Incidence of schizophrenia in Nottingham - A comparison of two cohorts, 1978-80 and 1992-94

Background Several studies have reported a decline of up to 50% in the incidence of schizophrenia over recent decades. We aimed to measure changes in the incidence and diagnostic patterns of first-episode psychosis by comparing two Nottingham cohorts, identified in two equal periods separated by 14 years. Method Two prospectively ascertained cohorts of first-episode psychotic disorder were identified over the time periods 1978-80 and 1992-94. The earlier cohort was of the World Health Organization De terminants of Outcome of Severe Mental Disorder (DOSMD) ten-country study. The later cohort was obtained using similar methodology. Both groups were diagnosed using ICD-10 diagnostic criteria and age-standardised incidence rates were compared. Results The standardised incidence rate for ail psychotic disorders rose slightly from 2.49 to 2.87 per 10 000 population per year, but the F20 classification fell significantly by over a third (1.41 to 0.87 per 10 000 per year). The second study group (1992-1994) included a greater diversity of psychotic diagnoses compared with the first, in particular an increased proportion of acute and drug-related psychoses. Conclusions Methodological considerations call for caution in interpreting such data, but we conclude that the significant fall in the narrowly defined diagnostic category of schizophrenia reflects a real change in the syndromal presentation of psychotic disorders.