Brief unipolar depressions: is there a bipolar component?

Recurrent brief depression is now recognised separately in the international classification of diseases (ICD 10). The disorder is characterised by short severe bouts of depression which recur frequently but erratically. In our series of patients the median duration of the depression is 3 days, with two thirds lasting between 2 and 4 days. The severity is often marked with a mean MADRS score of 30, and the episodes recurred 20 times a year. The disorder is easily separated from major depression which lasts 2 weeks or more, although, there is an unfortunate overlap group with major depression superimposed on the recurrent brief pattern. Those with "combined depression" have a higher suicide attempt rate. There should be little overlap with dysthymia since on average only 20% of the time is spent depressed, whereas dysthymia requires a minimum of 50%. However, in practice the qualification of the time spent depressed is imprecise in dysthymia so there is potential for misdiagnosis. There is little overlap with bipolar illness. In our series with follow up of up to 15 years, the conversion rate to bipolar illness is low at 3%. Almost all of these were found to have combined depression, which suggests that the rate for pure recurrent brief depression is very low. These data suggest that pure recurrent brief depression is a unipolar depressive illness.