It has long been documented that there are clinical differences between depression and bipolar disorder, but to date there has been no reliable study that shows differences in structural brain abnormalities in the two disorders.

A research team from the Institute of Psychiatry in London have published a meta analysis that investigates structural brain changes in depression, looking at primary research which assesses the effects of medication, demographic and clinical variables; and compares the findings with those of a meta-analysis of bipolar disorder studies.

The researchers searched a range of databases and identified studies going back to 1980.  They found 225 studies that used magnetic resonance imaging or x-ray computed tomography to compare brain structure in patients with depression. 143 studies measured common brain structures and were selected for meta-analysis.

They measured a number of different variables:

• Demographic and clinical data
• Mean structure size and standard deviation
• The proportion of patients and controls with an abnormality in brain structure

The meta analysis brought back some interesting results:

• Compared with the structure of a healthy brain, depression was associated with lateral ventricle enlargement; larger cerebrospinal fluid volume; and smaller volumes of the basal ganglia, thalamus, hippocampus, frontal lobe, orbitofrontal cortex, and gyrus rectus
• Patients during depressive episodes had significantly smaller hippocampal volume than patients during remission
• Compared with bipolar disorder patients, those with depression had reduced rates of deep white matter hyperintensities, increased corpus callosum cross-sectional area, and smaller hippocampus and basal ganglia
• Both disorders were associated with increased lateral ventricle volume and increased rates of subcortical gray matter hyperintensities compared with healthy controls

The researchers concluded:

The meta-analyses revealed structural brain abnormalities in major depressive disorder that are distinct from those observed in bipolar disorder. These findings may aid investigators attempting to discriminate mood disorders using structural magnetic resonance imaging data.

Kempton MJ, Salvador Z, Munafò MR, Geddes JR, Simmons A, Frangou S, Williams SC. Structural Neuroimaging Studies in Major Depressive Disorder: Meta-analysis and Comparison With Bipolar Disorder. Arch Gen Psychiatry. 2011 Jul;68(7):675-90. [PubMed abstract]