Long-term effects of deep brain stimulation for movement disorders: a literature-based analysis

Abstract

Deep brain stimulation has become an established therapy for various movement disorders but questions regarding its long-term effectiveness remain. This study was designed to evaluate the long-term effectiveness of deep brain stimulation for movement disorders refractory to current medical therapy based on published long-term studies. A review was carried out of all available studies with a minimum follow-up of 5 years of patients with deep brain stimulation for Parkinson's disease, essential tremor and dystonia. A total of 23 studies of deep brain stimulation for Parkinson's disease, 7 studies for essential tremor and 14 studies for dystonia were included. After a follow-up of at least 5 years, improvement of current motor scores could be observed in Parkinson's disease (subthalamic stimulation) by approximately 40%, by approximately 50% for essential tremor and by 60% for dystonia (mostly generalized forms). In Parkinson's disease, motor improvements tend to diminish over time due to progression of dysarthria, axial symptoms and other motor features less responsive to deep brain stimulation. Non-dopaminergic symptoms tend to progress and lessen the positive effects on the quality of life. There appears to be a subgroup of patients with essential tremor who show decreasing effectiveness of deep brain stimulation, probably related to disease progression. Currently, no single prognostic marker has been established to identify this subgroup. Most forms of secondary dystonia seem to respond more variably than primary generalized dystonia. Deep brain stimulation remains a relatively safe and effective therapy in carefully selected patients after long-term follow-up according to published data, although disease progression and other disease-specific factors seem to modify its effectiveness over time.