Chronic traumatic encephalopathy (CTE) changes the brain in ways you can see with the naked eye, under a microscope, and through the symptoms a person experiences while alive. The condition involves a buildup of abnormal tau protein that slowly damages and kills brain cells, typically after years of repeated head impacts. CTE can only be definitively diagnosed after death through autopsy, so “what it looks like” spans both the physical state of the brain and the behavioral and cognitive changes that unfold over years or decades.
What the Brain Looks Like at Autopsy
A brain with advanced CTE is visibly smaller than a healthy brain. The shrinkage is most pronounced in the frontal and temporal lobes, the areas responsible for decision-making, impulse control, memory, and emotional regulation. The ventricles, fluid-filled chambers deep inside the brain, are noticeably enlarged because the surrounding tissue has wasted away. The third ventricle in particular tends to be disproportionately widened compared to what you’d see in normal aging.
Several other structural changes are visible during autopsy. The septum pellucidum, a thin membrane separating the two ventricles, often develops holes or tears. The mammillary bodies, small structures involved in memory, are shrunken or destroyed. The corpus callosum, the thick band of nerve fibers connecting the brain’s two hemispheres, becomes noticeably thinner. Two pigmented brainstem regions that produce key neurotransmitters lose their color, a sign that the cells there have died off. In severe cases, the brain may also show old contusions from past injuries.
What CTE Looks Like Under a Microscope
The defining feature of CTE at the cellular level is a specific pattern of abnormal tau protein. In a healthy brain, tau helps stabilize the internal scaffolding of neurons. In CTE, tau becomes chemically altered (hyperphosphorylated), misfolds, and clumps together into tangles inside and around nerve cells. What makes CTE distinct from other brain diseases is where these tangles appear: clustered around small blood vessels at the bottoms of the brain’s grooves (sulci), the deepest folds of the cortex. This irregular, patchy, perivascular pattern is required for a CTE diagnosis and is detected using a specialized staining technique for phosphorylated tau.
Beyond tau, CTE brains show widespread damage to the nerve fibers (axons) that connect different brain regions. The protective insulation around these fibers breaks down, and proteins essential for maintaining axon structure are significantly reduced in moderate to advanced disease. Chronic inflammation is present throughout, and many CTE brains also contain deposits of TDP-43, another abnormal protein linked to cell death, as well as amyloid plaques similar to those seen in Alzheimer’s disease.
The Four Stages of CTE Pathology
Researchers classify CTE severity into four stages based on how far the tau pathology has spread through the brain.
In Stage I, the damage is minimal. Only one or two small clusters of tau tangles appear around blood vessels at the base of cortical grooves, most often in the frontal lobe. A person at this stage may have no symptoms at all, or only subtle ones like headaches or difficulty concentrating.
Stage II shows three or more of these tau clusters spread across multiple cortical regions. Tangles also appear in deeper brainstem structures involved in attention, arousal, and memory. This stage often correlates with mood changes and short-temper episodes.
By Stage III, the tau patches have grown larger and merged together. Tangles now extend into the superficial layers of the cortex and spread diffusely through memory-critical structures: the hippocampus, the amygdala, and surrounding areas. People at this stage typically experience noticeable memory problems, difficulty with planning and organization, and significant behavioral changes.
Stage IV represents the most severe form. Tau tangles are distributed throughout the entire cortex and have reached the cerebellum, the base of the brainstem, and even the spinal cord. Neurons in the pigmented brainstem regions have died, causing visible depigmentation. This stage is associated with profound cognitive decline, aggression, depression, and movement problems.
How CTE Symptoms Appear in Life
Because CTE cannot yet be diagnosed in a living person, what we know about its symptoms comes from people whose brains were later confirmed to have the disease at autopsy. The symptoms fall into four categories: cognitive, behavioral, mood, and movement changes.
Cognitive symptoms include trouble thinking clearly, progressive memory loss, and difficulty with planning or carrying out multi-step tasks. Behavioral changes often manifest as impulsive actions and aggression, sometimes dramatically out of character. Mood symptoms are common and can be severe: depression, emotional instability, apathy, substance misuse, and suicidal thoughts or behavior. In a meta-analysis of over 1,000 former contact sport athletes diagnosed with CTE after death, the suicide rate among those with the disease was 39%.
Movement symptoms tend to appear in more advanced stages. These can resemble parkinsonism, with changes in gait, coordination difficulties, and slowed movement. The timeline varies widely. Some people develop symptoms in their 30s or 40s, while others don’t show clear signs until decades after their last head impacts.
How CTE Differs From Alzheimer’s Disease
CTE and Alzheimer’s disease both involve tau protein tangles and can cause memory loss, which makes them easy to confuse. But the two diseases attack the brain in distinctly different patterns. In CTE, tau concentrates around blood vessels at the depths of cortical grooves, a pattern not seen in Alzheimer’s. The tangles also have a different molecular structure: cryo-electron microscopy reveals that tau fibers in CTE fold differently than those in Alzheimer’s.
The two diseases also target different parts of the hippocampus, the brain’s primary memory center. CTE produces high levels of tau in subfields called CA2, CA3, and CA4, while largely sparing another subfield called the subiculum. Alzheimer’s disease does the opposite, hitting CA1 and the subiculum hardest while leaving CA2 and CA3 relatively intact. This difference helps neuropathologists distinguish the two conditions at autopsy, though in life the symptoms can overlap considerably.
Who Gets CTE and How Common It Is
CTE has been found primarily in people with a history of repetitive head impacts: football players, rugby players, boxers, hockey players, soccer players, and military veterans exposed to blast injuries. A 2025 meta-analysis pooling data from eight studies of 1,000 former contact sport athletes found CTE in 53.7% of brains examined. Rugby players had the highest rate at 64.7%, followed by American football players at 53%. Elite athletes were more affected than amateurs, with prevalence of 72.8% versus 44.1%.
These numbers come with an important caveat: the brains donated for research tend to come from people who had symptoms or whose families suspected something was wrong. That skews the numbers upward. The true prevalence in the general population of former athletes is almost certainly lower, but exactly how much lower remains unknown.
Why CTE Still Can’t Be Diagnosed in Life
Despite years of research, CTE remains a diagnosis made only at autopsy through microscopic examination of brain tissue stained for phosphorylated tau. Standard brain imaging, including MRI and CT scans, can show atrophy and other structural changes in advanced cases, but these findings overlap with many other conditions and aren’t specific enough to confirm CTE.
The most promising avenue for living diagnosis is PET imaging using radioactive tracers designed to bind to tau. Several tracers originally developed for Alzheimer’s research have been tested on CTE brain tissue. Two of them, known as MK-6240 and PI-2620, show the most promise for detecting CTE tau in severe cases without binding to unrelated targets. But even these are still experimental, and they appear most useful in advanced disease. Detecting early-stage CTE in a living person remains out of reach for now, making the visible signs at autopsy and the symptom patterns observed during life the two pillars of what CTE currently “looks like.”