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

ABSTRACT

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 Determinants 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 all psychotic disorders rose slightly from 2.49 to 2.87 per 10000 population per year, but the F20 classification fell significantly by over a third (1.41 to 0.87 per 10000 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.