How Is CTE Diagnosed and Why It Requires Autopsy

Chronic traumatic encephalopathy, or CTE, can only be definitively diagnosed after death, through microscopic examination of brain tissue. The reason is deceptively simple: the defining feature of CTE is a specific pattern of abnormal tau protein deposits clustered around tiny blood vessels deep in the folds of the brain’s outer surface, and no imaging technology available today can reliably detect that pattern in a living person. While researchers have developed clinical criteria for a related condition called traumatic encephalopathy syndrome (TES) that can be assessed during life, a confirmed CTE diagnosis still requires an autopsy neuropathologist to physically slice, stain, and examine the brain.

What Pathologists Are Looking For

The hallmark of CTE is an accumulation of phosphorylated tau protein, often abbreviated as p-tau, in and around neurons and certain star-shaped brain cells called astrocytes. What makes CTE distinctive is not just the presence of abnormal tau, which shows up in several brain diseases, but where it appears and how it arranges itself. In CTE, the p-tau clusters around small blood vessels at the bottoms of the cortical sulci, the deep grooves that give the brain’s surface its wrinkled appearance.¹ This pattern is irregular and patchy rather than evenly spread across the brain, and it has a molecular structure unlike what is seen in aging, Alzheimer’s disease, or any other tau-related brain disease.²

A 2016 consensus meeting convened by the National Institutes of Health brought together neuropathologists to agree on what, exactly, counts as CTE under a microscope. The panel settled on that perivascular p-tau accumulation at cortical sulcal depths as the pathognomonic lesion, meaning the single feature that, when present, confirms the diagnosis. The neuropathologists who participated showed good agreement when they independently reviewed brain tissue cases, which gave the field confidence that the diagnostic criteria were reliable and not just a matter of individual interpretation.³

Why This Cannot Be Seen in a Living Brain

The lesion that defines CTE exists at a scale far below what any brain scan can resolve. We are talking about clumps of misfolded protein wrapped around capillaries that are fractions of a millimeter wide, sitting at the bottom of cortical folds that themselves are only a few millimeters deep. Standard MRI, even at its highest clinical resolution, shows brain structures in slices roughly a millimeter thick. That is not fine enough to distinguish CTE’s perivascular tau from the tau deposits seen in Alzheimer’s or normal aging, which can look similar at the macro level but differ in their precise location and cellular distribution.

Computational modeling has shed light on why the disease specifically targets the sulcal depths. When the brain experiences a rotational impact, the grooved geometry of the cortical surface creates complex stress waves. These waves interact and concentrate shear stress at the bottom of the sulci, producing forces there that are dramatically higher than in brain tissue without those folds.⁴ The regions of peak stress concentration in these models line up with the locations where CTE tau pathology is found at autopsy. So the disease’s signature pattern is essentially a map of mechanical damage written in protein, and reading that map requires a microscope, staining chemicals, and expertly prepared tissue sections.

Telling CTE Apart From Alzheimer’s Disease and Aging

One of the trickiest aspects of CTE diagnosis, and a major reason why autopsy examination is so important, is that CTE shares some molecular features with Alzheimer’s disease and normal brain aging. The tau tangles found in the neurons of CTE brains contain both three-repeat and four-repeat tau isoforms, which is also the case in Alzheimer’s. The chemical modifications on these tangles look similar too. If you were to examine a single tangle in isolation, you might not be able to tell which disease produced it.⁵

The distinction rests almost entirely on the pattern and distribution of pathology. In Alzheimer’s disease, tau tangles tend to appear first in the entorhinal cortex and hippocampus and then spread outward in a relatively predictable sequence. In CTE, the tau clusters around blood vessels at sulcal depths in an irregular, patchy pattern that does not follow Alzheimer’s staging. The astroglial tau in CTE has its own distinct character, resembling a type of age-related astrocyte change but appearing in locations and quantities that differ from what normal aging produces. A neuropathologist needs to see the whole spatial layout of the pathology, across multiple brain regions and at high magnification, to make the call. No blood test or brain scan currently provides that level of spatial and cellular detail.

