Biological Factors Associated With Susceptibility to Posttraumatic Stress Disorder

Rachel Yehuda, PhD1


Because only a proportion of persons exposed to traumatic events develop posttraumatic stress disorder (PTSD), it has become important to elucidate the factors that increase the risk for the development of PTSD following trauma exposure as well as the factors that might serve to protect individuals from developing this condition. Putative risk factors for PTSD may describe the index traumatic event or characteristics of persons who experience those events. Recent data have implicated biological and familial risk factors for PTSD. For example, our recent studies have demonstrated an increased prevalence of PTSD in the adult children of Holocaust survivors, even though these children, as a group, do not report a greater exposure to life-threatening (Diagnostic and Statistical Manual of Mental Disorders [DSM-IV] Criterion A) events. These studies are reviewed. It is difficult to know to what extent the increased vulnerability to PTSD in family members of trauma survivors is related to biological or genetic phenomena, as opposed to experiential ones, because of the large degree of shared environment in families. In particular, at-risk family members, such as children, may be more vulnerable to PTSD as a result of witnessing the extreme suffering of a parent with chronic PTSD rather than because of inherited genes. But even if the diathesis for PTSD were somehow “biologically transmitted” to children of trauma survivors, the diathesis is still a consequence of the traumatic stress in the parent. Thus, even the most biological of explanations for vulnerability must at some point deal with the fact that a traumatic event has occurred.

(Can J Psychiatry 1999;44:34–39)

Key Words: posttraumatic stress disorder, vulnerability, resilience, risk factors, biology, cortisol, children of Holocaust survivors

Posttraumatic stress disorder (PTSD) is a condition that can occur in people who have been exposed to extremely stressful or traumatic life events. Recent estimates suggest that as much as 14% of persons in the United States (US) will develop this condition at some point during their lives (1,2). The enormously high prevalence of this disorder reflects a society in which there is abundant exposure to traumatic events (1,2). Soon after the diagnosis of PTSD was established in 1980, however, it became clear that many trauma survivors did not develop this disorder following exposure to trauma (3–5). Among those who did develop PTSD soon after a traumatic event, most showed a remission of symptoms within a few months or years (6,7). It has also been recently observed that other psychiatric disorders such as major depression, panic, substance abuse, and eating disorders can also be precipitated by traumatic events (reviewed in 8). Thus, PTSD appears to represent a type of response, but by no means an inevitable or singular response, to traumatic events.

The acknowledgement of the limited prevalence of PTSD compared with the prevalence of traumatic events that can precipitate this condition has recently prompted a consideration of the factors that increase the risk for the development of PTSD. Putative risk factors for PTSD can be divided into several categories. They may include both factors describing the index traumatic event (severity or type of trauma) and characteristics describing those who experience these events (gender, prior experiences, personality characteristics). Although some risk factors for PTSD appear to be related to situational or “environmental” factors (such as sociodemographic status), data have recently emerged implicating biological and possibly genetic risk factors for PTSD.

The Problem With Discussions of Susceptibility

If there are risk factors for the development of PTSD, then trauma exposure is a necessary but not sufficient condition for the expression of this disorder. This constitutes a major change from the original intention of the diagnosis, which was to describe the long-term consequences resulting from traumatic stress exposure. Indeed, when the diagnosis was established, the major etiologic agent in PTSD was assumed to be the traumatic event. It was implicit that PTSD could occur in trauma survivors regardless of any pretraumatic considerations. In fact this was the main point of the diagnosis—to emphasize that trauma, by itself, can cause long-lasting symptoms. At the time of its appearance in the Diagnostic and Statistical Manual of Mental Disorders (DSM-III), this idea represented a critical turning point in how mental health professionals viewed the role of stressful events in the development of psychiatric symptoms (for review, see 9).

Prior to establishing the PTSD diagnosis, the enduring effects of a stressor were thought to occur as a result of anxiety or depressive or “traumatic” neurosis. The DSM-I and DSM-II made provisions only for acute symptomatic distress following adverse events (that is, using the categories of gross stress reaction and transient situational disturbance, respectively). These categories reflected the belief that a stressful event could produce psychological symptoms, but only for a relatively short time (9). The term “neurosis” implied that premorbid vulnerability accounted for prolonged responses to adverse events. As a result, many trauma survivors (and their therapists) felt that failure to overcome the effects of stress was likely due to a constitutional weakness on the part of the survivor. Indeed, the focus of treatment in “neurotic” individuals prior to the DSM-III was not on the effects of any event but rather on the constellation of factors that may have resulted in an inability to tolerate its effects (9).

