Can Concussions Cause White Matter Lesions?

Concussions can and do cause damage to white matter, the brain’s network of insulated nerve fibers, but the injury is often invisible on a standard hospital MRI scan. The disconnect between normal-looking clinical images and real underlying damage is one of the most important and misunderstood aspects of concussion science. Specialized imaging techniques reveal changes in white matter microstructure that conventional scans miss, and those changes correlate with symptoms, recovery time, and long-term outcomes in ways researchers are still mapping out.

Why a Normal MRI Does Not Mean Normal White Matter

If you have had a concussion and your MRI came back clean, that is actually the expected result. A large multicenter study using high-resolution 3-Tesla MRI found that white matter hyperintensities, the bright spots doctors look for on standard scans, appeared in about a third of concussion patients and about a third of uninjured controls, with no statistically significant difference between the two groups. Those hyperintensities also did not correlate with concussion symptom scores at any follow-up point.¹ In other words, those white spots on a routine scan are common in healthy people, and having more of them after a concussion does not reliably tell you anything about the injury.

This might sound like evidence that concussions leave white matter alone. It is not. Standard MRI is simply the wrong tool for the job. It detects large structural problems like bleeds, tumors, and strokes, but the damage a concussion inflicts on white matter is microscopic. The nerve fibers get stretched, their insulating myelin sheaths become disrupted, and the surrounding tissue swells with inflammation, all at a scale far below what a conventional scan can resolve.

What Advanced Imaging Actually Finds

Diffusion-based imaging techniques, which track the movement of water molecules through brain tissue, tell a very different story. Healthy white matter is organized in tight, directional bundles, so water flows along the fibers the way a train follows tracks. When those bundles are damaged, water starts leaking in directions it normally would not. Researchers quantify this with metrics that reflect the structural integrity of the tissue.

A systematic review of diffusion imaging studies in sport-related concussion found that in the weeks right after injury, results were mixed: some athletes showed increases and some showed decreases in standard diffusion metrics. But in athletes who were a month or more removed from their concussion, or who had a remote history of head injury, the pattern became more consistent. White matter tracts reliably showed signs of reduced structural integrity compared to uninjured controls.² The tracts affected include some of the brain’s major highways: the corpus callosum (which connects the two hemispheres), the corona radiata, the internal capsule, and the longitudinal fasciculi.

The severity of these white matter changes also appears to predict recovery. A study of collegiate athletes found that the microstructural properties of white matter tracts were significantly correlated with how long it took an athlete to return to play, and a classifier built from the 16 most affected tracts could distinguish slow recoverers from fast ones with high accuracy.³ Meanwhile, data from the large NCAA-DoD CARE Consortium showed that concussed athletes with better-preserved white matter integrity performed better on cognitive testing shortly after injury.⁴

Newer imaging methods are pushing even further. A technique called NODDI, which models water diffusion in a more detailed way than standard diffusion imaging, has found that concussion is associated with a reduction in the volume of water inside nerve fibers themselves. Those reductions were still present at the point when athletes were medically cleared to return to sport, suggesting damage persists beyond the window when symptoms resolve.⁵ In athletes with a remote history of concussion (more than six months prior), NODDI detected increased neurite density and more aligned fiber orientation, which may reflect a compensatory remodeling response rather than a simple return to baseline.⁶

How the Damage Happens

The brain is a soft organ floating in fluid, and a concussion subjects it to rapid acceleration and deceleration. White matter is especially vulnerable because its long, thin fibers stretch between distant brain regions and experience significant mechanical strain during impact. Laboratory research using computational models and tissue analysis has demonstrated that the distribution of strain across the brain during an impact closely predicts where axonal injury and myelin loss will show up afterward.⁷ The fibers at the boundary between gray and white matter, and within tracts that span long distances, tend to take the worst of it.

The initial stretch is only the beginning. Concussion also disrupts the blood-brain barrier, the tightly sealed lining of blood vessels that normally keeps blood proteins out of brain tissue. In experimental concussion models, researchers have observed multifocal blood-brain barrier leaks, even without any visible bleeding, and those leaks overlapped substantially with regions showing axonal damage in white matter.⁸ Once the barrier is compromised, blood-borne proteins seep into the tissue and get taken up by surrounding brain cells, especially astrocytes and neurons, which triggers an inflammatory cascade.

