Neurophysiologic Assessment of ET Patients Treated by Vim DBS

Essential tremor is a chronic and progressive neurological disease characterized by upper limb tremor. This is one of the most frequent movement disorders. Most of the time the disease worsens over the time, affecting patients' work abilities and in the most severe cases activities of daily living such as eating or dressing. For the most disabled patients, Deep brain stimulation (DBS) of the thalamic ventral intermediate median nucleus (Vim), a procedure consisting in an electrode implantation in a structure of the brain involved in tremor genesis, is the gold standard treatment. While this therapy is most of the time highly effective in alleviating the tremor, some subjects may exhibit gait impairment or upper limb coordination troubles years after the surgery, which are thought to be due to the involuntary stimulation of efferent cerebellar fiber tract. Unfortunately, this DBS induced side effect cannot be systematically avoided and may limit the possibilities of settings adaptation required to control the tremor. Surprisingly, while it could be a valuable therapeutic option for these patients suffering from DBS induced balance troubles, little is known about the effect of varying the rate of stimulation on the gait disorders associated with essential tremor and Vim DBS. The aim of or study is consequently to assess the effect of different frequency of stimulation on tremor, gait and balance disorders as well as on eye movements in patients uni or bilaterally stimulated in the Vim for a severe and medically intractable essential tremor. Patients followed at the National Hospital for Neurology and Neurosurgery (University College London Hospital) will be included. To better characterize the different symptoms, the investigators will use ataxia and tremor rating scale together with 3D gait motion analysis, oculography and computerized spiral test analysis. Our findings might lead to a better understanding of Vim-DBS associated gait disorders in essential tremor.