The establishment of the PTSD diagnosis provided, for the first time, a psychiatric category that legitimized the fact that traumatic events could produce chronic symptoms in otherwise “normal” individuals. It was hoped that this would relieve victims of the burden of feeling inadequate for succumbing to the effects of the traumatic event and would shift the emphasis of treatment to exploring the significance of traumatic events (summarized in 9). This conceptual history highlights the problem with susceptibility—particularly biological susceptibility—that it de-emphasizes the centrality of trauma as the true cause of posttraumatic symptoms and potentially invalidates the experience of survivors or, worse, blames them for their legitimate reactions to these events.

The Necessity of Discussing Susceptibility

It is essential now to discuss issues of vulnerability. At the time the diagnosis was established, the prevalence of either trauma exposure or PTSD was unknown. The DSM-III characterized a traumatic event as being “generally outside the range of usual human experience.” However, one of the major observations from epidemiologic studies was that exposure to markedly distressing, life-threatening events was common. Earlier studies found that about one-third of individuals experienced traumatic events, as defined by the DSM-III (5). Using the current definition of trauma in the DSM-IV, the most recent epidemiologic study estimated that about 90% of citizens in the US are exposed to at least 1 traumatic event during their lives, with many exposed to more than 1 traumatic event in their lives (1). Mysteriously, studies using the more narrow DSM-IV definition of trauma are associated with a greater prevalence of trauma than those that used the more liberal DSM-III definition. Nonetheless, the important point raised by such studies is that, regardless of how broadly or narrowly traumatic events are defined, only a proportion of those individuals exposed develop PTSD. Earlier studies estimated the population prevalence of PTSD to be between 1% and 14% (with roughly 25% of trauma survivors developing this disorder). Using more current definitions, PTSD was estimated to occur in about 14% of the US population, with women (18%) having a prevalence almost twice that of men (10%) (1,2).

It has become important to explore why only some individuals develop PTSD. One of the original intentions of the diagnosis was to define a traumatic stressor as an experience that would be so objectively horrible that it would be obvious to anyone, based on the fact that someone had undergone such an experience, why there would be resultant symptoms. If every trauma survivor developed similar symptoms following exposure, this would have served as further validation of the hypothesis of PTSD, that trauma caused symptoms. However, it became clear from epidemiologic studies of PTSD prevalence that the objective characteristics of events were not sufficient predictors of this condition, particularly the chronic subtype. More subjective characteristics have been considered. One of the first factors that was seriously considered as a putative predictor of PTSD was the subjective appraisal of the event by the survivor. Indeed, it seemed possible that some individuals could react to a life-threatening situation without feeling an internal loss of control and perceive a mastery over the event by the very fact of his or her survival. Such a person might feel triumphant, invigorated, or even invincible, rather than victimized and vulnerable.

Differences in the subjective response to traumatic events are often obscured by the observation that one of the most salient predictors of PTSD is the severity of the traumatic event. Epidemiologic studies show that events involving interpersonal victimization (sexual assault, torture) are associated with the highest rates of chronic PTSD, whereas lower magnitude events (motor vehicle accidents, life-threatening illness, disaster) are associated with lower rates of trauma (2). The prevalence of chronic PTSD among torture survivors such as prisoners of war and concentration camp survivors is 50% to 75% (10–13). In contrast, the prevalence rate of chronic PTSD in survivors of natural disasters is substantially lower (2,14,15).

The dose-response relationship between severity of the trauma and the subsequent development of PTSD provides an interesting paradigm for exploring issues of vulnerability. This relationship implies that vulnerability factors may be particularly important as one moves from greater to lesser severity in the spectrum of horror and catastrophe (Figure 1). Indeed, because there is a qualitative difference between being subjected to purposeful torture and being in a motor vehicle accident, even though both experiences may be associated with threat to life and physical and psychological injury, vulnerability factors may be more relevant in the induction of PTSD in response to the lower magnitude trauma. One strategy for studying risk factors in the development of PTSD would be to compare vulnerability factors in those experiencing high magnitude responses to moderately severe and extremely severe traumatic events.

yehuda.jpg - 105801 Bytes Figure 1. Relationship between vulnerability and stressor severity.

