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Causes of Posttraumatic Stress Disorder (PTSD)

PTSD may be considered a disorder, but it is based on an adaptive psychological process - the learned fear response, also known as fear conditioning - which serves to protect us from danger in unpredictable environments.

Fear is a very important emotion because it is what prevents us from exposing ourselves to dangerous situations. It makes sense that if someone happened to experience a very dangerous and traumatic event, that they should learn very quickly to be afraid of any such events in the future. PTSD is what happens when this learned fear response becomes too powerful and overwhelming for an individual to tolerate [ref, ref].

Biology of the Fight-or-Flight Response

The amygdala is the part of the brain that mediates the learned fear response, part of which involves our fight-or-flight response. If someone finds themselves in a potentially threatening situation, it is the amygdala that will identify, from among all the bits of information that one is perceiving, those elements that may represent danger. For example, if one were walking through a forest and suddenly saw a snake, the amygdala is the part of the brain that focuses on the snake and sounds the alarm that there may danger.

This alarm is our fight-or-flight response. The amygdala will cause stress hormones, like adrenaline and cortisol, to be released into the body, and these hormones will cause the heart to beat faster, breathing to speed up, the senses to sharpen, and various other physical changes in the body to occur that are all designed to make us ready either to confront the source of danger or to escape from it. This whole process is unconscious and happens extremely quickly; one may barely have time to realize that they are seeing a snake when they are already feeling their heart racing and their muscles tensing.

It is possible that we come into this world with certain signs of danger, such as the shape and movement of a snake, already programmed into our amygdala, so that the first time we confront a snake we automatically know to be afraid. Yet there are many other kinds of dangers in this world that we only learn from experience. Cars may not be a sign of danger to most people, but for someone who was involved in a very serious motor vehicle accident, cars can forevermore become identified as something very treacherous. After the car accident, the person's amygdala may have learned in a very powerful way that cars are dangerous, and thereafter, whenever it identifies a car in the environment, it signals the alarm and triggers the fight-or-flight fear response. This is the learned fear response.

Bringing Context to Dangerous Situations

The shortcoming of the amygdala is that it does not give much importance to the context in which information arrives. Just because someone got into a car accident while they were speeding and talking on their cell phone, it does not mean that they will necessarily have an accident if they drive more carefully. The amygdala ignores these subtle distinctions. Its role is to work very fast and signal the alarm as soon as danger is in sight. Trying to decide on the context of the information would just make this alarm slow and inefficient.

The medial prefrontal cortex and the hippocampus are brain structures that serve to balance the effects of the amygdala.

Once of the many functions of the medial prefrontal cortex is to help in the extinction (unlearning) of the learned fear responses.

The hippocampus organizes our memories into meaningful contexts; all of the memories of our life experiences, as well as the mental maps of the places that we know, are made possible by the functions of the hippocampus.

After surviving a serious car accident, putting that experience into the context of everything else that one knows about cars and traffic offers some perspective on the event and might help one to avoid concluding that all cars are dangerous all of the time. The functions of the hippocampus help to provide this context and perspective, while the medial prefrontal cortex serves to lessen the fear response that one might develop towards all cars in general.

The Neurobiology of PTSD

In PTSD the hippocampus and medial prefrontal cortex are under-active, while the actions of the amygdala take over. As a result, people with this condition often report that they have a very difficult time putting the memory of their traumatic event into any clear or meaningful context and in learning to see the event as something other than totally terrifying. Instead, the memory remains raw and overwhelming. Instead of being able to think about the event as they would any other happening in their life - within a context of things that came before and after, and with a bit of distance and perspective that comes with the passing of time - they remember this event with the full force of the raw fight-or-flight emotions, and can feel as if the entire event is repeating itself in real-time all over again each time it is recalled.

This is the reason why people with PTSD often feel like they are condemned to relive their traumatic memory over and over again through flashbacks, nightmares, and illusions and hallucinations. It is the fight-or-flight response that is at the root of many of the symptoms of reexperiencing and hyperarousal.

But PTSD is also more than just the fight-or-flight response. It is as if the person's mind somehow decides that the world is now a much more precarious place than they had realized before they experienced the traumatic event, and they now live in perpetual fear of when the next disaster might occur. The amygdala seems to become overactive as it constantly scans the environment for any signs of danger; this might explain some of the hyperarousal symptoms, where the person has trouble sleeping, is irritable and restless, overly vigilant, and startles easily. The hippocampus and medial prefrontal cortex, on the other hand, become under-active in PTSD and thus do not allow the learned fear response to diminish or the traumatic memory to be put into a more manageable perspective.

Studies have shown that the more active the amygdala becomes, the more symptoms a person with PTSD will experience, while the size and volume of the hippocampus is inversely correlated with PTSD symptoms [ref, ref]. In fact, it seems that it is the overactive amygdala that is responsible, at least in part, for suppressing the functioning of the hippocampus and medial prefrontal cortex via the action of stress hormones like cortisol [ref, ref].

As a result, people with PTSD tend to have exaggerated stress responses to all sorts of stressors, even ones unrelated to their traumatic event [ref, ref]. Their Hypothalamic-Pituitary-Adrenal (HPA) axis, which is responsible for controlling the circulation of cortisol (a stress hormone) in the body and which is influenced by the amygdala, is hyper-responsive. When these individuals are exposed to a stressful stimulus their levels of cortisol will shoot-up much higher than in people without PTSD, which suggests that they are primed to respond more vigorously to stressful situations, though at rest their cortisol levels will actually be lower than in other people [ref].

Risk Factors for PTSD

All of these findings nicely explain why people with PTSD suffer from their symptoms, but one important question that remains is why only certain people and not others will develop PTSD in response to a similar traumatic event.

Genetics of PTSD

Genetics offer one part of the answer to this question, as genes seem to account for a third of the risk of someone developing PTSD after experiencing a traumatic event [ref]. However, thus far it remains unclear as to which genes are involved in this process. There is some evidence that individuals who are susceptible to developing PTSD are those who already have smaller hippocampi before ever being exposed to a traumatic event, and that this may have a genetic basis [ref].

Gender and PTSD

Gender also seems to play an important role, as women tend to develop PTSD twice as commonly as men, even when confronted with similar kinds of traumatic events. So far this finding remains unexplained. It is possible that certain biological differences between men and women may explain this difference, but it is also possible that it is due to the fact that women's life experiences may differ from men's. For instance, women are more likely than men to be victims of physical and sexual assault, which are forms of trauma that carry a particularly high risk for women to develop PTSD [ref]. Women also have a higher rate than men of being sexually abused in childhood [ref].

Past traumas and the risk of PTSD

Having been traumatized in the past, especially when it comes to physical and sexual abuse in childhood, increases one's chances of developing PTSD after being exposed to a new traumatic event [ref]. This likely occurs because past traumas prime the amygdala and the HPA-axis to be more responsive to subsequent threatening events and to develop a more robust learned fear response [ref].

Lack of social support and PTSD

Lacking social support also makes it more likely that someone will develop PTSD following a traumatic event [ref]. This suggests that feeling supported by ones peers and family serves to temper the development of a strong learned fear response, perhaps by giving the individual confidence that they can better manage life's various challenges thanks to the help and support that they receive from others. On the other hand, seeing other people as sources of danger may undermine this sense of safety and support, which may explain why PTSD tends to be more severe and long lasting when the traumatic event was caused by intentional human actions, like torture or rape [ref].


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