Prevalence and characteristics of patients with severe mental illness and borderline intellectual functioning. Report from the UK700 randomised controlled trial of case management.

BACKGROUND Low cognitive ability and developmental delays have been implicated in the causation of mental illness. AIMS To examine the prevalence, socio-demographic characteristics, psychopathology and social functioning profiles of people with low intelligence and recurrent psychotic illness. METHOD A multi-centre randomised controlled trial of case management provided the opportunity to explore associations between mental illness and borderline intellectual functioning (assessed using the National Adult Reading test). RESULTS Overall prevalence of borderline intelligence was 18%. Significant positive associations were shown with: being Black Caribbean; having a father who worked in a manual occupation; lower educational achievement; having had special education; longer course of illness. Those with borderline intelligence had greater disability and were more likely to suffer extrapyramidal side-effects and show evidence of negative symptoms. Educational achievement, history of special education and social class were the best socio-demographic predictors of intellectual level. CONCLUSIONS Many patients who attend generic psychiatric services have considerable intellectual deficits. This may lead to difficulties in other domains of adaptive functioning, and merits further investigation as well as clinical vigilance.