Research report
Why does the lifetime prevalence of major depressive disorder in the elderly appear to be lower than in younger adults? Results from a national representative sample

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Abstract

Background

The explanation of the lower lifetime prevalence rate of major depressive disorder (MDD) in older adults compared to younger people in community surveys is debated. This study examines the hypothesis that the decrease of the lifetime prevalence of MDD in older adults may be due to an age-related difference in the lifetime prevalence of subthreshold hypomania and, to a lesser extent, to the increased rate of medical induced-depression.

Methods

Data were derived from the National Epidemiological Survey on Alcohol and Related Conditions (NESARC), a national representative sample of 43,093 adults of the United States population. We examined lifetime prevalence rates of pure MDD and MDD plus subthreshold hypomania (D(m)) by age, assuming that the lifetime prevalence of pure MDD in older adults would be similar to that in the youngest cohort, consequent to an inverse age-D(m) relationship. We further considered non-hierarchical MDD (i.e., general medical condition depressive disorders were not ruled out) with the same method.

Results

The lifetime prevalence of D(m) among depressed adults aged 65 years and over was substantially lower compared to the youngest group. When considering non-hierarchical MDD, the odds ratio of the lifetime prevalence estimates of non-hierarchical pure MDD in older adults compared to the youngest group appeared not significantly different from 1.

Conclusions

Findings indicate that the decrease of lifetime prevalence of MDD in older adults may be due to an age-related difference in the lifetime prevalence of subthreshold hypomania and, to a lesser extent, to the increased rate of medical induced-depression.

Introduction

Community surveys report a lower lifetime prevalence rate of major depression in older than in younger adults (Blazer and Hybels, 2005, Jorm, 2000). Most of the explanations for this finding stress the possibility that major depression is underestimated among the elderly. Suggested biases include (1) age-related differences in recall, favored by cognitive impairment as well as by depression itself, (2) cohort effect on mortality and suicide, and (3) selection biases linked to the household approach, where willingness to participate and admitting psychiatric symptoms in interview might not be identical according to psychiatric morbidity and age (Kessler et al., 2010). However, evidence for these methodological interpretations is weak (Ernst and Angst, 1995), leading some authors to propose that the lower estimated prevalence of depression in the elderly might be a true finding (Blazer and Hybels, 2005). Therefore, the explanations of the lower lifetime prevalence of major depression in older adults remain still debated.

One issue that complicates the estimation of the prevalence of major depression in older adults is that many physical disorders, which can induce depression (Bremmer et al., 2008, Salaycik et al., 2007), become increasingly prevalent in old age, making boundaries with depression sometimes unclear and raising the possibility that depression is underestimated. At the same time, late-life depression has been found to increase risk of some physical disorders (Bremmer et al., 2007, Petronijevic et al., 2008). Previous researches have consistently found that current prevalence of most mental disorders decreases with age (Blazer and Hybels, 2005) while the prevalence of many physical disorders increases (Drayer et al., 2005). However, a recent study (Kessler et al., 2010) reports that the association of major depressive episode (MDE) with comorbid mental disorders generally increases with age while the associations of MDE with comorbid physical disorders generally decreases with age. Findings from this latter study argue against the suggestion that the low estimated prevalence of MDE among older adults is due to increased confounding effect of physical disorders (Kessler et al., 2010).

Growing clinical and epidemiologic evidence supports the validity of distinguishing MDD plus subthreshold hypomania (D(m)) from pure MDD in the psychiatric classifications, recently acknowledged in the posted DSM-V update (Angst et al., 2010). Prior researches conducted in both clinical and general population (Angst et al., 2003, Angst et al., 2012, Hoertel et al., 2012a, Lewinsohn et al., 1995, Merikangas et al., 2008, Szadoczky et al., 1998, Zimmermann et al., 2009) suggest that subthreshold hypomania is present in 30% to 55% of individuals with MDD and associated with increased risk for suicide (Angst et al., 2003, Angst et al., 2010, Judd and Akiskal, 2003, Hoertel et al., 2013b), and a higher conversion rate to threshold-level bipolar disorder (Fiedorowicz et al., 2011, Zimmermann et al., 2009). Therefore, the lifetime prevalence estimate of D(m) in older adults could be lower than that of pure MDD and explain the low estimated prevalence of MDD in older adults.

