Twin study of illness history variables in psychosis

Background: Genetic factors are important in the aetiology of psychotic disorders, but it is unclear how far they influence aspects of illness history within psychoses. Aims: To investigate the extent, and type of familial aggregation for a range of illness history variables in psychosis. Methods: Two-hundred-and-twenty-four proband-wise twin pairs (106 monozygotic, 118 same-sex dizygotic), where probands had psychosis, were ascertained from the Maudsley Twin Register in London. We investigated the following illness history variables, rated using the OPCRIT checklist: age at onset; chronicity of course; mode of onset; psychotic/affective predominance; pre-morbid social adjustment; and the presence of a psychosocial precipitant. We used Mx statistical modelling software to analyse correlations of variables within pairs of monozygotic twins concordant for psychosis; and relationships between Variables in probands and risk of psychosis in monozygotic and dizygotic co-twins. Results: There was a high monozygotic within-pair correlation for age at onset (intra-class correlation = 0.9); moderate correlations for chronicity of course (polychoric correlation = 0.4) and psychotic/affective predominance (polychoric correlation = 0.5); and lower nonsignificant correlations for other variables. No variables consistently predicted risk of psychosis in co-twins. Conclusions: Illness history variables in psychosis show a broad range of familial aggregation. It is likely that familial influences are predominantly modifying effects, independent of susceptibility factors for psychosis. (C) 2009 Elsevier B.V. All rights reserved.