Dystonia is a movement disorder characterized by involuntary, sustained, often repetitive muscle contractions of opposite muscles that lead to abnormal twisting movements or odd postures. Essential tremor is a slowly progressive neurologic disorder characterized by the appearance of a tremor during the voluntary movement. High frequency stimulation of the ventral intermedius nucleus (Vim) of the thalamus, relay for the cerebellar output, is successfully used for the treatment of severe essential tremor. It occasionally induces adverse event such as balance disorders or cerebellar symptoms. The pathophysiology of dystonia or essential tremor is not fully elucidated. Dystonia and essential tremor are associated with dysfunction of the sensorimotor basal ganglia-cortical network and involvement of the cerebellum and cerebellar pathways has also been recently suggested. It seems that dystonia and essential tremor could be the result of basal ganglia or cerebellar dysfunction, or from dysfunction of structures controlled at the same time by the cerebellum and the basal ganglia.
methodology: We propose to study 30 patients having a primary dystonia (15 DYT11 genetically documented), 15 patients having an essential tremor without deep brain stimulation and 15 patients having an essential tremor with deep brain stimulation.
A group of 30 healthy volunteers will be recruited and tested according to the same modalities. They will be paired in sex and age. 30 patients having a Parkinson disease will be also tested.
The subjects will be seated in darkness facing a screen located 60 cm before their eyes, their chin on a chin strap and their forehead placed against a frontal support. Eye position is sampled at 500 Hz with a video-based monocular eye tracker (SMI, Germany). Each recording session start with a calibration test in which the subjects looked at nine consecutive targets covering the entire visual field, as used during the oculomotor paradigms: four experimental conditions: a visually guided saccade task, a pre-test, a backward adaptation task, and a post-test. The pre-test and post-test (40 trials each) are performed before and immediately after the backward adaptation task, in the same conditions, except that the target was extinguished when the velocity threshold (150°/s for 10 ms) is reached, instead of jumping to a new location. This avoided any post-saccadic visual feedback that would counteract the adaptive mechanism. Saccade adaptation is evaluated as the percentage change in the mean saccade amplitude between the pre-test and post-test.
Expected results:
* no or fewer alteration of the performance to the adaptation task in the Parkinson group than in the Essential Tremor group/ dystonia group.
* abnormal reactive saccade backward adaptation in the Dystonia group and Essential Tremor group, providing further neurophysiological evidence of cerebellar dysfunction.
