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

BackgroundIt 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.MethodsA 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.ResultsThe 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.ConclusionsFailure 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.