If lower magnitude traumatic events are more likely to lead to PTSD under conditions of increased vulnerability, then vulnerable individuals might also show exaggerated responses to “subthreshold” stressful events. Although the intent of the PTSD diagnosis was partly to differentiate between the transient consequences associated with chronic or everyday stress (divorce, job loss, chronic illness, occupational stress) and the persistent and more debilitating effects of overwhelming, potentially life-threatening stress (rape, torture, war), it may be that even everyday events could be “traumatic” to vulnerable individuals. In this context, it is interesting that adult children of Holocaust survivors, who are at increased risk for developing PTSD in response to DSM-IV Criterion A traumatic events, are also more likely to develop PTSD in response to non-Criterion A events such as divorce or death of a loved one, compared with demographically matched controls (16).

Prior Victimization and Other Pretraumatic Risk Factors

Cumulative lifetime stress, particularly a history of exposure to trauma, is a very important risk factor for PTSD (4,5). Prior victimization, especially in childhood, has been found to be a potent risk factor for PTSD following both rape (17) and combat (18,19). Conversely, good social support decreases the risk for PTSD (20).

It has been generally assumed that prior victimization is an “environmental” risk factor for PTSD. This would be particularly true if trauma exposure (that is, environmental events) were randomly distributed. In fact, in addition to there being risk factors for the development of PTSD, there are risk factors for exposure to certain types of traumatic events. For example, the lifetime prevalence of assaultive violence is twice as high in nonwhite versus white persons, in persons with low versus high education, and in persons with low versus high incomes. Interestingly, however, these risk factors were more likely to be important in younger rather than older individuals (21).

Some of the “environmental” risk factors may be biologically driven. For example, being female puts one at an increased risk for both sexual victimization and PTSD (1,2,22). To date, it is not known whether the increased risk in women for both trauma and PTSD is due to biological factors (that is, having to do with hormonal differences, for example) or experiential ones (that is, that being a woman may be associated with different environmental experiences).

Other risk factors for PTSD include having a history of behavioural or psychological problems. Persons with childhood conduct disorders (3) or adult avoidant, antisocial (23), or neurotic (1) personalities prior to the traumatic event have an increased risk for the development of PTSD (12). Several cognitive risk factors, including lower intelligence (24), have also been associated with increased risk for PTSD. These latter characteristics may reflect genetic diatheses yet also manifest early life experiences. The argument works both ways. It is easy to see, for example, how abuse early in life might lead to avoidance, sociopathy, conduct disorder, and lower intelligence as pretrauma cognitive impairments. Because traumatic events are not randomly distributed, the factors noted above might increase the risk for exposure to trauma, which in turn increases the risk for PTSD. Alternatively, these traits may constitute the type of biological vulnerabilities that make trauma survivors less likely to recover from the effects of adversity.

Biological Risk Factors

Other risk factors for PTSD have emphasized a possible role for a biological factors in contributing to risk for PTSD. There is now support from several lines of evidence for a possible genetic predisposition to PTSD. True and colleagues demonstrated a greater prevalence of PTSD in persons who had monozygotic twins as trauma survivors compared with dizygotic twins, demonstrating that as much as 30% of some PTSD symptoms appear to have a genetic basis (25). These findings imply that the increased prevalence in monozygotic twins is due to shared genes. Davidson and colleagues demonstrated that trauma survivors with PTSD were more likely to have parents and first-degree relatives with mood, anxiety, and substance abuse disorders compared with trauma survivors who did not develop PTSD (26).

Our group has demonstrated that children of Holocaust survivors are more likely to develop PTSD in response to traumatic events compared with demographically matched subjects whose parents did not have Holocaust experiences (16). Further, Holocaust survivors with PTSD are more likely to have children with PTSD compared with Holocaust survivors without PTSD (27).

The extent to which these study findings are indicative of truly biological or even genetic phenomena as opposed to environmental ones is not yet clear. Even twin studies do not always speak directly to the issues of genetics because of the large shared environment in families. In particular, the vulnerability for developing PTSD in a trauma survivor who has lived with a chronically mentally ill family member may reflect genetics, experience, or some combination. For example, in 1 of our studies, children of Holocaust survivors reported feeling chronically stressed from hearing stories about the Holocaust, having to witness their parents suffer chronic pain, feeling burdened by their parents’ expectations, or experiencing losses (like not having extended families or even grandparents) as a result of the Holocaust (16). Thus, the increased prevalence of PTSD in family members may reflect vulnerability owing to these experiences rather than to inherited genes. But even if the diathesis for PTSD were somehow “biologically transmitted” to the children, the diathesis is still a consequence of the traumatic stress in the parent. Thus, even the most biological explanation for vulnerability must at some point deal with the occurrence of the traumatic event.

