The practice of medicine has seen significant changes in recent years, the result of scientific advances in diagnosis and treatment, as well as more general changes in the pattern of disease in our ever-expanding population. Psychiatry itself has undergone more fundamental changes, due in part to recent modifications in classification (DSM-5, and forthcoming in ICD-11) and greater clinical subspecialisation. Moreover, for a variety of reasons comprising both well-elucidated factors and those that are poorly understood, the prevalence of mental disorders continues to rise, which has major implications for the healthcare budget of the nation.

My own specialty, the psychiatry of intellectual disabilities, has itself seen innumerable changes. It has risen in status from psychiatry's Cinderella subspecialty to one that embraces new technologies and scientific advances, in addition to benefitting from new legislation. This is acutely visible in relation to the care of individuals with autism spectrum disorder (ASD), which represents a significant component of the clinical workload of healthcare professionals in the psychiatry of intellectual disabilities. Advances in ASD's conceptualisation, diagnosis, comorbidity and treatment are increasingly incorporated into clinical practice, which includes the publication of evidenced-based guidelines. Generally speaking, the psychiatry of intellectual disabilities sits neatly on the interface of child and adolescent psychiatry, neurology and genetics, and, as a matter of course, it will be affected by developments in these specialisms.

Consequently, while we find ourselves at an exciting juncture in the care of the population of those with disorders of intellectual development (DID), a large body of literature exists that could impact on the clinical care we provide. By ‘we’ I do not refer principally to psychiatrists, but instead to all health and social care professionals working with this population. I believe that both clinicians and allied healthcare workers can benefit from research evidence being distilled down to the most important, clinically relevant points. Both can also profit from the availability of up-to-date references, as they provide the opportunity for broader and deeper reading, and it is with this in mind that the current volume was prepared.

Several of the chapters herein are revised and updated versions of previously published articles from Advances in Psychiatric Treatment.

The psychiatrist plays a pivotal role in multidisciplinary teams that provide support and services for adults with disorders of intellectual development (DID). More than simply a historical artefact, however, this undoubtedly reflects the significance of the behavioural, medical and neuropsychiatric ‘complications’ of DID in terms of their prevalence and complexity, as well as the dynamics of families and support systems. These points notwithstanding, the psychiatrist is only one member of the multidisciplinary team, and the complexity inherent in ‘dual diagnosis’, alongside the important contributions made by relational and environmental factors, demands a range of skills, as will become apparent during the course of this chapter.

The aim of this chapter is to provide an overview of the comorbidity of DID and additional psychiatric illness. The wider concept of ‘mental health problems’ includes the large category of ‘behaviours which are experienced as challenging’, which are the subject of Chapter 5, while the pharmacological treatment of psychiatric illness in this population is discussed by Deb in Chapter 7. Therefore, the present chapter will focus on epidemiology, aetiology, assessment and diagnosis, and will give some overriding principles to consider in designing a management plan.

Epidemiology of psychiatric disorder

Although estimates of the prevalence of psychiatric disorder among individuals with DID vary widely, as a general rule it seems to be higher than in the general population. The variation in prevalence figures reflects bias in ascertainment, the diagnostic methods used and the characteristics of the populations studied, including the sample size (Smiley, 2005). Moreover, the prevalence in institutionalised samples tends to be higher. The most robust studies in epidemiological terms are summarised in Table 3.1. Even within this small number of studies, different methodologies have been employed and the prevalence figures vary widely (e.g. as low as 10% in California in the 1990 study of Borthwick-Duffy & Eyman, and as high as 31.7% in the 2008 study by Morgan and colleagues in Perth, Australia). It should be pointed out that some studies have calculated point prevalence (the studies by Rutter et al, Lund and Cooper et al), while, by their very methodology, others must be considered estimates of lifetime prevalence (the study by Morgan et al and both US studies).

Epilepsy is a common, often chronic neuropsychiatric disorder that sits firmly between the disciplines of neurology and psychiatry. A detailed lexicon has evolved to capture the phenomenology of seizures, and there is increasing understanding of the complex relationship between seizures and associated psychological and behavioural manifestations, as well as frank psychiatric comorbidity. This has facilitated their earlier management. Moreover, research has continued to make significant progress in identifying genetic and metabolic factors that contribute to the aetiology of epilepsy.

Epilepsy is notably more prevalent among individuals with disorders of intellectual development (DID). Disproportionate severity in all the neuropsychiatric characteristics of epilepsy, compounded by unique challenges in diagnosis and management, occur in this population. In this chapter, those aspects of epilepsy that are most relevant to this population will be discussed.

Epidemiology

Epilepsy is a common neuropsychiatric disorder characterised by episodic disturbances of consciousness, sensorimotor function, behaviour and emotion resulting from paroxysmal abnormalities of the electrical activity of the brain. While the seizure itself is described by the symptomatic disturbance, the term ‘epilepsy’ is used when seizures become recurrent, more specifically when there are two unprovoked seizures more than 24 hours apart. The prevalence of epilepsy in the general population is 0.5–1.0% (Banerjee et al, 2009), and the lifetime prevalence is 1.5–5.0%. Among those with DID, however, this prevalence is increased severalfold. For example, among those with severe DID (IQ<50), a prevalence of 30% has been reported, and even among those with milder DID (IQ≥50), the prevalence is still relatively high, at 15% or more (Hannah & Brodie, 1998; Lhatoo & Sander, 2001; McGrother et al, 2006). The risk appears to be higher among those with additional neurological diagnoses, such as cerebral palsy (Singhi et al, 2003), and among those with autism spectrum disorder (ASD) (Spence & Schneider, 2009). As will be discussed below, the prevalence is also higher in association with particular genetic intellectual disability syndromes. Also of note is that the prevalence of the psychological, behavioural and psychiatric manifestations associated with epilepsy are more frequent among those with DID.