The perception of passive motion in Parkinson's disease

The perception of limb motion is a kinaesthetic property that is essential for voluntary motor control. This study examined the ability of patients with Parkinson's disease (PD) to detect the velocity of a passively moved limb. Eight patients with mild to moderate PD and eight age-matched healthy controls participated. They placed their forearm on a padded splint of a passive motion apparatus, which horizontally extended or flexed the elbow joint at velocities between 1.65 and 0.075 degrees /s (in steps of 0.15 degrees /s). Passive movement persisted until subjects detected arm motion and pressed a trigger held in the hand of their non-tested arm. Time until detection and associated arm displacement were recorded and subsequently adjusted for each subject's reaction time. We found that PD patients needed significantly larger limb displacements before they could judge the presence of passive motion. With decreasing passive motion velocity the detection time increased exponentially in both groups. Yet, the mean detection times of the PD group were 92-166% higher than in the control group for each of the 12 tested velocity conditions. Five of the eight patients were on Parkinsonian medication when tested. Yet, the degree of impairment in the PD group did not correlate significantly with the patients' levodopa equivalent dosage. Our results demonstrate that PD patients were impaired in the detection of passive forearm movements. This study complements a growing body of evidence indicating that various aspects of kinaesthesis (position sense, weight perception, passive motion sense) are affected even at early stages of PD. The impaired processing of proprioceptive signals likely contributes to motor symptoms in PD.