Researchers on Long Island say they've found physical evidence of post-traumatic stress disorder in World Trade Center first responders.

The study could change how the disorder is understood and even diagnosed.

Research shows difference in gray matter for PTSD patients

PTSD is diagnosed largely by symptoms, not science, but experts say a new study led by Stony Brook Medicine is offering physical proof that PTSD leaves lasting changes in the brain.

Sean Clouston, lead author of the new research, said they found clear differences in how the brain of someone with PTSD is structured.

"The gray matter in the responders with PTSD isn't as gray as it should be," he said. "It's lighter. It looks more like white matter."

Clouston said those blurred boundaries suggest disruptions in how the brain processes and controls traumatic memories, especially symptoms like flashbacks and re-experiencing.

"What we're trying to understand is, how is the brain holding onto those memories and why isn't it able to control them?" he said.

Researchers came up with the findings by using new technology to scan 99 World Trade Center first responders. These medical experts have been studying the same first responders for more than 20 years.

"[This study] builds on decades of research showing that PTSD doesn't just go away," Clouston said.

"It's a physical scar on their brains"

Research shows 23% of World Trade Center first responders developed PTSD.

"Heroes who dropped everything ... and now they're dealing with the aftermath," retired FDNY Deputy Chief Richard Alles said.

Alles said on 9/11, he arrived on the scene 20 minutes after the second collapse.

"Over two-thirds of the members of my own department were killed that day," he said.

Now, more than two decades later, the research puts science behind the trauma so many first responders have been living with.

"It's a physical scar on their brains," Alles said.

"Any step forward allows you to envision a future where people don't have to deal with this," Clouston said.