Predictive effects of previous episodes on the risk of recurrence in depressive and bipolar disorders.

Findings from several studies have suggested that the risk of recurrence increases with the number of previous episodes in depressive and bipolar disorders. However, a comprehensive and critical review of the literature published during the past century shows that in several previous studies, a progressive course of episodes may have been falsely shown, mainly because of various kinds of biases and because these studies have not considered individual heterogeneity in their analyses. Nevertheless, four recent studies, including two nationwide register studies from Denmark, a prospective 15-year multicenter study from the United States, and a prospective lifelong study from Zurich, Switzerland, generally have taken these drawbacks and pitfalls into account in the design and handling of data. In all four studies, an effect of episodes was found in depressive (four studies) and bipolar (three studies) disorders. It is concluded that the average risk of recurrence increases with the number of episodes in depressive and in bipolar affective disorders. Nevertheless, the course of illness in unipolar and bipolar disorders is heterogeneous, and the effect of previous episodes and its interrelation with other risk factors on the risk of relapse and recurrence warrants additional research.