Issues in the early diagnosis of Parkinson's disease.

The clinical diagnosis of Parkinson's disease (PD) is most difficult early in the disease when the signs and symptoms are most subtle. The differential diagnosis of PD includes a number of movement disorders with similar symptomatology (e.g., essential tremor, multiple system atrophy, vascular parkinsonism). In most published studies of PD, the disease is diagnosed simply by the presence of two of the three cardinal motor signs-tremor, rigidity, and bradykinesia-or by the presence of three of the four motor signs: tremor, rigidity, bradykinesia, and postural instability. However, there is an obvious need for better diagnostic criteria. Until discrete biologic markers are developed, the use of exclusion criteria may improve the accuracy of the presumptive diagnosis of PD.