The role and clinical significance of subsyndromal depressive symptoms (SSD) in unipolar major depressive disorder.

Analyses conducted in 10,526 community respondents investigated by the NIMH Epidemiological Catchment Area (ECA) Program, revealed the 1-month point prevalence of depressive symptoms and disorders in the general population, at the first ECA interview (Wave 1) to be 10%, as follows: 2.3% major depressive disorder (MDD); 2.3% dysthmic disorder (DD); 1.5% minor depressive disorder (MinD); and 3.9% subsyndromal depressive symptoms (SSD, defined as two or more depressive symptoms beneath the diagnostic threshold of MinD, DD or MDD). There appears to be two classes of SSD in this community sample: first, SSD, which occurred as an integral component of the course of unipolar major depressive disorder (MDD); and, second, SSD occurring spontaneously in non-unipolar depressed community subjects. In the first instance, SSD was frequently prodromal to episodes of MinD or MDD or residual to resolving episodes. Analyses also support the conclusion that SSD is a clinically significant, interepisode, depressive subtype of unipolar MDD, since SDD is associated with harmful dysfunction in five of six measures of adverse outcome, has a significantly increased prevalence of past histories of major depressive episodes, and an elevated lifetime prevalence of suicide attempts. Comparison of subsyndromal depressive symptomatology or depressive disorder diagnoses at Wave 1 with diagnoses obtained, 1 year later, at the Wave 2 interview, confirm the persistent and chronic nature of depression in this large representative sample of community respondents, in which 71% of subjects with depressive symptoms or disorders at Wave 1 continued to be symptomatic at Wave 2. In addition, subjects experienced a surprising degree of change in depressive symptom and disorder diagnoses during the 1-year observational window between Wave 1 and Wave 2, in which a remarkable percentage of individuals, who began the year in a depressive symptom or disorder diagnostic category, ended the year in another. This has led us to hypothesize that the typical clinical picture of unipolar MDD is dynamic and pleomorphic in nature, characterized by substantial symptomatic fluidity, in which patients frequently change diagnoses from one depressive subtype to another during their courses of illness.