Inter-episode mood lability in mood disorders: Residual symptom or natural course of illness?

Abstract

Residual depressive symptoms are common in mood disorders. Inter-episode mood lability (IML; i.e. frequent ups and downs of mood) is understudied as a possible residual symptom. The study aim was to find the frequency of IML, and to find if it was more likely to be a residual symptom or if it was instead part of the natural course of mood disorders. Consecutive 89 bipolar-II (BP-II) and 89 major depressive disorder (MDD) outpatients who were not on psychoactive drugs, were interviewed by the Structured Clinical Interview for the 4th edition of the Diagnostic and Statistical Manual of Mental Disorders as modified by Benazzi and Akiskal. Kraepelin's basic definition of IML (i.e. frequent up and down fluctuations of mood between episodes) was followed. IML was present in 48.3% of the patients, significantly more common in BP-II than in MDD (62.9% vs. 33.7%, P = 0.000). The sample of BP-II and MDD plus IML, versus the sample BP-II and MDD without IML, had significantly more BP-II, lower age at onset, longer illness duration, more depressive recurrences, more depressions with atypical features, more depressive mixed states, and more family history of mood disorders. Logistic regression of IML versus recurrences, controlled for duration of illness, found odds ratio =1.8, z =1.6, P =0.103. Forward stepwise multiple logistic regression of IML versus the variables found significant in the univariate analysis, showed that only BP-II (P = 0.002) and duration of illness (P = 0.015) were significant predictors of IML. IML was common in mood disorder outpatients. Its association with BP-II (an unstable disorder by definition) and duration of illness (but not with recurrences when controlled for illness duration), suggest that IML may be more likely to be part of the natural course of illness than the result of kindling induced by recurrences. Its association with depressive mixed state (a depression reported to be more difficult to treat) and the possibility that it may induce/facilitate recurrences (to be shown by prospective studies), support the need to better study IML for its possible important impact on treatment.