Imaging Study May Aid Detection of Concussive Brain Injury

A variant of magnetic resonance (MR) imaging may be more sensitive than conventional MR for detected blast-related axonal brain injury, according to a new study published online in the NEJM. The imaging variant, called diffusion tensor imaging, or DTI, measures anistrophy, or the directional diffusion of water in the brain—the idea being that water is more likely to diffuse along the path of intact axons. An abnormal DTI study would indicate a lack of directional flow, thereby suggesting some interruption or damage to axons—for instance, a shear injury from a concussive blast.

Among 63 military subjects who sustained a mild, uncomplicated, concussive-type brain injury (self-reported) in the recent line of duty, 18 (~30%) demonstrated abnormalities on DTI in 2 or more brain regions in this study. In all of these positive DTI cases except one, conventional MR images were normal. Observed DTI abnormalities, in contradistinction to those found historically in civilian cases, were more likely to be in the middle cerebellar peduncles (located in the back of the brain) and the orbitofrontal white matter (located in front of the brain)—areas "predicted to be vulnerable to primary blast injury," the study authors* reasonably conclude.

A major drawback of this study, however, is that none of the subjects had isolated primary blast exposure (meaning that they sustained other, extracranial injuries). Therefore the sole contribution of blast exposure to the DTI-detected brain abnormalities could not be determined. In addition, and most important, only about one third of subjects showed DTI abnormalities in the context of a normal MR study. Consequently the diagnosis of traumatic brain injury from a concussive blast remains a clinical one. It was also assumed, and the study results bear out this assumption in a qualified way (see the first sentence of this paragraph), that blasts, through their "shock energy" (as neurologist Allan Ropper writes in an accompanying editorial), are in and of themselves capable of producing brain injury.

Nevertheless, this study suggests that DTI is a promising method for detecting and studying concussive-type brain injury. Moreover, it can be "performed relatively quickly on the MRI scanners at US military and civilian hospitals." On a follow-up basis, specifically, DTI may aid the diagnosis and management of military personnel with concussive-type brain injuries, and it may provide some organic-based insight into post-traumatic stress disorder, the study authors (and Ropper) suggest.

* From Washington University, the Landstuhl Regional Medical Center in Germany, and the Walter Reed Army Hospital in DC.
Photo of aftermath of car bombing in Baghdad, August 2003, from the FBI web site.