By examining brain scans of a dozen professional soccer players from Germany, researchers found a pattern of damage that strongly resembled the injury seen in patients with mild traumatic brain injury, said Dr. Inga Katharina Koerte, a neuroradiologist at Boston's Brigham and Women's Hospital, who led the study.
Koerte and her colleagues focused on the athletes' white matter, the interior portion of the brain that carries signals from nerve cells to the spinal cord. They tracked the movement of individual water molecules within the brain tissue to see whether the atoms moved in a narrow linear pattern or in a random, diffuse pattern. Movement along a narrow track suggested the molecules were being hemmed in by healthy fibers. Diffusion, however, suggested that brain tissue had suffered some form of damage and could no longer restrict the movement of water molecules.
Using a high-resolution MRI technique called diffusion tensor imaging, the researchers observed microscopic changes in the frontal, temporal and occipital lobes - regions that control attention, visual processing, higher thinking and memory.
The research team also scanned the brains of 11 professional swimmers, who served as controls. Their brains showed much less damage.
"The origin of these results is not clear," said study senior author Martha Shenton, a neuroscience researcher at the VA Boston Healthcare System. Although frequent "heading" of the ball may be to blame, other factors - such as falling to the ground or goal posts or crashing into other players - could play a role as well, she said.
According to the U.S. Consumer Product Safety Commission, which maintains a national database of injuries, 40 percent of soccer concussions are the result of collisions between players, while roughly 13 percent are due to players heading the ball.
Though the study is small and preliminary, it is likely to fuel an already heated debate. While the danger of concussion among football has been widely studied, only recently have researchers begun to examine the risks faced by soccer players.
Statistics offered by the American Association of Neurological Surgeons suggest that soccer players, particularly young ones, are much less likely to suffer concussions than other athletes. In 2009, soccer players under the age of 15 suffered 8,392 concussions, according to the group. In contrast, bicyclists in that age group suffered 40,272 concussions; football players 21,878, and baseball and softball players 18,246.
However, the JAMA study focused on "sub-concussive" injuries, impacts there were not strong enough to cause a concussion. In selecting their soccer-player subjects, researchers included only men who had never reported suffering a concussion and had never been diagnosed with one by a physician.
None of the abnormalities seen in the study was apparent on conventional MRIs. Diffusion tensor imaging is revealing previously unobservable brain injuries, but it's unclear whether the white matter changes seen in this study would cause any problems, said Dr. Robert Harbaugh, director of the Penn State Institute of the Neurosciences, who was not involved in the study.
"It's way too early to make that next step and say, 'Well, if we see this, we should really worry about people playing soccer because they're going to get dementia at age 50,'" Harbaugh said. "I don't think we've got anything like that kind of information."
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