What CTE Looks Like During Life

People living with what turns out to be CTE typically show a combination of cognitive, behavioral, and mood problems. A study of 36 confirmed CTE cases found that cognitive difficulties appeared in almost all of them, while a triad of cognitive, behavioral, and mood impairments was common overall. Two somewhat distinct clinical profiles emerged: a younger-onset group whose earliest symptoms were behavioral or mood-related, and an older-onset group that presented first with cognitive decline.⁶

The trouble is that these symptoms overlap heavily with depression, post-traumatic stress disorder, Alzheimer’s, and other conditions that are far more common. A retired football player with memory problems and irritability might have CTE, but he might also have early Alzheimer’s, chronic depression, or the effects of substance use. Without the brain tissue to examine, clinicians cannot tell for certain.

This diagnostic gap led the National Institute of Neurological Disorders and Stroke to develop criteria for traumatic encephalopathy syndrome, or TES, essentially a clinical framework for identifying living people who may have CTE. A TES diagnosis requires substantial exposure to repetitive head impacts, core features of cognitive or neurobehavioral decline, a progressive course, and the ruling out of other explanations.⁷ The criteria also assign a provisional level of certainty that CTE pathology is present, based on how many features a person meets. But when researchers applied these criteria to living retired contact-sport athletes, the sensitivity appeared limited: relatively few met full criteria, suggesting the clinical framework may miss real cases.⁸

Experimental Tools Trying to Close the Gap

Researchers are actively pursuing ways to detect CTE-related changes in living people. The most promising avenue is tau PET imaging, which uses radioactive tracer molecules that bind to tau protein in the brain. A PET scanner then detects where the tracer has accumulated, producing a map of tau deposits.

Early results are cautiously encouraging but far from ready for clinical use. One study using a tracer called flortaucipir found that some patients with suspected CTE showed elevated signal in frontal and temporal brain regions, consistent with what would be expected in moderate-to-advanced CTE. But the tracer was unlikely to pick up early-stage disease.⁹ A study comparing flortaucipir PET scans taken near the end of life with postmortem brain examination in six former football players found a strong correlation between tracer uptake and actual tau density in several brain regions, but there was also overlap between CTE cases and non-CTE cases in some areas.¹⁰

A newer tracer, MK-6240, is also being tested. In a study of former NFL players, those athletes showed higher tracer uptake in the entorhinal cortex compared to controls, and higher uptake in certain regions correlated with worse memory and verbal fluency performance. But the signal was variable: more than half of the NFL players had no cortical signal at all.¹¹ These tracers were originally designed for Alzheimer’s-type tau, and whether they bind reliably to CTE-specific tau deposits is still being worked out. The technology shows enough promise that researchers are optimistic it could eventually contribute to a living diagnosis, but we are not there yet.

Researchers are also investigating blood-based and structural brain markers. Small studies have combined MRI measures of brain structure, metabolic PET scans, and blood levels of proteins like neurofilament light and total tau in former players who later came to autopsy.¹² Diffusion MRI has shown changes in the corpus callosum, the thick band of fibers connecting the brain’s hemispheres, that correlate with cumulative head-impact exposure in former professional football players.¹³ These markers track damage from repetitive head impacts, but they are not specific enough to confirm CTE itself. Damaged white matter and elevated blood proteins show up in many neurological conditions.

How CTE Staging Works at Autopsy

When a neuropathologist does confirm CTE, the disease is graded on a severity scale from Stage I through Stage IV, based on how widely the tau pathology has spread. In the earliest stages, the abnormal tau may be confined to a few small foci in the frontal cortex. By Stage IV, it has spread throughout the cortex, the hippocampus, and deeper brain structures. This staging matters because severity tracks with real-world decline. Stage IV CTE has been associated with roughly four and a half times the odds of dementia compared to brains without CTE, and Stage III with about double the odds.¹⁴

Recent research suggests CTE may not even be a single uniform disease under the microscope. One study identified pathological subtypes: in one subtype, the burden of tau in the cortex was more closely linked to functional decline during life, while other cases showed a different pattern of spread with a less clear relationship to symptoms.¹⁵ If CTE turns out to have genuine subtypes, it would further complicate any attempt to diagnose it with a single biomarker during life.

