Bipolar disorder
A disorder of mood instability in which the same reward-and-arousal circuits that fail low in depression periodically fail high, producing episodes of mania or hypomania that redefine the whole illness and its treatment.
First principles
Bipolar disorder is a disorder of mood-circuit instability, not just 'depression plus mania'
The monoaminergic and limbic-prefrontal circuits that set mood, drive and reward sensitivity can fail in either direction: hypoactivity produces depressive episodes, while a state of dysregulated hyperactivity (excess dopaminergic reward signalling with reduced prefrontal restraint) produces mania or hypomania. The defining feature of bipolar disorder is not the depression, which can look identical to unipolar depression, but the demonstrated capacity of the same circuitry to swing into pathological elevation. This is why a single confirmed episode of mania (or hypomania plus depression) is enough to make the diagnosis, and why the history of past mood elevation is the single most important question in any depressed patient.
Educational content pending clinical review. Not medical advice.