Intellectual disability (ID)

The concept of an ID is that some people have difficulty in caring for themselves due to deficits in intelligence. The current DSM–IVTR diagnostic uses the term ‘mental retardation’. The criteria are shown in Box 19.1.

Population surveys usually find that about 2% of the population have an ID. People with severe ID are more likely to be identified early in life and are more likely to have an identifiable aetiology. Below the age of 6, the diagnosis is based on delay in achieving developmental milestones. Those with less severe ID may not be identified until they start school, when poor academic performance becomes evident. Some may not be identified until secondary school and may present with oppositional behaviours and truancy due to their inability to meet the demands placed on them. The diagnosis is confirmed by demonstrating the IQ and functional deficits. At any age it is important to exclude medical problems that may be contributing to impaired intellectual function (e.g. hypothyroidism).

Aetiology

There are over 300 identified causes of ID and more are being identified as technology becomes more advanced, particularly in the field of genetics. In clinical populations the cause will be identified in about 10–15% of cases, but with more intensive investigation it is possible to determine the aetiology in up to a third of cases. Understanding aetiology is useful for the following purposes:

in the development of preventative strategies

in predicting the health needs of people with specific syndromes, including targeted screening and intervention programs, and

in understanding certain behavioural patterns and the development of particular psychiatric disorders that are associated with specific syndromes.

Autism and autism spectrum disorders

In the 1940s, Kanner initially described autism as a condition characterised by lack of verbal development, significant cognitive impairment, and a characteristic lack of interest in interacting with other people. Over time, the concept of a spectrum of autistic disorders has evolved, referring to a group of disorders with impairments of varying severity in three major areas, namely social interaction, communicative functioning and imagination.

These three functional deficits are referred to as the triad of impairments and can occur across a range of IQ scores. DSM–IVTR criteria are listed in Box 19.2.

In DSM–IVTR, the term pervasive developmental disorders (PDD) is used as the overarching term that covers the autistic spectrum disorders. This group includes five specified conditions, as shown in Box 19.3. See also Chapter 16 for a comparison between autism and Asperger’s disorder.

The earliest epidemiological studies used Kanner’s very narrow criteria and found a prevalence rate for autism of around 0.05%. Much higher rates (around 1%) have been recorded using current diagnostic criteria for autistic spectrum disorders. The diagnosis is a clinical one and consists of gathering evidence of impairments in the triad of domains. The manifestations of these deficits are broadly covered in the diagnostic criteria (see Box 19.1 above), but can present in many different ways depending on age, sex, IQ, personality and situational context. The more severe the autism, the earlier the diagnosis; the more subtle variations may never be formally identified.

In addition to the classical triad, there are some other features that are common but not essential for the diagnosis. These include odd responses to sensory stimuli, including both hypersensitivity (e.g. extremely good hearing) and hyposensitivity (e.g. a very high pain threshold), poor motor coordination, over or under activity, and abnormalities of mood and of biological functions (e.g. eating, drinking and sleeping). Physical disabilities such as epilepsy and sensory impairments are common.