When CTE Comes With Other Brain Diseases

CTE rarely exists in isolation. The same brains that show CTE tau pathology frequently harbor additional abnormalities. One of the most common co-pathologies involves a protein called TDP-43. In one early case series, TDP-43 deposits were found in the majority of CTE brains, spread across the frontal and temporal cortex, the medial temporal lobe, and deeper brain structures.¹⁶ Three athletes in that series developed a progressive motor neuron disease, with muscle weakness and wasting that resembled ALS, alongside their CTE. A larger study later found TDP-43 inclusions in over 40% of CTE cases and hippocampal sclerosis, a pattern of cell loss and scarring in the memory-forming region, in about 23%.¹⁷

These overlapping pathologies are another reason why autopsy remains essential. A person with CTE plus TDP-43 deposits plus hippocampal sclerosis is carrying three different disease processes in the same brain. No living-diagnostic tool can currently untangle that kind of complexity, and knowing which pathologies are present matters for understanding the disease and eventually developing treatments.

The Role of Neuroinflammation

The biological process driving CTE forward appears to involve a feedback loop between tau protein accumulation and brain inflammation. Microglia, the brain’s resident immune cells, become activated in response to repeated head impacts. One study found that the density of activated microglia was directly associated with both the duration of head-impact exposure and the amount of tau pathology present. The activated microglia appeared to drive further tau accumulation, and the tau in turn activated more microglia, creating a self-reinforcing cycle.¹⁸ This helps explain why CTE can progress for years or decades after a person has stopped playing contact sports: once the cycle is established, it sustains itself even without new injuries.

Does Starting Young Make Things Worse?

One of the more concerning findings in CTE research involves the age at which a person first begins absorbing repetitive head impacts. Studies of former NFL players found that those who started playing tackle football before age 12 performed significantly worse on tests of executive function, memory, and verbal IQ later in life, even after accounting for total years played.¹⁹ Diffusion MRI showed structural changes in the front portions of the corpus callosum, a region that is still actively developing during childhood, in the early-exposure group.²⁰

Among 211 participants with confirmed CTE at autopsy, every year younger a person was when they began tackle football predicted earlier onset of cognitive symptoms by about two and a half years. Those who started before age 12 experienced cognitive and behavioral symptoms roughly 13 years earlier than those who started at 12 or older. The earlier start did not, however, predict worse CTE pathology at autopsy, which suggests the vulnerability is about the developing brain’s response to impact rather than about accumulating more disease.²¹

Selection Bias and What We Still Cannot Measure

Nearly everything we know about CTE prevalence and severity comes from brain bank studies, meaning collections of brains donated by families who often suspected something was wrong. This creates an obvious sampling problem: brains donated because families noticed cognitive or behavioral decline are far more likely to show pathology than a random sample of, say, all retired football players. The high CTE rates reported in brain bank studies should not be read as population prevalence estimates.

Researchers have tried to address this statistically. One analysis used bias-adjustment methods and still found a genuine association between football playing and CTE diagnosis, but acknowledged that additional uncertainty was introduced by the need to estimate how likely different types of people were to donate their brains.²² The association held, but we cannot pin down how common CTE truly is among all people with a history of repetitive head impacts. That question probably cannot be answered until a reliable living diagnostic tool exists and large-scale screening studies become possible.

Genetic variability adds another layer. Two athletes with comparable exposure histories can end up with very different amounts of CTE pathology, which suggests that genes play a role in who develops the disease and how severe it becomes. Researchers have identified several candidate genes, including those involved in tau metabolism and neurodegeneration, but no genetic test can currently predict CTE risk with any useful accuracy. The interplay between impact exposure, genetics, age at first exposure, and individual brain anatomy means the disease is a moving target, one more reason why confirming its presence still demands the painstaking work of examining tissue after death.