Cortical structural signatures for early identification of bipolar disorder from depressive episode.

Objective: This study aims to explore specific biomarkers for the early identification of bipolar disorder (BD) by assessing cortical morphological features.

Methods: This study comprised 185 BD patients, 203 unipolar depression (UD) patients, and 257 healthy controls (HC). In addition, through follow-up, a cohort of 86 patients initially diagnosed with major depressive disorder (MDD) who later transitioned to BD were identified as having initial depressive episode BD (IDE-BD). The cortical structural indices were measured for all participants. General linear models were used to assess the deviations in cortical characteristics between each depressive episode group and HC. Cosine similarity was employed to analyze the similarity of cortical thickness deviations across the depressive episode groups. Additionally, differences in cortical thickness between the BD/IDE-BD and UD groups were analyzed, and the association with clinical characteristics was explored.

Results: The BD, IDE-BD, and UD groups share similar cortical thickness deviation patterns. Compared to HC, significant alterations in bilateral entorhinal cortex (EC) thickness were observed specifically in the BD and IDE-BD groups, while no such changes were observed in the UD group. Compared to UD, BD showed significant cortical thinning in the bilateral EC. In contrast, IDE-BD exhibited significant thickening in the bilateral EC compared to UD.

Conclusions: In this transdiagnostic study, we revealed shared and disease-specific cortical thickness deviation patterns between BD and UD. Additionally, we highlight bilateral EC thickening as a predictor for the early identification of BD from depressive episodes. Our findings contribute to the early identification of BD from depressive episodes.