Evidence for reduced dentate gyrus and fimbria volume in bipolar II disorder

Objectives: Dentate gyrus (DG)‐dependent inhibition of the stress response might play an important role in mood disorders. During stress, hippocampal projections traversing the fimbria, a white matter bundle on the hippocampal surface, inhibit the hypothalamic–pituitary–adrenal (HPA) axis. The aim of the present study was to measure the volumes of the DG–cornu ammonis 4 (DG–CA4) and fimbria in patients with bipolar II disorder (BD‐II) and healthy controls using a recently developed magnetic resonance imaging (MRI)‐based technique. Methods: Thirty‐seven individuals with a DSM‐IV diagnosis of BD‐II and 42 healthy controls underwent 3‐Tesla MRI. Hippocampal subfield volumes were estimated using a novel segmentation algorithm implemented in FreeSurfer. Results: In patients with BD‐II there was a significant reduction in the volume of the left [analysis of covariance (ANCOVA), F = 7.84, p = 0.006] and total (left + right) (F = 4.01, p = 0.047) DG–CA4 and left (F = 4.38, p = 0.040) and total (F = 4.15, p = 0.045) fimbria compared to healthy controls. Explorative analyses indicated a smaller left CA2–3 volume in subjects with BD‐II compared to healthy controls, and a reduced left fimbria volume in unmedicated patients compared to medicated patients and controls. Conclusions: Our results provide evidence for the involvement of the DG and fimbria in BD‐II. Longitudinal studies of the DG and fimbria with assessments of the HPA axis in BD‐II are warranted.