A new technology known as diffusion tensor imaging suggests that post-traumatic stress disorder, or PTSD, and traumatic brain injury—two common problems facing veterans returning from Afghanistan and Iraq—may both produce a type of damage barely recognized until now. That damage involves the disruption of the delicate axons that link brain regions and enable them to communicate with each other.
Most of the head injuries soldiers suffer are caused by explosions, which sometimes produce chronic and debilitating symptoms such as headaches, memory loss, mood disorders and sleep difficulties, even though conventional magnetic resonance imaging and computerized tomography scans show no damage to the brain.
Diffusion tensor imaging, however, can reveal that the myelin coating of some axons has been disrupted, which impairs their ability to transmit signals, or that some axons have been sheared away from their normal connections. Such damage could explain some of the symptoms experienced by soldiers who emerge from explosions with no obvious damage to their brain.
This type of injury also might explain some of the symptoms of post-traumatic stress disorder, according to research presented at the Society for Neuroscience conference.
“We think the primary blast injury is an axonal injury,” said researcher David L. Brody of Washington University in St. Louis. “The axonal tracts are disrupted.”
A conventional magnetic resonance imaging scan, which clearly reveals tumors, brain bleeding, strokes, multiple sclerosis lesions and damage to the blood-brain barrier, does not show damage to axons, he said.
However, diffusion tensor imaging, in which the magnetic resonance imaging scanner is tuned to detect the diffusion of water in the brain, reveals such damage.
Evidence supporting this theory was provided by recent experiments in which pigs subjected to explosions were found to have diffuse damage to their axons but almost no other brain damage.
Could axonal damage explain some of the symptoms of PTSD?
“That’s a really controversial issue,” Brody said. “The hyper-arousal that comes with PTSD could potentially be related to injury to those regulatory regions. So we’re looking very closely at the amygdala, which is involved in fear and arousal. Our hypothesis is that the amygdala’s regulatory circuitry could be injured in a blast injury.”
Diffusion tensor imaging also could transform the diagnosis and treatment of seemingly minor head injuries, said Marilyn Kraus of the University of Illinois at Chicago, one of the researchers behind another poster on diffusion tensor imaging and brain injury.
“A number of patients who have had mild head injuries have persistent deficits,” Kraus said. “They’re high-functioning, but they know they’re not the same.”
She believes that diffusion tensor imaging, which adds only about six minutes to a conventional magnetic resonance imaging scan, will help provide evidence of subtle physical trauma that causes such problems.