The largest-ever study of CTE has found a new link between playing contact sports, CTE, and developing Parkinson’s disease-like symptoms.

The Boston University study of 481 dead athletes reveals that most people with CTE developed parkinsonism, and CTE appears to be driving the parkinsonism symptoms in most cases.

Parkinsonism is a condition with symptoms similar to Parkinson’s disease — such as tremor, abnormal slowness of movements, or abnormal stiffness of one’s arms or legs. The condition has been associated with traumatic brain injury and CTE in boxers for a long time.

While Parkinson’s disease is associated with the buildup of proteins called Lewy bodies in brain cells, the Boston University researchers found that 76% of individuals with CTE and parkinsonism did not have Lewy body pathology.

“We were surprised to find that most individuals with CTE and parkinsonism did not have Lewy body pathology,” said Thor Stein, associate professor of pathology and laboratory medicine at BU and VA Boston Healthcare.

“Rather, subjects with parkinsonism were more likely to have more severe CTE-related brain cell death in a region of the brainstem important for controlling movement,” Stein added.

CTE is a degenerative brain disease, and the only known cause is repetitive head impacts, like those in contact sports.

A past study by the same BU research team found that duration of contact sports play is tied to developing Lewy body disease. However, the present study is the first to describe a link between contact sport participation, brainstem pathology, and parkinsonism in CTE.

“Increased CTE severity has been shown to be associated with longer duration of play,” said Daniel Kirsch, an MD/PhD student at BU and one of the first authors of the study.

“In this study, we found that an additional eight years of contact sport play was associated with 50% increased risk of more severe disease in a specific area of the brainstem that controls movement,” Kirsch added.

The subjects of the study had donated their brain to the Understanding Neurologic Injury and Traumatic Encephalopathy (UNITE) brain bank.

Those with parkinsonism were compared to those without to identify the types of pathologies that may explain why some individuals with CTE develop these symptoms, and to examine relationships with duration of contact sports play.

The researchers said this study shows the importance of understanding the long-term effects of repetitive head impacts, and the need for preventive measures in contact sports to mitigate the risk of neurodegenerative diseases like CTE and parkinsonism.