People with post-traumatic stress disorder (PTSD) often show differences in certain brain areas compared to healthy people, but it's been difficult for researchers to determine whether these differences are a cause or a consequence of the condition.
Now, a number of new studies may help researchers disentangle the condition's causes from its effects and, in doing so, bring a better understanding of how the disorder might be prevented or treated.
In a new review article, researchers draw upon these studies to piece together a new model for how the condition arises.
The model suggests that three factors are necessary in order for PTSD to develop: a person needs to have certain risk factors for the condition, he or she must be exposed to a traumatic event, and after that event, further changes to the brain need to occur.
With this new understanding of the condition, researchers may ultimately be able to predict who is at risk for PTSD before they experience a traumatic event, as well as treat people at the right time after trauma to prevent subsequent brain changes from occurring, thus keeping the disorder from progressing to its final form. [See Top 10 Mysterious Diseases].
"If the disease causes specific changes [in the brain], then treatment can cause the same change in the other direction," said Roee Admon, a researcher at Harvard Medical School in Boston who, along with his colleagues, proposed the new PTSD model in the July issue of the journal Trends in Cognitive Sciences.
However, the new model does not show a complete picture of how PTSD develops, and future studies are needed to validate and expand it, Admon said.
PTSD risk factors
According to the model, changes in two brain areas — the amygdala and the dorsal anterior cingulated cortex (dACC) — may predispose people to PTSD.
Both of these regions are involved in feeling and expressing fear, and both appear to be overactive in people with PTSD, even before they develop the condition.
In one study, brain scans of Israeli citizens before and after military deployment showed that those who went on to develop PTSD symptoms had high levels of amygdala activation before deployment.
Another study, which included pairs of twins in which one twin had PTSD and the other didn't, found that both had high levels of activity in the dACC.
The heightened activity in the amygdala and the dACC may contribute to one of the hallmarks of PTSD, called hyperarousal, which can cause people to be irritable or easily startled.
Genetic factors, or earlier life experiences, may lead to increases in activity in the amygdala and the dACC, the researchers said.
Consequences of trauma
Changes to another brain region, called the ventromedial prefrontal cortex, and the way it interacts with the brain's hippocampus, may come about only after a traumatic event, according to the model.
The brain's hippocampus helps form new memories, while the prefrontal cortex helps with "fear extinguishing," which is the ability to learn that a signal previously linked with an imminent threat is no longer linked with the threat, said Lisa Shin, a psychologist at Tufts University who conducted the study on twins.
"[Soldiers] have learned, very appropriately, that you have to pay attention to everything on the side of the road," because explosives are often placed there, Shin said, adding that once veterans return to civilian life, they may still find themselves concerned about objects at the roadside. "It takes them a long time to realize that those signals are no longer signals of explosives or threats," Shin said.
Experiencing a traumatic event may impair the connection between the hippocampus and ventromedial prefrontal cortex needed for fear extinguishing to occur, studies suggest. It may also promote two other classic symptoms of PTSD: reliving memories of trauma, and avoiding places, events, people or objects that are reminiscent of the traumatic event, according to the model.
Implications for treatment
If hyperarousal symptoms do turn out to be present before PTSD arises, it may be possible to screen for these symptoms among people considering professions in fields that come with a high risk of trauma exposure, such as the military or police force.
Admon stressed that "people that come out as vulnerable should not be rejected." Although these people may have risk factors for PTSD, they won't necessarily develop the condition, he said.
But people at risk for PTSD could be monitored, and treated right away, if they did experience a traumatic event. Immediate treatment may prevent the onset of the later brain changes in the hippocampus and ventromedial prefrontal cortex that prevent fear extinguishing, Admonsaid.
Currently, treatment is not given right away because most people who experience traumatic events don't go on to develop PTSD. Doctors wait until people show PTSD symptoms for at least three months before beginning treatment.
"By the time we start treating them, they already have a chronic disorder," Admon said.
Future studies should focus on what happens in the brain during PTSD treatment, and which brain changes are linked with successful treatment, Admon said.
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