A systematic literature review of resting state network--functional MRI in bipolar disorder.

BACKGROUND: Several studies using functional magnetic resonance imaging (fMRI) in bipolar disorder (BD) have been performed in the last decade. Some of them have applied novel neuroimaging techniques such as resting-state functional connectivity magnetic resonance imaging (rs-fcMRI).
METHODS: We reviewed the top-quality rs-fcMRI studies in BD available in the PubMed and Embase databases up to November, 2012 to identify brain activation networks and research techniques that may benefit future research.
RESULTS: We present and discuss the methods and findings of eight articles. Most of these studies used the regions-of-interest (ROI) and independent component analysis (ICA) methods, and some used approaches such as amplitude of low-frequency fluctuation (ALFF), restricted global brain connectivity (rGBC) and regional homogeneity (ReHO). The largest differences in their results were found in the connectivity of the medial prefrontal cortex (mPFC) and the anterior cingulated cortex with limbic-striatal structures, and in spatial extent in ReHo when studying the default mode network (DMN). LIMITATIONS: The heterogeneity of the analytical methods used to explore the resting-state network (RSN) and the characteristics of the sample of each study limit the conclusions.
CONCLUSIONS: Despite the variation among the results of the reviewed papers, they all support the cortico-limbic hypothesis and suggest that connectivity can be more complex and that intra-regional disturbances should also be studied. Recommendations for future studies include consideration of intra-regional disturbances, better control of confounding factors, use of larger scale methods, and a consensus regarding how to approach the study of resting-state networks and interpret the results obtained.