Subthalamic nucleus versus pedunculopontine nucleus stimulation in Parkinson disease: synergy or antagonism?

Stimulation of the subthalamic nucleus (STN) improves the cardinal features of Parkinson disease (PD). However, its efficacy on gait disorders is less satisfying in the long term. In recent years, the pedunculopontine (PPN) nucleus has emerged as a possible promising deep brain stimulation target for gait disorders in PD. In this review, we examine whether STN and PPN act synergistically or antagonistically. Results suggest that the combination of STN and PPN stimulations leads to a significant further improvement in gait as compared with STN stimulation alone, but additive effects on the classical motor triad are questionable. Thus, they highlight the specificity of STN stimulation over PPN's for the PD cardinal features and the specificity of PPN stimulation over STN for gait disorders. In addition, low-frequency stimulation of the PPN may improve alertness. The additive rather than potentiating effects of STN and PPN stimulations suggest that they may be mediated by distinct pathways. Nevertheless, considering the inconsistencies in published results regarding the influence of PPN stimulation on gait disorders, work is still needed before one can know whether it will convert into a standard surgical treatment and to decipher its place beside STN stimulation.