Bipolar II disorder requires at least one major depressive episode AND at least one hypomanic episode. Hypomania is the diagnostic anchor that distinguishes bipolar II from major depressive disorder. Without hypomania, the depression is unipolar; with hypomania, it is bipolar II.
Hypomania differs from mania in three ways: duration (at least 4 days vs at least 7 days for mania), severity (no marked impairment, no need for hospitalization, no psychotic features), and the patient's experience (often subjectively positive, even welcomed, rather than recognized as illness). Same symptom cluster as mania (DIGFAST) but at lower intensity and without catastrophic functional impact.
Bipolar II is among the most commonly missed psychiatric diagnoses. The depressive episodes bring patients to clinic; the hypomanic episodes don't. Patients often describe their hypomanic periods as "feeling great" — productive, creative, energetic, social, sleeping less but doing more. Why would they tell a doctor about that?
Effective screening requires specific questioning. Open questions like "have you ever felt different from your normal self?" miss most hypomania. Specific questions catch it: "Have you had periods of 4 or more days when you needed less sleep but had more energy than usual?", "Have you had periods when others noticed you seemed unusually energetic, talkative, or productive?", "Have you had episodes of rapid spending, increased sexual activity, or starting many projects at once?". Family history of bipolar disorder raises suspicion substantially.
Validated screening tools help: the Mood Disorder Questionnaire (MDQ) and Hypomania Checklist (HCL-32) have reasonable sensitivity. Neither is diagnostic by itself; structured clinical interview confirms.
Treatment of bipolar II differs from bipolar I in several ways. The depressive episodes are more prominent and more time-consuming than the hypomanic episodes — most bipolar II patients spend much more time depressed than hypomanic across their lives. Lamotrigine has particular evidence for depression-predominant bipolar II. Lithium remains valuable for suicide prevention and mood stabilization. Quetiapine, lurasidone, cariprazine for bipolar depression. Antidepressants only with mood stabilizer cover and close monitoring.
The clinical cost of missed bipolar II: years of incompletely treated mood disorder, antidepressant monotherapy that may worsen trajectory, missed opportunities for mood stabilizer-based treatment, accumulated functional cost. When recognition finally comes, treatment can be dramatically different and substantially better.