CTE, or chronic traumatic encephalopathy, is a progressive brain disease caused by repeated hits to the head. It develops when a toxic form of tau protein builds up in the brain, gradually killing nerve cells and causing problems with thinking, mood, and behavior that worsen over time. CTE can only be definitively diagnosed after death through brain autopsy, though researchers are working on ways to detect it in living people.
What Happens Inside the Brain
The defining feature of CTE is the abnormal accumulation of a protein called phosphorylated tau (p-tau) in brain cells. In a healthy brain, tau helps stabilize the internal structure of neurons. After repeated head impacts, tau becomes chemically altered and begins clumping around tiny blood vessels, particularly in the grooves of the brain’s outer surface. These clumps are found primarily inside neurons rather than other brain cell types, and they slowly spread as the disease progresses.
In early CTE, the damage is limited to the outer layers of the brain, often starting in the frontal cortex, which controls decision-making and impulse control. As the disease advances, toxic tau spreads into deeper structures: the hippocampus (critical for memory), the amygdala (involved in emotion), and eventually the brainstem, which governs basic functions like sleep and arousal. In the most severe cases, the brain visibly shrinks and loses tissue, particularly in the frontal and temporal regions.
What Causes CTE
Repeated head impacts are the established cause, but not in the way most people assume. The primary driver is cumulative exposure to head trauma, not the number of diagnosed concussions. About 16% of confirmed CTE cases had no recorded concussion history at all. The total years of exposure to repetitive hits, including the countless subconcussive impacts that never produce obvious symptoms, are what correlate most strongly with disease severity.
This makes CTE a concern well beyond professional sports. While American football players are the most studied group, CTE has been found in military veterans exposed to blast waves, boxers, hockey players, soccer players, and people with histories of domestic violence. In military personnel, the rapid back-and-forth acceleration of the head from blast exposure, sometimes called the “bobblehead effect,” appears to cause the same type of shearing damage to brain tissue as contact sports impacts.
How CTE Progresses
Researchers have identified four pathological stages of CTE, each corresponding to increasingly widespread tau damage and more severe symptoms.
In Stage I, only one or two small clusters of abnormal tau exist, usually in the frontal cortex. Most people at this stage have minimal symptoms, and dementia is uncommon, occurring in roughly 23% of cases studied.
By Stage II, tau deposits appear in multiple areas of the cortex, and pathology extends into deeper brain structures involved in attention and arousal. Dementia rates climb to about 47%. People at this stage often begin experiencing noticeable mood changes and difficulty with concentration.
Stage III brings widespread tau throughout the cortex and into the hippocampus, amygdala, and other memory-critical regions. The brain often shows visible shrinkage and enlarged fluid-filled spaces. About 65% of people at this stage have dementia, and memory loss becomes pronounced alongside worsening behavioral problems.
Stage IV represents the most severe disease. Tau pathology extends from the cortex all the way through the brainstem and into the spinal cord. There is significant loss of brain cells, particularly in the frontal and temporal lobes. Dementia rates reach 80 to 92%. Movement problems resembling Parkinson’s disease can emerge at this stage due to damage in the substantia nigra, a region that helps control motor function.
Symptoms in Daily Life
CTE symptoms generally fall into two categories: cognitive problems and behavioral or mood changes. Cognitive symptoms include difficulty with memory (especially forming new memories), trouble with planning and organization, and slowed thinking. Behavioral symptoms include impulsivity, explosive anger, emotional instability, and depression. Many people experience both, and the balance shifts over time as more brain regions become involved.
These symptoms typically appear years or even decades after the period of head trauma. The age when someone first starts taking hits to the head matters significantly for when symptoms show up. Research on football players with confirmed CTE found that every year younger a player began tackle football predicted symptoms appearing roughly 2.5 years earlier. Players who started before age 12 experienced cognitive and behavioral symptoms about 13 years earlier than those who started later. Notably, starting younger didn’t make the disease itself worse in terms of tau severity. It just moved up the timeline for when problems became apparent.
How CTE Differs From Alzheimer’s
CTE and Alzheimer’s disease both involve tau protein buildup, which is one reason they can be confused. But the pattern of damage is distinctly different. In CTE, tau tangles concentrate in the outer layers of the cortex (layers II and III) and cluster around blood vessels. In Alzheimer’s, tangles form in deeper cortical layers (V and VI) and don’t follow blood vessels.
The two diseases also affect the hippocampus differently. CTE damages multiple subregions of the hippocampus, while Alzheimer’s primarily targets just one (called CA1). CTE frequently involves the substantia nigra, producing Parkinson’s-like movement symptoms that are rare in Alzheimer’s. Alzheimer’s, on the other hand, is characterized by heavy amyloid plaque buildup, a second type of protein deposit that is typically absent or minimal in CTE. Clinically, CTE tends to present with mood disturbances and behavioral problems earlier in its course, while Alzheimer’s more commonly begins with pure memory loss.
Who Gets CTE
The most extensive data comes from the Boston University CTE Center brain bank, which has diagnosed CTE in 345 of 376 donated brains from former NFL players, a rate of 91.7%. That number sounds alarming, but it comes with an important caveat: families who donate brains to CTE research are far more likely to do so because their loved one showed symptoms. This means the true rate among all football players is certainly lower, though it remains unknown.
For context, a separate Boston University study examined 164 brains donated through the Framingham Heart Study, a general population sample. Only one of those 164 brains (0.6%) showed CTE, and that person had played college football. This confirms that CTE is strongly linked to repetitive head impact exposure rather than being a common finding in the general population.
Diagnosing CTE in Living People
A definitive CTE diagnosis still requires examining brain tissue after death. However, a set of clinical criteria called Traumatic Encephalopathy Syndrome (TES) allows doctors to identify people who likely have CTE while alive. A TES diagnosis requires three things: a history of substantial repetitive head impacts, core symptoms involving cognitive decline or behavioral dysregulation (or both) that are progressive, and ruling out other conditions that could fully explain the symptoms.
Researchers are actively pursuing brain imaging and blood tests that could detect CTE directly. Tau-targeting PET scans are the most studied approach, but results have been mixed. The first-generation tracers lacked specificity, binding to multiple proteins rather than just tau. A commonly used tracer called flortaucipir showed some differences between former NFL players and controls at the group level, but individual measurements overlapped too much to be useful for diagnosis. Newer tracers show promise in matching the expected pattern of CTE tau distribution, but laboratory studies on actual CTE brain tissue have shown little to no binding. No blood test or imaging scan can reliably detect CTE at this time.
Duration of Exposure and Total Risk
The single strongest predictor of CTE severity is how many years someone spent absorbing repetitive head impacts. This holds true regardless of the sport or activity. Interestingly, the total number of diagnosed concussions does not independently predict how severe the tau pathology becomes. This reinforces that the thousands of smaller, unremarkable hits accumulated over years of play are doing most of the cumulative damage, not the handful of impacts dramatic enough to cause recognized concussion symptoms.
Duration of play did not significantly predict when symptoms appeared, but age of first exposure did. This suggests that younger brains may be more vulnerable to the initial effects of repetitive impacts, potentially because the brain is still developing. For a child who starts tackle football at age 8 versus age 14, the disease may not end up being more severe, but the window of healthy years before symptoms emerge shrinks considerably.