Biological Alterations in High-Risk Groups

In the last 10 years, the field of biological studies of trauma and PTSD has grown rapidly, and there are now several strong candidates for biological “markers” of PTSD. Trauma survivors with PTSD have shown differences in several neuroendocrinological, neurochemical, psychophysiological, and neuroanatomical measures compared with trauma survivors without PTSD and nonexposed comparison subjects (28).

It has been widely assumed that the biological changes in trauma survivors with PTSD are a result of trauma exposure and secondarily of PTSD. However, without any knowledge of the biological alterations in a particular trauma survivor prior to trauma exposure, it is impossible to know with certainty whether biological changes observed in PTSD truly reflect consequences of traumatic stress exposure or rather represent an underlying biological vulnerability for PTSD.

Studies of individuals who are believed to have a greater susceptibility to PTSD may be key to exploring this issue. In a landmark study, Resnick and colleagues measured cortisol levels during the immediate aftermath (that is, within several hours) of rape. Lower cortisol levels were observed in women who had histories of rape or assault compared with women who did not have this risk factor (29). It was the risk factor of prior trauma that was associated with a different neuroendocrine response to a subsequent traumatic event. Interestingly, the alteration observed was consistent with observations of individuals who have chronic PTSD. On the basis of this observation, we wondered if any biological variable associated with PTSD could be observed in individuals at risk for PTSD before they experienced a focal traumatic event. We previously hypothesized that adult children of Holocaust survivors represent a high-risk group for PTSD because of the increased PTSD prevalence in this group (16). We are, therefore, interested in exploring biological alterations in adult children of Holocaust survivors and have previously reported on preliminary findings of cortisol levels in this group (30,31).

Cortisol is a hormone that is released by the adrenal gland. In response to stress, several biological systems are activated in order to allow the body to become mobilized for the “fight-or-flight” reaction (32). During stress, the brain also signals the pituitary gland to stimulate the release of cortisol from the adrenal gland. The function of cortisol in response to stress is to contain the other biological reactions (that is, increased gluconeogenesis, inhibition of tissue repair, immunosuppression) that have been activated to respond to the short-term demands of the stressor. If cortisol did not facilitate the termination of these other reactions, they would do long-term damage to the body. Therefore, it is possible to think about cortisol as an “antistress” hormone. A person’s inability to produce cortisol in sufficient amounts in response to stress would have adverse consequences.

Under conditions of acute and chronic stress and in certain types of psychiatric disorders associated with stress (such as major depression), cortisol levels are elevated (33–35), but this sometimes reflects that the hypothalamic–pituitary–adrenal (HPA) axis has grown resistant to the effects of cortisol. The dexamethasone suppression test (DST) has been used as a probe of the HPA axis (36). Dexamethasone is a synthetic glucocorticoid that mimics the effects of cortisol to test the effectiveness of the HPA axis in shutting down the stress system. Under normal conditions, the administration of dexamethasone results in a suppression of the body’s own cortisol. Dexamethasone acts at the level of the pituitary to shut down subsequent release of cortisol in much the same way as cortisol would control its own release. The decline in cortisol following dexamethasone indicates that the negative feedback of cortisol is intact and the body is capable of responding to stress hormones (cortisol). However, under conditions in which the pituitary–adrenal system has grown resistant to the negative feedback effects of cortisol, such as is in depression, dexamethasone may fail to shut down cortisol levels (that is, causing them to be higher than they would normally be if negative feedback inhibition were functioning properly). A failure to suppress cortisol levels in response to dexamethasone administration (cortisol nonsuppression) usually implies a reduced sensitivity of the cortisol receptors on the pituitary gland.

Trauma survivors with PTSD show a different cortisol response from that observed under conditions of acute and chronic stress and in disorders such as major depression. Studies in various trauma survivors have shown that cortisol levels are lower in survivors with PTSD compared with normal controls and persons with other psychiatric diagnoses (37–42). Further, trauma survivors with PTSD respond to the administration of dexamethasone by suppressing their cortisol levels to a greater extent than normals do (43–45).

