The relationship of major depressive disorder to bipolar disorder: continuous or discontinuous?

Recent studies have questioned current diagnostic systems that split mood disorders into the independent categories of bipolar disorders and depressive disorders. The current classification of mood disorders runs against Kraepelin's unitary view of manic-depressive insanity (illness). The main findings of recent studies supporting a continuity between bipolar disorders (mainly bipolar II disorder) and major depressive disorder are presented. The features supporting a continuity between bipolar II disorder and major depressive disorder currently are 1) depressive mixed states (mixed depression) and dysphoric (mixed) hypomania (opposite polarity symptoms in the same episode do not support a splitting of mood disorders); 2) family history (major depressive disorder is the most common mood disorder in relatives of bipolar probands); 3) lack of points of rarity between the depressive syndromes of bipolar II disorder and major depressive disorder; 4) major depressive disorder with bipolar features such as depressive mixed states, young onset age, atypical features, bipolar family history, irritability, racing thoughts, and psychomotor agitation; 5) a high proportion of major depressive disorders shifting to bipolar disorders during long-term follow-up; 6) a high proportion of major depressive disorders with history of manic and hypomanic symptoms; 7) factors of hypomania present in major depressive disorder episodes; 8) recurrent course of major depressive disorder; and 9) depressive symptoms much more common than manic and hypomanic symptoms in the course of bipolar disorders.