Original articleA second look at comorbidity in victims of trauma: the posttraumatic stress disorder–major depression connection
Introduction
The definition of posttraumatic stress disorder (PTSD) in DSM-III and in subsequent DSM editions American Psychiatric Association 1980, American Psychiatric Association 1987, American Psychiatric Association 1994 links a specific syndrome with a specific class of stressors, catastrophic or traumatic events that are distinguished from ordinary stressful life events, such as loss of job or marital discord. Although stressors, those that qualify for the definition of PTSD and those that do not, might precipitate major depression, major depression occurs independent of stressors and, unlike PTSD, does not require an etiologic event as an essential part of its definition. High rates of comorbid major depression in persons diagnosed with PTSD have been reported Davidson et al 1991, Green et al 1992, Helzer et al 1987, Keane and Wolfe 1990, Kessler et al 1995, McFarlane and Papay 1992. Studies that examined the temporal order in PTSD and major depression have suggested that the lifetime association of the two disorders might be explained by several causal pathways. First, pre-existing major depression increases a person’s susceptibility to the PTSD-inducing effects of traumatic events Breslau et al 1997, Bromet et al 1998, Connor and Davidson 1997. Second, PTSD increases the risk for the first onset of major depression Breslau et al 1997, Kessler et al 1995. There is also evidence that major depression increases the risk for exposure to traumatic events (Breslau et al 1997), as it does for exposure to ordinary stressful life events Kendler et al 1993, Kendler et al 1999. The finding that PTSD increases the risk for major depression, taken together with the evidence that major depression increases the risk for PTSD following trauma, suggests the possibility of a shared diathesis, an underlying vulnerability to both PTSD and major depression (Breslau et al 1997). That personal vulnerabilities play a critical role in the PTSD and the depressive effects of stressors is clear, for only a minority of persons who report the occurrence of a trauma or a stressful event develop PTSD or become depressed, respectively Breslau and Davis 1987, Davidson et al 1991, Kendler et al 1995, Kessler 1997, Yehuda and McFarlane 1995.
An alternative explanation that might account for the PTSD–major depression comorbidity is that the traumatic events that lead to PTSD might also increase the risk for major depression (McFarlane and Papay 1992). The hypothesis that traumatic events increase the risk for major depression, independent of their PTSD effects, would be supported if we found a significantly higher incidence of major depression in persons exposed to trauma who did not develop PTSD, as compared with persons who were not exposed. Such evidence would suggest that the depressive effects of traumatic events might have a distinct pathway, separate from that of PTSD. Conversely, evidence of an increased risk for the subsequent onset of major depression in exposed persons with PTSD, but not in exposed persons who did not develop PTSD, would suggest that PTSD might cause major depression or that the two disorders share a common underlying vulnerability.
In this study we examine this hypothesis using data from the Epidemiologic Study of Young Adults in southeast Michigan (Breslau and Davidson 1991). We estimate the risk for first-onset major depression in persons with PTSD and in persons exposed to traumatic events with no PTSD sequelae, with persons who were not exposed to traumatic events serving as a reference. In addition, we examine data from the National Comorbidity Survey (NCS; Kessler et al 1995) to address the concern that traumatic events might vary in their potential for inducing major depression, just as they vary in their potential for inducing PTSD. It might be argued that the traumas experienced by persons who developed PTSD are different from those experienced by exposed persons who did not, and that it is the differences in the types of traumas that might account for any observed differences in the risk for major depression in persons with PTSD versus exposed persons without PTSD. The larger sample of the NCS, as compared with the sample of the Epidemiologic Study of Young Adults, offers an opportunity to estimate the risk for major depression by history of exposure and PTSD, holding constant trauma type. The structured diagnostic interviews used in these studies do not measure severity of trauma within the same type of trauma; however, it should be emphasized that the consistent evidence of differences in the risk of PTSD across trauma types (e.g., Breslau et al 1998a, Kessler et al 1995, Norris 1992) indicates that the classification of traumas by type is an important index of the potential to induce PTSD.
Section snippets
The Epidemiologic Study of Young Adults
A sample of 1200 persons was randomly selected from all 21- to 30-year-old members of a large health maintenance organization in southeast Michigan. Personal interviews were conducted in 1989 with 1007 (84%), and follow-up interviews were conducted in 1990, 1992, and 1994. Complete follow-up data are available on 974 persons (97%). At baseline the sample was 62% female, 80% white, 45% married, and 29% college educated. (Detailed information on the sample has been previously reported—e.g.,
Results
We first summarize results on pre-existing major depression, to provide a more complete picture of the relationship of major depression with exposure to trauma and PTSD (Table 1). Calculating with Cox proportional hazards models with time-dependent covariates, we found that the adjusted HR of exposure to trauma was higher in persons with pre-existing major depression, as compared with persons with no pre-existing major depression (HR = 2.12; 95% CI 1.39, 3.25). In the subset of exposed persons,
Discussion
Analysis of lifetime data gathered at baseline indicates that pre-existing major depression increased the risk for subsequent exposure to traumatic events twofold and increased the risk for PTSD among exposed persons more than threefold. With respect to our key question (do traumatic events increase the risk for major depression independent of their PTSD effects?), we found that the risk for major depression was 2.8 times higher (relative to persons who were not exposed) in exposed persons who
Acknowledgements
Supported by Grant No. MH 48802 from the National Institute of Mental Health, Bethesda, Maryland (NB). Breslau et al 1991
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