Motor initiation and execution in essential tremor and Parkinson's disease.

Clinical differentiation of essential tremor (ET) from idiopathic Parkinson's disease (iPD) is based on the lack of akinesia and bradykinesia. Nevertheless, early tremor-predominant iPD often is difficult to distinguish from ET. Motor initiation and execution in ET, iPD, and normal control (NC) subjects were investigated. Individuals with iPD, ET and NC performed a reaction-time wrist flexion and extension task. Motor performances were similar between ET and iPD and both were different than normal control subjects. Both the patients with iPD and ET had longer reaction times and slower movement velocities than NC subjects. This may help to explain some of the difficulties in distinguishing patients with these two diseases. The similarities of motor performance suggest that while ET and iPD may be separate disease entities, they may share similar pathogenic motor mechanisms from the perspective of an integrated motor system that drives the motor cortex.