The hypersuppression of cortisol in response to dexamethasone suggests that the cortisol receptors in PTSD are more sensitive (43). Importantly, the hypersuppression is the opposite of the nonsuppression response to dexamethasone observed in depression (36). These and other results suggest that, unlike depressed patients who seem relatively unresponsive to the environment, trauma survivors with PTSD may be exquisitely sensitive to external events and may hyperrespond, even to nondangerous environmental stimuli (40).

In a pilot study, we observed that offspring of Holocaust survivors as a group tend to have lower cortisol levels than nonreferred children of parents not exposed to trauma (that is, normal controls) (30,31). The low levels in offspring were comparable to what we had observed in other groups of trauma survivors with PTSD, but there was great within-group variation (30). Cortisol levels in offspring were low, particularly if the subject had been exposed to traumatic stress, met the diagnostic criteria for PTSD, or reported significant life stress associated with their Holocaust upbringing.

The observation that some children of Holocaust survivors show lower cortisol levels compared with demographically comparable subjects provides the first demonstration of a biological alteration similar to that occurring with PTSD in a putative high-risk group. The task at hand is to systematically test whether other biological alterations associated with PTSD might also represent risk factors. However, since the environment actually alters many biological parameters, even genetic ones, such demonstrations do not necessarily address heritability but rather inform as to the kinds of parameters that might be associated with risk. Indeed, family studies may ultimately demonstrate intergenerational effects of stress and trauma as the most potent of risk factors due to the persistent neurobiological changes of traumatic events.

Susceptibility Versus Resilience

Exposure to trauma may be associated with many different types of outcomes, one of which is PTSD. This response is associated with specific risk factors. We should consider the different types of factors associated with different responses, particularly since prospective, longitudinal studies clearly show that most trauma survivors do not develop any psychiatric disorder in the acute or chronic aftermath of an event (8,46). It may be appropriate to explore the nature of those who do not develop any psychiatric disorder—the less vulnerable, stress-resistant trauma survivors. On one level, resistance to PTSD may be a characteristic that is malleable by traumatic experience. Individuals who are invulnerable in certain situations may become more vulnerable with repeated stress exposure. Ultimately, knowledge not only about vulnerability but of the factors that augment or erode resistance may be of substantial benefit to trauma survivors.

Clinical Implications

Limitations

Acknowledgements

This work was supported by National Institute of Mental Health (NIMH) grant R0-2 MH49555 and Merit Review Funding.

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Résumé

Étant donné que ce ne sont pas toutes les personnes exposées à des événements traumatisants qui développent le syndrome de détresse post-traumatique (SDPT), il importe d’élucider les facteurs susceptibles d’augmenter le risque de développement du SDPT par suite de l’exposition à un traumatisme de même que les facteurs pouvant éviter cet état. Les facteurs de risque hypothétiques de SDPT ont trait à l’intensité des événements traumatisants ou aux caractéristiques des personnes qui les vivent. Les données récentes font appel à des facteurs de risque biologiques et familiaux de SDPT. Par exemple, nos études récentes révèlent une prévalence accrue du SDPT chez les enfants adultes des survivants de l’Holocauste, bien que ceux-ci, en tant que groupe, ne déclarent aucune exposition accrue à des événements mettant la vie en danger (critère A du Manuel diagnostique et statistique des troubles mentaux). Ces études sont examinées de plus près. Il est difficile de déterminer dans quelle mesure la vulnérabilité accrue au SDPT des membres de la famille de personnes ayant survécu à un traumatisme est liée à des facteurs biologiques ou génétiques, par opposition à des facteurs existentiels, puisque ces membres partagent un même milieu de vie. Entre autres, il se peut que les membres à risque comme les enfants soient plus vulnérables au SDPT s’ils ont été témoins de la grande douleur qu’éprouve un parent souffrant de SDPT chronique, que s’ils ont hérité de certains gènes. Même si la diathèse du SDPT est, d’une façon ou d’une autre, « transmise biologiquement » aux enfants de survivants de traumatismes, elle demeure la conséquence de la détresse traumatique du parent. Par conséquent, même les explications strictement biologiques de la vulnérabilité doivent, à un moment donné, reconnaître le fait qu’un événement traumatisant s’est produit.

Manuscript received October 1998.

1Professor of Psychiatry, Mount Sinai School of Medicine; Director, PTSD Program, Bronx Veterans Affairs, Bronx, New York.

Address for correspondence: R Yehuda, Psychiatry 116A, 130 West Kingsbridge Road, Bronx, NY  10468

email: yehuda.rachel@Bronx.VA.gov

Can J Psychiatry, Vol 44, February 1999