A chronic course of MDD in early adulthood is best characterized by a composite index of early onset age, recurrent episodes, and lengthy duration of ill time. The chronic course index provides thorough coverage of MDD course while also reflecting the overlap between course indicators. Predictive validity of the index from adolescence through early adulthood was supported. Based on these findings, the index may provide the basis for a systematic approach to operationalizing MDD course and assisting in integrating findings across future investigations.
MDD recurrence and duration have been examined frequently as indicators of a chronic course. The present findings suggest that early onset represents another integral feature of chronicity. It should be noted that an earlier onset allows for a greater amount of time in which recurrences or lengthy episodes may occur. Nevertheless, the long prospective follow-up of this study (approximately 15 years) lends support to the notion that early onset is indeed a feature of a chronic course, not simply an artifact of a short follow-up.
It is somewhat surprising that dysthymia and minor depression course indicators did not load on the course factor. This may have resulted in part from the relatively low lifetime prevalence rates of dysthymia (8.0%) and minor depression (16.9%), or perhaps because we restricted our sample to probands with a positive lifetime history of MDD. Alternatively, it may suggest that the presence and/or course of less severe manifestations of depression are unrelated to the course of syndromal MDD, at least in early adulthood. It is also possible that they may be more strongly linked to MDD course among individuals with a later onset or as depressed individuals progress through adulthood.
Using the three course indicators in our sample, recommended criteria for ‘definite’ and ‘probable’ chronic courses of MDD up to age 30 were presented. Of note, our empirical, bottom-up approach to developing an index led to similar designations as others' rational, top-down approach. Our definition of a probable chronic MDD course roughly corresponds to the subtype described in the Genetics of Recurrent Early Onset Depression project (GenRED; Mondimore et al. 2006
), in that individuals display early onset and either recurrent episodes or a single lengthy episode. Moreover, the prevalence (29%) of definite or probable chronic depression in the present sample was similar to the prevalence (35%) of the early onset recurrent subtype in GenRED. We encourage future research to cross-validate indicators of the chronic course index and to explore whether the suggested criteria are appropriate in diverse samples. It may be that the three indicators consistently reflect a chronic course, but that the suggested cut-points need to be modified based on sample-specific characteristics (e.g. raising onset age criterion for depressed older adults).
An additional aim of the present study was to identify early adulthood outcomes associated with of a chronic course of MDD while addressing course measurement limitations of prior studies. The findings suggest that, among individuals with a positive history of MDD, a chronic course predicts greater impairment in young adulthood in multiple domains. Consistent with past research (e.g. Markowitz et al. 1992
; Berndt et al. 2000
; Gilmer et al. 2005
), individuals with a chronic course were likely to experience depressive episodes of greater severity, more episodes or a longer total duration of co-morbid psychiatric disorders, more severe current depressive symptoms, parental history of MDD, and to have lower incomes. They were also more likely to report impaired relationships with family members, impaired work performance, poorer coping skills, negative cognitive styles such as low optimism and low self-esteem, and to experience a higher number of stressful life events.
It is unsurprising that individuals with a chronic MDD course were more likely to undergo treatment. The findings of an association with treatment utilization, even controlling for greater MDD severity and co-morbid psychiatric disorders, suggest that the course of MDD contributed uniquely to individuals' decisions to seek treatment. Even among those who sought treatment, however, a chronic course continued to predict poorer functioning. It is likely that basic treatment approaches will not suffice for individuals with chronic MDD courses. Intensive, evidence-based approaches specifically tailored for chronic MDD (e.g. Keller et al. 2000
; Schatzberg et al. 2005
) may hold more promise in reducing its course and attendant psychosocial impairments. Given the overlap between MDD course and substance use disorder episodes and anxiety disorder duration respectively, treatment providers may also assess and intervene for these co-occurring disorders (Watkins et al. 2006
; Zimmerman et al. 2006
The strengths of this study include the prospective, longitudinal design with repeated assessments over approximately 15 years, the large sample of depressed persons, and the application of empirical procedures to developing an internally consistent course index. Despite these strengths, several limitations deserve mention. First, probands were oversampled on the basis of early onset depression and were only 30 years old at the final assessment point. These methodological aspects restrict our findings to relatively early onset MDD chronicity. Many probands will be likely to experience future, possibly lengthy episodes. It is unclear whether the features and outcomes of MDD course up to age 30 remain the same through adulthood. Second, data were not available on the course of subthreshold, inter-episode or residual depressive symptoms, as were included in a previous measure of chronicity (Mondimore et al. 2006
). As such, the chronic course index is limited to syndromal MDD. Consideration of other course features in the refinement of the empirical index represents a worthwhile endeavor for future research. Third, retrospective bias in reporting the number and duration of episodes between assessment waves is a concern given the 6-year intervals from T2 to T3 and T3 to T4. Fourth, we separately examined number of episodes and duration of co-morbid psychiatric disorders, rather than creating empirical indices of the chronicity of those disorders. Fifth, limited data were available regarding the modalities, timing, and duration of mental health treatment utilization. This prevents conclusions regarding the efficacy of treatment in modifying the impact of chronic MDD, but does present a naturalistic view of mental health treatment utilization in a community setting. Additional research with older samples and with more detailed information on treatment utilization is encouraged.
In summary, this report has provided the empirical basis for an index of the long-term course of MDD. Using the index, a more chronic course up to early adulthood is associated with outcomes such as more severe and co-morbid psychopathology, parental MDD history, and psychosocial impairment. In addition to refining understanding of the phenomenology and outcomes of a chronic course, we hope the index will provide a common metric for research on the long-term course of MDD.