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.