Bradykinesia is not a "systematic" feature of adult-onset Huntington's disease; implications for basal ganglia pathophysiology.

Our goal was to determine whether bradykinesia is present in choreic adult-onset Huntington's disease (HD) patients, and determine the impact of chorea on their voluntary movements. We recorded whole-body involuntary movements (WBIM) and voluntary motor acts simultaneously, using a magnetic tracker system, in 15 choreic HD patients and 15 healthy age- and gender-matched control subjects. Participants were asked to perform two distinct tasks; a manual-tracking (MT) task yielding a measure of chorea intrusion during accurate movements, and a rapid alternating movement (RAM) task, yielding measures of bradykinesia. Results show that patients with HD presented with deviations from the target that hindered their ability to match the target velocity during the MT task. Furthermore, error in performance was correlated with the amplitude of whole-body chorea (Rho=0.67), illustrating the deleterious effect of chorea during accurate movements. However, patients with choreic HD presented with significantly higher RAM range and velocity than matched controls, therefore ruling out the idea that bradykinesia is a systematic feature of HD even when chorea is predominant. The present results imply that patients may have benefited from an intact direct pathway ("select ON" pathway in the focused attention model of basal ganglia function) that allowed them to supersede any dysfunctions associated with the progressive alteration of the "control function" (striatal-globus pallidus-subthalamic) pathway responsible for generating the chorea. Finally, the present results suggest that patients with adult-onset HD having chorea would greatly benefit from improved treatments aiming at reducing their involuntary movements while maintaining proper motor function.