Mental health disorders in adults with intellectual disabilities. Methods of assessment and prevalence of mental health disorders and problem behavior

Objective: The diagnostic manual ‘DC-LD: Diagnostic criteria for use with adults with learning disabilities/mental retardation’ (DC-LD) was developed in an effort to increase the accuracy of diagnosis of mental disorders in adults with intellectual disability (ID), as the standard diagnostic criteria for mental disorders do not fully apply to people in this group. Currently, there is a need for assessment methods that incorporate these diagnostic criteria to gain more knowledge of the prevalence of specific mental disorders and problem behaviors. The first paper of this thesis investigates psychometric properties of the recently developed ‘Psychopathology Checklists for Adults with Intellectual Disability’ (P-AID). In the second paper, prevalence rates and co-morbidity of mental disorders in a population sample of adults with ID using the PAID were investigated. The main focus in the third paper was to investigate the relationship between level of ID and symptoms of specific mental disorders and problem behavior. In doing this, the effect of several other relevant factors regarding the association between level of ID and mental disorders were analyzed and discussed. Methods: There were 901 people registered with administratively defined intellectual disability who were eligible for the study. This comprises 0.27% of the total adult population in the investigated area of Rogaland and Sunnhordland, Norway. Information from 593 individuals was returned (66%). Staff members from group homes were used as informants. Mental disorders and problem behavior were screened for using the PAID. The instrument comprises checklists for dementia, psychosis disorder, depression, mania, agora phobia, social phobia, specific phobia, generalized anxiety and panic anxiety as well as eight types of problem behavior. The internal consistency and inter-rater reliability of the checklists were investigated, as was the factor solution of the P-AID. Comparisons of prevalence rates between studies were conducted using chi-square analysis. The associations between the predictors and symptoms of mental and behavioral disorders were studied using a hierarchical linear regression model with forced entry. Results: Reliability and validity of P-AID: The overall findings in paper I indicate acceptable psychometric properties of the P-AID; reliability measures of the P-AID checklists were substantial with few exceptions. The inter-rater reliability was, however, affected by level of intellectual disability. The construct validity of the instrument was found to be strong because among other things, the factor structure of the P-AID corresponds to the conceptual structure of the DC-LD. Prevalence of mental disorders: In our sample, 43% showed indices of either a mental disorder or problem behavior, whereas 35% of the sample showed indices of mental disorders. Problem behavior was present in 20% of the participants. Among those with mental disorders, anxiety disorder (16%) and depression (12%) were most common. Analysis of co-morbid disorders showed that 22% of the sample had one mental disorder or problem behavior diagnosis, 10% had two diagnoses and 12% had three or more diagnoses. This is similar to co-morbidity figures found in people with lifelong conditions of mental disorder. Association between level of ID and symptoms of mental disorders: Level of ID contributed to the explained variance of symptoms of all disorders except mania, and was both linearly and curve-linearly related to symptoms of mental disorders dependent on the type of disorder that was considered. The factors age and negative life events were related to increased symptoms of one or more of the disorders. Some disorders were typically more prevalent in people with ID such as Down’s syndrome and autism. Conclusion: The psychometric properties of the P-AID were found to be acceptable. Moreover, the P-AID provides a more detailed description of symptoms and possible diagnoses than the other available screening instruments. Although some revisions may be needed, these findings support the use of P-AID as a screening instrument in psychiatric epidemiological research and in the initial clinical assessment of mental disorders with adults with ID. Overall, mental disorders was found to be more common in adults with ID compared with adults in the general Norwegian population. However, not all disorders were more prevalent in this group. Adults with ID were shown to have similar burden of co-morbidity as other people with lifelong conditions of mental disorders. Level of ID was found to be independently associated with symptoms of mental disorders in adults with ID. The direction of the relationship varied in the different disorders. Symptom levels of psychopathology tended to peak between severe and moderate ID in most disorders