Background: Clinical practice guidelines increasingly recognize the heterogeneity associated with major depressive episodes (MDE), e.g. through strategies such as watchful waiting. However, the implications of episode heterogeneity for long-term prognosis have not been adequately explored.
Methods: In this project, we used data from a Canadian longitudinal study to evaluate recurrence risks for MDE after an initial episode in the mid-1990s. This study collected data from a community cohort between 1994/1995 and 2008/2009 using biannual interviews. Characteristics of the index episode: syndromal versus sub-syndromal, duration of symptoms, and indicators of seriousness (activity restriction, high distress or suicidal ideation) were recorded. The ability of these variables to predict MDE recurrence was explored using proportional hazards modeling. Additional analyses using generalized estimating equations were used to assess robustness.
Results: Even brief, sub-syndromal episodes not characterized by indicators of seriousness were associated with an increased risk of subsequent MDE. However, episodes meeting diagnostic criteria for MDE, those lasting longer than four weeks and those associated with indicators of seriousness were associated with much higher recurrence risk. Sub-syndromal episodes associated with these characteristics generally predicted subsequent MDE as strongly as the occurrence of MDE itself.
Limitations: The data source did not include assessment of all potentially relevant covariates. The assessment of MDE used an abbreviated instrument.
Conclusions: Brief sub-syndromal episodes of depression are not usually targets of acute treatment, but such episodes have implications for subsequent MDE risk. Episode characteristics identify a range of outcomes that have potential implications for long-term management.
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