Identification of unilateral elbow-joint position is impaired by Parkinson's disease

We have compared the ability of patients with idiopathic Parkinson's disease (PD) with that of control subjects to identify unilateral elbow-joint position, in the absence of direct vision of the arm, by visual reference to a graduated angular scale, placed beside the elbow, across a range of test angles of 90-108 degrees. The positioning of the subject's elbow was achieved under either passive (subject relaxed, Experiment 1) or active (subject contracting, Experiment 2) conditions. PD patients' performance (while on L-dopa medication) with the elbows on the sides of "worse" and "better" motor signs was compared with that of controls with, respectively, the left and right elbows. In both experiments, (a) both the individual, overall mean unsigned (with respect to direction) error averaged across all test angles (accuracy), and the SD about this mean (precision), were significantly larger on each side among PD patients than among controls, and (b) the subjective ranges of values employed by PD patients were substantially compressed, on average, by comparison with those of controls. Within-group analyses revealed that (a) among control subjects, but not among PD subjects, individual, overall mean unsigned errors, on each side, averaged across test angles, were significantly smaller under active than under passive conditions, and (b) the subjective ranges employed by PD patients, but not by controls, under active conditions significantly exceeded those under passive conditions. We conclude that these results are generally consistent with the notion that PD impairs unilateral elbow-joint position sense.