Interest of Prior Relaxation on the Recordings of the SomatoSensory Evoked Potentials

Many factors influence the quality and duration of SomatoSensory Evoked potentials (SSEP). Some are related to the technique : type of electrodes, intensity of stimulation, stimulation site ; others are related to the patient : poor state of relaxation of the patient generating muscle artefacts; hypersensitivity of the patient to electrical stimuli making the examination unpleasant; examination time considered too long. However, the patient's relaxed state is essential to the quality of the signal . Thus the duration of an examination is very variable : between 30 and 75 minutes for the upper limbs.... For several months, the investigators provide relaxation to some patients just before the examination. Results seem to be positive. The investigators aim to study the effect of relaxation session on the quality of the SSEP recordings. Fifteen-minutes relaxation session will be provided by a nurse trained to the relaxation. The investigators will compare two groups : one group with one relaxation session before the examination and one group without relaxation session before the examination. The investigators chose to study the influence of relaxation session on (i) the artifacts rejection rate by the machine for the recordings of SSEP by stimulation of the median nerve to the upper limbs (ii) the duration of the examination (iii) the patient experience