ABSTRACT
The three classical cardinal signs of PD are tremor, rigidity, and akinesia. It is currently accepted that the presence of at least two of these three signs is required for the diagnosis of PD. Asymmetric onset and levodopa responsiveness are additional useful diagnostic criteria. Postural instability is considered a fourth cardinal sign but is less useful for early diagnosis, as it typically appears only later in the course of PD and is such a common early feature of other forms of parkinsonism.
