The effect of subthalamic nucleus deep brain stimulation on precision grip abnormalities in Parkinson's disease

We have studied grip force performance in a group of 10 patients who were in a stable state after implantation of bilateral stimulating electrodes in the subthalamic nuclei (Stn) to counter drug-resistant or drug-induced symptoms of advanced Parkinson's disease. The patients were required to use a precision grip to lift an object which recorded grip force development and lift dynamics. Lifting was performed with stimulation on and with stimulation off under optimal medication. Post-operatively, dyskinesia was absent in all patients in both conditions, but in the ‘off’ state the patients showed the profound bradykinesia and excessive levels of grip force development associated with Parkinson's disease from its early stages. In the stimulation ‘on’ state both the rate of grip force development and the speed of the lifting phase were increased significantly. The excessive levels of grip force present in the stimulation ‘off’ state, and present from the early stages of the disease, however, were even more marked with Stn stimulation on. It is suggested that this results from a failure to modify stored motor programs developed over a long period under the influence of bradykinesia, leading to an inappropriately prolonged duration of grip force development when this influence is removed by Stn stimulation. Thus although Stn stimulation achieved a dramatic improvement in the mobility of the patients in general, and in the dynamics of hand movements specifically, by improving rates of force development and lifting dynamics, it does not restore, and may even worsen, the ability to match lifting parameters to actual conditions.