Kinetic tremor during tracking movements in patients with Parkinson's disease

This study was undertaken to analyse kinetic tremor during a compensatory tracking task performed with the index fingers. Patients with Parkinson's disease (PD) (n=21) and control subjects (n=30) were tested using a laser system transducing displacement. All participants underwent a clinical examination. Nine characteristics quantifying the tracking task and tremor were applied to the processed displacement or velocity signal. The discriminating power of each characteristic was evaluated using differences between group means (p values), maximum percentage discrimination, and number of outliers in the patient group. All nine characteristics showed significant differences between means of the two groups using Welch-modified t-tests for unequal variances. The most discriminating characteristics reflected differences in the frequency distribution of the movement (proportional power in the 3-4Hz range, harmonicity and median frequency). These differences were rarely visible and did not correlate highly with tremor amplitude or with clinical ratings of tremor. Control subjects had residual spectral power from the tracking oscillations leaking above 3Hz, while most patients had a small 'kinetic tremor' in the 7-12Hz range. The maximum discrimination was moderate, 63% in the best case. Combining representative information about proportional power during posture (recorded in the same subjects) and tracking gave a much higher discrimination (90%) with respect to the 96.7th percentile of the control group. These results suggest that information coming from postural and kinetic tremors can be combined to isolate a subclinical feature of PD symptomatology. This feature could be used to re-evaluate the classic distinction made between the akineto-rigid and tremulous forms of PD and is independent of tremor amplitude.