There is controversy and lack of agreement regarding approaches to the classification and description of the phenomenology of psychopathology in young people with mental retardation.^ One approach is the application of DSM-IV(7) or the ICD-10(8) systems to the findings of a clinical assessment. The reliability and validity of this approach when applied to children with mental retardation has still to be fully established. Many of the diagnostic categories in these systems require information from the young person on their emotions and perceptions such as the experience of persistent intrusive and inappropriate thoughts that cause anxiety in obsessive-compulsive disorder, or the presence of delusions and hallucinations in schizophrenia. Some diagnoses require a judgement that the symptoms are inconsistent with the developmental level of the child as is the case for attention-deficit hyperactivity disorder ( ADHD).(7) These clinical judgements become increasingly difficult, if not impossible, in young people with more severe levels of mental retardation and language impairment. The recognition of some comorbid conditions also creates diagnostic problems. For example, DSM-IV(7) specifies that the diagnosis of either ADHD or separation anxiety disorder should not be made 'exclusively during the course of a Pervasive Developmental Disorder,' and the diagnosis of Asperger's disorder cannot be made in a person who also meets the criteria for schizophrenia. These current restrictions on dual diagnosis should not limit our capacity to describe fully the range of presenting symptoms and offer appropriate treatment. For example, the use of stimulant medication might prove effective in a child with autism who also has severe ADHD symptoms.
Some patterns of emotional and behavioural disturbance are specific to persons with mental retardation. For example the DSM-IV (7) specifies the diagnosis of stereotypic movement disorder with or without self-injurious behaviour. Other patterns of emotional and behavioural disturbance that occur exclusively in persons with mental retardation probably await definition. (9,10) Developmental level and intellectual ability influence the presentation of symptoms at least in adult psychiatric patients. For example, adult patients with mild mental retardation, compared with patients with higher intellectual functioning, are more likely to have externalizing symptoms such as avoidance of others, or assaultive and aggressive behaviours. (H) If the patient is psychotic, they are more likely to experience hallucinations without delusions. These findings are consistent with studies of psychopathology in children in general, therefore children with mental retardation might be even more prone to have externalizing symptoms such as disruptive conduct, withdrawal, and attention and impulse control problems. (12> In a population study of mentally retarded children, self-absorbed, autistic, and withdrawn behaviours were more common in those children with severe disability whereas anxiety, disruptive, and aggressive behaviours were more common in those children with milder levels of mental retardation. (310)
The World Health Organization has recently attempted to tackle some of these issues by producing draft ICD-10 guidelines for the psychiatric assessment of persons with mental retardation, but these still require clinical trials and further development. (13>
Another approach to the taxonomy of psychopathology in young people with mental retardation is the use of standardized informant questionnaires that rate disturbed emotions and behaviour. Information from these instruments can be examined using factor analysis to produce dimensions of disturbance that may have clinical utility.(14> Examination of the factor analysis of each of six rating scales in current use identified six relatively consistent groupings of disturbance: aggression-antisocial behaviour, social withdrawal, stereotypic behaviours, hyperactive disruptive behaviours, repetitive communication disturbance, and anxiety fearfulness.(6) The review included only one questionnaire specifically designed and validated for use in children and adolescents with mental retardation, the Developmental Behaviour Checklist. ^ Another behavioural rating scale for children with mental retardation is the Nisonger Child Behaviour Rating Form which also has six factor subscales which cover similar dimensions of disturbance to those above.(16) Therefore, taxonometric questionnaires can assist in diagnosis, assessment, and management of relatively common emotional and behavioural problems, but they may be less effective in detecting uncommon disorders.
The existing multiaxial classification systems of DSM and ICD should form the basis of diagnosis of psychiatric disorder in young people with mental retardation, but can be usefully supplemented with taxonometric information gathered from informant questionnaires. The combined approach will improve clinical assessment, communication between clinicians, and research.
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