Bipolar II disorders are diagnosed as the cause for the appearance of both depressive and hypomanic episodes during the clinical course. However, identifying a hypomanic episode, which is operationally defined in the DSM-5 and has no natural boundary with other (hypo) manic statuses, is sometimes difficult. Bipolar II disorder would constitute a heterogeneous clinical entity, unless an autochthonous hypomanic episode that persists independent of the situation were properly assessed and differentiated from a hypomanic change appearing in response to the situation or within the range of temperament. On the other hand, a depressive episode as defined in the DSM-5 is also a heterogeneous clinical entity. In fact, melancholic and atypical features are often seen in the depressive phases of bipolar disorder. However, they are opposite in terms of vegetative symptoms: the former includes insomnia and appetite loss, and the latter includes hypersomnia and hyperphagia. Excitatory elements play important roles in the formation of melancholic symptoms, whereas atypical depression lacks excitatory elements, but appears before or after a manic episode. From the viewpoint of life stage, the clinical picture of bipolar II disorder varies depending on the integration of the personality structure (ability to form melancholia) and the quantity of manic elements. Adolescents with a low integration of personality structure tend to show bipolar II disorder superimposed on a cyclothymic temperament. Young adults tend to present with bipolar II disorder with anxiety/agitation-predominant or mild inhibition-predominant depression. Middle-agers tend to show bipolar II disorder with typical inhibition-predominant depression. Based on the mania-depression ratio at each life stage, more manic episodes/mixed states are seen among younger people and more depressive episodes are seen in middle-age-onset affective disorders. Overall, cases involving transition from monopolar depression to bipolar II disorder and from bipolar II to bipolar I disorder are observed in the long-term clinical course. Therefore, rather than adhering to the strict diagnosis of bipolar II, clinicians should treat patients individually by assessing their age, personality, previous clinical course, manic elements, and situation.
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