Risk of developing psoriasis in patients with schizophrenia: a nationwide retrospective cohort study.

Background Schizophrenia is a complex disease which proceeds from an interaction between genetic background and environmental factors. Recent studies showed T helper 17 (Th17) signalling, which is the main downstream immune response of psoriasis, is activated in schizophrenia. Objective To investigate whether patients with schizophrenia have higher risk of psoriasis. Methods In this nationwide retrospective cohort study, we analysed the 1 million enrollees' cohort from Taiwan's National Health Insurance Research Database. Psoriasis and schizophrenia were ascertained by International Classification of Diseases, 9th revision, Clinical Modification coding. The study cohort was comprised of enrollees diagnosed with schizophrenia during the period from 1 January 1996 through 31 December 2010, while the comparison population consisted of enrollees who had not been diagnosed with schizophrenia during the study period. Hazard ratio (HR) and 95% confidence interval (CI) were calculated for the risk of psoriasis associated with schizophrenia using Cox proportional hazard regression. Results The adjusted HR of psoriasis associated with schizophrenia was 2.32 (95% CI = 1.81-2.98). After 15 years, the cumulative incidence of psoriasis in patients with schizophrenia and comparison population was 2.82% and 1.17%, respectively. The Kaplan-Meier curves for the cumulative incidence of psoriasis in individuals with and without schizophrenia differed significantly (P < 0.0001, log-rank test). Conclusions Patients with schizophrenia have higher risk of psoriasis, which may be due to common genetic susceptibilities and/or immunologic mechanisms in both diseases. Th17 signalling and pro-inflammatory cytokines may act as a link between these two diseases and are potential therapeutic targets for schizophrenia.