A18 18..18

The population prevalence of intellectual disability (also known as learning disability in UK health services) is approximately 1%, rather similar to the estimate of lifetime population risk for schizophrenia. The risk of psychiatric illness, particularly schizophrenia, is thought to be increased in people with intellectual disability. Morgan et al (pp. 364–372) linked two large population-based case registers in Australia to examine the overlap between intellectual disability and psychiatric illness. They found that a third of those with intellectual disability had psychiatric illness; the prevalence of schizophrenia was three times higher than the population estimates, with lower or equivalent rates of bipolar disorder and depression. The authors suggest that this overlap may reflect a common aetiology, acknowledging that other studies have shown that lower intellectual performance in adolescence is a risk factor for the later development of schizophrenia. Kelleher et al (pp. 378–382) examined an adolescent sample for the presence of psychotic symptoms, another putative risk factor for the development of subsequent psychotic illness. Six per cent of their sample reported experiencing at least one positive psychotic symptom, and these individuals were more likely to report having been abused during childhood and identified as both a bully and a victim of bullying. Kelleher et al propose that traumatic childhood events may contribute to the development of psychotic symptoms and eventual psychotic illness. Another acknowledged risk factor for psychotic illness is the use of cannabis in adolescence; however, whether it has an impact on the outcome of established psychotic illness is unclear, despite a widespread clinical belief that it contributes to a detrimental outcome. Zammit and colleagues (pp. 357–363) examined this question through a systematic review of the effects of cannabis on outcome in psychosis. They found that cannabis use was associated with increased relapse and decreased treatment adherence. However, links with increases in psychotic symptoms were more varied and their somewhat surprising conclusion was that there is insufficient evidence of cannabis use having a detrimental effect on outcomes in psychotic illness.