Larry Johnson spent his best seasons as a bruising workhorse for the Kansas City Chiefs, piling up touches few running backs in NFL history have matched. Now 46, Johnson says a different kind of toll has set in: memory lapses severe enough that he sometimes cannot find his own car in a parking lot, and he believes the cause is chronic traumatic encephalopathy, the degenerative brain disease linked to repeated head impacts.
“I could literally go into a store and 30 minutes later come out and not know where my car is parked,” Johnson said, according to the Kansas City Star. Johnson first said nearly a decade ago, in 2017, that he believed he had CTE, and the Star reports he has also described episodes of becoming disoriented while driving. CTE is currently defined as a brain disease believed to be caused by repeated head injuries, and according to the Mayo Clinic it causes the death of nerve cells in the brain.
There is no way to confirm Johnson's belief. As Mass General Brigham notes, CTE can only be definitively diagnosed after death, through direct examination of brain tissue — meaning no blood test, scan, or exam can currently confirm it in a living person, whatever their symptoms.
A Hall-of-Fame Workload, Decades Later
Johnson was the Chiefs' first-round pick, taken 27th overall in 2003, and went on to make the Pro Bowl in both 2005 and 2006, according to ESPN. He played for Kansas City from 2003 through 2009. Over the 2005 and 2006 seasons combined, Johnson carried the ball 752 times and caught 74 passes for 826 total touches, per the Star's reporting — including 457 touches in 2005 alone, a single-season total the paper notes ranks second-most in NFL history.
That workload is central to why Johnson's belief resonates beyond his own case. The same questions have followed other former Chiefs players. The family of former Chiefs offensive lineman Jordan Devey, who died by suicide this summer at age 38, believes he had CTE, the Star reports. Devey's parents have said they suspect the disease may have contributed to his mental health struggles, according to Deseret News, which notes Devey played in the NFL from 2013 to 2020, appearing in 44 games with 21 starts, and won a Super Bowl with the New England Patriots in 2015.
Researchers Say a Living-Patient Test May Be Close
The search for a way to detect CTE while someone is still alive has been underway for years and may be nearing a breakthrough, researchers say. “I think we're close,” Michael Alosco said, as reported by ESPN. Alosco's team has followed more than 300 former football players, along with Alzheimer's patients who never played the sport, since the first phase of their work began in 2015, and researchers are now looking for patterns to emerge from testing in what has become the second phase of the project.
That work has since expanded into the DIAGNOSE CTE Research Project-II, a five-year, NIH-funded multicenter study designed to build on those earlier findings, according to UCSF's clinical trials registry. The study plans to examine 225 former football players alongside 75 controls and 50 people with Alzheimer's disease, with participants completing a single visit that includes clinical exams, MRI scans, blood draws and tau PET imaging at one of five research centers.
Other avenues are being explored in parallel. National Geographic reports that scientists are testing brain scans, blood tests and other biomarkers as possible ways to flag CTE during life, including an experimental tracer called OXD-2314 that has produced a signal in living people in areas where CTE would be expected — though researchers caution the scans remain too preliminary to serve as an actual diagnosis. Preliminary findings reported separately by Science News suggest blood levels of a protein called eMTBR-tau243 rose alongside the severity of confirmed CTE cases, though that work has not yet been peer-reviewed and needs confirmation in a larger dataset.
What's Known From Deceased Players' Brains
Much of what is understood about CTE's prevalence among NFL veterans still comes from brain tissue studied after death. Mass General Brigham researchers found that at least 25% of football players who appeared in at least one NFL game and died between 2016 and 2021 had CTE, while a separate account relayed by National Geographic cited a BMJ study estimating the figure at 24.5% for the same period — a discrepancy the sources do not reconcile. Mass General Brigham also reported that of 878 former players who died in that window, 235 donated their brains, and 215, or 91.4%, of those donated brains were found to have the disease.
A 2023 Boston University CTE Center report, cited by Science News, found CTE pathology present in more than 90% of brains examined from a sample of 376 former NFL players. Separately, a Boston University study published in JAMA Network Open and detailed by WCVB examined 33 NFL players who had taken a baseline cognitive test while alive and were later diagnosed with CTE through autopsy, part of the broader effort to connect living symptoms to the disease eventually confirmed after death.
For now, researchers say clarity from a reliable living-patient test could eventually help inform how doctors approach current and former players experiencing CTE-associated symptoms — the kind Johnson describes living with today. Until such a test exists, his belief that he has the disease remains just that: a belief, shaped by symptoms, but unconfirmed by any medical standard currently available.
