Epidemiology of mental health problems in adults with learning disability: an update

Epidemiological studies measuring prevalence rates and factors associated with mental health problems in adults with learning disability have produced very different and sometimes contradictory results, mainly because of methodological problems. Consequently, much of the epidemiology of mental health problems in this population is still unknown, although improved methodology and additional information are emerging. This article describes the methodological difficulties of studies, the reported prevalence rates in the context of these difficulties and what is currently known about the factors associated with mental health problems in this population. Box 1 Key methodological challenges • The definition of learning disability • The definition of mental health problems • The representativeness of study samples • The method of identification and assessment of cases • The use of appropriate diagnostic criteria Epidemiology of mental health problems in learning disability 215 Advances in Psychiatric Treatment (2005), vol. 11. http://apt.rcpsych.org/ Accurate epidemiological information can be obtained only by using reliable and valid measurements on population-based samples. The ideal method would be to screen everyone in a certain area for learning disability and then to further screen those identified for mental health problems using clinical assessment and appropriate diagnostic criteria, as discussed below. However, this would be costly and time consuming. A few studies have used population-based samples, but they are limited by small sample sizes. Definition and identification of mental health problems In prevalence studies, many researchers have used terms such as mental illness, mental disorder, psychiatric illness, psychiatric disorder, emotional problems and behavioural disorder without detailed definition. Some have excluded personality disorder and behavioural disorder from their reported prevalence rates, whereas others have included them. This difference in the types of disorder counted can have a considerable effect on reported prevalence rates and makes the comparison of studies complicated.