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In a nutshell
Being symptom-free doesn’t mean the brain has fully healed. Even after athletes are medically cleared to return to play, advanced brain scans show persistent changes in blood flow and white matter structure.
Some brain changes last up to a year after concussion. Reduced blood flow in key brain regions and disrupted white matter pathways were still detectable 12 months after athletes resumed sports.
Current concussion protocols may miss hidden recovery issues. Standard tests might clear athletes too soon, overlooking ongoing physiological changes that could affect brain function and long-term health.
TORONTO — When an athlete gets a concussion, they typically follow a standard recovery path: rest, a gradual return to activities, and medical clearance once symptoms disappear. This approach, used everywhere from high school teams to professional leagues, assumes the brain has healed when symptoms vanish and basic cognitive tests return to normal. But what if that assumption is wrong?
New research published in Neurology challenges this conventional wisdom with a sobering discovery: substantial brain abnormalities continue long after athletes receive medical clearance to return to their sports. The findings suggest our current methods for determining when it’s safe for athletes to resume competition might be missing something crucial.
Before and After Brain Imaging Reveals the Truth
The research team from St. Michael’s Hospital and the University of Toronto tracked 25 varsity athletes who suffered concussions, comparing their brain scans from before injury through various recovery stages and up to a year after medical clearance. They discovered that even after symptoms disappeared and athletes were cleared to play, their brains showed ongoing abnormalities in blood flow and white matter structure.
Some of these changes remained detectable a full year after athletes returned to sports, suggesting that what doctors consider “recovery” might actually be a new brain state quite different from how things worked before the injury.
Underlying damage in the brain is often still present when athletes take the field again. (© Crystal light – stock.adobe.com)
Rather than comparing concussed athletes to different people (which introduces all sorts of variables), the team collected baseline brain scans from 187 university athletes before any injuries happened. This allowed them to compare each concussed athlete’s brain to their own pre-injury state, a major advantage over previous studies. Of those 187 athletes, 33 later suffered concussions during their athletic careers, and 25 were successfully followed with brain imaging after their injuries.
The researchers used two advanced brain imaging techniques. First, arterial spin labeling (ASL) measured cerebral blood flow (CBF), essentially how much blood travels to different brain regions. Second, diffusion tensor imaging (DTI) examined the microstructural features of white matter, the brain’s communication highways connecting different regions. DTI produces measurements called mean diffusivity (MD) and fractional anisotropy (FA), which tell us about the health and organization of white matter fibers.
What’s Happening Inside the “Recovered” Brain
At medical clearance, athletes showed substantial decreases in cerebral blood flow, especially in brain regions handling emotional processing, decision-making, memory, and sensory integration. These functions are essential for athletic performance and everyday life. These blood flow reductions didn’t bounce back over time; they remained decreased a year later.
White matter also showed ongoing abnormalities. After concussion, athletes had changes in several brain regions, particularly the corona radiata and internal capsule, major white matter pathways that connect different brain areas and carry signals from the brain to the spinal cord. These changes indicate disruption to the structural integrity of white matter tracts, potentially affecting how efficiently different brain regions talk to each other.
When compared to normal brain variability in uninjured athletes, only the blood flow changes consistently distinguished concussed athletes from controls. This highlights that blood flow alterations may be especially important markers of incomplete recovery.
The researchers also noticed that changes in blood flow, but not white matter properties, correlated with how long it took athletes to recover clinically. Athletes who needed more time to be cleared for return to play showed more abnormal blood flow within brain regions critical for memory formation.
Rethinking Return-to-Play Protocols
Just because a player is clinically cleared, their brains may not be fully healed. (Rocketclips, Inc./Shutterstock)
These ongoing brain changes happened despite the absence of obvious symptoms. By the time athletes were cleared to return to play, they reported feeling fine and performed normally on cognitive tests. There were no reported instances of symptoms coming back after return to play. Yet beneath this apparent clinical recovery, noteworthy physiological changes continued.
