Pallidotomy and bradykinesia: Implications for basal ganglia function

Background and Objective: The scientific rationale for pallidotomy as a treatment for PD is that the lesion will reduce excessive tonic inhibition of the thalamus, thereby allowing movement to proceed more normally. If true, then PD patients who move slowly while on medication should increase movement speed following pallidotomy. To test this we used a simple motor task to determine if pallidotomy leads to an improvement in 'on' motor performance when those movements are impaired before surgery. Methods: Nine patients with PD performed elbow flexion movements 'as fast as possible' while they were 'on' before and I month after pallidotomy. Patients with mild PD and healthy control subjects were also tested. Results: The clinical effects of pallidotomy were typical of those found in other studies. 'Off' Unified Parkinson's Disease Rating Scale scores improved and dyskinesias were reduced. Although before surgery the patients were far slower while they were 'on' than the groups of mild PD patients and healthy control subjects, there was no change in mean peak velocity while they were 'on' after pallidotomy. There was no change in other mean 'on' motor performance measures such as peak acceleration, peak deceleration, initiation time, and symmetry. There was a decrease in the variability of peak acceleration, symmetry, and initiation time. Conclusion: Despite the clinical efficacy of pallidotomy while patients were 'off,' bradykinesia of elbow flexion movements while patients were 'on' is not affected by pallidotomy. Therefore, we conclude that the bradykinesia observed in this experiment is due to a mechanism other than excessive tonic inhibition of the motor thalamus. Our results are consistent with the idea that pallidotomy reduces the noise from the abnormally functioning basal ganglia.