Voluntary motor impairment is a functionally important aspect of Huntington's disease (HD). Therefore, quantitative assessment of disturbed voluntary movement might be important in follow-up. We investigated the relation between quantitatively assessed daytime motor activity and symptom severity in HD and evaluated whether assessment of daytime motor activity is a responsive measure in the follow-up of patients. Sixty-four consecutive HD patients and 67 age- and sex-matched healthy controls were studied. Daytime motor activity was recorded using a wrist-worn activity monitor that counts all movements during a period of five consecutive days. Patients were rated clinically for voluntary motor impairment, dyskinesias, posture & gait, depression, cognitive impairment and functional capacity. Follow-up was available from 40 patients (mean follow-up 2.0 years) and 29 controls (mean follow-up 5.9 years). Despite chorea, patients had less daytime motor activity than controls (P < 0.005). This hypokinesia correlated with impaired voluntary movements (r = 0.37; P < 0.01), disturbed posture & gait (r = 0.38; P < 0.005) and especially with reduced functional capacity (r = 0.51; P < 0.0005). During follow-up, hypokinesia remained unchanged in clinically stable patients, but became worse in those whose functional disability progressed (P < 0.005). Hypokinesia seems a core symptom of HD which is related to functional capacity. Actimetric assessment of hypokinesia is responsive to disease progression and can be used as an objective tool for follow-up.

Functional disability of patients with Huntington's disease (HD) is determined by impairment of voluntary motor function rather than the presence of chorea. However, only few attempts have been made to quantify this motor impairment. By using a simple reaction time paradigm, we measured the time needed for movement initiation (akinesia) and execution (bradykinesia) in 76 HD patients and 127 controls. Akinesia and bradykinesia were already evident in early stages and increased linearly with increasing disease stage. Quantified motor slowness correlated with clinical impairment of voluntary movements but also with cognitive impairment and medication use. In patients without severe cognitive impairment, quantified motor slowness reflected clinical motor impairment more purely. During 1.9 years follow-up (range, 0.8-3.8 years), quantified akinesia and bradykinesia progressed concomitantly with progression of clinical impairment of voluntary movements, cognition, and functional capacity. However, rate of change in motor slowness did not discriminate between patients whose disease stage remained stable and those whose disease stage progressed. We conclude that the reaction time paradigm may be used to quantify akinesia and bradykinesia in HD, at least in patients without severe cognitive impairment. Although reaction and movement times increased in time, these measures failed to detect functionally important changes during our follow-up period.