Chronic traumatic encephalopathy (CTE) was found in 110 of 111 (99 percent) brains of deceased former National Football League players that were donated to scientific research, according to a study published in the medical journal JAMA. CTE, a neurodegenerative brain disease, was also neuropathologically diagnosed in 177 of the 202 players studied across all levels of play (87 percent). It was found in three of the 14 high school players and 48 of the 53 college players.

CTE is typically found in individuals who have been exposed to repeated head trauma, including veterans and football players. CTE can only be diagnosed with an autopsy. The JAMA study focused on football as the primary exposure to head trauma, whether or not the individual had exhibited symptoms while living. The study acknowledged the lack of a comparison group without which the study cannot offer an estimate on the overall risk of brain injury due to participation in football.

Although CTE cannot be formally diagnosed until after death, individuals may experience symptoms during their lifetime, such as depression or anxiety.

The Four Stages of CTE

The study identified four stages of CTE severity. Stages one and two are considered mild. Stages three and four are considered severe. Individuals who experienced behavioral mood symptoms during their lifetime were more likely to have findings of mild disease. Those individuals with mild disease were also more likely to have died by suicide. Those with severe disease were more likely to have experienced cognitive symptoms, such as memory loss. Future studies will examine the pathology’s relationship to clinical symptoms.

What do the CTE studies mean for sports today?

Dr. Jeffrey Kutcher, national director of the Sports Neurology Clinic at the Core Institute, who was not involved in the study, noted that many of the brains examined in the study came from players who played decades ago. At that time, awareness, medical protocols, and equipment to prevent brain injuries were not the same as today. Kutcher stressed that playing sports is an individual decision, taking into consideration “what the arc of their life is going to be, what their health is going to be at the end of their career.”

One NFL player who apparently decided to play it safe and prioritize brain health over football reacted to the study by retiring from the NFL at age 26. Baltimore Ravens player John Urschel has been pursuing a doctorate in math at M.I.T. during the off season.