Nearly a quarter of people who experience severe trauma go on to develop PTSD (or post-traumatic stress disorder), and a new study could help doctors better understand how one leads to the other. In time, this might lead to improved treatments, too.

As a type of anxiety disorder, PTSD comprises four main groups of symptoms, which can include flashbacks and physical reactions to them, heightened anxiety, emotional numbness, and avoiding anything that is associated with the traumatic event.

The new research indicates that PTSD develops along two separate tracks: one involving fear conditioning and intrusive thoughts, and one involving symptoms associated with depression.

Those two tracks could be separately identified and treated, the team behind the study says. Based on this evidence, it's "critically important" that potential PTSD sufferers are diagnosed and given help as early as possible.

"Getting PTSD is not like the flu where you wake up one day with a virus and feel sick," says psychologist Matthew Price, from the University of Vermont. "It's a complex system where a range of symptoms develop, build on themselves and influence each other over time.

"After about a month, the die is cast, so to understand and prevent PTSD, it's very important to map the dynamics of how things develop early on."

Getting information on the causes and development of PTSD is very tricky – after traumatic events, most people are keen to get on with their lives as quickly as possible, and that doesn't involve extended stays in hospital or answering questions about what happened.

In the case of this study, the researchers used a new approach, involving questions asked over text message in the first 30 days after a traumatic event. The 80 participants in the study were free to answer as and when they felt able to.

The questions and answers provided a day-to-day picture of how people were reacting to the events they'd experienced. A statistical technique called short-term dynamic modelling was then used to tease out patterns in the responses.

In particular, the researchers wanted to know which symptoms acted as major influencers, which symptoms were developed and strengthened by those influencers, and which symptoms appeared independently of the influencers.

"For one series of symptoms, the symptom chain looked a lot like fear conditioning," says Price. "People first had intrusive, unpleasant thoughts about what happened to them, which led them to avoid doing things that remind them of their trauma, and that avoidance led to hypervigilance.

"Depression wasn't influenced by other symptoms and wasn't an influencer; it was off on its own and self-perpetuating."

That separation of fear conditioning and depression gives doctors two targets in the future. In full-blown PTSD, fear conditioning and depression are very closely linked, suggesting there's a window of opportunity before that happens.

Fear conditioning can be tackled by different forms of exposure therapy, for example, while depression can be treated through more cognitive-based methods.

And as difficult as it is to get people to think about PTSD in the days after a traumatic event, these methods could prove crucial in stopping it from appearing. Those with strong reactions to reminders of their trauma could be good candidates for treatment, the researchers suggest.

While there's still a lot we don't know about PTSD, scientists are learning more about how fearful thoughts impose on the brain, and the ways in which traumatic experiences actually rewire our thinking. This latest piece of research is another step forward.

"This research is trying to piece together what this process may look like as it unfolds so that we can start to develop treatments that might be able to deliver it in this very acute phase," says Price. "There is much more work to do."

The research has been published in Depression and Anxiety.