White Matter Abnormalities in Bipolar II and Unipolar Depression - Evidence from Fixel-Based Analysis.

Area of Science:

• Neuroimaging
• Psychiatry
• Neuroscience

Background:

• Diagnosing bipolar II disorder (BD-II) during depressive episodes is challenging, impacting clinical outcomes.
• A lack of BD-II-specific neuroimaging studies hinders understanding of its pathophysiology.
• This study investigates white matter (WM) integrity differences between BD-II and unipolar depression (UD).

Purpose of the Study:

• To compare WM integrity in individuals with BD-II, UD, and healthy controls (HC) using fixel-based analysis (FBA).
• To identify shared and distinct WM abnormalities between BD-II and UD.
• To explore the relationship between illness characteristics and WM alterations in BD-II.

Main Methods:

• Fixel-based analysis (FBA) was used to compare fibre density (FD), fibre cross-section (FC), and fibre density and cross-section (FDC) in 72 WM tracts.
• Participants included 33 individuals with BD-II, 50 with UD, and 51 HC.
• Sensitivity analyses examined unmedicated participants and the impact of medication status.

Main Results:

• Both BD-II and UD groups exhibited reduced FD in left parieto-occipito-pontine (POPT) and striato-occipital (ST-OCC) tracts.
• BD-II showed significantly lower FD than UD in the left arcuate fascicle (AF) and bilateral superior longitudinal fasciculi I and II (SLF-I and II).
• In BD-II, longer illness duration correlated with reduced FD, while more depressive episodes correlated with higher FDC.

Conclusions:

• Shared and distinct WM abnormalities exist in BD-II and UD, particularly in tracts relevant to visuomotor and executive functions.
• BD-II demonstrates more extensive WM alterations compared to UD.
• Findings suggest potential degenerative (illness duration) and compensatory (depressive episodes) neurobiological mechanisms in BD-II, warranting longitudinal investigation.