Muhammad Ali's death: Can head injuries cause Parkinson's?

Muhammad Ali poses with gloves in this undated portrait.Mandatory Credit: Action Images / Sporting Pictures/File Photo EDITORIAL USE ONLY. - RTSFZ2A

Boxing champion Muhammad Ali lived with Parkinson's disease for three decades before his death on Friday (June 3) at the age of 74, and many have wondered whether Ali's boxing career caused him to develop the neurological disorder.

Although it's likely that frequent head injuries played a role in the boxer's Parkinson's disease, certain genes may have also increased his susceptibility to the disease, experts said.

"[It's] likely his repeated head injuries contributed to his Parkinson's disease," said Dr. Barbara Changizi, a neurologist at The Ohio State University Wexner Medical Center, who was not involved with Ali's treatment. But given how young Ali was when he was diagnosed with the disorder — the boxer was 42 — there's a "strong chance that genetics played a significant role as well," Changizi said. The average age of Parkinson's onset is 60 years old, according to the Michael J. Fox Foundation. [10 Things You Didn't Know About the Brain]

In patients with Parkinson's disease, the brain cells that produce a chemical called dopamine start to die off. Because dopamine is important for the control of muscle movement, Parkinson's patients experience symptoms such as tremors, slowed movements and muscle stiffness.

In most cases, the exact reason that the dopamine-producing cells start to die is not known, Changizi said. But certain genes appear to increase people's risk of developing the disease at a relatively young age. For this reason, "when you have young onset disease, we are more suspicious" of genetics contributing, Changizi said.

Still, head trauma has also been linked with Parkinson's disease. In a 2013 review study, researchers found that people with head trauma that resulted in a concussion were 57 percent more likely to develop Parkinson's disease, than people who never experienced such head trauma.

Head injuries can cause inflammation in the brain, which may lead to changes in cells and brain structures that contribute to Parkinson's, Changizi said.

And injuries that specifically damage the part of the brain that contains dopamine-producing cells, called the substantia nigra, can also lead to Parkinson's, Changizi said.

After Ali's death, some people asked on social media sites whether Ali could have also suffered from chronic traumatic encephalopathy (CTE), a brain disease found in athletes such as football players who have experienced repeated blows to the head.

But Changizi said, "Parkinson's disease would be enough to explain a lot of his symptoms."

The official cause of Ali's death was septic shock, according to news sources. Septic shock is a complication that can happen in people who have an infection. It can involve bodywide inflammation, and a dangerous drop in blood pressure. Ali had been hospitalized a week before he died for a respiratory infection, sources reported.

Original article on Live Science.

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