Traumatic brain injury (TBI) is a serious public health problem in the United States. Each year, an estimated 1.7 million people sustain a TBI, and of these, 275,000 are hospitalized.

The severity of TBI may range from "mild" (a brief change in mental status or consciousness) to "severe" (comatose state). The effects resulting from TBI may include short-term and long-term functional changes affecting memory, reasoning, sensation and mood.

That said, anxiety disorders (such as depression) rank right up there with health problems to be reckoned with and are the most common mental illness in the U.S. They affect 40 million adults (18 percent of the U.S. population) and are the leading cause of disability for ages 15-44 years.

Anxiety disorders develop from a complex set of risk factors, including genetics, brain chemistry and personality, and they are more prevalent in women than men. Increasing evidence indicates that major depressive disorder (MDD) is usually accompanied by altered white matter in the prefrontal cortex, the parietal lobe and the limbic system.

Interestingly, white-matter brain abnormalities in some patients closely resemble abnormalities found in patients who have experienced a mild TBI, according to new research presented by University of Pittsburgh School of Medicine researchers at the annual meeting of the Radiological Society of North America (RSNA).

White matter in the brain is made up of long, finger-like fibers projecting from nerve cells and is covered by a whitish fatty material. Gray matter, the part of our brain without the fatty covering, holds our knowledge. White matter is what connects different regions of gray matter, allowing different parts of the brain to communicate with one another.

Saeed Fakhran, M.D., assistant professor of radiology at Pitt, and his team wanted to determine if a trauma to the brain could be found in imaging as an underlying cause of depression or anxiety in certain patients. They looked at a group of patients who had depression and anxiety following their injury to see if they could identify specific areas in the brain that were injured. They were looking for commonalities shared by patients with depression and anxiety disorders caused by brain trauma and those with nontraumatic depression.

Fakhran and his team examined MRI scans performed in 74 concussion patients from 2006-14 using an advanced technique called diffusion tensor imaging. Diffusion tensor imaging allows doctors to visualize the white matter and look for places where the white matter may be injured, resulting in decreased connections in the brain and post-concussion symptoms.

In patients with depression, researchers found injured regions in the reward circuit of the brain, which has also been found to be abnormal in patients with nontraumatic MDD. Greater injury to the reward center of the brain correlated with a longer recovery time, similar to patients with nontraumatic MDD.

Identifying such similar injuries in mild TBI patients with depression and MDD may suggest a common pathophysiology in both traumatic and nontraumatic depression that may help guide treatment. In developing treatments for any disease, the first step is understanding the cause and proving any links or common pathways such as that between post-traumatic depression and depression in the general population that could potentially lead to effective treatment strategies for both diseases.

The researchers agreed that their project was limited by its retrospective nature and moderate sample size. Because so few concussion patients undergo imaging, future research could benefit from following a larger group of patients.

Moreover, findings of the Pitt researchers didn't include irritability — the third neuropsychiatric symptom they set out to study — causing them to determine that not all such post-concussion symptoms appear to result in discrete white-matter injuries. It also was difficult to determine if pre-existing brain abnormalities rendered certain patients more susceptible to depression or anxiety.

Looking at patients with depression following concussion, the researchers found three specific areas in which those patients had injury compared to other concussion patients without depression. And what was really interesting was that those three areas are also three areas that are injured in people who are depressed without any history of a concussion.

One implication of the study is that treatment should be focused on the depression and less on concussion.