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Mild traumatic brain injury (TBI) disrupts the brain's "default mode" in a way that shows up on functional MRI and correlates with symptoms, researchers found.

Compared with normal controls, patients with posttraumatic symptoms soon after mild TBI showed reduced connectivity in some regions, and increased connectivity in others within the network that the brain uses during inactivity for information processing and maintenance.

These abnormalities were associated with cognitive dysfunction, depression, anxiety, fatigue, and post-concussion syndrome, reported Yulin Ge, MD, of the NYU School of Medicine in New York City, and colleagues in the December issue of Radiology.

Conventional findings on imaging with CT or MRI typically show up normal for these patients who lost consciousness briefly, had transient memory problems or disorientation, or both after head trauma, even though 20% to 30% develop long-term symptoms.

The study findings "suggest that resting-state functional MR imaging can be used as an additional clinical tool for detecting subtle brain injury that is not apparent with conventional MR imaging," Ge's group wrote.

If longitudinal studies confirm these results, functional MRI monitoring of the default-mode network could track progression and recovery in mild TBI, they suggested.

This network is called the brain's default mode, because the nodes that comprise it are activated when the brain isn't doing any tasks that require attention or decision making. Its components are:

• The posterior cingulate cortex, which has its main role in memory encoding, consolidation, and environmental monitoring
• Medial prefrontal cortex, which plays a part in self-relevance, rapid error identification, and social functions
• Precuneus
• Inferior parietal

Problems with this network have been seen in Alzheimer's disease, autism, and schizophrenia.

To see what role it plays in mild TBI, Ge and colleagues compared 23 affected patients with posttraumatic symptoms within 2 months of injury and 18 age-matched healthy controls using resting-state functional MRI to map out energy expenditure across the brain while awake.

The mild brain injury group showed significantly decreased connectivity in the posterior portion, but increased connectivity in the anterior portion of the default-mode network compared with the controls (P<0.01).

The increased connectivity was seen primarily in the anterior medial prefrontal cortex region, while the decreased connectivity was primarily in the posterior medial cingulate cortex and parietal regions.

These opposite impacts were significantly correlated (P=0.03), "suggesting that these opposite within-network changes are essentially associated."

Cognition and symptoms appeared tied to these brain connectivity abnormalities.

Patients' scores on the Trail Making Test B, which is "a measure of executive functioning to assess mental flexibility, specifically, the ability to shift rapidly between cognitive sets," significantly correlated with the decreased activity in the posterior regions of the default-mode network (multiple comparisons corrected P=0.02).

Poorer anterior medial prefrontal cortex connectivity correlated with a higher degree of clinical symptoms, including anxiety, depression, fatigue, and post-concussive symptoms ( P<0.01).

Thus, the increased connectivity seen overall in that region may reflect a compensatory mechanism in TBI, the researchers explained.

"Increased medial prefrontal cortex activation has been reported previously in moderate and severe traumatic brain injury and is hypothesized to represent brain neuroplasticity operating in recovery and neural repair after injury," they wrote.

"The abnormally increased medial prefrontal cortex usage over the long run, however, might lead to persistent psychologic symptoms, such as the depression, anxiety, and fatigue seen in these patients," they explained.

The group noted the limited scope of the study in not looking at functional connectivity in other networks in the brain. Also, the difference between patients and controls didn't reach statistical significance with one of the analysis methods, which may have been due to the relatively small sample size, but still requires some caution in interpretation.

Also, disrupted connectivity in the default-mode network could have been caused by structural problems, such as axonal injury, which wasn't looked at in the study, they acknowledged.

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