The evidence reviewed above strongly suggests that late-life SubD, howsoever defined, is prevalent, worsens quality of life and health of older adults, and impacts healthcare systems via increased service utilization and cost expenditure. Epidemiological research on late-life SubD demonstrated a pattern observed in late-life MDD: increasing prevalence moving from community to PC to LTC settings (community [10%], PC [25%], medical inpatients [30%+], LTC [c. 45–50%]). SubD was consistently at least 2–3 times more prevalent than MDD across all settings. When gender data were reported, the female:male diagnostic ratio was similar to but perhaps slightly lower than the 2:1 ratio in MDD. When assessed simultaneously, SSD prevalence was generally higher than MinD prevalence. The most consistently identified risk factors for late-life SubD in prospective studies were increased medical burden, disability, decreased social support, female gender, and neurological illnesses (e.g. Parkinson’s disease, stroke, AD).
Research on the longitudinal course of late-life SubD indicates risk of developing MDD to be generally 8–10% per year, with possibly higher risk in LTC and among persons with CHD. Studies from various settings indicate the prognosis (e.g., odds of remission) are better for late-life SubD than for MDD, although a majority (median 73%) of older adults with SubD remained symptomatic during follow-up. Some studies indicated the diagnostic stability of late-life SubD categories was relatively low but did not suggest either that SubD has a benign course or that it is an invalid diagnosis. Indeed, a fluctuant course is similar to results from a study of the 12-year course of MDD among adults aged up to 79 which demonstrated that affected persons were symptomatic 60% of the 12-year period, and when symptomatic, 73% met criteria for SubD, not MDD (Judd et al., 1998
The negative health consequences associated with late-life SubD are on par with those seen in many chronic medical and psychiatric conditions, including increased healthcare utilization and expenditure, cognitive impairment, physical health decline, increased disability, and increased suicidal ideation. Most studies among general populations of older adults do not support an association between SubD and increased mortality, although SubD has been linked to increased mortality in certain medical conditions. Disability emerged as a robust risk factor for and
consequence of late-life SubD. Please see Bruce (2001)
for a review of the complex interplay between depression and disability.
Data also suggest that SubD exists on a spectrum with established depressive disorders (MDD and DYS). Similarities between late-life MDD and SubD have been demonstrated in gender ratio (Beekman et al., 1995
), affective disorder family history (Beekman et al., 1995
), and associations with a number of similar adverse outcomes (Hybels et al., 2001
). Also, variables, such as cognitive impairments (Costa et al., 2006
; Elderkin-Thompson et al., 2003
), worse symptomatic and functional outcomes (Lyness et al., 2006
), ADL impairments (Schneider et al., 2000
), and lack of social support (McCusker et al., 2005
), which were associated with late-life depression diagnoses, demonstrated levels of correlation with SubD intermediate between correlations reported for MDD and ND.
Some limitations of this review warrant discussion. We could not conduct a meta-analysis due to data heterogeneity (especially in the criteria used to define “less than major” depression) which complicated data synthesis. The DSM-IV criteria for MinD were the most replicable and consistent among the various criteria of SubD reviewed, although several studies did not apply these criteria rigorously. Aside from MinD, the most common diagnosis used was SSD, defined based on various criteria (Judd et al., 1994
; Lavretsky and Kumar, 2002
; Lyness et al., 2007
), or scoring above an established threshold on a depression rating scale.
Concerns for the effects of heterogeneous SSD criteria on conclusions in this review are partially tempered by a study (Lyness et al., 2007
) demonstrating a preponderance of similarities in variables associated with three different criteria for SSD. Our review did not include data on early or middle adulthood SubD, limiting extrapolation of findings to other age groups.
As psychiatry moves toward DSM-V, establishing consensus on terminology, definitions and criteria for SubD should be an important goal. This will allow for creating an agenda on future SubD research. Priority areas should include studies of incidence, prevalence, and longitudinal course of SubD in various clinical settings, diverse geographical areas, and cultural/socioeconomic groups. While some late-life SubD risk factors (e.g., female gender, medical illness, functional disability, and poor social support) seem well established, others remain to be identified, in order to guide effective prevention efforts (Reynolds, 2008
). More sophisticated health economic studies of the effects of SubD would assist in formulating optimal healthcare policy. Based on existing literature, there is certainly evidence to make a case for developing interventions targeted at prevention of SubD (van't Veer-Tazelaar et al., 2009
; Vahia et al., 2010
). This could potentially be a simple and low-cost means of reducing global morbidity and mortality among older adults.
It would be important to ascertain whether late-life SubD is associated with other forms of psychopathology as a risk factor, consequence, or comorbidity. Existing studies suggest that SubD is associated with both threshold and subthreshold anxiety disorders (Kvaal et al., 2008
; Palmer et al., 1997
) in older adults—Geisselmann et al. (2001)
reported that 32% of older adults with SSD experienced significant anxiety symptoms (though not specifically DSM disorders). SubD is also prevalent among older persons with schizophrenia (Zisook et al., 2007
; Diwan et al., 2007
). While.a comprehensive review of comorbidity of SubD and sunsyndromal anxiety or other psychiatric conditions is beyond the scope of this work, we believe that an exploration of the course of illness and outcomes associated with psychiatric cormorbidity is warranted. Depressive symptoms have also been associated with higher cardiovascular and cerebrovascular morbidity, as well as higher mortality in the elderly, These associations may have plausible biological underpinnings that merit further research (Schulz et al., 2000
; Schulz et al., 2009
As SubD becomes a focus of increased research, our understanding of the continuum of mood pathology from “normal” to major depression will be enhanced. Based on existing research, a need to clarify the terminology associated with subsyndromal forms of depression is clear. This will be an important step in facilitating more valid and reliable research in this area. Finally, research advances in understanding the neurobiology of late-life SubD and development of psychosocial and pharmacological treatments are necessary to guide evidence-based treatment algorithms. Such advances could save millions of older adults from the morbidity associated with depressive symptoms that, in spite of their notable prevalence and impact, are often overlooked, under-diagnosed, and under-treated.