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 inhi bition of the thalamus, thereby allowing movement to proceed more norm ally. If true, then PD patients who move slowly while on medication sh ould increase movement speed following pallidotomy. To test this we us ed a simple motor task to determine if pallidotomy leads to an improve ment in ''on'' motor performance when those movements are impaired bef ore surgery. Methods: Nine patients with PD performed elbow flexion mo vements ''as fast as possible'' while they were ''on'' before and 1 mo nth after pallidotomy. Patients with mild PD and healthy control subje cts were also tested. Results: The clinical effects of pallidotomy wer e typical of those found in other studies. ''Off'' Unified Parkinson's Disease Rating Scale scores improved and dyskinesias were reduced. Al though before surgery the patients were far slower while they were ''o n'' 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 performan ce measures such as peak acceleration, peak deceleration, initiation t ime, and symmetry. There was a decrease in the variability of peak acc eleration, symmetry, and initiation time. Conclusion: Despite the clin ical efficacy of pallidotomy while patients were ''off,'' bradykinesia of elbow flexion movements while patients were ''on'' is not affected by pallidotomy. Therefore, me conclude that the bradykinesia observed in this experiment is due to a mechanism other than excessive tonic i nhibition of the motor thalamus. Our results are consistent with the i dea that pallidotomy reduces the noise from the abnormally functioning basal ganglia.