Cortical Thickness and Surface Area Abnormalities in Bipolar I and II Disorders

Objective Although bipolar II disorder (BD II) is not simply a mitigated form of bipolar I disorder (BD I), their neurobiological differ-ences have not been elucidated. The present study aimed to explore cortical thickness (CT) and surface area (SA) in patients with BD I and BD II and healthy controls (HCs) to investigate the shared and unique neurobiological mechanisms of BD subtypes. Methods We enrolled 30 and 44 patients with BD I and BD II, respectively, and 100 HCs. We evaluated CT and SA using FreeSurfer and estimated differences in CT and SA among the three groups (BD I vs. BD II vs. HC). We adjusted for age, sex, educational level, and intracranial volume as confounding factors. Results We found widespread cortical thinning in the bilateral frontal, temporal, and occipital regions; cingulate gyrus; and insula in patients with BD. Alterations in SA, including increased SA of the pars triangularis and decreased SA of the insula, were noted in pa-tients with BD. Overall, we found BD II patients demonstrated decreased SA in the right long insula compared to BD I patients. Conclusion Our results suggest that decreased SA in the right long insula is crucial for differentiating BD subtypes. Psychiatry Investigation 2021;18(9):850-863