Changes in brain due to concussions persist up to 1 year after athletes medically-cleared to return to play, St. Michael’s study reveals
Dr. Tom Schweizer and Dr. Nathan Churchill at St. Michael’s Hospital, Unity Health Toronto.
A groundbreaking new study from St. Michael’s Hospital provides the most direct evidence to date that concussions in athletes cause changes to the brain that persist beyond medical clearance. The study, published in Neurology on March 12, 2025, directly compared athletes’ brain scans before and after injury, showing that the brain has not fully returned to its baseline when they’re cleared to return to play, and when the athlete otherwise shows no significant signs or symptoms of injury.
“There is growing recognition that concussions can have significant, long-term effects on brain health. Our study challenges the assumption that the brain has fully returned to its pre-injury state at the time of medical clearance to return to play,” said Dr. Nathan Churchill, study co-author and senior research associate at St. Michael’s. “While athletes may appear to be clinically recovered, our findings reveal that their brains may still be healing, which could have implications for long-term brain health and functioning.”
The St. Michael’s study is notable for its unique longitudinal design. Using advanced MRI, the research team followed a group of university athletes pre- and post-concussion to track changes in their brain after injury up to one year after return to play. By contrast, most concussion studies compare the brains of concussed athletes against the brains of a control group of non-concussed athletes.
“By using advanced neuroimaging, our study answers a fundamental question of whether or not a brain injured through concussion has truly returned to its ‘normal’ pre-injury baseline at the time of medical clearance,” said Dr. Tom Schweizer, study co-author and scientific research director of the Brain Health and Wellness Pillar at Unity Health Toronto.
Study design
The researchers enrolled 187 varsity athletes from the University of Toronto at the start of their athletic seasons. All participants were screened to ensure they had no history of neurological disorders, psychiatric disease or sensory/motor impairments. Demographic and clinical data were collected for each participant and advanced MRI was conducted to determine a baseline of each athlete’s brain physiology.
Of this group, 25 athletes went on to have a concussion. These concussed athletes were re-evaluated and had MRI at four different time points after their injury: a) 1-7 days post-concussion, b) at the time of medical clearance and return to play, c) 1-3 months after return to play and d) 1 year after return to play. Twenty-seven non-injured athletes were included as a matched control group to account for any natural variation that might occur in the brain in the absence of concussion.
The researchers examined the brain images to measure any significant changes in blood flow to the brain, and changes in white matter related to injury and inflammation. Cerebral blood flow and white matter integrity are essential to brain health. Cerebral blood flow delivers oxygen and nutrients to brain tissue, and white matter acts as the brain’s “wiring system,” connecting different areas of the brain and enabling the transmission of signals. Any interruption of cerebral blood flow or damage to white matter due to injury can affect cognition, emotions, and can cause debilitating physical symptoms, such as headache, dizziness, and fatigue.
Key findings
Compared to baseline, MRI detected significant post-concussion changes to the brain, including decreased cerebral blood flow and signs of injury in the brain’s white matter. These changes to the brain persisted up to one year after the athletes had been medically cleared to return to play.
Changes in blood flow, in particular, substantially exceeded normal variations in blood flow seen in athletes without concussion over the course of a year. This suggests that the brain’s ability to regulate blood flow is particularly vulnerable to injury.
A change in cerebral blood flow was strongly correlated with longer recovery times.
The study provides new insights into the long-term brain changes that happen after concussion, helping to understand why some athletes with repeated concussions have gone on to show significant declines in brain health, cognition and mental health later in life, the researchers say. It also adds to the ongoing debate surrounding safe concussion management, particularly in varsity and professional sports with routine body-to-body contact.
“There are many benefits associated with sport and physical activity and when someone chooses to participate in sport, I think many individuals are aware that there is always a risk of injury, including concussion. This study brings awareness to the ongoing changes in the brain that can last beyond symptom resolution and return to play, and can help athletes understand the importance of removing themselves from sport and seeking medical attention,” said Kyla Pyndiura, PhD candidate and lab manager at the Centre for Sport-Related Concussion Research, Innovation, and Knowledge at the University of Toronto, who helped to recruit athletes to the study.
The study authors say there is a need for further research that tracks these changes and their long-term consequences for brain health.
By Marlene Leung
Photo by Katie Cooper
Share this Article
