SAN DIEGO—Diffusion tensor imaging (DTI) may help neurologists detect axonal injury before any symptoms appear in an athlete who has had a concussion, said Jeffrey J. Bazarian, MD, MPH. Neurologists can maximize the technique’s sensitivity by comparing it with a DTI scan taken at baseline, he noted at the 65th Annual Meeting of the American Academy of Neurology.
A pattern of decreased fractional anisotropy and elevated mean diffusivity on DTI suggests axonal loss. The opposite pattern, which suggests axonal edema, may occur at the same time. “We still don’t know the clinical significance of these changes,” said Dr. Bazarian, Associate Professor of Emergency Medicine at the University of Rochester Medical Center in New York. The relationship of these changes to short-term cognitive outcome and neurologic function is uncertain. Similarly unclear is the relationship of these changes to the development of chronic traumatic encephalopathy.
Water Movement Can Reveal Axonal Swelling
Axonal injury is the primary pathophysiologic process that occurs after brain injury. After concussion, the brain rotates and creates shear forces along the length of the axon. “If the axon gets stretched at the right threshold of stretch force, little pores form along the axon, and things start to fall apart,” said Dr. Bazarian. The axon swells, stops functioning, and may disconnect itself from the neuron over time.
CT and MRI scans cannot show axonal injury, but DTI can by illustrating the movement of water in the brain. A DTI scan measures fractional anisotropy, which indicates the degree to which water’s movement in the brain is straight, and mean diffusivity, which reveals the amount of water movement in the brain. “If axons swell up, then we would guess that the straightness of water motion would go up and the overall diffusivity would go down,” said Dr. Bazarian. “Conversely, if the axon degenerated, just the opposite would occur. This might be a nice way to indirectly see axonal swelling or axonal loss.”
Concussion Correlated With Changes on DTI
Dr. Bazarian and his colleagues examined seven high school football players to test whether DTI detects axonal injury after concussion. Each player underwent a DTI scan at the beginning of the football season and another scan at the end of the season. One player had a concussion, and six players served as controls. Players took cognitive exams before and after the football season. During the season, players used a diary to record the number of times they had been hit in the helmet. The investigators used wild bootstrapping to look for statistically significant differences between voxels before and after injury.
The player with concussion was cognitively worse than the other players, said Dr. Bazarian. The investigators found no cognitive difference between the other six players and nonathletes. All of the players were hit in the helmet between 50 and 400 times during the season.
Compared with the other players, the player with concussion had the greatest proportion (3%) of significant changes in fractional anisotropy and mean diffusivity in his brain after the season ended. The controls had nearly as much white-matter change as the player with concussion. The amount of change in fractional anisotropy and mean diffusivity in the players correlated with the number of times they reported being hit in the head and to the increases in their postconcussion symptom score at the end of the season. For the player with concussion, any part of the brain with post-season changes in fractional anisotropy also had changes in mean diffusivity, which suggested that DTI was detecting axonal injury, said Dr. Bazarian.
DTI May Indicate a Threshold for White Matter Damage
In a follow-up study, Dr. Bazarian and colleagues gave 10 college football players helmets with sensors that recorded blows stronger than 10 g. Players wore the helmets during practices and games and underwent a DTI scan at the beginning of the season, at the end of the season, and after six months of rest. Players also underwent cognitive and physical exams at these same three time points. Investigators used wild bootstrapping to compare each player’s three scans.
All players had increases and decreases in fractional anisotropy and mean diffusivity at the end of the season. The percentage of the brain with decreased fractional anisotropy correlated well with almost all of the helmet readings and with some neurologic outcomes. “That may be the important metric, in terms of axonal injury,” said Dr. Bazarian. Three players had a greater proportion of brain with decreased fractional anisotropy than the other players and the controls. Postseason changes correlated well with balance and some aspects of cognition, said Dr. Bazarian.