The corpus callosum plays a pivotal role in inter-hemispheric transfer and integration of information and is a relatively understudied structure in bipolar disorder. Magnetic resonance studies have reported callosal abnormalities in this condition but findings have been inconsistent. Structural changes affecting the CC may underlie functional abnormalities in bipolar disorder and could contribute to, or explain the pathophysiology of the condition.

A systematic review was carried out to identify, appraise and summarise magnetic resonance studies which compared callosal areas in bipolar disorder with an unrelated control group. The findings were then synthesised using random effects meta-analysis. Consideration was given to a number of variables to explain heterogeneity.

Five case-control studies were identified. Bipolar patients showed reduced callosal areas and the effect size revealed a statistically significant effect: (- 0.52, 95% CI = -0.82, - 0.21). Bipolar patients showed reduced callosal areas in comparison to healthy volunteers. There was no statistically significant heterogeneity in the studies included (I2 = 0.15, P = 0.3) and no evidence of publication bias was detected (Egger test P value = 0.5). No significant effects were noted in meta-regression analysis with reference to age (P = 0.9), gender expressed as percentage of male subjects (P = 0.6), and year of publication (P = 0.4).

Findings from this study indicate that callosal areas are reduced in bipolar disorder and suggest that a failure to integrate information across the hemispheres may contribute to the pathophysiology of the disorder.