Even in the absence of a concussion, repeated blows to the head during a season of US football or ice hockey appear to be associated with changes in brain white matter and may also be linked to reduced cognition, a new study shows.
The study, published online December 11 in Neurology, was led by Thomas W. McAllister, MD, Geisel School of Medicine at Dartmouth College, Hanover, New Hampshire.
"A lot of attention has been paid to the effects on the brain of concussion, but there has not been much study on repetitive impacts to the head which do not cause concussion," Dr. McAllister commented to Medscape Medical News. "Sometimes the magnitude of these impacts can exceed those that caused concussion."
The study found a small increase in diffusivity in the brain white matter of the athletes involved in contact sports vs athletes participating in noncontact sports... And the contact sport athletes who performed worse than expected on cognition tests at the end of the season had greater changes in their white matter.
Dr. McAllister said, "We cannot say at this point that these are causal links, only that we have found associations between head impact and white matter changes and between white matter changes and poor cognition."
Awareness Should Be Expanded
"The difference in diffusivity we saw was very small — 0.005. We don’t know exacly what it means," he noted. "Our take-home messge is that just because someone hasn’t had a concussion doesn’t mean they have not experienced brain changes which may be associated with reduced cognition. We can’t make any recommendations from this data at present; only to say that we might need to expand awareness of brain changes to groups of people who engage in contact sports and have repeated head impacts but do not suffer concussion."
He added: "The contact athletes in this study were engaged at a pretty intense level, playing American football or ice hockey 5 or 6 times a week. They could have sustained about 500 to 600 impacts over the course of a season. That is a lot. I would say this data could also apply to other sports where there are head impacts, such as boxing, martial arts fighting and heading a football."
The study involved 80 nonconcussed varsity football and ice hockey players who wore helmets that recorded the acceleration time history of the head following impact, and 79 non–contact sport athletes. Assessment occurred before the season and shortly after the season with diffusion tensor imaging and neurocognitive measures.
When the reserachers analyzed the data across the whole time period, they found small but significant differences in white matter diffusivity between the contact and noncontact athletes. Measures of head impact exposure correlated with white matter
diffusivity measures in several brain regions, including the corpus callosum, amygdala, cerebellar white matter, hippocampus, and thalamus. The magnitude of change in corpus callosum mean diffusivity postseason was associated with poorer performance on a measure of verbal learning and memory.
"It appears to be a subtle effect," Dr. McAllister commented. "We are not sure how long it lasts. But it does appear to be linked to the frequency and magnitude of impacts to the head and may have an effect on cognition. There might also be a subgroup of people who are particularly sensitive in this regard."
Some Good News
There was no difference between the contact and noncontact athletes at the start of the season. "That is the good news," Dr. McAllister said. "If there were cumulative changes over years of sustaining head impacts, we would expect to see widespread differences in white matter diffusivity between the two groups of athletes at the start of the season, but this was not the case."
He explained that initially it was thought that there would be a specific g value for the magnitude of impact to the head that would be the threshold for concussion and /or brain changes, but that does not now appear to the case.
"We are not sure why one blow causes concussion while a similar blow at a different time or in another person does not. It may be to do with the direction or rotation of the blow. And it may be that context matters — how many high-impact hits have been received in a set time — there may be time window of vulnerability."
Dr. McAllister said this study was novel in that it involved 2 different cohorts of athletes, 2 different time points, and data on biomechanical exposure and cognition.
"The next step is to to determine how long these changes last and try and pin down if there is a subgroup with heightened vulnerability and what is causing that," he added.
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