Differential response of speed, amplitude, and rhythm to dopaminergic medications in Parkinson's disease.

Clinicians must consider multiple aspects of movement including speed, amplitude, hesitations, fatiguing, and arrests in movement when assigning a single severity score in the Unified Parkinson’s Disease Rating Scale motor subscale (UPDRS-III). The modified bradykinesia rating scale (MBRS) was introduced to independently rate the movement impairments of speed, amplitude, and rhythm.1 Each of these movement components is given a score from 0 to 4 for each of tasks 23–25 of the UPDRS-III (finger taps, hand grasps, rapid alternating movements, respectively). The MBRS has demonstrated inter- and intrarater reliabilities similar to those of the UPDRS.2 Accurate assessment of movement impairment is necessary to ascertain the motor state and monitor response to standard and experimental therapeutic interventions. Several studies have shown that certain therapies may differentially improve specific components of movement impairment. For example, amplitude improves during finger taps postpallidotomy, but not speed or rhythm.3 Also, amplitude but not speed improves during bimanual compared with unimanual finger tapping.1 An example of the complexities in interpreting studies using the “bradykinesia- related” items of the UPDRS-III as an independent outcome in clinical trials is illustrated by a trial of a serotonin reuptake inhibitor antidepressant, which determined that “citalopram did not affect rigidity and tremor, but significantly improved bradykinesia and finger taps.”4 These findings seem to contradict clinical experience and several reports of potentially detrimental effects of SSRIs on motor function in patients with PD.5–7 Also, although subthalamic deep brain stimulation has been shown to be efficacious for ameliorating PD motor symptoms including “bradykinesia,” 8,9 it is unclear whether specific components of movement are improved.9–11 A recent pilot study demonstrated that levodopa normalizes bradykinesia to a greater extent than hypokinesia.12 Therefore, we sought to evaluate motor function and response to dopaminergic medication in patients with PD with various impairments in speed, amplitude, and rhythm of movement.