.
Collateral information
Family members tend to know the long-term baseline, while paid staff can give detailed recent observations. Use more than one source and record what is missing.