Repetitive hits to the head that are below the threshold for causing a concussion may still cause changes in the brain, a small study of soccer players found.
On average, elite male soccer players -- who often use their heads to direct the ball -- had a range of changes in the white matter deep inside their brains compared to a group of competitive swimmers who were unlikely to have repetitive brain trauma, according to Dr. Inga Koerte of Harvard Medical Schools Psychiatry Neuroimaging Laboratory in Boston and colleagues.
Those differences were observed even though none of the participants in either group had a history of concussion, Koerte and colleagues reported in a research letter in the Nov. 14 issue of the Journal of the American Medical Association.
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Although it is possible that frequent heading of the ball could explain the impairments in the soccer players, "differences in head injury rates, sudden accelerations, or even lifestyle could contribute," the authors wrote.
Previous studies have shown that repetitive traumatic brain injury can have negative long-term consequences -- including impaired white matter integrity -- but the effects of frequent subconcussive head impacts are less clear.
Koerte and colleagues explored the issue using high-resolution diffusion tensor imaging, which can detect changes in white matter structure.
The study included 12 right-handed male soccer players from an elite-level soccer club in Germany. All had trained since childhood for a professional career. Their mean age was 19.7 and they had been playing for an average of more than 13 years.
The control group consisted of 11 competitive swimmers who were matched by age, handedness, and sex. Their mean age was 21.4 and they had trained for an average of more than 9 years.
None of the participants in either group had a history of concussion or any neuropsychiatric disorders.
All underwent diffusion tensor imaging to measure markers of mild traumatic brain injury and axonal and myelin pathology.
After adjustment for age and years of training, the soccer players showed increased radial diffusivity in multiple brain areas. A widespread increase in radial diffusivity also has been seen in patients with mild traumatic brain injury and suggests possible demyelination, according to the researchers.
A neuroradiologist found no abnormalities in structural images of the brain.
The researchers could not exclude the possibility that heading the soccer ball resulted in the changes in white matter architecture, but they said that the reason remains unclear.
As an alternate explanation, they noted that "soccer players showed increased axial diffusivity in the absence of increased radial diffusivity limited to the corpus callosum, possibly resulting from specialized training or neuroinflammation."
The authors acknowledged that the study was limited by the small sample size, the use of a single cross-sectional evaluation, and the lack of information on functional outcomes.