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 kills 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, and there is currently no cure.
What Happens Inside the Brain
The hallmark of CTE is the accumulation of a misfolded form of tau protein, called phosphorylated tau (p-tau), in the brain. Tau normally helps stabilize the internal structure of nerve cells, but repeated head trauma causes it to become chemically altered and clump together. These clumps are toxic. They collect around small blood vessels deep in the folds of the brain’s outer surface, a pattern unique to CTE and distinct from other brain diseases.
About 95% of the damaged cells in CTE lesions are neurons, not the supportive glial cells that also populate the brain. This means CTE is fundamentally a disease of nerve cell destruction. In early stages, the damage stays confined to the outer layer of the brain, particularly in the frontal lobes. As the disease progresses, it spreads inward to structures critical for memory, emotion, and basic body functions: the hippocampus, the amygdala, and eventually the brainstem.
Repetitive Hits, Not Just Concussions
One of the most important findings in CTE research is that diagnosed concussions are not the primary driver. The total years of exposure to head impacts, including the countless smaller hits that never cause obvious symptoms, is what matters most. These subconcussive impacts, the routine collisions in football, hockey, soccer, and military combat, accumulate over time. In fact, 16% of confirmed CTE cases had no documented history of concussion at all, meaning that subconcussive hits alone were enough to trigger the disease.
The number of years spent playing contact sports correlates with the severity of tau buildup in the brain. Someone who played football from age 10 through college accumulated far more total head impacts than someone who started at 18, even if the younger starter never had a diagnosed concussion.
Who Gets CTE
CTE has been found in professional and amateur athletes across many contact sports, as well as in military veterans exposed to blast injuries. The most striking data comes from Boston University’s CTE Center, which has diagnosed CTE in 345 of 376 former NFL players whose brains were studied after death, a rate of 91.7%. That number carries an important caveat: these brains were donated, often because the person or their family suspected something was wrong, so the sample is not random. Still, the finding underscores how common the disease is among people with long careers in high-impact sports.
CTE is not limited to elite athletes. It has been identified in high school and college football players, rugby players, boxers, wrestlers, and soccer players. The common thread is years of repetitive head impact exposure.
Symptoms and How They Progress
CTE symptoms tend to appear in one of two patterns depending on the person’s age. In younger people, typically in their late 20s to early 30s, the first signs are usually mood and behavioral changes: depression, anxiety, impulsive decisions, and aggression that seems out of character. In older individuals, around age 60, the dominant symptoms are cognitive. Memory problems, difficulty with planning and organizing, and confusion that can eventually progress to full dementia.
These two patterns likely reflect where the disease is most active in the brain at a given stage. Researchers have mapped CTE into four stages of severity:
- Stage I: Isolated clusters of damaged cells in the frontal lobes. Symptoms, if present, are mild and easy to overlook.
- Stage II: Damage spreads across the frontal, temporal, and parietal lobes. Mild brain swelling may be visible, along with early mood and behavioral symptoms.
- Stage III: Widespread damage reaches the hippocampus (critical for memory), amygdala (involved in emotion), and deeper brain structures. Cognitive problems become noticeable, and brain tissue begins to visibly shrink.
- Stage IV: Severe, widespread degeneration with significant brain shrinkage, particularly in the frontal and temporal lobes. Dementia is common. Loss of neurons is extensive.
By stage IV, the physical changes in the brain are dramatic. The tissue connecting the two brain hemispheres thins noticeably, memory structures atrophy, and pigmented brain regions involved in movement and mood regulation lose their color, a sign that their cells have died.
How CTE Differs From Alzheimer’s
CTE and Alzheimer’s disease both involve tau tangles in the brain, and both can cause dementia, which makes them easy to confuse. But the two diseases are pathologically distinct. In CTE, tau tangles concentrate in the superficial layers of the brain’s cortex and cluster around blood vessels. In Alzheimer’s, tangles appear in the deeper cortical layers and follow a different distribution pattern.
Alzheimer’s also consistently features amyloid plaques, sticky protein deposits found outside neurons. These plaques appear in only about 40% of CTE cases and, when present, tend to be a different type. People with CTE are more likely to show Parkinson’s-like movement problems because the disease attacks a brain region called the substantia nigra, which Alzheimer’s largely spares. People with Alzheimer’s, by contrast, tend to show cognitive decline as the dominant early feature, with mood disturbances coming later if at all.
The two diseases can also co-occur. Researchers have documented cases where CTE and Alzheimer’s overlap, particularly in older individuals, creating a mixed picture that complicates diagnosis even further.
Why It Can Only Be Diagnosed After Death
There is currently no approved test to confirm CTE in a living person. The defining feature of the disease, p-tau deposits around blood vessels deep in the brain’s folds, can only be seen under a microscope during autopsy. Researchers have been working on alternatives. Brain imaging with specialized PET scans can detect tau, and blood tests measuring different forms of phosphorylated tau are showing promise in early studies. One approach uses tiny cell-derived particles called exosomes that cross from the brain into the bloodstream, potentially carrying signals of what is happening inside the brain.
Some blood markers may even help distinguish CTE from Alzheimer’s. Lower levels of certain phosphorylated tau forms in the blood, combined with a history of head trauma and cognitive symptoms, may point toward CTE rather than Alzheimer’s. But none of these tools have been validated for clinical use yet. For now, doctors use a clinical framework called Traumatic Encephalopathy Syndrome to identify people who likely have CTE based on their history of head impacts and their pattern of symptoms.
Reducing the Risk
Because CTE results from cumulative head impacts over time, the most effective prevention strategy is reducing the total number of hits a person’s brain absorbs across their lifetime. This is especially important for children and adolescents, whose brains are still developing.
Several practical steps can lower exposure. In youth sports, coaches can enforce rules that limit head contact, teach proper technique to avoid unnecessary collisions, and penalize dangerous play. Many youth football leagues have adopted limits on full-contact practices. Some sports medicine experts advocate delaying participation in tackle football until age 14, since the total years of exposure is the strongest predictor of CTE severity.
For athletes already in contact sports, shorter careers mean less cumulative exposure. Improved helmet technology can reduce the force of individual impacts, though no helmet can prevent the brain from moving inside the skull during a collision. The most protective choice remains reducing the frequency and intensity of head impacts across a lifetime, not relying on equipment alone.