The multiple phenotypes of corticobasal syndrome and corticobasal degeneration: implications for further study

Corticobasal degeneration (CBD) is a complex neurodegenerative disorder which nomenclature of which its nomenclature and characterization continues to evolve. The core clinical features that have been considered characteristic of the disorder include progressive asymmetric rigidity and apraxia, with other findings suggesting additional cortical (e.g., alien limb phenomena, cortical sensory loss, myoclonus, and mirror movements) and basal ganglionic (e.g., bradykinesia, dystonia, and tremor) dysfunctions. The characteristic findings at autopsy are asymmetric cortical atrophy that is typically maximal in the frontoparietal regions, as well as basal ganglia and nigral degeneration. Microscopically, abnormal accumulations of the microtubule-associated tau protein are found in both neurons and glia, and this disorder is now considered one of the "tauopathies." CBD was initially thought to represent a distinct clinicopathologic entity. Recent studies have shown considerable clinicopathologic heterogeneity, leading some to use the term "corticobasal syndrome" (CBS) for the constellation of findings initially considered characteristic of the disorder, and the term "corticobasal degeneration" for the histopathologic disorder. In this review, the multiple phenotypes/syndromes associated with CBD pathology, and multiple diseases associated with the CBS, are presented. The clinicopathologic heterogeneity in CBS/CBD and the implications of this heterogeneity on clinical practice, on understanding the focal/asymmetric cerebral degeneration syndromes, and on future research are all reviewed.