Unipolar depression with racing thoughts: a bipolar spectrum disorder?

Major depressive disorder (MDD) with racing/crowded thoughts is understudied. Kraepelin classified 'depression with flight of ideas' in the mixed states of his manic-depressive insanity. The aim of the study was to test whether MDD with racing/crowded thoughts was close to bipolar disorders. Consecutive 379 bipolar-II disorder (BP-II) and 271 MDD depressed outpatients were interviewed using the Structured Clinical Interview for DSM-IV, the Hypomania Interview Guide, and the Family History Screen, by a senior psychiatrist in a private practice. Intra-depression hypomanic symptoms were systematically assessed. Mixed depression was defined as a major depressive episode (MDE) plus three or more intra-MDE hypomanic symptoms. MDD with racing/crowded thoughts was compared to MDD without racing/crowded thoughts on classic bipolar validators (young onset age, many recurrences, atypical and mixed depression, bipolar family history). Frequency of MDD with racing/crowded thoughts was 56.4%. MDD with racing/crowded thoughts, versus MDD without racing/crowded thoughts, had significantly lower age at onset, more MDE severity, more psychotic, melancholic, atypical, and mixed depressions, and more bipolar family history. Of the intra-MDE hypomanic symptoms, irritability, psychomotor agitation and distractibility were significantly more common in MDD with racing/crowded thoughts. Compared to BP-II on bipolar validators, validators were less common in MDD with racing/crowded thoughts. MDD with racing/crowded thoughts seemed to be a severe variant of MDD. MDD with racing/crowded thoughts versus MDD without racing/crowded thoughts, and versus BP-II, had significant differences on bipolar validators, suggesting that it may lie along a continuum linking MDD without racing/crowded thoughts and BP-II.