That inflammation is a second wave of injury. Activated immune cells in the brain, called microglia, shift into an aggressive state and release inflammatory molecules. If left unchecked, this neuroinflammation accelerates the breakdown of myelin and damages the tight junction proteins that hold the blood-brain barrier together, creating a self-reinforcing cycle. Research into blocking this inflammatory pathway has shown that reducing neuroinflammation after experimental brain injury preserves white matter and improves nerve conduction in both myelinated and unmyelinated fibers.⁹

The Timeline of White Matter Changes

White matter damage from concussion is not static. It evolves over days, weeks, and months, and the trajectory does not always match how the person feels. One longitudinal study of college athletes tracked diffusion imaging at two days, two weeks, and two months after concussion. The most pronounced changes appeared within the first 72 hours, with a statistical difference from controls. By two weeks, the concussed athletes looked indistinguishable from healthy controls on imaging. But at two months, there was a trend toward abnormality again, suggesting that what looks like recovery at two weeks may actually be a temporary normalization before a longer-term process unfolds.¹⁰

A larger study tracked concussed athletes from 24 to 48 hours after injury and beyond the point when they reported feeling symptom-free. The extent of affected white matter did decrease over time, but part of the corpus callosum showed persistent abnormalities across all time points measured. Athletes whose white matter was more disrupted in the acute window took longer to recover and had worse symptom scores.¹¹ Another longitudinal study found that concussed athletes still had elevated diffusion metrics in several white matter tracts at every visit after injury, with no clear evidence of a return to baseline within the study period, and proposed that these abnormalities could serve as an objective biomarker of concussion outcome.¹²

The pattern that emerges is sobering. Feeling better does not necessarily mean the brain’s white matter has healed. This is a key reason many concussion researchers argue that return-to-play decisions based solely on symptom checklists may send athletes back too soon.

How White Matter Damage Relates to Symptoms

The connection between white matter integrity and the symptoms people experience after concussion is now well established across multiple studies and populations. At 12 months after a mild traumatic brain injury, widespread reductions in white matter integrity across many pathways were associated with poorer functional, cognitive, and emotional outcomes.¹³ In patients with persistent post-concussion syndrome, the severity of symptoms correlated with diffusion abnormalities in specific tracts including the corpus callosum, the uncinate fasciculus, and the internal capsule.¹⁴

The relationship between white matter and symptoms is not always straightforward, though. A study of veterans who sustained blast-related mild brain injuries found that white matter abnormalities were significantly associated with the physical symptoms of post-concussion syndrome, like headaches and dizziness, even after accounting for PTSD. But the same abnormalities did not show a strong connection to cognitive or emotional symptom severity.¹⁵ That disconnect matters clinically, because it means someone whose main complaints are memory problems or anxiety after a concussion may have very real brain injury that does not show up the same way on current white matter imaging.

Subconcussive Hits Add Up

You do not need a diagnosed concussion to accumulate white matter damage. This is one of the more alarming findings in recent concussion research. A study of youth football players tracked head impacts over a single season using helmet-mounted sensors, then compared pre- and post-season brain imaging. Players who absorbed more cumulative rotational acceleration showed significantly decreased white matter integrity in several tracts, and the relationship was dose-dependent: more impact exposure meant more change.¹⁶ None of these players were diagnosed with a concussion during the season.

The cumulative nature of subconcussive impacts is drawing increasing attention. A review of the evidence found that these repeated low-grade hits, accumulated over an active sports career, can produce measurable changes to brain health and cognitive function.¹⁷ The implication is that a single concussion is not the only mechanism by which contact sports affect white matter. The day-to-day impacts of practice and play may be contributing to a slow accumulation of injury that never triggers acute symptoms but still reshapes the brain’s wiring.

Sex Differences in White Matter Response

Men and women do not appear to sustain concussion-related white matter damage in the same way, and this has practical implications for diagnosis and recovery. Fiber-based analysis comparing male and female athletes after sport-related concussion found that male athletes showed significantly increased fiber density in the cingulum, a major tract involved in attention and emotion, at both 48 hours and two weeks after injury. Female athletes showed no such increase. No differences between sexes were observed in uninjured control athletes, which means the disparity was specific to the injury response.¹⁸

A separate study looking at a broader mild traumatic brain injury population found that male patients had significantly reduced white matter integrity in the uncinate fasciculus bilaterally compared to female patients and controls, and this correlated with a longer time to symptom resolution. Being male and having lower uncinate fasciculus integrity both independently predicted a recovery period exceeding three months, while the initial severity of symptoms at the time of injury did not predict recovery time at all.¹⁹ These findings suggest the biology of white matter recovery after concussion differs between sexes in ways that go beyond how bad the initial injury looks or feels.

