Corticobasal degeneration

The core clinical syndrome of CBD—corticobasal syndrome (CBS)—is defined by progressive asymmetric rigidity and apraxia, often accompanied by cortical sensory loss and the alien limb phenomenon, where a limb moves without the patient's intention¹. Other common features include myoclonus, dystonia, and aphasia. However, the clinicopathological correlation is imperfect: many patients with autopsy-confirmed CBD present with a phenotype resembling progressive supranuclear palsy or a frontal behavioral variant, while some with CBS have Alzheimer's disease pathology². Diagnosis is primarily clinical, supported by MRI showing asymmetric frontoparietal atrophy, and dopamine transporter imaging may show reduced uptake in the basal ganglia. There are no fluid biomarkers in routine use, though research on CSF tau and neurofilament light is ongoing.

Pathologically, CBD is characterized by neuronal and glial inclusions composed of hyperphosphorylated four-repeat (4R) tau protein, with distinctive astrocytic plaques and ballooned neurons (achromatic cells) in the cortex and basal ganglia. The tau aggregation is similar to that in progressive supranuclear palsy, but the regional distribution and cell types differ. Most cases are sporadic, but rare familial forms have been linked to mutations in the MAPT gene, which encodes tau. Genome-wide association studies have identified the MAPT H1 haplotype as a risk factor, and a common variant in MOBP has also been associated with CBD³. These findings suggest that CBD shares genetic risk with other tauopathies.

There is no disease-modifying therapy for CBD; management is symptomatic and supportive. Levodopa is often tried but provides only modest or transient benefit in a minority of patients. Botulinum toxin injections can relieve focal dystonia, and physical and occupational therapy help maintain function. Speech therapy and swallowing management are important as dysarthria and dysphagia progress. The disease is relentlessly progressive, with a median survival of about 6–8 years from symptom onset, though this varies widely. Death typically results from complications of immobility, such as pneumonia or sepsis.

Beyond the classic motor syndrome, CBD can present with prominent cognitive or behavioral symptoms, such as non-fluent aphasia or executive dysfunction, which may dominate the early course⁴. The alien limb phenomenon is not exclusive to CBD and can occur in other conditions, including stroke and Creutzfeldt-Jakob disease. Historically, the term "corticobasal degeneration" was first used in 1968 by Rebeiz and colleagues, who described three patients with a "corticodentatonigral degeneration with neuronal achromasia"⁵. In research, CBD has been used as a model for studying tau propagation, and recent work has explored the use of tau PET imaging to track disease in vivo. Additionally, some patients with CBD have been found to have co-existing TDP-43 pathology, suggesting a mixed proteinopathy in a subset of cases.

Tauopathy

A class of neurodegenerative diseases characterized by abnormal accumulation of tau protein in the brain.

Alien limb phenomenon

A complex motor disorder in which a limb performs involuntary, seemingly purposeful movements that the patient does not initiate or control.

Apraxia

The inability to perform learned, purposeful movements despite having the physical ability and desire to do so.

4R tau

Tau isoforms that contain four microtubule-binding repeat domains; these are the predominant isoforms aggregated in CBD.

MAPT gene

The gene encoding microtubule-associated protein tau, located on chromosome 17; mutations cause frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17).