Seasonal influences on admissions for affective disorder and schizophreniain Ireland: a comparison of first and readmissions

Although genetic and environmental factors operating before or around the t ime of birth have been demonstrated to be relevant to the aetiology of the major psychoses, a seasonal variation in the rates of admission of such pat ients has long been recognised. Few studies have compared first and readmis sions. This study examined for seasonal variation of admission in the major psychoses, and compared diagnostic categories by admission status. Patient s admitted to Irish psychiatric inpatient facilities between 1989 and 1994 with an ICD-9/10 diagnosis of schizophrenia or affective disorder were iden tified from the National Psychiatric Inpatient Reporting System (NPIRS). Th e data were analysed using a hierarchical log linear model, the chi-square test, a Kolmogorov-Smirnov (KS) type statistic, and the method of Waiter an d Elwood. The hierarchical log linear model demonstrated significant intera ctions between the month of admission and admission order (change in scaled deviance 28.77, df = 11, P < 0.003). Both first admissions with mania, and readmissions with bipolar affective disorder exhibited significant seasona lity. In contrast, only first admissions with schizophrenia showed signific ant seasonal effects. Although first admissions with mania and readmissions with bipolar disorder both show seasonality, seasonal influences appear to be more relevant to onset of schizophrenia than subsequent relapse. (C) 19 99 Editions scientifiques et medicales Elsevier SAS.