by Janice L. Krupnick, Ph.D.
Professor of Psychiatry
Georgetown University School of Medicine
An important consideration in understanding posttraumatic stress disorder (PTSD) is its frequent co-occurrence with other psychiatric disorders. Population-based surveys of individuals who have been diagnosed with PTSD show that these persons have rates of 62% to 92% of other types of psychological disorders. In a major study of veterans who had served in Viet Nam (Kulka et al.1990), 99% suffered from another psychiatric disorder. While disorders such as panic disorder, other anxiety disorders, and substance abuse and dependence frequently co-occur with PTSD, the disorder that most commonly co-occurs is major depressive disorder. In the National Comorbidity Survey (Kessler, Sonnega, Bromet, Hughes, & Nelson, 1995), a major study investigating the prevalence of different types of psychiatric disorder in the United States, major depressive disorder was found to co-occur with PTSD in almost one-half of cases. Among men with PTSD, 47.9% had co-occurring major depression; among women 48.5% of those with PTSD also suffered from major depression.
Some observers suggest that it is not surprising that so many people who are diagnosed with PTSD also meet criteria for a diagnosis of major depression because many of the symptoms for both disorders overlap. For example, a number of symptoms of PTSD, such as diminished interest in previously enjoyed activities, problems with sleep, restricted range of emotions, and difficulty with concentration, are the same symptoms that commonly occur in depression. Some researchers have even suggested that one dimension of PTSD might be called a depressive cluster.
Until recent years, the role played by trauma exposure has not been central in the study of major depressive disorder. A number of studies, e.g., Burnam et al. (1988), Cascardi, O’Leary, & Schlee (1999), however, have shown that exposure to trauma is a risk factor not only for PTSD but also for major depression. One study of depressed elderly patients that included a short screen for PTSD found that 42% of these depressed patients also had PTSD. In a two-site study of depression treatments, DeRubeis and colleagues (2005) found that 17% of their study participants also had PTSD.
The relationship between PTSD and major depression is beginning to be clarified, particularly with regard to the way that exposure to trauma relates to these disorders both jointly and individually. The National Comorbidity Survey (cited above) found that, based on retrospective data, PTSD usually came first. Some studies (Koenen et al., 2002) showed that major depression was a risk factor for the later development of PTSD while other studies (Breslau et al., 2000) indicated that PTSD was a risk factor for later depression. Researchers who have looked closely at the relationship between PTSD and major depressive disorder think that the tendency to develop both disorders most likely indicates a joint vulnerability with regard to trauma exposure. This means that individuals who develop major depression in response to trauma are essentially the same subset of people who also develop PTSD. Trauma exposure without PTSD is not associated with higher rates of depression, although O’Donnell and colleagues (2004) found some evidence for a “depression only” response within the first three months after exposure to trauma. This suggests that there are two different groups of individuals, those who develop depression alone in response to trauma exposure and those who develop both depression plus PTSD.