Children’s Brains Are Still Building White Matter

The developing brain is in the middle of an active construction project. Throughout childhood and adolescence, white matter tracts are gaining myelin, becoming more organized, and pruning unnecessary connections. Concussion during this window does not just damage existing structures; it can disrupt the developmental process itself. A study following children over two years found that the youngest children with a concussion showed less growth in superficial white matter neurite density than their uninjured peers. The effect was particularly pronounced in girls and was associated with increased internalizing behavior scores, meaning symptoms like anxiety and withdrawal.²⁰

This is a qualitatively different kind of harm than what adults experience. An adult who sustains white matter damage is losing ground from an established baseline. A child whose white matter maturation slows or stalls may end up with a different baseline altogether, with potential consequences for cognitive development and emotional regulation that unfold over years rather than weeks.

Blood Tests That Track White Matter Injury

One of the most promising developments in concussion science is the emergence of blood-based biomarkers that reflect what is happening to white matter. Neurofilament light (NfL) is a protein released when nerve fibers are damaged. In traumatic brain injury, serum NfL levels correlate with measures of brain atrophy on MRI and with diffusion-based estimates of white matter injury.²¹

In former professional athletes, NfL levels were positively correlated with diffusion abnormalities in the corpus callosum and the fornix, two tracts commonly affected by repetitive head impacts. More strikingly, NfL measured at a first visit predicted the amount of white matter deterioration observed at a two-year follow-up, suggesting the blood test may capture an ongoing degenerative process.²² Research has also begun examining how blood biomarkers like NfL and diffusion MRI measurements interact in the acute phase of mild brain injury, with the goal of identifying distinct recovery trajectories early on.²³ A simple blood draw within days of a concussion could eventually help clinicians estimate the likelihood and pace of white matter recovery, rather than relying purely on how the patient feels.

Genetic Vulnerability and the APOE Connection

Not everyone’s white matter responds identically to the same force of impact, and genetics appears to play a role. The APOE ε4 gene variant, best known for increasing Alzheimer’s disease risk, has attracted attention in traumatic brain injury research for years. A study of 200 Iraq and Afghanistan war veterans found a significant interaction between carrying the APOE ε4 variant and exposure to close-range blast events. Veterans who carried ε4 and had been near an explosion showed more white matter abnormalities than those with the same blast exposure but without the gene variant. Among ε4 carriers, the number of close-range blasts predicted the number of white matter disruptions in a dose-dependent fashion. The relationship remained significant even after controlling for the severity of their traumatic brain injuries overall.²⁴

This does not mean APOE ε4 carriers should panic at every bump to the head. The study population experienced intense blast exposures, and the findings are specific to that context. But the broader implication is worth noting: your genetic background may influence how much white matter damage you accumulate from head impacts and how well your brain repairs it. As genetic testing becomes more common, this kind of information could eventually factor into individualized risk assessments for athletes and military personnel.

Animal Models Confirm What Imaging Suggests

One reason the field has moved with growing confidence on the question of white matter injury is that animal studies provide tissue-level confirmation of what imaging detects in living humans. In a pig model of mild traumatic brain injury, researchers found that the corpus callosum showed a roughly 21% increase in fractional anisotropy from one to three months after injury, far exceeding the modest change seen in uninjured animals. The internal capsule on the injured side showed decreased integrity, which may indicate demyelination.²⁵ The pig brain is reasonably similar to a human brain in terms of its white-to-gray matter ratio and gyral complexity, which makes these findings more translatable than rodent experiments.

These animal results also help explain the sometimes confusing pattern seen in human studies, where diffusion metrics can go up or down depending on the time window and the specific tract. In the acute phase, swelling and inflammation can cause water to pool in new directions. Weeks later, the breakdown of myelin sheaths changes the picture again. Months out, compensatory reorganization may shift metrics yet again. The biology is messy because the injury is not a single event but a cascading sequence of mechanical damage, inflammation, repair, and remodeling that plays out over very different timescales.