Chronic traumatic encephalopathy (CTE) is a progressive brain disease that damages mood, thinking, behavior, and eventually motor function. It develops after years of repeated hits to the head, and it worsens over time as abnormal proteins spread through the brain. CTE cannot currently be diagnosed with certainty during a person’s lifetime; confirmation requires examining brain tissue after death.
How CTE Changes the Brain
CTE starts with a specific type of damage: a protein called tau, which normally helps stabilize brain cells, becomes chemically altered and begins clumping in places it shouldn’t. These toxic clumps first appear around small blood vessels at the bottom of the brain’s folds (the grooves on the brain’s surface), particularly in the frontal cortex, the region responsible for decision-making, impulse control, and personality. This pattern is distinct from Alzheimer’s disease, where tau spreads more evenly across the cortex. Researchers at Indiana University have also confirmed that the physical shape of the misfolded tau protein in CTE is structurally different from Alzheimer’s tau, with an unidentified molecular element attached to CTE tau that doesn’t exist in Alzheimer’s.
As the disease progresses, these protein clumps spread outward from their original clusters into surrounding brain tissue, then deeper into regions involved in memory and emotion, including the hippocampus and amygdala. In the most advanced cases, tau pathology is widespread across nearly the entire cortex. This spreading pattern explains why CTE symptoms start subtle and grow more severe over years or decades.
What CTE Does to Mood and Behavior
The earliest noticeable effects of CTE are often emotional, not cognitive. Because the disease typically begins in the frontal lobe, it disrupts the brain’s ability to regulate emotions and impulses before it impairs memory or thinking. People in the early stages commonly experience irritability, depression, and social withdrawal. Some develop explosive anger, violent outbursts, or severe mood swings that cycle between crying, laughing, and rage within short periods.
These changes can be devastating to relationships and daily life. Case studies of former professional football players describe patterns that emerged in their early 30s: deepening depression, suicidal thoughts, impulsivity, and uncontrollable emotional reactions like rocking, pacing, or laughing inappropriately. The behavioral symptoms often appear years or even decades after the person’s last head impact, which makes CTE easy to misdiagnose as a psychiatric condition like major depression or bipolar disorder.
What CTE Does to Thinking and Memory
Cognitive decline typically follows the mood and behavioral symptoms, though the timeline varies. Problems with concentration and attention often come first, followed by short-term memory loss and difficulty with executive functions like planning, organizing, and following through on tasks. In later stages, these deficits progress to full dementia, with severe memory loss and an inability to function independently.
Researchers have identified four pathological stages of CTE based on how far the tau protein has spread. In Stage I, the disease may produce no symptoms at all, or only lingering post-concussion symptoms like headaches. Stage II often brings noticeable mood and behavioral changes. By Stage III, the frontal and temporal lobes show widespread damage, and cognitive problems become harder to ignore. Stage IV involves severe, widespread brain degeneration and typically corresponds with dementia.
Physical and Motor Symptoms
In advanced CTE, the damage extends beyond thinking and mood into motor function. Symptoms resemble Parkinson’s disease: tremors, slowed movement, a shuffling or wide-based gait, slurred speech, and a flat, expressionless face. Some people develop muscle weakness, stiffness, and involuntary twitching. Persistent dull headaches and dizziness are also common, particularly earlier in the disease. These motor problems reflect damage spreading into deeper brain structures and, in some cases, the spinal cord.
Who Gets CTE
CTE is not caused by a single concussion or even a few of them. The CDC notes there is no strong evidence that occasional head impacts lead to CTE. Instead, the disease is associated with long-term, repeated exposure to head impacts, including the countless subconcussive hits (ones that don’t cause obvious concussion symptoms) that accumulate over years of contact sports, military service, or other activities involving regular blows to the head.
The numbers from the largest brain bank study are striking. Boston University’s CTE Center has diagnosed CTE in 345 of 376 former NFL players whose brains were donated for research, a rate of 91.7%. By contrast, a study of 164 brains from the general population found CTE in just one person (0.6%), who happened to be a former college football player. These figures come with an important caveat: the brains donated to the bank were often donated because the person showed symptoms during life, so the 91.7% figure doesn’t mean that percentage of all NFL players develop CTE. But it does confirm that prolonged exposure to repeated head impacts carries serious risk.
A CDC study found that youth tackle football players ages 6 to 14 sustained 15 times more head impacts per practice or game than flag football players, and 23 times more high-magnitude (hard) impacts. Duration of play matters: the total accumulation of hits over a career appears to be a more important risk factor than the number of diagnosed concussions. Why some people with extensive head-impact histories develop CTE while others don’t remains unclear, and genetics, sex, and other individual factors likely play a role.
How CTE Is Detected
The only definitive diagnosis of CTE currently requires examining brain tissue under a microscope after death, looking for that characteristic pattern of tau protein clumped around blood vessels in the cortical grooves. For living patients, a set of clinical criteria called Traumatic Encephalopathy Syndrome (TES) was developed by the National Institute of Neurological Disorders and Stroke to identify people who likely have CTE. A TES diagnosis requires three things: a history of substantial repeated head impacts, core symptoms of cognitive decline or behavioral problems (like explosive anger, impulsivity, or memory loss) that represent a clear change from the person’s baseline and are getting worse over time, and ruling out other conditions that could fully explain the symptoms.
Researchers are working on brain imaging tools that could detect CTE tau in living people. A PET scan tracer called Flortaucipir, already approved for detecting Alzheimer’s-related tau, is being studied for its ability to identify the distinct tau patterns of CTE. Blood-based biomarkers are also under investigation. For now, though, a living diagnosis remains provisional.
How CTE Differs From Alzheimer’s
CTE and Alzheimer’s disease share some overlapping symptoms, particularly memory loss and dementia, but they are distinct diseases. CTE tau starts in specific spots around blood vessels at the bottom of brain folds and spreads unevenly. Alzheimer’s tau distributes more diffusely across the cortex. The molecular structure of the tau itself is different between the two diseases. CTE also tends to produce prominent behavioral and mood symptoms earlier in its course, while Alzheimer’s more commonly presents with memory loss first. And CTE requires a history of repeated head trauma; Alzheimer’s does not.
Both diseases involve the same six forms of the tau protein, which is one reason distinguishing them in living patients remains challenging. The behavioral overlap with frontotemporal dementia, depression, and other psychiatric conditions adds further complexity, making the history of head impacts a critical diagnostic clue.