Is there a continuity between bipolar and depressive disorders?

Background: Recent studies questioned the current categorical split of mood disorders into bipolar disorders ( BP) and depressive disorders (MDD). Methods: Medline database search of papers from the last 10 years on the categorical-dimensional classifcation of mood disorders. Various combinations of the following key words were used: mood disorders, bipolar, unipolar, major depressive disorder, spectrum, category/categorical, classifcation, continuity. Only English language clinical papers were included, review papers were excluded, similar papers selected by quality. The number of papers found was 1,141. The number of papers selected was 109. Results: The continuity/spectrum between BP (mainly BP-II) and MDD was supported by the following findings: (1) high frequency of mixed states (mixed mania, mixed hypomania, mixed depression, i.e. co-occurring depression and noneuphoric manic/hypomanic symptoms) because opposite polarity symptoms in the same episode do not support a hypomania/mania-depression splitting; (2) MDD was the most common mood disorder in BP probands' relatives; (3) no bimodal distribution of distinguishing symptoms between BP and MDD; (4) bipolar signs not uncommon in MDD; (5) many MDD shifting to BP; (6) many lifetime manic/hypomanic symptoms in MDD; (7) correlation between lifetime manic/hypomanic symptoms and MDD symptoms; (8) hypomania factors in MDD; (9) MDD often recurrent; (10) similar cognitive style. The categorical distinction between BP (mainly BP-I) and MDD was supported by the following findings: (1) BP more common in BP probands' relatives; (2) lower age at BP onset; (3) females as common as males in BPI, more common than males in MDD; (4) BP-I depression more atypical and retarded, MDD depression more sleepless and agitated; (5) BP more recurrent. Conclusions: Focusing on mood spectrum's extremes (BP-I vs. MDD), a categorical distinction seems supported. Focusing on midway disorders (BP-II and MDD plus bipolar signs), a continuity/spectrum seems supported. Results seem to support both a categorical and a dimensional view of mood disorders.