The Brain’s Protective Barrier May Remain Damaged Years After Athletes Retire

Their sporting careers had ended about 12 years earlier on average. Yet specialized MRI scans still showed areas where the boundary between blood and brain allowed more contrast agent to pass than expected.

The athletes with the most extensive abnormalities also performed worse on tests of memory and executive function—the mental skills used to plan, sustain attention, and switch between tasks. The study cannot show that barrier damage caused the cognitive problems, but it identifies a biological process that may help connect repeated head impacts with later brain dysfunction.

The boundary that stabilizes the brain

Capillaries in the brain are more tightly controlled than blood vessels in most other organs. Their cells are closely sealed and work with surrounding cells to form the blood–brain barrier. This is not an impenetrable wall: oxygen, nutrients, and selected signaling molecules cross in a regulated way, while many blood proteins, toxins, and immune cells are kept out.

Head trauma can temporarily increase the barrier’s permeability. What remained uncertain was whether this disturbance could persist for many years after an athlete stopped playing, and whether it was related to brain function.

The researchers examined 47 retired athletes from collision and combat sports and compared them with 15 people without comparable exposure to repetitive head impacts. Vascular permeability was measured using dynamic contrast-enhanced MRI. A contrast agent is injected into the bloodstream, and a series of scans tracks how readily it leaves blood vessels and enters surrounding tissue.

The athletes had been retired for roughly 12 years on average. Even after that interval, they showed greater blood–brain barrier disruption as a group than the comparison participants.

Extensive abnormalities in 17 athletes

The team identified 17 athletes in whom regions of increased permeability covered a particularly large portion of the brain. This subgroup performed worse on memory and executive-function tests than athletes with less extensive abnormalities. They also had lower volumes in some brain regions.

Self-reported concussion history did not clearly explain who had the most severe barrier disruption. That finding is consistent with the possibility that diagnosed concussions capture only part of the relevant exposure. Numerous impacts that never produce obvious acute symptoms may also matter. The study, however, did not directly measure each athlete’s cumulative mechanical exposure over the course of a career.

Common blood markers of nervous-system injury offered limited information. A different association stood out: athletes with poorer cognitive performance had a higher proportion of circulating monocytes, innate immune cells involved in inflammatory responses.

Gene-activity analysis in these cells pointed to dysregulation of the complement system. Complement is a network of innate immune proteins that helps eliminate microbes and damaged cells. When activated excessively or in the wrong context, the same machinery can injure the body’s own tissues.

In postmortem brain samples from people with neuropathologically confirmed chronic traumatic encephalopathy, the researchers found deposits of the membrane attack complex—the terminal component of complement—around cerebral blood vessels. Analysis of individual cell nuclei also suggested altered communication between vascular endothelial cells and microglia, the brain’s resident immune cells.

A possible mechanism, not a diagnostic test

The findings support a plausible sequence. Repeated impacts may injure small blood vessels, leaving the barrier abnormally permeable. Blood-derived molecules and immune signals could then gain greater access to brain tissue, sustaining inflammation and gradually interfering with neural networks.

Several steps in that sequence remain unproven. This was an observational snapshot, so it cannot establish which came first: vascular damage, immune dysregulation, or poorer cognition. The cohort was small and included athletes from different sports with different exposure histories.

Increased permeability on MRI is also not a diagnostic test for chronic traumatic encephalopathy. CTE still cannot be confirmed in a living person with the same certainty as it can through postmortem neuropathology. The brain-tissue samples in this study came from different individuals, not from the athletes who underwent MRI.

The study shifts attention from neurons alone to the system that maintains their environment. The lasting consequences of repeated head impacts may persist partly in the brain’s blood vessels and in the barrier meant to keep neural tissue stable and protected.