Our findings confirmed a key aspect of our hypothesis: Patients with SSD had poorer outcomes than non-depressed subjects in terms of psychiatric symptoms and functional status. In many cases the SSD outcomes were not significantly different from the outcomes of minor and even major depression, e.g., with regard to one-year lagged overall depressive symptom severity, suicidal ideation, psychic anxiety, cognition, and medical burden. These results build on prior work regarding the clinical significance of SSD by demonstrating that poorer outcomes include symptoms of anxiety, suicidal ideation, and cognitive functioning, as well as overall depressive symptom severity and functional status. Also, our work is the first to our knowledge to explore differences in outcome of three definitions of SSD. Specifically, SSD-A and SSD-B identified subjects with poorer psychiatric and functional outcomes than SSD-C.
Of interest, SSD was associated with poorer outcomes in the basic self-care tasks assessed by the PSMS, but not the IADL's higher-order activities that clinical experience suggests might be the earlier and more prominent ‘casualties’ of depressive disorders. It is not clear how to understand this finding. One might wonder whether SSD captured symptoms of medical illnesses rather than depression per se, but SSD was not associated with outcomes of medical burden (CIRS) or physical disability (KPSS). Future work should examine the association of SSD with more specific functional outcomes, assessed by multiple methods including performance-based measures (33
), to better understand the complex relationships among SSD, medical illnesses, and functional status.
Prior work in this cohort demonstrated varying weekly depressive symptom trajectories for patients with symptom severity at study intake in the SSD to minor depression range: some such patients improved, some worsened, and some remained at the same symptom level over 2-year follow-up (16
). The weekly trajectory methodology used in the prior analyses corresponded most closely to the SSD-C definition in the present analyses. However, the outcomes of the SSD-A and SSD-B groups in this study also were heterogeneous. While future research might usefully examine fluctuations in phenomenology over time, it would seem unlikely that broad SSD group definitions will usefully distinguish different outcome trajectories.
The findings regarding cognition merit comment, in light of prior work in this cohort demonstrated that major and minor depression were associated with poorer outcome in Trails B and Δ Trails time (albeit not in the other cognitive measures) (34
). In this study, SSD-A in particular was worse than non-depressed and generally similar to major and minor depression in regard to several cognitive outcomes. SSD-A also appeared to predict poorer outcomes in other outcome domains as compared to SSD-C and, to a lesser extent, SSD-B. Thus the SSD-A definition identified a subgroup with an overall worse prognosis, rather than a specific association with cognitive decline.
Our study limitations must be acknowledged. The sample was predominantly white and relatively ‘young-old;’ findings may not generalize to other populations. While the use of 1-year lagged analyses improved power and accuracy, the findings might differ over longer periods of follow-up. And our explorations of the relative outcomes of three SSD definitions were limited by the use of overlapping subjects drawn from the same study population.
Still, our data strongly suggest that clinicians should be vigilant in caring for patients with depressive symptoms that fail to meet diagnostic criteria for major or minor depression, monitoring for persistent or worsening depressive symptoms including suicidality, anxiety, cognitive impairment, and functional decline. Researchers studying SSD can identify at-risk subjects using any of the three SSD definitions employed here, albeit recognizing that some definitions may capture patients with poorer outcomes than others. In addition to observational studies, research must test innovative interventions, whether framed as treatments for SSD or as preventive interventions to reduce the development of major and minor depression, suicidality, or functional disability over time. Such approaches likely will need to include psychosocial as well as pharmacological components (35
), and ultimately will prove crucial to reducing the public health burden of depression in later life (36