Thalamic, subthalamic nucleus and internal pallidum stimulation in Parkinson's disease.

The limits of drug therapy in severe forms of Parkinson's disease have lead to a renewal of functional neurosurgery of the basal ganglia and the thalamus. Deep brain stimulation (DBS) of these structures was developed with the aims of reducing the morbidity of surgery and of offering an adaptative treatment. DBS was first applied to the thalamus in patients with severe tremor. Tremor of the hemibody is greatly reduced by stimulation of the contralateral electrode in 85% of the cases. There is little change in other symptoms. However, motor fluctuations and dyskinesias are a more frequent problem than severe tremor; in attempt to treat these symptoms, DBS has recently been applied to the subthalamic nucleus (STN) and the internal pallidum (GPi). STN stimulation greatly decreases off motor symptoms and motor fluctuations, which allows a reduction of drug dosage and consequently of dyskinesias. GPi stimulation decreases dyskinesias in most patients, but the effect on off motor symptoms is more variable from one series to another, from very good to nil. The severe morbidity of DBS applied to these 3 targets is low. Comparative studies of the cost and the efficacy of DBS and lesions applied to these different targets are now required.