Traumatic brain injury (TBI) has been in the news lately because it is prevalent among veterans of the wars in Iraq and Afghanistan. The two papers chosen as best in the field on TBI by Russell Packard, MD, a headache and TBI expert in Palestine, TX, are not directly related to injuries sustained in the military, but do shed light on the impact of even mild TBI on brain structure and function.
The first study, reported in the March 18 Neurology, shows that diffusion tensor imaging (DTI) may detect subtle but clinically important changes in the brain after mild traumatic brain injury (MTBI) event when CT or MRI do not detect an abnormality.
MTBI, defined as loss or change of consciousness for less than 30 minutes with post-traumatic amnesia for less than 24 hours, is relatively common among adolescents: the documented incidence of TBI-related emergency department (ED) visits is 66.1 per 100,000 among 15-to-19-year-olds, wrote lead author Elisabeth A. Wilde PhD, of Baylor College of Medicine, and colleagues. And that number is probably an underestimate, because many cases never come to the ED.
Among patients who do report to the ED and exhibit normal CT findings, many experience lingering symptoms, such as poor memory and concentration, headaches and dizziness, and depression or irritability. These symptoms may resolve within a few weeks or months, but in a few cases the symptoms persist, with serious quality-of-life implications.
“This dissociation during the initial weeks and months after MTBI between normal conventional imaging results in patients with cognitive dysfunction and postconcussion symptoms has remained enigmatic for clinicians and investigators,” the authors pointed out.
Given the tenacity of some symptoms despite normal CT scans, and the neuropathologic evidence from fatal accidents of axonal damage in the corpus callosum and elsewhere, Dr. Wilde and her colleagues tried to determine if other imaging methods might detect changes that CT missed.
They performed DTI on 10 adolescents who had closed-head MTBI, with loss of consciousness lasting less than 10 minutes. The adolescents all had negative CT and scored 15 on the Glasgow Coma Scale, indicating normal cognitive function, and also had DTI within six days of the injury. Each patient was paired with a control without MTBI who was matched for age, sex, and the patient's pre-MTBI intellectual status.
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“DTI is acquired on a standard MRI scanner is far more sensitive to white matter injury than conventional MRI,” Dr. Packard told Neurology Today. It measures anisotropic diffusion, the orderly movement of water molecules along nerve fibers when the myelin sheath is intact. Anisotropic diffusion is measured through fractional anisotropy (FA), in which FA values range from 0 (completely free diffusion) to 1 (completely anisotropic diffusion). Higher values imply more severe white matter disruption. The investigators also recorded radial diffusivity (RD), a measure of diffusion perpendicular to the normal orientation of axons, and apparent diffusion coefficient (ADC), the overall average measure of diffusion.
On group analysis, the MTBI patients had significantly higher FA values (p=0.038), and lower ADC (p=0.006) and RD (p=0.010) values than the control subjects. These findings suggest cytotoxic edema, Dr. Packard explained. The patients also scored higher on tests of emotional distress than the controls. The authors noted that cognitive, affective, and somatic symptoms of MTBI all correlated with DTI findings in the corpus callosum.
All in all, this study suggests that “with more sophisticated analysis, there do seem to be recognizable patterns of brain injury, even after a ‘mild’ injury,” Dr. Packard explained.
BRAIN CHANGES AFTER TBI
The second study, reported in the March 4 Neurology, further explores the relationship between brain changes after TBI. In the Toronto Traumatic Brain Injury Study, lead author Brian Levine, PhD, of the Rotman Research Institute at Baycrest, Toronto, performed MRI on 69 patients from Canada's largest trauma center (the Sunnybrook Health Sciences Centre), one year after mild, moderate, or severe TBI, as well as 12 uninjured controls matched for age and sex. The control group had a significantly higher education level.
MRI demonstrated a stepwise, dose-response relationship between TBI severity and brain parenchymal volume, the authors reported. Volume loss was seen in both grey and white matter. The changes did not differ significantly between patients with moderate or severe TBI, but the loss was significantly more in those two groups than in people with mild TBI, and brain volume in people with mild injuries was significantly less than in the controls.
Nearly every brain region they studied was affected, although the frontal, temporal, and cingulate regions showed the most severe effects. As Dr. Packard explained, “this study was meaningful to me because it could differentiate all levels of TBI severity, even in mild TBI, where traditional CT and MRI are often normal.” •
Wilde EA, McCauley SR, Levin HS, et al. Diffusion tensor imaging of acute mild traumatic brain injury in adolescents. Neurology 2008;70:948–955.
Levine B, Kovacevic N, Black SE, et al. The Toronto traumatic brain injury study: Injury severity and quantified MRI. Neurology 2008;70: 771–778.