Intellectual disability and psychiatric disorders : more than a dual diagnosis

Background: Despite the difficulties associated with establishing a diagnosis of a mental illness in persons with intellectual disability, most authors agree to say that those persons are at high risk of developing comorbid serious mental illness but the prevalence of psychiatric disorders in this population varies widely. The main reason for this variation lies in the difficulty to diagnose intellectual disability and psychiatric disorders at the same time. The aim of the present study is to investigate the association between severity of intellectual disability and prevalence of psychiatric and somatic disorders in an adult population with intellectual disability treated in the Psychiatric Unit of Mental Development (UPDM) in Geneva, Switzerland. Methods: The present study is based on the analysis of the medical record of all ambulatory patients of the UPDM treated in March 2008. This population presents at least a dual diagnosis of intellectual disability associated with psychiatric disorders. Results: Data show that 59.1% of the total sample has behavioural disorders and this percentage increases with severity of intellectual disability since it is higher in persons with severe and profound intellectual disability (79.7%). Furthermore, 48.2% of our sample has psychiatric disorders and this percentage is higher for persons with mild intellectual disability (59.5%). The most frequent psychiatric diagnosis associated to intellectual disability are pervasive developmental disorders (27.4%) and its prevalence is higher in the severe and profound intellectual disability level (66.1%), while schizophrenia and disorders of adult personality are significantly more frequent in the mild intellectual disability level (20.4% and 23.0% respectively). Furthermore, 31% of the sample have somatic disorders and its prevalence is higher in persons with severe and profound intellectual disability (55.9%). Considering the total prevalence of all diagnoses, our results reveal that 65% of our sample have more than a dual diagnosis and that this is more frequent in persons with severe and profound intellectual disability (84.7%) compared to persons with mild intellectual disability (54.8%). Conclusions: The total prevalence of all psychiatric disorders decreases with severity of intellectual disability. These data are consistent with other studies, which found a lower prevalence of psychiatric disorders in participants with severe and profound intellectual disability. Our results also reveal that the total prevalence of all diagnoses increases with severity of intellectual disability, which is consistent with the literature since some authors underline the presence of multiple pathologies associated with intellectual disability. In general, our results are encouraging and suggest a progress in defining more precise diagnostic methods.