CTE (chronic traumatic encephalopathy) is a progressive, degenerative brain disease caused by repeated head impacts over time. Over the last few decades, it has gained increasing attention due to high-profile media coverage, feature films, and documentaries, as it is most commonly associated with contact sport athletes and military veterans exposed to repetitive brain trauma.
While researchers have learned much about CTE, it can currently only be definitively diagnosed after death, which complicates the legal process for sufferers and their families. Understanding what causes CTE, its symptoms, stages, diagnosis, and potential legal implications can help those affected make informed decisions.
What Causes CTE?
CTE develops from repetitive head impacts, including both diagnosed concussions and subconcussive blows, which are hits to the head that don’t cause immediate concussion but contribute to cumulative brain damage later on.
A common misconception about CTE is that it can be caused by a single concussion. Recent research, including that of a 2025 Mount Sinai study, suggests that isolated traumatic brain injuries rarely lead to CTE, and the biggest risk factor arises from repeated exposure to head impacts.
It is believed that repetitive trauma, often from hundreds of thousands of impacts over the years, causes a buildup of abnormal hyperphosphorylated tau protein, sometimes referred to as CTE tau protein. These deposits spread throughout the brain, damaging neurons and disrupting normal brain function.
Research published by Dr. AnnMcKee and colleagues in Acta Neuropathologica concludes that over 97% of published CTE cases involved individuals with a known exposure to repeated head impacts, such as those experienced playing contact sports. Those sports most commonly associated with CTE include:
- Football
- Boxing
- Ice hockey
- Soccer
- Wrestling
- Rugby
CTE has also been identified in military veterans exposed to repeated blast injuries and in individuals subjected to long-term physical abuse.
What Is the Difference Between a Concussion and CTE?
A concussion is an acute brain injury caused by a specific impact. CTE is understood to develop after hundreds of thousands of head impacts sustained over the course of many years. While it’s true that concussions do contribute to the risk of CTE, researchers recognize that it is repetitive subconcussive impacts that play the most significant role in CTE development.
CTE Symptoms
A challenging aspect of CTE is the fact that symptoms often take years, or even decades, after the trauma occurred, to appear. This makes diagnosis and treatment of the disease very difficult, and is a primary reason that CTE cases are both medically and legally complex.
Early Symptoms of CTE
Research, including that conducted by the Mayo Clinic, suggests that CTE symptoms often involve changes in mood, behavior, and emotional regulation in their early stages, including:
- Depression
- Anxiety
- Irritability
- Mood swings
- Aggressive behavior
- Impulsivity
- Difficulty controlling emotions
Many individuals experience these symptoms during their 20s or 30s, although timelines vary significantly.
Later Symptoms of CTE
As the disease progresses, cognitive impairment becomes more prominent. Later-stage symptoms, which often appear in patients’ 40s-60s, may include:
- Memory loss
- Confusion
- Impaired judgment
- Difficulty planning and organizing
- Problems with concentration
- Progressive dementia
In the advanced stages of the disease, individuals may also experience motor symptoms such as tremors, balance difficulties, slowed movement, and speech problems. Some patients even develop severe psychiatric symptoms, including suicidal thoughts.
The Four Stages of CTE
Dr. Ann McKee and colleagues developed a four-stage system to classify CTE, which was later adopted by a consensus panel for the National Institute of Neurological Disorders and Stroke (NINDS).
Stage I
Stage I involves small, isolated accumulations of abnormal tau protein, typically in the brain’s frontal regions.
Symptoms may be minimal or completely absent, with mild headaches, attention difficulties, or subtle behavioral changes experienced by some individuals.
Stage II
During Stage II, tau pathology spreads to additional areas of the brain. At this stage, symptoms may include:
- Depression
- Mood instability
- Impulsive behavior
- Increased irritability
Many affected individuals still function independently at this stage.
Stage III
In Stage III, tau pathology expands into regions such as the hippocampus and amygdala, which play critical roles in memory and emotion. Symptoms often include:
- Memory impairment
- Cognitive decline
- Executive function difficulties
- Increased emotional instability
At this stage, daily functioning may become noticeably impaired.
Stage IV
Stage IV represents the most severe form of the disease.
Tau pathology becomes widespread throughout the brain and may extend into additional structures involved in movement and cognition.
Individuals may experience:
- Severe dementia
- Profound memory loss
- Significant behavioral changes
- Motor impairment
- Loss of independence
Research indicates that higher CTE stages are strongly associated with dementia and severe neurological impairment.
How Is CTE Diagnosed?
CTE diagnosis is one of the most complex aspects of the disease, as it can currently be made only by examining brain tissue after death and identifying the characteristic pattern of tau protein deposits.
This limitation led researchers to develop a related clinical framework known as Traumatic Encephalopathy Syndrome (TES), which describes the pattern of cognitive, behavioral, and neurological symptoms that may be associated with CTE during life. Patients are evaluated using:
- Medical history
- Cognitive testing
- Neurological examinations
- Brain imaging
- Psychiatric assessments
If you or a loved one is experiencing symptoms consistent with CTE, a neurologist can evaluate symptoms and rule out other conditions, even though a definitive CTE diagnosis is not yet possible during life.
Can CTE Be Diagnosed While Alive?
In short, no, CTE cannot be definitively diagnosed in living patients. However, researchers are actively working to develop methods to achieve this, including PET imaging targeting the tau protein and blood-based biomarkers such as p-tau.
Many of these efforts are currently being led by the NIH-funded DIAGNOSE CTE Research Project and Boston University CTE Center. While encouraging progress has been made, no test has been widely accepted as a definitive diagnostic tool for living patients.
CTE Research: What We Know Now
Encouragingly, CTE research over the past decade has accelerated dramatically. According to the Boston University CTE Center, researchers identified CTE in 345 of 376 former NFL players studied, or approximately 91.7% of donated brains examined.
Another landmark study published in JAMA found evidence of CTE in in 91% of college football players and 21% of high school players. Researchers in BU’s UNITE brain bank have also found that over 40% of contact sport athletes under 30 had CTE, including amateur-only athletes.
It’s important to note that these findings require context. Brain bank studies often select individuals whose families have donated because symptoms are already present. Therefore, these studies don’t reflect the prevalence of CTE among all former athletes. Community-based research suggests that CTE is actually rare, affecting less than 1% of individuals without significant exposure to repetitive head trauma.
CTE and Legal Options for Former Athletes
Medical understanding of CTE has greatly evolved, and with it, more questions have been raised regarding institutional responsibility.
Former athletes and their families are increasingly concerned that schools, leagues, governing bodies, or sports organizations adequately warn participants about the risks associated with repetitive head impacts. Questions often arise relating to concussion management, return-to-play protocols, medical oversight, and long-term athlete safety policies.
Former college athletes, their families, and surviving family members of deceased athletes may have legal options available, including personal injury or wrongful death claims. However, it may be important to seek legal guidance early on, due to the variance in filing deadlines per state, when CTE symptoms emerge so many years after exposure.
For more information about legal claims involving college athletics and head injuries, visit our NCAA head injury litigation resource.