It has been widely argued that the multiple head injuries Muhammad Ali sustained during his boxing career could have caused his Parkinson's disease. In reality, the science behind it is much more nuanced.

Experts' theories vary on whether head trauma is linked to the disease.

In short, Parkinson's kills off the neurons in the brain that produce dopamine, which leads to issues with movement.

Ali was diagnosed with Parkinson’s in 1984, three years after he retired. His doctors did not connect the disease to injuries he received from boxing. 

Marie-Francoise Chesselet, professor of neurology at UCLA with several publications on Parkinson's, said there are studies "that show that a severe traumatic brain injury [TBI] can increase the risk of developing Parkinson's disease later in life".

However, the studies are not conclusive since other factors, such as environment or genetics, can also factor in as causes, she said.  

The life of Muhammad Ali

Chesselet used the example of the pesticide paraquat to further elaborate. Sometimes exposure to paraquat shortly after traumatic brain injuries can increase the chances of developing Parkinson’s disease later on in life.

"In the case of Ali, people have suspected for a long time that his symptoms may be related to the repeated head injuries he has sustained in his life, but argue whether it is related to Parkinson's disease or not. This will be settled if an autopsy is conducted," Chesselet said.

On the other hand, Dr Beckie Port, senior research communications officer at Parkinson’s UK, said currently there is "no evidence that single, mild or moderate injury to the head leads to Parkinson's".

Even though there is a recent study linking severe and repeated head injuries to a higher risk of developing Parkinson's, Port maintains "the risk, although higher, would still be very low".

Dr Omar al-Agnaf, director of neurological disorders at the Qatar Biomedical Research Institute, said there are enough studies to support the hypothesis that brain trauma increases the risk for Parkinson’s.

"We believe that boxing could lead to Parkinson's disease," he said.

Source: Al Jazeera