The ongoing decreases in cerebral blood flow are especially worrisome, as they may reflect continuing secondary injury processes, including metabolic problems, inflammation, and disruption of the blood-brain barrier. Such blood flow issues can worsen injuries through decreased protein synthesis and increased oxidative stress, potentially leading to long-term brain changes.
For athletes, coaches, and parents, these findings call into question the current return-to-play guidelines. While the study doesn’t offer specific recommendations for changing protocols, it does point to the potential value of using advanced brain imaging in concussion management to better track individual recovery patterns.
Unlike a broken bone that shows clear healing on an X-ray, brain recovery involves multiple systems that may heal at different rates. Symptoms might disappear before the underlying physiology has fully recovered, giving a misleading impression of healing.
What This Means for Concussion Management
While focused on university athletes (average age about 20 years), these findings raise important questions about concussion management across age groups and settings, from youth sports to professional leagues to non-sports-related concussions. The ongoing brain changes documented in these young, healthy athletes raise concerns about how concussions might affect more vulnerable groups, such as children with developing brains or older adults.
The next time you hear that an athlete has been “cleared” following a concussion, remember that while they may feel fine and ace their clinical tests, their brain might still be on a longer healing journey, one that continues long after they’ve returned to competition.
Paper Summary
Methodology
The researchers recruited 187 university athletes and performed baseline MRI scans before any injuries occurred. Those who later suffered concussions (25 athletes) were scanned at several points: during symptoms (median 5 days post-injury), at medical clearance (averaging 22 days post-injury), 1-3 months after return, and one year after return. A control group of 27 uninjured athletes was also rescanned at their next preseason baseline. The study used arterial spin labeling to measure cerebral blood flow and diffusion tensor imaging to assess white matter structure, comparing each athlete’s post-concussion brain to their own pre-injury baseline.
Results
When athletes were cleared to return to play, they showed decreased blood flow in multiple brain regions (approximately 9 mL/100g/minute below baseline), particularly in areas handling emotion, decision-making, and memory. They also showed altered white matter measurements in specific tracts. Blood flow reductions persisted up to one year post-clearance, while some white matter changes partially normalized. Only blood flow changes reliably distinguished concussed athletes from controls. Changes in blood flow correlated with clinical recovery time, with slower-recovering athletes showing greater abnormalities in memory-related brain regions. Despite these ongoing brain changes, athletes reported no symptoms at return to play and performed normally on standard concussion tests.
Limitations
Data loss occurred over time, particularly at the one-year follow-up. The control group maintained normal activity while concussed athletes had periods of reduced activity, creating different comparison conditions. Potential undisclosed concussions in control athletes couldn’t be ruled out. The sample size (25 concussed athletes) limited analysis of individual recovery patterns. Since the study focused on university athletes (average age 20), findings might not generalize to children, older adults, or non-sports concussions.
Discussion and Takeaways
This study demonstrates that physiological brain recovery continues well beyond clinical symptom resolution and medical clearance. Current return-to-play protocols may not account for ongoing brain healing, raising questions about potential vulnerability to re-injury or cumulative effects of multiple concussions. Persistent blood flow reductions may reflect ongoing secondary injury processes that could affect long-term brain health. The findings suggest a more cautious approach to concussion management might be warranted, potentially incorporating advanced neuroimaging to better assess recovery.
Funding and Disclosures
The study was supported by a Canadian Institutes of Health Research (CIHR) Project Grant (FRN: 159506). The researchers reported no relevant conflicts of interest that might influence the findings or interpretation.
Publication Information
“Post-Concussion Brain Changes Relative to Pre-Injury White Matter and Cerebral Blood Flow: A Prospective Observational Study” was published in Neurology (Volume 104, Number 7) on March 12, 2025. The research was conducted by Nathan W. Churchill, Michael G. Hutchison, Simon J. Graham, and Tom A. Schweizer from St. Michael’s Hospital, Unity Health Toronto, and affiliated institutions.