Course of depression in a 10-year prospective study: Evidence for qualitatively distinct subgroups

The course of depressive disorders can vary considerable, with some individuals exhibiting a chronic course and poor outcomes, while others have a more episodic course and better outcomes. However, it is unclear whether degree of chronicity is continuous or reflects qualitatively distinct subgroups. Using data from a 5-wave, 10-year, naturalistic study of 127 depressed outpatients, we examined whether depression chronicity lies on a continuum or manifests natural boundaries. Spline regression was used to test 7 continuous and discontinuous models of the relationship between depression during the first follow-up interval and multiple outcomes at subsequent follow-ups. In order to further validate the findings, we also created empirically derived subgroups based on the results of the spline regression analyses and compared them on baseline clinical characteristics and long-term outcomes. There was a clear and consistent discontinuity indicating that for higher levels of chronicity during the first 30-month period, depression was linearly related to outcome; in contrast, for lower levels of chronicity, depression in the initial interval was unrelated to subsequent outcomes. The findings were strikingly consistent across the 4 follow-up evaluations using multiple outcomes and goodness-of-fit indices. In addition, the chronic group--as defined by the first follow-up period--exhibited more baseline chronic depression, anxiety and personality disorders, family history of dysthymia, and childhood adversity, and was more likely to attempt suicide and be hospitalized during follow-up, than the nonchronic group. Results suggest that there are qualitatively distinct classes of patients with more and less chronic depressions, and support the utility of longitudinal course as a means of parsing depression into more homogeneous subgroups.