Atypical parkinsonism refers to a group of neurodegenerative disorders that have some features similar to those of Parkinson disease but have some different clinical features, a worse prognosis, a modest or no response to levodopa, and a different pathology (eg, neurodegenerative disorders such as multiple system atrophy, progressive supranuclear palsy, dementia with Lewy bodies, and corticobasal ganglionic degeneration).