Does testing for bimodality clarify whether the bipolar disorders are categorically or dimensionally different to unipolar depressive disorders?

Background: It has been held that if bipolar disorder is categorically distinct, it should differentiate from unipolar depressive disorders by showing bimodality or a 'zone of rarity' in bipolar symptom scores. Two previous studies have failed to demonstrate bimodality. We undertook a third study.

Methods: A total of 1106 patients attending the Black Dog Institute Depression Clinic completed the Mood Disorders Questionnaire (MDQ), in addition to undergoing clinical assessment by an Institute psychiatrist.

Results: The distributions of scores for the total number of hypomanic symptoms endorsed by unipolar and bipolar patients were both skewed, with the bipolar group endorsing a high number of hypomanic symptoms and the unipolar group endorsing few symptoms--and so giving the impression of an 'even' distribution generated by two quite distinctly differing sub-groups. However, formal statistical analyses involving mixed modelling provided no clear evidence that a bimodal distribution provided a better fit to the data than a unimodal one.

Conclusions: Failure to statistically demonstrate a 'point of rarity' did not marry with visual inspection of the plotted data--which clearly suggested two groups putatively capturing those with bipolar and unipolar disorders respectively. The paper considers some limitations to the emphasis on 'bimodality' in differentiating potentially differing conditions.