Repeated hits during contact sports could cause damage to the brain years before the development of chronic traumatic encephalopathy, a study funded by the National Institutes of Health revealed Wednesday.

Young boys wear pads and helmets as they take part in a football camp with former NFL players in Beijing in 2018. An NIH study evaluated the brains of athletes in contact sports under the age of 51 and found signs of neuron loss well before the typical signs of CTE. File Photo by Stephen Shaver/UPI UPI

Sept. 17 (UPI) -- Repeated hits during contact sports could cause damage to the brain years before the development of chronic traumatic encephalopathy, a study funded by the National Institutes of Health revealed Wednesday.

Doctors have long known that repeated, mild hits to the head experienced by combat veterans and athletes -- particularly boxers, and football, soccer and hockey players -- can lead to CTE, a neurodegenerative disease. CTE can only be diagnosed after death through an autopsy of the brain, but doctors can be reasonably certain a patient with hallmark symptoms has the disease, including memory loss, cognitive impairment, impulsive behavior, aggression, mood changes, and troubles with walking, balance and speech.

Scientists officially typically CTE by studying tau protein accumulation in nerve cells near blood vessels in the brain's folds.

The new study, though, took at look at the brains of younger athletes under the age of 51 and found there were other changes in the same area of the brain, even in those who had no tau buildup. This part of the brain showed a 56% loss of neurons in athletes exposed to repetitive head impacts.

The study also found that these younger athletes' brains had increased activation in immune cells calls microglia; those who played contact sports for longer had a greater activation of microglia.

The NIH said these findings suggest that damage to neurons can happen in athletes years before the tell-tale tau markers develop.

"This study underscores that many changes in the brain can occur after repetitive head impacts," said Walter Koroshetz, director of the NIH's National Institute of Neurological Disorders and Stroke.

"These early brain changes might help diagnose and treat CTE earlier than is currently possible now."

The study was published Wednesday in the journal Nature.

Dozens of professional have been formally diagnosed with CTE after postmortem studies of their brains, including NFL player and TV personality Frank Gifford; New England Patriots tight end -- and convicted murderer -- Aaron Hernandez; and NHL defenseman and enforcer Derek Boogaard.