Hall of Fame quarterback Brett Favre’s announcement that he has Parkinson’s disease has renewed attention to the possible link between collision sports and the risk of brain disease.

Favre revealed his diagnosis on Tuesday while testifying before Congress about his alleged misuse of taxpayer dollars. The hearing focused on a scandal involving Mississippi Social Security, but the topic of Favre’s health came up because he discussed losing an investment in a company he believed was making a “groundbreaking concussion treatment.”

“I’m sure you understand why it’s too late for me, as I was recently diagnosed with Parkinson’s,” Favre said.

Favre played 20 seasons in the National Football League, primarily for the Green Bay Packers, and retired 13 years ago. In a 2022 interview with the radio show “The Bubba Army,” he estimated he had suffered thousands of concussions.

“Every time my head hit the grass, there was a ringing or stars going off, flashing lights, but I could still play,” he said in that interview. “That’s what makes the concussion thing so scary. It’s the little injuries that do the damage.”

Sports that involve repeated collisions, such as football, boxing and rugby, carry a risk of concussions and other head injuries.

“We know the brain can only handle so much, and when you have this kind of trauma — not just sports-related, but any kind of repetitive trauma to the brain — we know it’s going to affect the brain long-term,” said Shannon Shaffer, a nurse practitioner at the Cleveland Clinic and liaison to Rune Labs, a software and data analytics company focused on neurology.

Early research has established a likely link between these activities and Parkinson’s, as well as other neurodegenerative diseases such as Alzheimer’s, amyotrophic lateral sclerosis (ALS), and chronic traumatic encephalopathy (CTE). Multiple studies have shown that a single concussion can increase a person’s risk of being diagnosed with Parkinson’s by more than 55%.

The NFL did not immediately respond to a request for comment on Favre’s diagnosis and the connection between football and neurodegenerative diseases.

Last year, a study found that a history of football was associated with a 61 percent higher chance of having Parkinson’s symptoms or a diagnosis of the disease. A 2018 study also found that repeated head blows from years of contact sports were associated with precursors to Parkinson’s.

Hannah Bruce, one of the authors of the 2023 study, says it can be difficult to determine what role football played in a case like Favre’s.

“It’s hard to say whether it’s exactly because of football or whether it’s just increasing his risk or chances of developing Parkinson’s disease, or whether there are other factors at play,” said Bruce, who conducted the study at the Boston University Alzheimer’s Disease Research Center. “But we’re seeing more football players coming forward. That certainly tells us something.”

Neurologists treating boxing legend Muhammad Ali concluded in 2022 that his medical history supported a diagnosis of young-onset Parkinson’s disease, but they could not link the disease to head trauma. Ali died in 2016.

Parkinson’s is a neurodegenerative disease that causes nerve cells in the brain to weaken and die. It can lead to tremors, muscle stiffness, impaired balance, and difficulty walking and talking. To reduce symptoms, patients often take prescription medications or receive deep brain stimulation, which sends electrical pulses to nerve cells in the brain.

Repeated trauma to the brain is known as a potential trigger for the disease, as it can cause inflammation that leads to the death of more nerve cells over time. But it is one of several risk factors, including older age, an underlying genetic susceptibility and exposure to certain pesticides.

According to Dr. Kevin Crutchfield, a neurologist at Hackensack Meridian Health, doctors typically rule out other factors before linking Parkinson’s disease to football injuries.

“The standard response is not, ‘Oh, you played football, you have Parkinson’s,’” he said.

Crutchfield added that tens of thousands of men have played in the NFL and that there hasn’t been a flood of Parkinson’s diagnoses among them. But the disease isn’t always easy to diagnose, and symptoms don’t necessarily develop quickly after a head injury.

Thor Stein, director of molecular research at Boston University’s CTE Center and a co-author of the 2023 study, said CTE could in some cases be a trigger for Parkinson’s. Many former football players have shown signs of the brain disease, which can only be diagnosed postmortem.

“The more times you get these repetitive blows to the head, the longer you’re exposed to them, the greater your risk is of developing a number of different degenerative diseases, including both CTE and Parkinson’s disease,” Stein said. “And we recently found, surprisingly, that in the people who develop these symptoms, it’s often not the typical pathology, it’s often CTE that’s causing their Parkinson’s symptoms.”

In a July study of brain donors diagnosed with CTE, Stein and his co-authors found that nearly a quarter showed signs of parkinsonism.

Favre said in a 2021 interview with TODAY that he wasn’t sure if he had CTE.

Crutchfield said it’s important for players to take breaks after a head trauma to prevent more serious brain injuries.

“The brain is much more susceptible to reoccurring blows if your brain hasn’t had enough time to recover,” he said. “So athletes hiding a concussion because they want to keep playing is not good.”