During stereotaxy in nonanesthetized patients with Parkinson's disease or essential tremor, tremor was suppressed by test stimulation of the thalamic ventralis intermedius nucleus (Vim) at high frequency (130 Hz). Based on this finding, the authors have used chronic stimulation of the Vim as a permanent treatment of tremor for the past 10 years in 134 patients with movement disorders (Parkinson's disease, 91; essential tremor, 23; various dyskinesias and dystonias, 20). Based on experimental data, this method has been extended in the past 4 years to two other targets, the subthalamic nucleus (STN) (51 patients) and the internal pallidum (GPi) (12 patients). Stereotactically implanted chronic electrodes were connected to a programmable stimulator. Results depended on the indication and on the target. Parkinson's disease was the best indication, and, depending on the accurate placement of the electrode, tremor was totally controlled by Vim stimulation (which has no effect on akinesia and rigidity) in 85% of patients. Stimulation of the GPi was extremely effective on levodopa-induced dyskinesias but only moderately effective on the other symptoms. Stimulation of the STN, however, controlled all three symptoms and allowed the decrease or suppression of levodopa, which then alleviated dyskinesias. In other types of movement disorders, including dystonia, these targets were not thoroughly evaluated for deep brain stimulation The stimulation of any of the three targets induced minor side effects, which were well tolerated, adjustable, and immediately reversible. There was no operative mortality, and permanent morbidity was observed in 3 of 197 patients, including all targets. The mechanisms of action of stimulation are not fully understood and are probably different depending on the target. In all cases, however, inhibition of cellular activity or of neural network function could be induced by jamming a retroactive loop in the Vim or by shutting down neurotransmitter release in the STN and the GPi. Mechanisms within the same target also are probably different for suppression of tremor or other symptoms. Chronic stimulation of these three targets, which is reversible, adaptable, and well. tolerated even in bilaterally operated (119 of 197) and elderly patients, has been validated by several years' use in hundreds of patients around the world and should replace ablative methods in the regular surgical treatment of Parkinson's disease and essential tremor. The low rate of permanent complications, the minor side effects (when any) and their immediate reversibility, the possibility of bilateral implantation in the same session, and the long-term persistence of symptom relief support the use of chronic stimulation of the Vim, the STN, or the GPi when surgery is indicated for the treatment of Parkinson's disease and essential tremor, and even more so when a bilateral procedure is necessary. STN stimulation, however, is Likely to become the first choice because of its efficacy in treating all major symptoms of Parkinson's disease.
