Intellectual disability is defined by the World Health Organization (WHO) as: ‘a significantly reduced ability to understand new or complex information and to learn and apply new skills (impaired intelligence) resulting in a reduced ability to cope independently (impaired social functioning)’, and begins before adulthood, with a lasting effect on development. People with an intellectual disability can develop any of the mental illnesses common to the general population; however, they are up to three times more likely to develop a mental illness. This predisposition to psychiatric illness can occur due to a variety of reasons, including associated genetic syndromes, brain injury, and sensory impairments. People with an intellectual disability are also more likely to have negative psychosocial experiences, such as deprivation, abuse, separation/loss events, low self-esteem, and financial disadvantage, and consequently the ensuing effects of these can affect their mental health. The psychiatric assessment of a person with an intellectual disability broadly covers the same as that of the general population; however, a different approach at times is needed to adapt to the individual’s communication skills. It may be necessary to complete history taking from a family member or carer, and an MSE may even have to be completed solely on observable behaviours. It is also more pertinent to focus on any co-existing medical conditions, such as epilepsy which is present in 25–30% of people with an intellectual disability. The WHO states that the true prevalence of intellectual disability is close to 3%. The vast majority of these people (85%) have mild intellectual disability defined as an IQ of 50–69 points. Many of these people can and do access mainstream services (with or without additional support). In whichever service you work, doctors and medical students will encounter people with intellectual disabilities, and an awareness of their needs is essential.
The intellectual disability of trisomy 21: differences in gene expression in a case series of patients with lower and higher IQ
