Thalamic tremor: correlations with three-dimensional magnetic resonance imaging data and pathophysiological mechanisms.

Tremor associated with a single focal thalamic lesion has rarely been reported. Furthermore, the exact localization of the lesions is difficult to determine because of the imprecision of "conventional" radiology (computed tomography scan and/or "standard" magnetic resonance imaging). The aim of this study was to identify which thalamic structures are involved in tremor associated with a single focal thalamic lesion. We selected two patients who presented with unilateral postural and kinetic tremor of the upper limb related to a localized thalamic infarction. Three-dimensional T1-weighted magnetic resonance imaging sequence (MP-RAGE sequence) was used to determine the precise topography of the lesions by stereotactic analysis using the atlas of Hassler. The lesions were located within the pulvinar, the sensory nuclei, the mediodorsal nucleus, and the ventral lateral posterior nucleus (according to the classification of Hirai and Jones), the latter including the ventral intermediate nucleus (Vim according to the classification of Hassler). However, the Vim was spared. The subthalamic area, which can induce tremor, was not involved. After having compared the topography of the lesions with the clinical findings, we suggest that thalamic tremors may result from the interruption of the cerebellar outflow tract to the Vim within the thalamus.