CTE: What We Know — A Clear Look at Chronic Traumatic Encephalopathy

Why It Matters

CTE has received a lot of attention in news and sports media, which has helped push concussion safety forward. But the coverage has also created two common misunderstandings: that any concussion will eventually cause CTE, and that mood or memory changes after a head injury must mean someone has it. Neither is supported by the evidence — most such changes have other explanations, and many are treatable.

Key Facts at a Glance

• A tau-based brain disease. CTE is defined by abnormal tau protein, typically around small blood vessels in specific brain regions (McKee et al., Brain, 2013).
• Confirmed only after death. No test can definitively diagnose CTE in a living person (CDC, 2024).
• Linked to repeated impacts, not single concussions. The CDC notes that "there is no strong evidence that getting one or more concussions… leads to CTE."
• Higher rates in contact-sport athletes. A JAMA study of 202 deceased former football players found CTE in 87% of donors, including 99% of former NFL players (Mez et al., JAMA, 2017).
• Selection bias matters. Brain donor cohorts are not random; donors often had symptoms in life. Population prevalence is likely much lower.
• Not the same as dementia. CTE has its own neuropathological definition, though it can coexist with Alzheimer's and other conditions.

What CTE Is — and Isn't

CTE is a neuropathological diagnosis: it is defined by what pathologists see in brain tissue, not by a symptom checklist in life. The core finding is an unusual pattern of tau protein around small blood vessels deep in the folds of the cortex (McKee et al., Brain, 2013).

CTE is not:

• A diagnosis a living person can receive with certainty. Symptoms can raise concern, but they cannot confirm CTE.
• An automatic outcome of one or several concussions (CDC, 2024).
• The same as post-concussion syndrome. PCS describes ongoing symptoms in the weeks or months after a concussion; CTE is a distinct pathological process. See post-concussion syndrome symptoms.
• The same as Alzheimer's disease. The pathology differs, even though both involve abnormal tau.

Who Has Been Studied Most

Research to date has focused on groups with long-term, repeated head-impact exposure: contact-sport athletes (particularly in football, ice hockey, boxing, soccer, and rugby), military service members with blast and impact exposure, and domestic-violence survivors with repeated head injuries.

The most cited study is the Mez et al. 2017 JAMA analysis of 202 deceased former football players who donated their brains. CTE was found in 177 of 202 donors (87%), with rates rising by level of play — 21% of high school players, 91% of college players, and 99% of former NFL players. The Concussion Legacy Foundation maintains the UNITE Brain Bank that supports much of this work.

These numbers are striking, but cautions matter. Brains were often donated because the family had concerns about cognition or mood in life — that is selection bias. The studies do not tell us what fraction of all athletes develop CTE. For a wider view, see how repeated concussions can affect brain health.

The Four Stages Described by Researchers

A 2013 Boston University study proposed a four-stage pathological framework based on the spread of tau in donated brains (McKee et al., Brain, 2013):

• Stage I: Focal tau pathology; reported symptoms often included headache and attention difficulties.
• Stage II: Wider cortical tau; reported symptoms included depression and memory complaints.
• Stage III: More widespread pathology; executive function difficulties were more commonly reported.
• Stage IV: Widespread tau and cortical atrophy; dementia was frequently described.

These stages describe what pathologists see in tissue — not a clinical staging tool for living people. The symptom descriptions came from after-death family interviews and cannot prove cause and effect with the underlying pathology.

Why Diagnosis in Life Remains Hard

There is no blood test, MRI sign, or PET scan that can confirm CTE in a living person today. Researchers have proposed a clinical research syndrome called traumatic encephalopathy syndrome (TES) to describe the in-life features sometimes associated with CTE pathology, but this is a research framework — not a confirmed diagnosis. The Concussion Legacy Foundation and the CDC both emphasize that having symptoms commonly described in CTE — such as mood swings or memory complaints — does not mean a person has the disease. The 2023 international consensus on concussion in sport likewise notes that the potential long-term effects of repeated head impacts remain an active area of research (Patricios et al., BJSM, 2023).

Many other conditions can produce similar symptoms — depression, anxiety, sleep disorders, medication side effects, vascular disease, and other forms of cognitive change. Most are treatable, and several are reversible. That is one reason a careful clinical evaluation, rather than self-diagnosis, is the right starting point.

How Athletes, Veterans, and Families Can Think About Risk

A few practical principles fit the current evidence:

• Reduce exposure where possible, especially in youth.
• Take concussions seriously. Follow medical guidance after any head injury — see the cognitive effects of a concussion.
• Consider a baseline. A baseline test is most useful in groups with ongoing exposure; see baseline cognitive testing for athletes.
• Track changes over time. Trends in attention, memory, and processing speed can flag a change worth discussing with a clinician.
• Do not self-diagnose. Cognitive or mood changes have many causes, most of them not CTE.
• Get help for symptoms now. Depression, anxiety, sleep disturbance, and post-concussion symptoms can often be treated.

The Cleveland Clinic similarly notes that repeated concussions can raise CTE risk, while a single concussion does not typically cause permanent damage.

What the Science Still Cannot Answer

Important questions remain open. Why do some people exposed to repeated impacts develop CTE pathology while others do not? Genetics, age at first exposure, and total impact dose are all under study (CDC, 2024). How common is CTE in the general population? Prior studies suggest it is likely uncommon without documented repeated exposure, but precise rates are not known. Research on tau PET imaging and fluid biomarkers is moving forward, but no in-life test is approved for CTE diagnosis today.

What Happens Next

Public conversation about CTE will continue, and so will the science. The most useful position is between dismissing the question and assuming the worst. For most people, the right next step is a thoughtful clinical evaluation of current symptoms, attention to treatable causes, and — for those with ongoing impact exposure — sensible monitoring of cognition over time.

Taking the Next Step

For a broader view of how head impacts affect thinking and recovery, start with our concussion and traumatic brain injury overview.

If you want a structured way to monitor attention, memory, and processing speed over time and share trends with your clinician, explore how Orena's at-home cognitive test works.