Objective: Episodes of bipolar disorder are defined as depressive or manic, but depressive and manic symptoms can combine in the same episode. Coexistence or rapid alternation of depressive and manic symptoms in the same episode may indicate a more severe form of bipolar disorder and may pose diagnostic and treatment challenges. However, definitions of mixed states, especially those with prominent depression, are not well established.
Method: The authors performed literature searches for bipolar disorder, multivariate analyses, and the appearance of the terms "mixed" in any field; references selected from the articles found after the search were combined after a series of conferences among the authors.
Results: The authors reviewed the evolution of the concept of mixed states and examined the symptom structure of mixed states studied as predominantly manic, predominantly depressive, and across both manic and depressive episodes, showing essentially parallel structures of mixed states based on manic or depressive episodes. The authors analyzed the relationships between mixed states and a severely recurrent course of illness in bipolar disorder, with early onset and increased co-occurring anxiety-, stress-, and substance-related disorders, and they used this information to derive proposed diagnostic criteria for research or clinical use.
Conclusions: The definitions and properties of mixed states have generated controversy, but the stability of their characteristics over a range of clinical definitions and diagnostic methods shows that the concept of mixed states is robust. Distinct characteristics related to the course of illness emerge at relatively modest opposite polarity symptom levels in depressive or manic episodes.