Bipolar affective disorder is a condition where there are periodic swings of mood periods of months or years between manic episodes and depressed episodes.

The American Psychiatric Association's Diagnostic and Statistical Manual (DSM-IV) distinguishes between bipolar I and bipolar II disorder:

• Bipolar I disorder:
  • lifetime prevalence of around 0.4-1.6%
  • characterised by episodes of depression, mania or mixed states separated by periods of normal mood
    • features of mania include elevated, expansive, euphoric mood, irritability and hyperactivity, decreased need for sleep, disorganised behaviour, delusions, hallucinations and significant (often severe) functional impairment
• Bipolar II disorder
  • lifetime prevalence around 0.5%
  • do not experience mania but have periods of hypomania, depression or mixed states
    • hypomania, as defined by DSM-IV, is characterised by milder elevation of mood and overactivity (lasting at least 4 days) without psychotic features or significant functional impairment

In both bipolar I and bipolar II disorder, depression tends to predominate over elevated mood in the overall course of the illness

Note also that at some stage in their illness, around 14-53% of patients are reported to develop 'rapid cycling' - defined as four or more manic, hypomanic, depressive or mixed episodes occurring within 12 months.

Reference:

1. Drug and Therapeutics Bulletin 2005; 43(4):28-31.