Several news items this week put professional football and head trauma back in the spotlight.
On Monday, a study showed that one-third of former National Football League (NFL) players believed they had chronic traumatic encephalopathy (CTE).
On Tuesday, former NFL quarterback Brett Favre revealed he has Parkinson's disease.
Favre said five specialists told him they believed it was likely due to the hits he suffered while calling signals in the NFL.
"They all said the same thing," he said in an interview with . "'If it's not in your family' -- and there's none on either side of my family -- 'then the first thing we look at is head trauma.' Well, hell, I wrote the book on head trauma."
The day after Favre's announcement, former Minnesota Vikings quarterback Tommy Kramer posted on that he had been diagnosed with dementia.
CTE, Parkinson's-like symptoms, and dementia have been associated with repeated head hits. In disorders like Parkinson's and dementia, etiology often is unknown and questions about causality can't be answered easily.
But an abundance of evidence suggests there are links between playing American football or other contact sports -- particularly at the elite level -- and increased risks of developing neurodegenerative diseases, especially CTE, later in life.
"There is a dose-response relationship between the number of years and the level of football played and risk for having CTE neuropathology, as well as for having more severe disease," said researcher Michael Alosco, PhD, of the Boston University CTE Center.
"CTE is a neurodegenerative disease that is triggered by exposure to repetitive head impacts that occur during activities like American football," Alosco told MedPage Today. "It's both the frequency and the intensity of head impacts that are the drivers of the disease."
Historical studies have tied head trauma from boxing to Parkinson's-like features later in life, Alosco noted. "This literature can be dated back to the 1920s," he said. "More recently, there have been increasing efforts to better understand whether repetitive head impacts from American football might influence risk for other neurological conditions besides CTE."
A study in 2023, for example, showed that participating in American football was tied to higher odds of . "Again, the association strengthened as the years and level of play increased," Alosco pointed out. And a 2024 autopsy study showed that nearly 25% of 481 deceased athletes with CTE had parkinsonism symptoms before they died; nearly all had played American football.
Studies of pro soccer players also have tied repetitive head impacts to subsequent brain disorders. In Sweden, for example, elite soccer players had a 46% higher risk of neurodegenerative diseases like Alzheimer's and dementia compared with the general population. In Scotland, dementia deaths were greater among former pro players than matched controls.
"We see associations between CTE neuropathology and and dementia," Alosco said. "However, dementia in CTE can also be the result of mixed neuropathologies, as it is frequently co-morbid with other brain diseases like Alzheimer's disease or Lewy body disease."
"We are also increasingly seeing a strong link between repetitive head impacts and injury to the white matter," he continued. "We are still in the beginning stages of understanding how repetitive head impacts from American football might influence risk for Parkinson's disease and other neurological conditions."
"We have strong evidence for the relationship between repetitive head impacts and CTE," Alosco observed. "But, do these head impacts increase susceptibility to other brain diseases? Do they occur independent of, or in parallel with, CTE? What are the different pathologies that repetitive head impacts can induce, and why are certain ones triggered in some individuals and not others? Why are some people resilient or resistant to the effects of repetitive head impacts? These are questions we still need to answer."