Postulating that lifetime pure MDD in older adults is as prevalent as in the youngest adults, one potential explanation is an age-related difference in the lifetime prevalence of subthreshold hypomania, resulting in a low estimated lifetime prevalence of depression in older adults.

Therefore, the aim of the present study was to examine the impact of subthreshold hypomania on the lifetime prevalence of MDD by age, using a large (n=43,093), nationally representative of the U.S. general population sample, the National Epidemiological Survey on Alcohol and Related Conditions (NESARC). We first examined lifetime MDD prevalence by age to confirm that an inverse age-MDD relationship does exist in the NESARC. We then examined lifetime prevalence rates of pure MDD and D(m) by age, assuming that the lifetime prevalence of pure MDD in older adults would be similar to that in the youngest cohort, consequent to an inverse age-D(m) relationship. We further considered lifetime non-hierarchical MDD (i.e., general medical condition depressive disorders were not ruled out), and applied the same method. We hypothesized that the decrease of the lifetime prevalence of MDD in older adults may be due to an age-related difference in the lifetime prevalence of subthreshold hypomania and, to a lesser extent, to the increased rate of medical induced-depression.

Section snippets

Method

The 2001–2002 NESARC is a nationally representative survey of the adult population of the United States conducted by the U.S. Census Bureau under the direction of the National Institute on Alcoholism and Alcohol Abuse (Grant et al., 1995). The NESARC target population was the civilian noninstitutionalized population, aged 18 years and older, residing in households and group quarters in the 50 states and the District of Columbia. Face-to-face personal interviews were conducted with 43,093

DSM-IV diagnostic interview

All psychiatric diagnoses were made according to the DSM-IV criteria with the Alcohol Use Disorder and Associated Disabilities Interview Schedule-DSM-IV Version (AUDADIS-IV), a valid and reliable fully structured diagnostic interview designed for use by professional interviewers who are not clinicians (Grant et al., 1995, Grant et al., 2004). The test–retest reliabilities of AUDADIS–IV measures of DSM–IV mood disorders, including hypomania and mania, were fair to good, kappa ranging from 0.42

Statistical analyses

Weighted percentages with their standard errors measuring prevalences of MDD, pure MDD and D(m) were estimated using cross-tabulations across four age groups (18–34, 35–49, 50–64, 65+). Because of the weighting and clustering used in the NESARC design, all statistical analyses were performed using the Taylor series linearization method, a design-based method implemented using SUDAAN, version 10 (RTI International, Research Triangle Park, N.C.). Logistic regression analyses were used to study

Results

The estimated lifetime prevalence of MDD in the total sample was 13.23% (SE=0.30, n=5695). This estimate varied significantly across the four age groups considered here (Wald F=58.78, p<0.001) due to a much lower estimated prevalence among respondents in the oldest age group than in younger age groups (Table 1). The estimated lifetime prevalence of pure MDD and D(m) were respectively 10.70% (SE=0.24, n=4644) and 2.53% (SE=0.12, n=1051). The odds ratio of the lifetime prevalence estimates of

Discussion

To our knowledge, this is the first study examining lifetime prevalence rates of major depression by age, when distinguishing pure MDD from MDD plus subthreshold hypomania (D(m)). Consistent with prior research (Blazer and Hybels, 2005, Jorm, 2000, Kessler et al., 2010), we found as expected an inverse age–MDD relationship. However, as hypothesized, our findings reveal that age-related difference in the lifetime prevalence of pure MDD, although still significant, was lower than that of MDD,

Role of funding source

We declare that there has been no financial support for this work.

Conflict of interest

No conflict declared.

Acknowledgment

We thank the National Institute on Alcohol Abuse and Alcoholism (NIAAA) for making their dataset publicly available.

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