By Dr. Claudia Muñoz, MD, MPH — Board-Certified Neurologist & Epileptologist, Medical Expert Witness
Questions about repeated head impacts in athletes come up in personal injury, school and league liability, and workers' compensation matters. A recent study, reported by the National Institutes of Health (NIH), adds new detail about what may happen in the brain after years of contact-sport play. This article summarizes what the researchers found, what the study cannot tell us, and what it means for case evaluation.
About the Study
Researchers examined brain tissue after death from 28 men aged 20 to 51, using single-cell analysis of the sulci, the grooves on the brain's surface. The men fell into three groups:
8 who had not played contact sports
9 who had played American football or soccer and did not have chronic traumatic encephalopathy (CTE)
11 who had played contact sports and were diagnosed with low-stage CTE
The study was published in Nature in 2025 and funded in part by the NIH's National Institute on Aging and National Institute of Neurological Disorders and Stroke, along with the U.S. Department of Veterans Affairs.
What the Researchers Found
In the tissue examined, contact-sport athletes showed several differences from the non-athletes, whether or not they had CTE:
More inflammatory immune cells. Microglia, the brain's first-responder immune cells, were more numerous.
Changes in blood vessel genes. Gene activity in the brain's blood vessels was altered.
Lower neuron density. Neuron density in the sulci was lower, and fell further with more years of play.
Neuron loss ahead of CTE markers. The loss did not track with levels of p-tau, the protein associated with CTE. The authors read this as a sign that neuron loss may begin before p-tau builds up in these regions.
Differences in cell signaling. A signaling molecule produced by microglia, TGFB1, appeared to play an important role, and signaling differed between athletes with and without CTE.
The authors' central point: athletes without CTE can still show substantial changes in the brain.
What the Study Does Not Show
The findings are important, and they have clear limits. A careful reading matters in either direction:
A small sample. Twenty-eight men cannot represent all athletes, and the authors note that more studies in more people are needed.
Studied after death. CTE can currently be diagnosed definitively only after death. The study cannot show how these changes develop in a living person.
Tissue, not symptoms. The measures were laboratory findings in brain tissue. The article does not describe imaging, symptom reports, or cognitive testing.
Group patterns, not individual causation. The work does not establish that a particular head impact caused a particular person's symptoms or condition.
Mechanisms are still being worked out. The molecular processes behind CTE are not yet fully understood.
Why It Matters for Detection and Prevention
The NIH summary notes that understanding these changes, and eventually detecting them during life, could support better prevention and treatment. That is a goal for future research, not a current clinical tool.
Exposure history belongs in the record. Years of play, position, level of competition, and diagnosed concussions all matter.
A diagnosed concussion and repeated impacts are different questions. The evidence and the analysis differ for each.
Clinical evidence carries the weight in an individual case. The symptom timeline, evaluations at the time, exam findings, and testing results matter more than group-level research.
Research supports general principles only. It helps explain why repeated impacts are taken seriously. It does not establish what happened in a specific case.
Each case depends on its specific facts and medical history. This article summarizes general research findings, not an opinion on any specific case.
Work With a Neurologist Expert Witness
If you are handling a case involving an athlete with a history of repeated head impacts, a thorough neurological review can clarify what the records, exam, and testing do and do not support.
I provide independent medical examinations, medical record review, expert report writing, and deposition and trial testimony for plaintiff and defense attorneys handling traumatic brain injury cases.
📩 doctor.claudia@gmail.com | info@drclaudiamunoz.com
Dr. Claudia Muñoz, MD, MPH Neurologist | Epileptologist | Medical Expert Witness
Source: National Institutes of Health, NIH Research Matters, "Effects of repeated head trauma in young athletes," summarizing Butler MLMD et al., Nature, 2025.
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