Background: Depressive mixed state (DMX) is understudied, although this diagnostic concept may be of clinical and theoretical importance. Our goal was to provide preliminary evidence of the inter-episode stability of DMX. The inter-episode stability is known to be an important validator for establishing a distinct clinical entity.
Methods: Out of depressive patients consecutively hospitalized at our institute, those who experienced two or more hospitalizations due to discrete depressive recurrences during a 6-year period were selected. All depressive episodes were directly observed and assessed using a standardized rating instrument in terms of eight intra-episode manic symptoms (flight of idea, logorrhea, aggression, excessive social contact, increased drive, irritability, racing thoughts, and distractibility). Assessments for subsequent episodes were performed blindly to those for previous episodes within each patient.
Results: The inter-episode stability of categorical DMX diagnoses and the number of intra-episode manic symptoms was moderate but significantly high. Approximately 50% of patients with DMX in the index episode obtained a DMX diagnosis in the second episode. Approximately 40% of the total variance of the number of intra-episode manic symptoms was explained by agreements across several depressive episodes. Depressive patients who experienced a diagnostic switch from unipolar to bipolar disorder had a higher frequency of DMX and a greater number of intra-episode manic symptoms in the index as well as subsequent episodes.
Limitations: All consecutive patients were not followed up. Bipolar I and II patients were combined due to a small number of bipolar II patients in this sample.
Conclusion: The inter-episode stability of DMX may not be so high as is required for establishing a distinct clinical entity. However, the findings strongly suggest that some depressive patients have a long-lasting liability to DMX. It is important to determine whether such a liability to DMX is mediated by affective temperaments, as was originally hypothesized by Akiskal [J. Clin. Psychopharmacol. 16 (1996) 4S-14S]. DMX may be a risk factor to the diagnostic switch from unipolar to bipolar disorder.