Understanding the Concussed Brain and Its Recovery

Researchers have more questions than answers about the effects of sudden, repeated impacts

BY MARY HENDERSON September 02, 2025

Churchill

Lipton

Investigators across the country, investigating the brain and the mechanisms at play during mild traumatic brain injury (mTBI), are tackling a myriad of questions, from when and if the brain recovers to how to prognose a patient's outcome.

In a recent study published in Neurology, a research team from St. Michael’s Hospital and the University of Toronto addressed one question routinely asked by athletes, coaches and team trainers: how long should an athlete take a break from their sport following a concussion?

Unlike many studies that compare athletes’ imaging results post-concussion with those of a normative control group, the study compared images of the athlete's own brain taken before and after the injury.

"We wanted to get a handle on brain changes that last beyond symptom resolution and medical clearance to return to play," said the study's lead researcher, Nathan Churchill, PhD, an adjunct professor in the Department of Physics at Toronto Metropolitan University.

Large Study Cohort with Pre- and Post-Imaging

In the study, 187 varsity athletes from various collegiate sports underwent specialized imaging as part of their pre-season fitness assessment. During the season, 25 athletes who sustained concussions were re-imaged multiple times, including at time of injury, at return-to-play (RTP) which is typically one- to three-weeks post-concussion, again at one- to three-months post-concussion and one year later.

The imaging protocol included arterial spin labeling to quantify resting cerebral blood flow (CBF), diffusion tensor imaging (DTI) to measure mean diffusivity and fractional anisotropy.

"We also had a control arm where we re-scanned athletes who did not have concussions to get a sense of what normal brain variability looks like from year to year," Dr. Churchill said.

The analyses showed significant, spatially extensive changes in all MRI parameters at RTP relative to baseline. CBF to the frontoinsular area of the brain dropped significantly following concussion, and athletes who took longer to recover also showed reduced CBF in the medial temporal region.

DTI analyses found significant increases in mean diffusivity and decreases in fractional anisotropy of white matter, which increased in spatial extent and magnitude at RTP before gradually declining one-year post-RTP. Importantly, among all the changes measured, only post-concussion CBF exceeded normal longitudinal variability seen in athletes who were not concussed.

"In general, recovery is probably more prolonged than anyone acknowledges. There can be persistence of injury mechanisms that a person can't feel and won't be visible on standard medical images."

—MICHAEL L. LIPTON, MD, PHD

Mechanisms of Brain Recovery

"In general, recovery is probably more prolonged than anyone acknowledges," said Michael L. Lipton, MD, PhD, professor of radiology and biomedical engineering at Columbia University in New York City. "There can be persistence of injury mechanisms that a person can't feel and won't be visible on standard medical images."

Dr. Lipton said microscopic injuries to axons, such as misalignment or micro-perforations, aren't necessarily devastating by themselves, but they provoke a cellular and metabolic cascade over time that can go awry and cause further injury.

"For the proportion of the population with concussion and permanent injury, there is likely a progression of the inflammation and other processes including excitotoxicity in which excessive glutamate causes neuronal calcium overload leading to damage or death of nerve cells," he said. "Patients who weather this initial storm, essentially recover with normalization of their neurons before these processes cause irreversible damage."

Dr. Lipton said recovery occurs along one of two paths: either the neurons recover fully, or the brain’s neuroplasticity compensates for lost function. What's happening for an individual patient is challenging to parse out.

What does brain recovery look like, and how do we measure it? That is a question many are trying to answer. Comparing athletes who have experienced a concussion to healthy controls has provided insight into the structural and functional abnormalities associated with concussion. However, translating that generalized knowledge into individualized treatment plans remains elusive.

Shutterstock image of translucent head with brain inside

Cumulative Effect of Head Impacts

In Dr. Churchill's study, each of the 25 collegiate athletes who had concussions eventually self-reported as asymptomatic and were medically cleared to play again.

"Although all athletes had returned to regular activities with full symptom resolution, the long-lasting brain changes raise concerns about additive effects and what happens if the athlete gets another concussion," Dr. Churchill said. "This is where we get a peek into what could be a precursor to the long-term pathological processes we see at the extreme end of sports-related brain injuries."

Dr. Churchill’s concerns echo a growing body of research focused on what standard clinical evaluations might miss. At Columbia University, Dr. Lipton's Translational Neuroimaging Laboratory focuses on repetitive head impacts—both concussive and sub-concussive. He said the Neurology study demonstrates that clinical assessments like those used in RTP protocols are a crude way to determine the presence of pathologic processes.

"An asymptomatic patient with brain trauma can have significant underlying pathology," Dr. Lipton said. "Moreover, a concussed individual in the recovery phase is much more vulnerable to subsequent impacts."

Experts are hopeful that we'll soon have better RTP guidelines with pharmacologic and non-pharmacologic interventions to help with recovery, especially for those with persistent concussive symptoms. For now, researchers have more queries than definitive data on the highly complex brain and the effects of sudden or repeated impacts.

Research has shown that changes can occur in the brain after a football season, but it remains unclear whether those changes are reversible or how much exposure is too much. Experts emphasize the need to continue investigating these questions to inform better interventions and protect young athletes.

For example, eliminating head impacts during practice could eliminate more than half of head impact exposure. Dr. Churchill said, "Our long-term goal is to use neuroimaging to get a clear, objective view of the risks and benefits of participating in sports following a concussion, and particularly of when and where the risk gets too high—but at the moment, we are still just scratching the surface."