Autopsies of deceased boxers and pro football players have long confirmed that repeat head injuries can lead to a devastating brain condition known as chronic traumatic encephalopathy (CTE).

Now, research supports the notion that contact sports can also raise the odds for a Parkinson's-like disease, called parkinsonism, in athletes already affected by CTE.

In the new study, "subjects with parkinsonism were more likely to have more severe CTE-related brain cell death in a region of the brainstem important for controlling movement," noted study lead author Dr. Thor Stein. He's an associate professor of pathology and laboratory medicine at Boston University.

The study involved postmortem examination of the brains of 481 deceased athletes. It was published July 15 in the journal JAMA Neurology.

As the researchers described it, parkinsonism carries some of the hallmarks of classic Parkinson's disease — tremor, slowed movements, abnormal stiffness in the limbs.

The two conditions differ, however, in how they manifest in brain tissue.

People with Parkinson's disease typically show a buildup of proteins called Lewy bodies in their brain cells, but the new research found no such buildup in the brains of about three-quarters of people with CTE and parkinsonism.

"We were surprised to find that most individuals with CTE and parkinsonism did not have Lewy body pathology," Stein noted in a university news release.

So, what's driving the parkinsonism? According to Stein, athletes with CTE showed signs that the CTE was killing off brain cells in brain areas crucial to coordinated movement.

And the more head injuries a player suffered, the worse the damage.

“Increased CTE severity has been shown to be associated with longer duration of play,” said study first author Daniel Kirsch.

“In this study, we found that an additional eight years of contact sport play was associated with 50% increased risk of more severe disease in a specific area of the brainstem that controls movement," said Kirsch, who is an MD/PhD student at the university.

According to the researchers, the new insights should further research into how repetitive head hits affect brains long-term, and perhaps bring better ways of preventing CTE brain damage.