WASHINGTON — At least a quarter of National Football League players may end up with a degenerative brain disease, according to a new study published yesterday that lays bare the extreme toll of America’s favourite sport.

Chronic traumatic encephalopathy (CTE) is caused by repeated head impacts and its symptoms include trouble with thinking and memory loss, difficulty walking, emotional instability and suicidal thoughts.

It has previously been linked to contact sports including American football and boxing, and found in military members exposed to explosive blasts.

But since CTE can only be confirmed in a post-mortem brain exam, the true extent among NFL players has remained unclear.

In the new study, led by Daniel Daneshvar, a professor at Harvard Medical School, and published in the British Medical Journal, researchers looked at the brains of dead players who had played at least one NFL game in the mandatory plastic helmet era that began in 1949.

The main finding comes from players who died between 2016-2021, when brain donations were most frequent. Of the 878 deaths during this period, 235 had their brains donated and examined, and 215 had confirmed CTE.

That means, at minimum, the rate of CTE was 24.5 per cent if none of the brains that weren’t examined would have shown CTE. The maximum estimate was 97.7 per cent, and the true figure likely lies somewhere in the middle.

The researchers also assessed severity of CTE and found a connection between high grade CTE and dementia before death, with the latter ascertained by the players’ medical notes and interviews with family members.

“Together, these findings indicate that NFL players have a higher prevalence of CTE at death than has been previously shown in multiple community-based studies,” wrote the authors.