Degenerative Disease Found in 99 Percent of NFL Players' Donated Brains

New research published Tuesday in the Journal of the American Medical Association has strengthened the link between American football and traumatic brain injuries, finding a degenerative brain disease in 99 percent of deceased NFL players' brains that were donated to the research, and 87 percent of brain donations from all former players.

The study, which is the largest of its kind, examined 202 brains of former football players at all levels (from high school to the NFL) for signs of chronic traumatic encephalopathy (CTE), a brain disorder linked to repetitive head trauma. The brains of the deceased players came from a donation bank maintained by the VA Boston Healthcare System, Boston University, and the Concussion Legacy Foundation.

Researchers diagnosed CTE in 87 percent of the former players; of the 111 former NFL players in the study, 110 (or 99 percent) had CTE.

"This study more than doubles the number of cases reported in the literature of CTE," study author Jesse Mez, an assistant professor of neurology at Boston University's School of Medicine, told Time. "It suggests, with a lot of caveats, that this is probably not a rare disease—at least among those who are exposed to a lot of football."

The first thing researchers looked for was the main pathological marker of CTE—a buildup of an abnormal protein called tau in the brain, which can disable neuropathways and cause symptoms like memory loss, confusion, impaired judgement, aggression, depression, anxiety, impulse control issues, and sometimes even suicidal behavior, according to CNN.

More From Tonic: Football's Most Dangerous Rivalry

While abnormal tau buildup is also a marker for other degenerative brain diseases like Alzheimer's, researchers can distinguish CTE from the location of the protein's buildup—mainly near smaller blood vessels and near the bottom of the brain's sulci (they're what look like folds in the brain), reports Science Magazine.

In the new JAMA study, researchers found that the more football a person played, the more the severity of their CTE progressed. Longtime football players, like ones who competed throughout college and in the NFL, were also more likely to have other degenerative abnormalities in the brain compared to those who only played in high school: 86 percent of former NFL players had a severe form of the disease, as did 56 percent of college players. Three out of the 14 high school players examined had only a mild form of the disease. CTE wasn't found at all in the brains of two men who only played football before high school.

In addition to examining the donated brains, researchers also interviewed the families of the deceased players, concluding that, regardless of the severity of CTE, all of the men experienced the mood, behavioral, or cognitive symptoms of the disease; and nearly all of them (96 percent) experienced a progression of those symptoms.

Men with a milder form of CTE were found to have died younger—at about 44 years old—often due to suicide, while men with a more severe form died later in life—around age 71—from symptoms of dementia or trouble with involuntary movements like swallowing. That's because, as researchers note, milder versions of the disease are more closely linked to behavioral symptoms, while severe versions often cause cognitive limitations. Researchers also note that the behavioral and mood symptoms could be linked to neuroinflammation or axonal injury (an injury to the brain cells).

Despite the size of the study and its results, however, there are still limitations: For one, all of the brains examined were submitted due to clinical symptoms displayed while the former players were still living that may have suggested CTE. Because of that, the study lacks a comparison group that represents all college- or professional-level football players with and without symptoms. Another issue raised is that many of the brains found to have CTE came from those who played football before the year 2000, when medical protocols and measures to prevent brain injuries were less prevalent.

Though more research is needed, and study authors stress that the rate of CTE in this study can't be used to estimate how prevalent the disease is in all football players, it helps researchers better understand CTE. As Mez told Time, it begins "to suggest a relationship between football and this disease, and that's an important step for research that will look at this in the future."

Read This Next: Should Heading Be Banned From Soccer?