White matter can be negatively impacted due to repeated head impacts during contact sports. Such injuries occur even when there is not a clinically diagnosed concussion. These are results from several studies released in the past few months. The studies show impact to cognition and length of recovery in collegiate football and hockey athletes. In addition to those studies, another identifies issues with vestibular and balance due to white matter changes.
White matter facilitates communication between areas of the brain. Changes are identifiable through the use of Diffusion Tensor Imaging (DTI), which is a form of Magnetic Resonance Imaging (MRI). White matter changes do not show on standard MRIs. The DTI tests found lower fractional anisotropy (FA) levels.
One study found more impacted white matter for athletes participating in contact sports vs. non-contact sports over the course of a season  and that “there was relationship between how hard and often you hit your head.”  Another study found that time between seasons was not adequate time to recover for some who had experienced repetitive head impacts during a football season.  In both of those studies, participants did not experience a concussion.
In each of these studies, the corpus callosum was affected. The corpus callosum divides and connects the left and right hemispheres of the brain. It’s also a major location of white matter. How, then, does impacted white matter affect communication between the brain hemispheres? This is a question I have. In reviewing information online, I found several items which may provide some clues – and directional pointers on where to look for answers.
First, there’s a treatment to stop seizures in some epileptic patients: corpus callosotomy surgery. This cuts the corpus callosum to reduce seizure electrical signal transmissions between the hemispheres. Second, a new study was just released that found that people born without a corpus callosum were able to forge new avenues of communication to bridge the two hemispheres. 
My thoughts upon finding those two items are twofold. First, I think there’s likely quite a bit of information available regarding white matter and communication related to corpus callosotomy surgery. There must be something there to provide some direction. Second, this new study might indicate that plasticity could perhaps help with recovery. (Or perhaps that’s wishful thinking on my part.)
I’m not saying that those items infer that they result from a concussion. However, because concussion is yet a mystery, I think it’s helpful to look at existing data and research and apply that as needed. So, if DTIs are finding issues with white matter in the corpus callosum, I think it’s beneficial to look at existing information and apply what might be relevant to concussion.
Moving on, discussion of the studies follows.
Researchers conducted preseason and postseason cognitive testing using the California Verbal Learning Test. The test included memory, learning, and attention. At preseason testing, the study found no difference between the two participant groups. After the season ended, the reverse was true. The researchers did see changes over the season, but could not determine cumulative effects. The contact sport athletes didn’t perform as well on the test. Those athletes that played contact sports had more white matter impact. The athletes did not have concussion during the season (they would have been excluded from the study). What the study indicated was that white matter was impacted even though a person did not have a concussion. They found that “there was a relationship between how hard and how often you hit your head.” 
A main affected area was the corpus callosum.  The corpus callosum is the area of the brain between the left and right hemisphere. It connects the two hemispheres and facilitates communication between them. 
Research Institution: Dartmouth College
Researchers: Thomas McAllister MD, James Ford PhD, Laura Flashman PhD, et al
Objectives: “To determine whether exposure to repetitive head impacts over a single season affects white matter diffusion measures in collegiate contact sport athletes.” 
- Division 1 university
- Did not have a concussion when tested pre-season or during the study
- 80 Contact sport athletes (football and ice hockey)
- 80 Non-contact sport athletes (track and field, cross-country running): those that “did not hit their head on a regular basis but were competing at a very high level as well” 
Affected Brain Areas
Corpus callosum, amygdala, cerebellar white matter, hippocampus, and thalamus
Continual head impacts over the course of a season can cause injury even if a person doesn’t sustain a diagnosed concussion. These repetitive head impacts (RHI) cause result in subconcussive injury and white matter changes. Those are the findings of this study from the University of Rochester. Notably, the study also found that the changes didn’t resolve after an extended period. Upon followup testing six months after cessation of contact sport activities, white matter changes were still evident.
Research Institution: University of Rochester
Researchers: Jeffery Bazarian MD et al
Objectives: “1) characterize the magnitude and persistence of RHI-induced white matter (WM) changes; 2) determine their relationship to kinematic measures of RHI; and 3) explore their clinical relevance.” 
- Division III university
- Did not have a concussion during the study
- 10 football players
- 10 controls (non-athletes)
Affected Brain Areas
Vestibular and Balance
This study found vestibular issues with white matter. Difficulties with balance are a common symptom for concussion. This study abstract doesn’t identify the age group of the participants, so it may or may not include collegiate athletes. However, the findings are worth consideration, I believe.
Research Institution: University of Pittsburgh Medical Center
Researchers: Lea Alhilali M.D, Karl Yaeger MD, Michael Collins PhD, Saeed Fakhran MD
Objectives: “To determine if central axonal injury underlies vestibulopathy and ocular convergence insufficiency after mild traumatic brain injury (TBI) by using tract-based spatial statistics (TBSS) analysis of Diffusion Tensor Imaging (DTI)” 
Affected Brain Areas
Cerebellar area (controls balance and movement) and fusiform gyri (visual fields and spatial orientation)
“Patients with vestibular symptoms had white matter injury in the cerebellar area, which is known to control balance and movement, and also in the fusiform gyri, a brain area that integrates the visual fields of the left and right eye and is important to spatial orientation,” she said.
The findings appear to show a connection between vestibulopathy and regional brain damage, Dr. Alhilali added.
Update Note: I worked on this post through 2014 and published it publicly on July 3, 2014 when I launched this site. I had been waiting until I finished migrating content from my older site.
 “Effect of head impacts on diffusivity measures in a cohort of collegiate contact sport athletes” Neurology December 11, 2013
 Podcast, Thomas McAllister MD “Effect of head impacts on diffusivity measures in a cohort of collegiate contact sport athletes” Neurology.org January 7, 2014
 Corpus Callosum Healthline.com
 Lea Alhilali M.D, Karl Yaeger MD, Michael Collins PhD, Saeed Fakhran MD “Detection of Central White Matter Injury Underlying Vestibulopathy after Mild Traumatic Brain Injury” Radiology April 2014
 Bazarian JJ, Zhu T, Zhong J, Janigro D, Rozen E, et al. (2014) Persistent, Long-term Cerebral White Matter Changes after Sports-Related Repetitive Head Impacts. PLoS ONE 9(4): e94734. doi:10.1371/journal.pone.0094734
 Tanya Lewis “People Missing Brain Wiring Form Unique Neural Connections” livescience.com May 12, 2014