12.2 Epidemiology of intellectual disability and coexisting disorder

Defining a case

• Intellectual disability requires impaired intellectual functioning, impaired adaptive functioning and onset during development.

• Adaptive functioning means independence in communication, self-care, social life and practical activities, judged against age and culturally appropriate expectations.

• IQ alone is not enough, because anxiety, sensory impairment, language or unsuitable norms can lower test scores.

• The International Classification of Diseases, 11th revision (ICD-11), places mild impairment two to three standard deviations below the mean and moderate impairment three to four below.

• Adaptive functioning carries most weight where cognitive testing is unreliable, and failing to complete a test does not itself establish severity.

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Occurrence and severity

• Incidence peaks at school age, around 10 to 14 years, because mild disability is often recognised only when school demands rise.

• Borderline intellectual functioning (IQ about 70 to 84) carries a raised risk of later psychiatric disorder, including schizophrenia.

• Theoretical prevalence, modelled from intellectual and adaptive impairment, is around 2% of UK adults, depending on method.

• Administrative prevalence, counted from service registers, is lower because mild disability is often missed after school.

Specific reading difficulty

• Developmental dyslexia is identified more often in boys than in girls.

• About 16% of Japanese schoolchildren read at least a year behind the level expected for their age.

Genetic causes

• Down syndrome is the commonest genetic cause of intellectual disability, but it is almost always a sporadic nondisjunction event rather than inherited.

• Fragile X syndrome is the commonest inherited cause and second only to Down syndrome among genetic causes. It is an FMR1 CGG repeat expansion affecting about 1 in 4,000 males.

• Autosomal causes, usually arising de novo and including the imprinting disorder of 15q11-q13, affect males and females equally.

• X-linked and sex chromosome syndromes are skewed towards one sex.

• Lesch-Nyhan syndrome is caused by deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT), the purine salvage enzyme, and produces excess uric acid.

Maternal age and Down syndrome

The risk at live birth rises slowly through the twenties and steeply after about 35.

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Outcomes in Down syndrome

• Respiratory infection, chiefly pneumonia, is the leading cause of death.

• Alzheimer pathology (plaques and tangles) is present in over 90% of people by age 40, because trisomy 21 gives an extra copy of the amyloid precursor protein gene.

• Clinical dementia lags behind the pathology and is typically diagnosed at about 55 years, years earlier than in the general population.

Epilepsy and autism

• Epilepsy affects about 20% of people with intellectual disability, against under 1% of the general population.

• Epilepsy rises with severity, affecting about 15 to 20% in mild disability and 30 to 50% in severe and profound disability.

• Autism affects about 20 to 30% of adults with intellectual disability known to services, against about 1% of the general population.

• Conversely, intellectual disability is the condition most often co-occurring with autism, present in about a third of autistic people.

• A full sibling of a child with idiopathic autism has a recurrence risk of roughly 10 to 20%.

• Among adults with intellectual disability, autism keeps a clear male predominance, while the marked childhood male excess of attention deficit hyperactivity disorder (ADHD) narrows and is not consistently seen.

Mental ill health in adults with intellectual disability

Cooper and colleagues (Glasgow, 2007) showed that point prevalence depends heavily on the diagnostic criteria used. The historical classifications below identify the study methods; the estimates should not be relabelled as ICD-11 findings.

• Diagnostic overshadowing is attributing new symptoms to the disability itself, so coexisting conditions are missed.

• Depression is reported more in mild to moderate than in severe to profound disability, more in institutions than family homes, and more in older than younger people.

Why rates are high and hard to measure

• Rates run high through shared biological causes, trauma, deprivation, exclusion, unmet communication needs and physical illness.

• Constipation, dental pain or fractures may present as behaviour change rather than a complaint.

• Proxy reports from carers miss internal symptoms, because behaviour is more visible than inner experience.

• Specialist and institutional samples give higher estimates than population samples, because they select for complexity.

Mortality

• LeDeR, the English mortality review programme, documents substantial premature death and avoidable care failures in people with intellectual disability.

• Respiratory disease, including aspiration pneumonia, and epilepsy contribute, but the leading recorded cause varies by year and population.