Bradykinesia, characterized by slowness and decreased amplitude of movement, is often considered the most important deficit in Parkinson's disease (PD). The current clinical rating of bradykinesia in PD, based on the motor subscale of the Unified Parkinson's disease Rating Scale (UPDRS-III), does not individually weigh the impairments in speed and amplitude of rapid alternating movements. We sought to categorize movement in PD to determine whether speed and amplitude have different relationships to current measures of motor impairment and disability. Categories of speed and amplitude (normal, slow/low, and very-slow/very-low) were ascertained using an electromagnetic tracking device. Amplitude was disproportionally more affected than speed in the "off" state. UPDRS-III and the Schwab & England disability scale were worst in patients with very impaired amplitude and best in patients with normal amplitude. A similarly graded relationship was not found for categories of speed impairment. The examiner clinical global impression of change mirrored "off" state amplitude but not speed categories. Levodopa, however, normalized speed to a greater extent than amplitude. Our observations suggest that amplitude and speed impairments may be associated with different functional aspects in PD and deserve separate clinical assessment.
(c) 2009 Movement Disorder Society.