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Current psychiatric nosology and clinical experience now recognizes several forms of mood disturbance that differ from typical depressive disorders (unipolar depression). Subsumed under the heading of the bipolar spectrum of mood disorders, these conditions include bipolar I disorder, bipolar II disorder, dysphoric mania, rapid cycling, and secondary (or induced) mania.
Other conditions that are related by their clinical characteristics to the bipolar spectrum are cyclothymia and unipolar II disorder. In day-to-day practice, these conditions are easier to describe than to define, owing to their heterogeneous often overlapping symptomatology.
In brief, bipolar I disordered patients have episodes of anergic/retarded depression and episodes of full-blown manic excitement. The episodes of mania and depression may alternate, but often do not. The key to diagnosis is the presence, sometime during the lifetime course of the illness, of a spontaneous full-blown manic syndrome: expansiveness, grandiosity, euphoria, flight of ideas, pressured speech, irritability, increased energy, decreased need for sleep, and, frequently, psychotic symptoms. Bipolar I disorder, previously termed manic-depressive insanity, was the condition that Kraeplin differentiated from the schizophreniform disorders in his classic work of the early twentieth century (Kraeplin 1913, 1921).
In bipolar II disorder, patients have depressive episodes, but never develop the fulminant manic syndrome: instead there are spontaneous periods of hypomania. Thus, the key to differentiating bipolar I from bipolar II disorder is the presence (or absence) of spontaneous manic episodes. Bipolar II disorder is now clearly differentiated from bipolar I disorder in DSM IV (American Psychiatric Association, 1994).