The announcement of Parkinson's disease by Hall of Fame quarterback Brett Favre draws renewed attention to the possible links between collision sports and the risk of brain disease.

Favre revealed his diagnosis Tuesday while testifying before Congress about his alleged misuse of taxpayer money. The hearing focused on a welfare scandal in Mississippi, but the topic of Favre's health came up as he discussed losing an investment in a company he said was developing a “groundbreaking concussion drug.”

“I'm sure you'll 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, mostly for the Green Bay Packers, and ended his career 13 years ago. In a 2022 interview with the radio show “The Bubba Army,” he estimated that he had suffered thousands of concussions.

“Every time my head hit the turf, the bell would ring or the flashing lights would start, but I could still play,” he said in that interview. “That's the scary thing about concussions. It's the seemingly minor ones that do the damage.”

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

“We know the brain can only take so much, and with trauma like this – not just sports-related trauma, but any kind of repeated trauma to the brain – we know it's going to be compromised in the long run,” says Shannon Shaffer, a nurse at the Cleveland Clinic and liaison at Rune Labs, a software and data analytics company specializing in neurology.

Early research has found a likely link between these activities and Parkinson's disease, as well as other neurodegenerative diseases such as Alzheimer's disease, amyotrophic lateral sclerosis (ALS), and chronic traumatic encephalopathy (CTE). Several studies have found that a single concussion can increase a person's risk of developing Parkinson's disease 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 previous football career was linked to a 61% higher risk of Parkinson's symptoms or diagnosis of the disease. A 2018 study also found that repeated head impacts from years of playing contact sports were linked to precursors of Parkinson's.

Hannah Bruce, an author of the 2023 study, said it can be difficult to determine what role, if any, football played in a case like Favre's.

“It's hard to say if it's directly due to football or if it just increased his risk or likelihood of developing Parkinson's or if there are other factors at play,” said Bruce, who conducted the research at Boston University's Alzheimer's Disease Research Center. “But we're seeing more and more football players coming forward. That definitely says something.”

Neurologists who treated boxing star Muhammad Ali concluded in 2022 that his medical history supported a diagnosis of early-onset Parkinson's disease, but they could not link the disease to head trauma. Ali died in 2016.

Parkinson's is a neurodegenerative disease in which nerve cells in the brain weaken and die. This can cause tremors, muscle stiffness, balance problems, and difficulty walking and speaking. To relieve symptoms, patients often take prescription medications or receive deep brain stimulation, which sends electrical impulses to nerve cells in the brain.

Repeated trauma to the brain is considered a possible 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 many risk factors, including older age, genetic predisposition and exposure to certain pesticides.

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

“The default 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 to date there has not been a flood of Parkinson's diagnoses among them. But the disease is not always easy to diagnose and symptoms do not necessarily appear soon after a head injury.

Thor Stein, director of molecular research at Boston University's CTE Center and co-author of the 2023 study, said CTE may be a trigger for Parkinson's in some cases. Many former football players have been found to show signs of the brain disease, which can only be diagnosed post-mortem.

“The more frequently you get these repeated blows to the head and the longer you are exposed to them, the greater your risk of developing a number of different degenerative diseases, including both CTE and Parkinson's,” Stein said. “And recently, surprisingly, we've found that in the people who develop these symptoms, it's often not the typical pathology, but 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 Parkinson's disease.

In a 2021 interview with TODAY, Favre said he wasn't sure if he had CTE.

Crutchfield said it is important for players to take rest after head trauma to avoid more serious brain injuries.

“The brain is much more vulnerable to repeated impacts if it hasn't had enough time to heal,” he said. “So it's not good for athletes to hide a concussion because they want to keep playing.”