Is there a familial overlap between schizophrenia and bipolar disorder?

Background: Most investigators accept that schizophrenia and bipolar disorders are distinct entities. The proponents of continuum model have challenged this dichotomy model. Methods: Information about the first-degree relatives of probands with DSM-IV diagnosis of schizophrenia (n=90), bipolar disorder (n=90), and epilepsy (n=60) was collected by using the Family Interview for Genetic Studies (FIGS). A trained psychiatrist blind to the status of index probands obtained the information. Morbid risk in relatives was calculated using abridged Weinberg’s method of age correction. Results: Rates of schizophrenia and bipolar disorder were elevated in the relatives of schizophrenia and bipolar probands, but there was no evidence of coaggregation. The risk for major depression was significantly elevated in the relatives of schizophrenia probands and was comparable to the risk in the relatives of bipolar probands. Limitations: Family history method was used to obtain information about relatives. Schizoaffective disorder patients were not included in the study and this may have amplified the distinction between schizophrenia and bipolar disorder. Conclusions: The findings suggest that schizophrenia and bipolar disorders are familially independent, but there could be a familial relationship between the predisposition to schizophrenia and to major depression.