What distinguishes bipolar disorder from unipolar depression is that the affected person experiences not only states of depression but also mania. The depression episodes in these two disorders are too similar to be distinguished.

This is a review of studies comparing unipolar and bipolar depression, with focus on neurobiology, genetics and psychosocial features: does the evidence support diagnosing bipolar and unipolar depressions as the same disorder or different?

Literature review with articles selected on Pubmed and medlink from 1990 to 2011 that had depression, unipolar depression, bipolar depression, affective disorder as keywords.

There are overlapping genes for Unipolar and Bipolar Depression.Brain imaging reveal an atrophy of the prefrontal cortex and basal ganglia and a decrease in blood flow and metabolism in both types of depression. The amygdale become more active in depressive states. in unipolar depression hippocampus shows atrophy but in bipolar disorder this result is controversial.Theres is evidence of decreased concentration of serotonin and norepinephrine in unipolar and bipolar depression. Patients with bipolar depression have a decreased function of dopamine D2 postsynaptic in pituitary and an enhanced function of 5-HT2A receptors.Psychosocial research found the same social triggers for both diseases. However, low social support is connected only with unipolar depression. After remission of depression, bipolar patients show higher self-esteem.

These data supports the hypothesis of an approximation of unipolar and bipolar depression as belonging to a spectrum of depressive illness. Hence, Mania could be a comorbid disorder. This hypothesis needs, however, more comparative studies.