SummaryStereotactic thalamotomy of the VIM (ventral intermediate) nucleus is considered as the best neurosurgical treatment for Parkinsonian and essential tremors. However, this surgery, especially when bilateral, still presents a risk of recurrence and neurological complications.We observed that acute VIM stimulation at frequencies higher than 60 Hz during the mapping phase of the target suppressed the tremor of Parkinson's disease (PD) and essential tremor (ET). This effect was immediately reversible at the end of the stimulation. This was initially proposed as an additional treatment for patients already thalamotomized on the contralateral side, and then extended as a regular procedure for extra-pyramidal dyskinesias.Since January 1987, we implanted 126 thalami in 87 patients (61 PD, 13 ET, 13 dyskinesias of various origins). Deep brain stimulation electrodes were stereotactically implanted under local anaesthesia, using stimulation and micro-recording to delineate the best site of stimulation. Electrodes were subsequently connected to implantable programmable stimulators. The optimal frequency was around 130 to 185 Hz.The results (evaluated by a neurologist from 0 = no effect to 4 = perfect relief) are related to the type of tremor. Altogether, 71 % of the 80 patients benefited from the procedure with grade 3 and 4 results. In 88% of the PD cases, the results were good (grade 3) or excellent (grade 4) and stable with time. Rigidity was moderately for a long improved but akinesia was not. The same level of improvement was observed in 68% of the ET patients and only in 18% ofthe other types of dyskinesias. A rebound effect was observed in 30% of the ET patients in whom the long term results decreased. In all patients, adverse effects were mild and always reversible. There was no operative morbidity. The mechanism of action of electrical VIM stimulation is still unknown but could involve a jamming-based effect. However, the high rate of success, the extremely low morbidity, the reversibility and adaptability of chronic VIM stimulation makes this procedure safer than traditional thalamotomy, especially when bilateral surgery is indicated.

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