Psychopathology is 2-3 times more common in intellectually disabled (ID) children than in the general population.^(1,2) Psychiatric disorder is the most common source of additional handicap causing loss of educational, recreational, and social opportunity, burden for carers and cost to the community. Numerically, the size of this problem is approximately equal to schizophrenia, but is less well-recognized due to diagnostic over shadowing in which psychiatric disorder is not differentiated from ID as a separate condition open to diagnosis and treatment.⁽²⁾ Although there is probably a significant reduction in overall prevalence of psychiatric disorders from approximately 43 per cent of children to 37 per cent of young adults with ID, psychopathology if present in childhood is likely to persist.⁽²⁾ The profile of disorders varies from childhood into young adult life with the prevalence of attention-deficit hyperactivity symptoms decreasing, the frequency of symptoms of depression increasing and the prevalence of anxiety remaining stable with maturation.⁽²⁾

There are two approaches to the description and classification of psychopathology in young people with ID. First is the application of DSM and ICD diagnostic criteria. The reliability and validity of this approach is not well established when applied to children with ID.⁽³⁾ Young people with more severe ID and language impairment are unable to report abnormalities of their emotions, thoughts, and perceptions, which are criteria for conditions such as obsessive-compulsive disorder (OCD) and schizophrenia. Some diagnoses, for example attention-deficit hyperactivity disorder (ADHD)⁽⁴⁾ require a judgement that symptoms are inconsistent with developmental level, which in a child with ID is delayed relative to chronological age. The DSM-IV TR⁽⁴⁾ specifies that either ADHD or separation anxiety disorder should not be made ‘exclusively during the course of a pervasive developmental disorder’. These restrictions on comorbid diagnosis should not limit the necessity to describe the range of presenting symptoms and offer appropriate treatment, for example the use of stimulant medication in a child with autism and severe ADHD symptoms. Developmental level and degree of cognitive impairment influence the presentation of symptoms. For example, children with ID are more likely than children in general, to have externalizing symptoms such as disruptive, aggressive, impulsive, or avoidant behaviours; if psychotic to experience hallucinations without delusions; or if depressed to present with irritability and stereotypies. Self-absorbed, autistic, and withdrawn behaviours are more common in children with severe ID whereas anxiety, disruptive, and aggressive behaviours are more likely in children with milder levels of ID.⁽²⁾ Some patterns of psychopathology recognized by DSM-IV TR⁽⁴⁾ are specifically associated with more severe ID such as ‘stereotypic movement disorder with or without self-injurious behaviour’. Other emotional and behavioural disturbances seen in people with ID receive non-specific, atypical, or not otherwise specified classifications and await better definition. Recent attempts to produce diagnostic criteria for psychiatric disorders in people with ID (the draft ICD-10 guidelines for the psychiatric assessment
of persons with mental retardation,⁽⁵⁾ the Royal College of Psychiatrists diagnostic criteria for psychiatric disorders for use with adults with learning disabilities⁽⁶⁾ and the DSM-IV TR for intellectual disability⁽⁷⁾ are mainly designed for use with adults and require clinical validation).

The second approach to the definition of psychopathology in young people with ID is the use of informant questionnaires which rate disturbed emotions and behaviour. Factor analysis produces subscales which have clinical utility and refer to dimensions of disturbance such as disruptive/antisocial behaviours, social withdrawal, self-absorbed behaviours, communication disturbance, and anxiety/depression. Two reliable questionnaires validated for use in children and adolescents with ID are the Nisonger Child Behaviour Rating form⁽⁸⁾ and the Developmental Behaviour Checklist.⁽⁹⁾

The multiaxial classification system of DSM or ICD, revised for use in people with ID, should form the basis of diagnosis of psychiatric disorder in young people with ID, but are usefully supplemented with standardized information gathered from informant questionnaires.