Chronic traumatic encephalopathy, or CTE, is a progressive brain disease caused by repeated hits to the head over months or years. It leads to a buildup of a toxic protein in the brain that slowly destroys nerve cells, producing symptoms that range from mood swings and impulsive behavior to severe dementia. CTE can only be definitively diagnosed after death through brain autopsy, though researchers are working toward ways to detect it in living people.
What Happens in the Brain
CTE is defined by an abnormal accumulation of a protein called tau around the small blood vessels deep in the brain’s folds. In a healthy brain, tau helps stabilize the internal structure of nerve cells. After repeated head trauma, tau becomes chemically altered (hyperphosphorylated), clumps together, and begins killing neurons and the support cells around them.
What makes CTE distinct from other brain diseases is where and how this damage appears. The toxic tau clusters start at the depths of the brain’s grooves, particularly in the frontal lobe, the region responsible for decision-making, impulse control, and emotional regulation. Over time, the damage spreads outward and deeper, eventually reaching areas that govern memory, movement, and basic body functions. This spreading pattern explains why symptoms tend to worsen and change character as the disease progresses.
How CTE Differs From Alzheimer’s
CTE and Alzheimer’s disease can look similar in late stages, but they start differently and affect the brain in distinct ways. Alzheimer’s typically begins with short-term memory loss. CTE, by contrast, tends to first show up as depression, mood instability, and episodes of explosive anger, with memory and thinking problems developing later.
Under a microscope, the differences are even clearer. Alzheimer’s is characterized by widespread brain shrinkage and the consistent presence of amyloid plaques, sticky protein clumps found in only about 40% of CTE cases. The tau tangles in CTE sit in the outer layers of the brain’s cortex (layers II and III), while in Alzheimer’s they concentrate in the deeper layers (V and VI). CTE also heavily involves a brain structure called the substantia nigra, which controls movement, something rarely seen in Alzheimer’s. That’s why some people with advanced CTE develop Parkinson’s-like symptoms such as tremors and stiffness.
The Four Stages of Disease
Researchers have mapped CTE into four pathological stages based on how far the tau damage has spread. These stages were defined through autopsy, not during life, so they describe the physical progression of the disease rather than a clinical checklist.
In Stage I, only one or two small clusters of abnormal tau appear around blood vessels in the frontal cortex. This stage is found in the youngest brain donors and may produce no obvious symptoms or only subtle changes in mood. By Stage II, three or more of these lesions have appeared across multiple brain regions, and the damage has reached deeper structures involved in attention and arousal.
Stage III marks a significant turning point. The tau pathology becomes widespread, reaching the hippocampus and surrounding memory structures. Visible brain shrinkage, enlarged fluid-filled spaces, and structural abnormalities become apparent on gross examination. This is the stage where cognitive decline typically becomes noticeable to the person and those around them. In Stage IV, severe shrinkage affects the entire brain. Neurons are dying in large numbers throughout the cortex, brainstem, and even the spinal cord. Each one-stage increase in CTE severity corresponds to a 64% increase in the odds of dementia before death.
It’s the Cumulative Hits, Not Just Concussions
One of the most important findings in CTE research is that diagnosed concussions are not the main driver of the disease. The total years of exposure to repetitive head impacts, including the countless subconcussive hits that never cause obvious symptoms, are what matter most. In studies of confirmed CTE cases, 16% of individuals had no recorded history of concussion at all, meaning the accumulation of smaller, routine impacts was enough to trigger the disease.
The number of years spent playing contact sports, not the number of concussions, is significantly associated with more severe tau pathology. This shifts the conversation away from concussion management alone and toward the sheer volume of head contact a person experiences over a career or lifetime.
Who Is at Risk
Professional football players are the most-studied population. The Boston University CTE Center has diagnosed CTE in 345 of 376 former NFL players whose brains were donated for research, a rate of 91.7%. That number comes with an important caveat: families who donate brains to a CTE research center are far more likely to do so because their loved one showed symptoms, so this figure does not represent the rate among all NFL players.
CTE has also been found in athletes from soccer, ice hockey, rugby, boxing, and wrestling. Military veterans are another population of concern due to blast exposure and combat-related head trauma, though research here is less conclusive. One study of 70 military veterans and 93 civilians found no definitive CTE in either group, with only about 4 to 7% showing possible early features of the disease. The true prevalence in veterans remains unclear.
Why It Can’t Yet Be Diagnosed in Life
There is currently no validated test to confirm CTE in a living person. The National Institute of Neurological Disorders and Stroke has published diagnostic criteria based on symptoms related to memory and executive function, but these are approved only for research use, not clinical care. The challenge is that CTE’s hallmark, perivascular tau deposits at the depths of the brain’s grooves, can only be seen through microscopic examination of brain tissue after death.
Researchers at Boston University and other institutions are actively developing brain imaging tools and blood-based biomarkers that may eventually detect the specific tau signature of CTE during life. Until those tools are validated, anyone experiencing cognitive or behavioral changes after years of head trauma receives a clinical evaluation that can identify treatable conditions and rule out other causes, but cannot confirm or rule out CTE with certainty.
Reducing Risk in Youth Sports
Because total years of head impact exposure are the strongest predictor of CTE, reducing contact early in life has become a major focus. The CDC highlights that flag football may be a safer alternative to tackle football for children under 14, based on data showing significantly fewer head impacts in non-contact programs.
For youth tackle football programs that continue, specific measures make a meaningful difference. Reducing contact practices from three or more days per week to two or fewer cut head impacts during the high school football season by 42%. The American Academy of Pediatrics recommends teaching position-specific and age-appropriate tackling and blocking techniques that avoid using the head as the point of contact. These aren’t just guidelines for organized leagues. They reflect a core principle: every head impact avoided during childhood and adolescence is one fewer contribution to a cumulative total that may matter decades later.