Diagnosis and Differential Diagnosis of Parkinson Disease.

Parkinsonism is one of the most common neurologic disorders in the aging population. Although Parkinson disease (PD) is the most common cause, there is a lengthy differential diagnosis. The diagnosis of PD hinges on recognizing its typical features, including bradykinesia, rest tremor, unilateral onset, cogwheel rigidity, and beneficial and sustained response to levodopa. Equally important is to be familiar with the "red flags," which are features not expected with PD and suggest an alternative diagnosis, usually a parkinsonian syndrome. In general, it is best to have the diagnosis confirmed by a neurologist, especially one with expertise in movement disorders.