Intellectual disability: severity levels, epidemiology, etiology, clinical features, course, and principles of treatment and care

01Definition and overview

Intellectual disability has three parts: intellectual functioning significantly below average, usually about two standard deviations or more below the mean (an IQ below about 70); significant limits in adaptive behaviour, the conceptual, social and practical skills of everyday life; and onset during the developmental period. An IQ score alone never makes the diagnosis.

The names have changed as each became an insult: ICD-10 uses mental retardation (F70 to F79), ICD-11 disorders of intellectual development (6A00), and DSM-5-TR intellectual developmental disorder (intellectual disability). In the UK, learning disability means the same thing, while a learning difficulty usually means a specific problem such as dyslexia, with normal general intelligence.

ICD-10 adds a fourth character for the degree of behavioural impairment, from none to significant. ICD-11 and DSM-5 grade severity by adaptive behaviour, and ICD-11 tells severe from profound by adaptive behaviour alone, since IQ tests cannot measure reliably at that level. ICD-11 also has a provisional category for children under about 4, or people with sensory or physical impairments, who cannot yet be assessed properly; DSM-5 uses global developmental delay for children under 5. An IQ of 70 to 84 is borderline intellectual functioning, not a disorder.

02Etiology and pathophysiology

A specific cause is found in about 80 per cent of severe and about half of mild intellectual disability. Severe disability is usually biological and spread evenly across social classes; mild disability is commoner with social deprivation and often runs in families without a single cause. Grouped by timing:

The syndromes worth knowing:

Rett syndrome, an X-linked cause in girls, is covered with the pervasive developmental disorders. Iodine deficiency remains the commonest preventable cause of intellectual disability worldwide.

03Clinical features

Severe disability is usually recognised in infancy or early childhood, often with a known syndrome; mild disability is often noticed only at school, when learning falls behind.

Comorbidity

Physical and mental illness are commoner, and more likely to be missed:

• Epilepsy affects about a fifth of people with intellectual disability, and a third or more of those with severe or profound disability; seizures may be hard to tell from behaviour, and sudden death in epilepsy is a leading cause of early death.
• Autism is present in roughly a fifth to a third, more often in severe disability, and ADHD is common.
• Sensory impairments: poor sight and hearing are common and often undetected.
• Physical problems: swallowing difficulty and aspiration, gastro-oesophageal reflux, constipation, which can progress to fatal bowel obstruction, obesity or underweight, osteoporosis, dental disease, hypothyroidism (especially in Down syndrome), and dementia, early in Down syndrome.
• Mental illness is two to three times commoner: depression, anxiety, psychosis (schizophrenia about three times the general rate), bipolar disorder and dementia. A third or more have a mental health problem at any time if challenging behaviour is counted.

In more severe disability, mental illness shows through change: withdrawal, loss of skills, poor sleep and appetite, tearfulness or aggression in depression; overactivity and disinhibition in mania. The history comes largely from carers who know the person's usual self, and diagnostic criteria adapted for intellectual disability (the DC-LD in the UK, the DM-ID-2 in the US) help.

Behaviour that challenges

Challenging behaviour (aggression, self-injury, destruction, stereotypies, pica, smearing) affects about 1 in 10 to 1 in 6 people in services, more often in severe disability, autism and some syndromes. It is behaviour, not a diagnosis, and usually has a function: to escape a demand, to get attention or an object, sensory stimulation, or the expression of pain, distress or illness.

04Diagnosis and management

Making the diagnosis and finding the cause

1. 1

   History

   Pregnancy, birth and neonatal course, milestones, any loss of skills, seizures, family history and consanguinity.
2. 2

   Examination

   Growth and head circumference, dysmorphic features, skin signs of neurocutaneous disorders, a neurological examination, and always vision and hearing.
3. 3

   Measurement

   A standardised IQ test (Wechsler scales) and an adaptive behaviour scale (Vineland), interpreted with culture, language and sensory problems in mind.
4. 4

   Cause

   Genetic tests first, then others as the picture suggests; a clinical geneticist and genetic counselling for the family.

Chromosomal microarray is the first-tier genetic test, finding the deletions and duplications too small for a karyotype in about 1 in 7 to 1 in 5 children with unexplained disability, with FMR1 testing for fragile X. Exome or genome sequencing, increasingly offered next or first (in England, whole genome sequencing is in the national test directory for intellectual disability), finds a cause in many more. Metabolic tests are done for regression, consanguinity, episodes of decompensation or organomegaly; thyroid function and, in boys, creatine kinase are cheap and useful; MRI for an abnormal head size, focal signs or seizures; EEG for suspected seizures. Finding the cause explains the disability to the family, guides screening for associated problems and informs the risk in future pregnancies.

