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The bipolar spectrum is the continuous range of depressive diseases, ranging from bipolar disorder to unipolar depression. The concept of the bipolar spectrum is similar to that of Emil Kraepelin's original concept of "manic depression", although that term has come to be used as a (now deprecated) synonym for bipolar disorder, and is now hardly ever used in its original meaning.

A simple nomenclature system was introduced in 1978, although there are others, by Angst, J., et al, to easier label individuals' affectedness within the spectrum, following a clinical study by the Psychiatric University Clinic of Zürich.

Points on the spectrum using this nomenclature are denoted using the following codes:

• 'M' severe mania
• 'D' severe depression (unipolar depression)
• 'm' less severe mania (hypomania)
• 'd' less severe depression

Thus, 'mD' represents a case with hypomania and major depression. 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 ('MD') or vice versa ('DM').

On this scale, major 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'.

Unipolar hypomania ('m') without accompanying depression is not observed in the medical literature. There is speculation as to whether this condition may exist in the general population, but that their successful social functioning means that they are not regarded as anything other than high-achieving normals, keeping them out of sight of the mental health profession.

Although it is officially considered a personality disorder rather than an affective/mood disorder, some experts advocate adding borderline personality disorder (BPD) to the bipolar spectrum. BPD has a lot of similarities to rapid-cycling bipolar type II and other depressive disorders, and many patients show a positive response to the same types of medication.

• Angst J, Felder W, Frey R, Stassen HH. Arch Psychiatr Nervenkr. 1978 Oct 9;226(1):57-64. The course of affective disorders. I. Change of diagnosis of monopolar, unipolar, and bipolar illness. (PMID 708227)