Force overflow and levodopa-induced dyskinesias in Parkinson's disease

We assessed force coordination of the hand in Parkinson's disease and its relationship to motor complications of levodopa therapy, particularly to levodopa-induced dyskinesias (LID). We studied two groups of Parkinson's disease patients with (Parkinson's disease + LID, n=23) and without levodopa-induced dyskinesias (Parkinson's disease-LID, n=10), and age-matched healthy controls. The motor score of the Unified Parkinson's Disease Rating Scale, a dyskinesia score and force in a grip-lift paradigm were assessed ON and OFF levodopa. A pathological increase of forces was seen in ON-state in Parkinson's disease + LID only. In Parkinson's disease + LID, the force involved in pressing down the object before lifting was significantly increased by levodopa (by 61%, P<0.05). An overshooting of peak grip force by 51% (P<0.05) and of static grip force by 45% (P<0.01) was observed in the ON- compared with the OFF-drug condition. In contrast, no excessive force was found in Parkinson's disease-LID. Peak grip force in ON-state was 140% (P<0.05) higher in Parkinson's disease + LID than in Parkinson's disease-LID, while static grip force was increased by 138% (P<0.01) between groups. Severity of peak-dose dyskinesias was strongly correlated with grip force in ON-state (r=0.79 with peak force, P<0.01). No correlation was observed between forces and the motor score as well as with the daily dose of dopaminergic medication. Force excess was only observed in patients with LID and motor fluctuations. A close relationship was seen between the overshooting of forces and dyskinesias in the ON-drug condition. We postulate that both LID and grip force excess share common pathophysiological mechanisms related to motor fluctuations.