Predominant polarity as a course specifier for bipolar disorder: a systematic review

Background
Predominant polarity (PP) is a proposed course specifier for bipolar disorder, which was not incorporated in the DSM-5 as a descriptor for the nosology of bipolar disorder (BD). Here we perform a systematic review of original studies about PP. 

Methods
A computerized search of MEDLINE/Pubmed, EMBASE and Web of Science databases from inception to October 6th, 2013 was performed with keywords, including ‘bipolar disorder’, ‘polarity’ and ‘predominant polarity’.

Results
A total of 19 studies met inclusion criteria. A unifying definition and conceptualization for PP is lacking. A PP is found in approximately half of BD patients. Most studies that included type I BD patients found the manic PP to be more prevalent, while studies that included type II BD participants found a higher prevalence of depressive PP. The depressive PP has been consistently associated with a depressive onset of illness, a delayed diagnosis of BD, type II BD and higher rates of suicidal acts. The manic PP is associated with a younger onset of illness, a first episode manic/psychotic and a higher rate of substance abuse. Evidence suggests that PP may influence responses to acute treatment for bipolar depression. Furthermore, evidences indicate that PP should be considered for the selection of maintenance treatments for BD.

Limitations
There are few prospective studies on PP. There were disparate definitions for PP across studies.

Conclusions
The concept of PP provides relevant information for clinicians. Future studies should investigate the genetic and biological underpinnings of PP.