Principles of care

Care is lifelong, person-centred and multidisciplinary: psychiatrists of intellectual disability, paediatricians, psychologists, speech and language therapists, occupational therapists, physiotherapists, dietitians, nurses and social workers, with the family at the centre.

• Communication: plain words, easy-read material, pictures, signing systems such as Makaton, and symbol systems such as PECS; the person is spoken to directly, with carers helping, and is offered time alone.
• Education and development: early intervention, special educational support, and planned transitions to adult services, work and housing.
• Family: support, respite and planning for the time when ageing parents can no longer care.
• Rights: relationships, sexuality and parenthood are supported; abuse and exploitation are more common, so safeguarding is part of care.

Physical health and reasonable adjustments

In England, everyone with a learning disability is offered an annual health check by their GP from the age of 14, with a health action plan. The law (the Equality Act 2010 in Great Britain, the UN Convention on the Rights of Persons with Disabilities across Europe) requires services to make reasonable adjustments: longer or first appointments, a quiet place to wait, easy-read letters, a hospital passport describing the person's needs and how they show pain, learning disability liaison nurses, and a flag on the record. A learning disability is never on its own a reason for a do-not-attempt-resuscitation decision.

Mental illness and behaviour that challenges

Mental illness is treated as in anyone else, adapted to the person: CBT and other therapies adjusted to the level of understanding, and drugs started low and increased slowly, because sensitivity to side effects and paradoxical reactions are commoner. For behaviour that challenges, the order is:

1. Functional assessment: what happens before, during and after the behaviour (ABC charts), and what it achieves for the person.
2. Treatment of what lies beneath: pain, constipation, epilepsy, side effects, sensory problems, mental illness.
3. Positive behaviour support: changes to the environment, routines and communication, teaching new skills, and agreed responses to the behaviour.
4. Medication only if these have not worked in a reasonable time, the risk to the person or others is severe, or for a coexisting illness: usually an antipsychotic such as risperidone, at a low dose, reviewed after 3 to 4 weeks and stopped if there is no clear benefit by 6 weeks.

Overmedication is a known harm. An estimated 30,000 to 35,000 adults with a learning disability in England were taking an antipsychotic, an antidepressant or both without a diagnosed condition, which prompted NHS England's STOMP programme in 2016 (and STAMP for children). In a randomised trial for aggression in intellectual disability, placebo did at least as well as risperidone or haloperidol. Every psychotropic drug needs a recorded reason, consent or a best-interests decision, monitoring for side effects, and review, ideally every 3 months and at least every 6, with gradual reduction when it is no longer needed. Restraint and seclusion are a last resort.

Capacity and consent

Capacity is presumed, specific to each decision and never decided by a diagnosis. Under the Mental Capacity Act 2005 in England and Wales, a person lacks capacity for a decision only if an impairment of the mind or brain leaves them unable to understand the relevant information, retain it, use or weigh it, or communicate a choice, after all practicable help has been given: simple words, pictures, time and the best time of day. A person with capacity may make an unwise decision. If capacity is lacking, a best-interests decision is made, consulting family and carers, choosing the least restrictive option, with an independent advocate if there is no one else; a relative cannot consent for an adult unless appointed as their attorney or deputy for health decisions. Much of Europe relies on guardianship, which the UN Convention pushes towards supported decision-making.

05Epidemiology

of the population have intellectual disability

about 1per centof the population have intellectual disability

of them have the mild form

85per centof them have the mild form

have Down syndrome, more with older mothers

1 in 700–1000birthshave Down syndrome, more with older mothers

Prevalence is about 1 per cent in meta-analyses, higher in low- and middle-income countries, where infection, malnutrition, iodine deficiency and perinatal injury are commoner. Males are affected about one and a half times as often, partly from X-linked causes such as fragile X (about 1 in 4,000 males and 1 in 8,000 females). Fetal alcohol spectrum disorder is estimated at about 2 per cent of children in Europe, the highest of any WHO region. Down syndrome live births are fewer where prenatal screening is widespread. Many adults with mild disability are never identified by services.

06Prognosis

Intellectual disability is lifelong but not fixed: skills keep developing with education and support, adaptive functioning can improve, and many people with mild disability live independently, work and have families. A progressive course suggests a degenerative cause, such as Rett syndrome or a storage disorder, or a new problem such as epilepsy, depression or dementia.

Life expectancy is shorter. The 2024 LeDeR report found a median age at death of about 63 years for adults with a learning disability in England, 19 years earlier than the general population, and 39 per cent of deaths were from avoidable causes. The commonest causes are respiratory disease, especially aspiration pneumonia, circulatory disease, cancers diagnosed late, epilepsy and bowel obstruction from constipation. People with Down syndrome now live to about 60 on average, and most develop Alzheimer's disease by their sixties, so a baseline assessment of their usual abilities in early adulthood makes later decline easier to recognise.