Intellectual disability is a descriptive term not a condition in itself and although the number of identifiable disorders that alter neurodevelopment and create an associated intellectual disability is steadily increasing, we are only beginning to understand the many ways in which the altered neural substrate modifies the course and presentation of coexisting psychiatric disorders. Save in the case of relatively common conditions, such as Down syndrome or fragile-X syndrome, studies in intellectual disability usually involve patients with ill-defined, heterogeneous aetiologies grouped on the basis of their degree of intellectual impairment rather than aetiology. It is clear, however, that adults with intellectual disability are susceptible to the whole range of psychiatric disorders seen in the general population. In addition, our understanding of behaviour phenotypes has developed over recent years; this concept has helped us understand some of the links between genetics and behaviour (O'Brien & Yule, 1995).

The term intellectual disability is used throughout this chapter because it has been adopted by the Royal College of Psychiatrists and is used internationally.

The evidence base in this area of ECT use is composed almost entirely of case reports. The limited nature of this evidence, compounded with specific issues regarding diagnosis and consent, partially explains why ECT seems to be used rarely in people with intellectual disability.

Diagnostic issues

Principles of psychiatric assessment in this population are similar to those in general adult and child psychiatry; particular attention is given to the person's level of communication and understanding, developmental history, direct observation and information from informants, as well as the exploration of associated disabilities.

The predominant view is that psychiatric disorder can be reliably diagnosed using standard diagnostic classifications in people with mild intellectual disability (Meins, 1995; Hurley, 2006). However, diagnosis is more difficult in those with a more severe level of intellectual disability. Diagnostic criteria are very much language-based, so they are less relevant to people with significant communication difficulties. It is then much more difficult, if not impossible, to assess cardinal features of psychiatric disorder such as low self-esteem, guilt (Hemmings, 2007), delusions or hallucinations. The presence of intellectual disability will alter the way that signs of psychiatric disorder manifest themselves. Psychiatric diagnosis can be difficult because of the frequent assumption that symptoms could be part of the presentation of the intellectual disability because of ‘diagnostic overshadowing’ (Santosh & Baird, 1999).