Increased morbid risk for schizophrenia in families of in-patients with bipolar illness

Abstract

Background. It has been reported that relatives of probands with severe, psychotic forms of bipolar illness have increased rates of schizophrenia but not the relatives of individuals with milder, non-psychotic forms of disorder. In this study, we examined the prevalence of psychiatric disorders in the first degree relatives of a sample of 103 inpatients with bipolar disorder and in a matched control sample of 84 healthy individuals. Method. Relatives of cases and controls were interviewed using the FH-RDC to determine familial morbid risk for schizophrenia and bipolar disorder. Age- and sex-adjusted morbidity risks were calculated in both samples according to the method of Strömgren. Results. The morbid risks for both bipolar disorder (4.9%) and schizophrenia (2.8%) were higher in relatives of patients than in relatives of controls (0.3% and 0.6% respectively). The relative risks were 14.2 [95% confidence interval (CI)=3.1–64.2] for bipolar disorder and 4.9 (95% CI=1.3–18.8) for schizophrenia. Relatives of women with early onset of bipolar illness had the highest morbid risks for both bipolar illness and schizophrenia. The presence of more than one patient with bipolar disorder in a family increased the risk for schizophrenia nearly fourfold (RR=3.5, 95% CI=1.2–10.2). There was no additional effect of presence of psychotic features. Conclusion. Our results suggest that the transmission of psychosis is not disorder-specific. Bipolar illness characterised by a high familial loading is associated with increased risk of schizophrenia in the relatives.

Introduction

The Kraepelinian distinction between schizophrenic and manic-depressive psychoses is still an enduring tradition in psychiatry. Debates about the adequacy of the distinction should consider the prominent contribution of family studies. Recent findings suggest that the familial liability to schizophrenia is, at least in part, a liability to develop psychosis (Erlenmeyer-Kimling et al., 1997, Kendler et al., 1993, Taylor, 1992). If this were so, one would expect a higher morbid risk (MR) of schizophrenia in the relatives of bipolar disorder at the severest and psychotic end of the disease spectrum. In addition, one would expect, in analogy with findings in relatives of patients with schizophrenia (Sham et al., 1994, Verdoux et al., 1996), a higher familial morbid risk for schizophrenia in bipolar patients with early onset, poor prognosis and female gender. In order to examine these hypotheses, we conducted a familial MR study in a clinically and biologically homogeneous sample of bipolar inpatients and a group of sociodemographically matched controls.