Background Bipolar Disorder (BD) is a leading cause of disability worldwide and factors contributing to its burden include chronic relapsing course, comorbidity, suicide risk, and early age at onset (AAO). In particular, recent investigation has shown that BD onset may occur earlier than previously believed, even though whether BDI and II are different in such regard is still debated. Reduced samples may, moreover, limit the confidence in the published studies, with geographic issues, in turn, representing potentially conditioning factors. The present review was aimed to select and analyze large sample studies comparing AAO in BDI vs II patients. Methods A PubMed literature search was performed, considering English-written articles published up to December 2015, comparing AAO in BDI vs II patients with sample size≥100 subjects per group. Results Seventeen studies were considered suitable for revision, with 8 studies reporting statistically significant differences and 9 not. Among studies reporting statistically significant differences, mostly conducted in Europe, 6 showed an earlier AAO in BDI, while 2 in BDII subjects. Limitations Only studies with large samples included, considering AAO as a continuous variable, and providing a comparison between the bipolar subtypes. Conclusions Our findings suggest that AAO per se does not seem to reliably differentiate BDI from BDII patients and that such variable should likely be investigated in the context of other clinical characteristics, in order to assess its overall influence over BD course. Geographic factors may, in turn, play a potential role with future investigation warranted to further explore this specific issue.
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