Corticobasal degeneration (CBD) is caused by the abnormal buildup of a protein called tau inside brain cells. Specifically, a particular form of tau accumulates in neurons and support cells across the cerebral cortex, basal ganglia, and brainstem, progressively destroying them. The only well-established risk factor is advanced age, and no environmental toxins or infections have been linked to the disease. Most cases appear to arise spontaneously, with no clear inherited pattern.
How Tau Protein Damages the Brain
Tau is a normal protein that helps stabilize the internal scaffolding of brain cells, known as microtubules. Think of microtubules as tiny structural beams that give a cell its shape and help transport nutrients. In CBD, tau undergoes a chemical change called hyperphosphorylation, meaning too many phosphate groups attach to it. This altered tau loses its grip on the microtubules, causing them to break down. Worse, the freed tau molecules clump together, forming toxic deposits inside the cell.
CBD specifically involves a version of tau called 4-repeat (4R) tau, one of several natural variants the brain produces. This same 4R form is also the culprit in progressive supranuclear palsy (PSP), a related but distinct condition. What sets CBD apart under a microscope is where and how the tau accumulates. In CBD, tau deposits form characteristic structures called astrocytic plaques inside star-shaped support cells called astrocytes. These plaques are the pathological hallmark of CBD and do not appear in other tau diseases. Additional tau deposits show up as thread-like tangles in white matter and as coiled bodies inside another type of support cell, oligodendroglia.
The damage is widespread. Abnormal tau infiltrates gray and white matter across the cortex, the basal ganglia (deep brain structures involved in movement), the diencephalon (which relays sensory signals), and the upper brainstem. Compared to PSP, CBD tends to produce more extensive cortical damage, which is why it often causes problems with higher-level thinking, language, and skilled movements rather than primarily affecting balance and eye movement.
Why Tau Goes Wrong in the First Place
This is the central unanswered question. The cellular process that triggers tau to become hyperphosphorylated in sporadic cases of CBD remains unknown. One line of evidence points to the brain’s immune cells, called microglia. Research has described a mechanism in which signaling from activated microglia enhances tau hyperphosphorylation in neurons during neuroinflammation. In other words, the brain’s own inflammatory response may push normal tau toward its toxic form, though this has not been confirmed as the definitive trigger in CBD.
The astrocytic plaque appears to be the earliest pathological change, with neurons and oligodendroglia becoming affected only in more advanced stages of the disease. This suggests the disease process may begin in astrocytes before spreading to other cell types, though researchers are still working out why astrocytes are the first to accumulate abnormal tau.
Genetic Factors
Most cases of CBD are sporadic, meaning they occur without a family history. However, genetic factors do play a role in a subset of cases. When researchers look at patients who present with the clinical picture associated with CBD (called corticobasal syndrome), the most commonly identified gene is GRN, accounting for roughly 48% of genetically determined cases. Mutations in the MAPT gene, which provides the blueprint for tau protein, account for about 16%. Other implicated genes include C9ORF72 (10%) and PRNP (7%).
Having a mutation in one of these genes does not guarantee someone will develop CBD. These mutations are found in a small minority of cases overall. For the vast majority of people diagnosed, no specific genetic cause can be identified.
Why CBD Is Hard to Pin Down
Part of the challenge in understanding what causes CBD is that the disease doesn’t always look the same from person to person. CBD is technically a pathological diagnosis, meaning it can only be confirmed with certainty by examining brain tissue after death. During life, doctors diagnose a clinical pattern called corticobasal syndrome (CBS), but the relationship between the two is not one-to-one.
Not everyone with CBD pathology presents with the classic corticobasal syndrome. Some people with CBD instead develop progressive difficulty with language, a frontal-lobe behavioral syndrome, or a movement disorder resembling PSP. The specific symptoms depend on where in the brain the tau deposits are heaviest. Conversely, the clinical picture of corticobasal syndrome can be caused by several different underlying diseases, including Alzheimer’s disease, PSP, and a form of frontotemporal degeneration involving a different protein called TDP-43.
Diagnostic criteria developed by Armstrong and colleagues attempted to address this complexity by broadening the recognized clinical presentations and introducing categories of “probable” and “possible” CBD. Even so, validation studies found that only about 47% of pathologically confirmed CBD cases met criteria for probable CBD at initial presentation, rising to 68% by the time of last clinical assessment. This means a significant number of people with CBD are not correctly identified during life.
Age as the Primary Risk Factor
Advanced age is the only confirmed risk factor for CBD. Symptoms typically begin in the 60s or 70s. No environmental exposures, toxins, infections, or lifestyle habits have been linked to the disease. Researchers have investigated potential environmental triggers and found no evidence supporting a connection.
While no proven prevention exists, maintaining overall brain health through regular physical activity, cognitive engagement, and a balanced diet is generally recommended for reducing the risk of neurodegenerative diseases broadly, even though none of these measures have been shown to specifically prevent CBD.
Advances in Identifying CBD During Life
Historically, the only way to know for certain that someone had CBD was through a brain autopsy. That is beginning to change. Biomarker-based classification can now detect different types of protein pathology in living patients using spinal fluid tests and specialized brain imaging. Spinal fluid analysis can measure levels of amyloid proteins (to rule out Alzheimer’s), and newer PET scans can visualize tau deposits directly in the brain. A marker of nerve cell damage called neurofilament light chain, measured in spinal fluid, provides additional information about how much degeneration has occurred.
These tools allow doctors to sort patients with corticobasal syndrome into biological subgroups, something that was previously only possible after death. This matters because identifying the specific protein driving someone’s disease is essential for selecting the right treatment, particularly as therapies targeting tau directly move through clinical trials.