ACUTE EFFECTS OF LEVODOPA ON WRIST MOVEMENT IN PARKINSONS-DISEASE - KINEMATICS, VOLITIONAL EMG MODULATION AND REFLEX AMPLITUDE-MODULATION

Acute changes in motor performance due to levodopa were evaluated by a series of four motor tests unified by their focus on wrist flexion-ex tension movements. Subjects with idiopathic Parkinson's disease were e valuated with this battery of tests before (OFF) and after their usual morning dose of levodopa (ON). The test battery consisted of (i) repe titive self-paced movement in which velocity was to be maximized; (ii) visually guided tracking of a sinusoid and a II square wave; and (iii ) art assay of stretch reflex modulation during volitional sinusoidal tracking. The maximal wrist joint velocity of self-paced reciprocating flexion and extension movements increased after levodopa (ON), withou t significant changes in the movement period or amplitude. In the two tracking tasks, some subjects improved as evident by a lower root mean square (rms) error, bur in similar numbers of subjects the rms error increased. Overall, the rms error peak velocity or peak movement ampli tude did not change after levodopa in either tracking task. Significan t and consistent changes did occur after levodopa in an assay of refle x modulation during error-constrained tracking (Johnson et al., Brain 1992; 114: 443-60). The amplitude of volitional EMG increased after le vodopa, with a concurrent reduction in reflex EMG. These changes are c onsistent with the noted increase in movement velocity. These results show that the effects of levodopa on movement velocity were not consis tently translated into increased accuracy. The changes in the long lat ency reflex gain argue for a central control of this reflex, mediated by structures sensitive to levodopa. Finally, the results show that th e quantitative evaluation of levodopa therapy cannot be unidimensional , but requires a battery of motor tests as undertaken in this study.