Mood episodes are associated with increased cortical thinning: A longitudinal study of bipolar disorder type II.

Objectives: Previous studies found evidence for thinner frontotemporal cortices in bipolar disorder (BD), yet whether this represents a stable disease trait or an effect of mood episodes remains unknown. Here, we assessed the reproducibility of thinner frontotemporal cortices in BD type II, compared longitudinal changes in cortical thickness between individuals with BD type II and healthy controls (HCs), and examined the effect of mood episodes on cortical thickness change.

Methods: Thirty-three HCs and 29 individuals with BD type II underwent 3T magnetic resonance imaging at baseline, as published previously, and 2.4 years later, at follow-up. Cross-sectional and longitudinal analyses of cortical thickness were performed using Freesurfer, and relationships with mood episodes from baseline to follow-up were assessed.

Results: Individuals with BD type II had thinner left and right prefrontal and left temporal cortex clusters at follow-up (all corrected P < 0.001), consistent with baseline results. Both groups showed widespread longitudinal cortical thinning, and patients had increased thinning in a left temporal cortex cluster compared to HCs (corrected P < 0.001). Patients with more (>2) depressive episodes between baseline and follow-up had greater left temporal cortical thinning than patients with fewer depressive episodes (corrected P < 0.05). In addition, patients with more depressive episodes had greater thinning in bilateral ventromedial prefrontal clusters relative to HCs (uncorrected P < 0.05), yet these results did not survive correction for multiple comparisons.

Conclusions: Together, these findings support reduced frontotemporal cortical thickness in BD type II and provide the first preliminary evidence for an association between depressive episodes and increased cortical thinning.