The term emphasises the idea of a 'bipolar spectrum' of mood disorders ranging from unipolar depression through to bipolar disorder.
Points on the spectrum are often denoted using the following codes:
- 'M' severe mania
- 'D' severe depression
- 'm' less severe mania (hypomania)
- 'd' less severe depression
Thus 'mD' represents a case with major depression and hypomania. A further distinction is sometimes made in the ordering of the letters, to represent the order of the episodes, where the patient's normal state is euthymic, interrupted by episodes of mania followed by depression or vice versa.
On this scale, unipolar depression would be denoted as 'D'.
Unipolar mania ('M') is, depending on the authority cited, either very rare, or nonexistent with such cases actually being 'Md'.
See Goodwin and Jamison, 'Manic Depressive Illness' for a detailed discussion.
There is speculation as to whether some high-achieving individuals are actually 'm', with their succesful social functioning keeping them out of sight of the mental health profession.