Topodiagnostic significance of hemiataxia: a MRI based mapping analysis

Aim: To investigate the incompletely understood topodiagnostic significance of hemiataxia we performed a voxel-based statistical analysis of MRI in patients with acute brainstem infarction. Methods: Over a period of three years we prospectively recruited 49 patients with acute hemiataxia due to isolated unilateral brainstem infarction documented on high-resolution MRI following a fixed protocol. For statistical analysis individual MRI lesions were normalized and imported in a three-dimensional voxel-based model of the human brainstem. Results: Correlation analysis revealed an association of hemiataxia with lesions of two different sites: 1) the ipsilateral rostral and dorsolateral medulla oblongata affecting the ventral and dorsal spinocerebellar fibre tracts (p<0,001), 2) the contralateral ventral pons, especially when hemiataxia was accompanied by motor hemiparesis (p<0,001). Conclusions: Hemiataxia with lesions of the medulla oblongata was attributed to an involvement of the dorsal spinocerebellar tract, which conveys afferent information from the ipsilateral arm and leg. Based on animal studies, the ventral spinocerebellar tract conveys information of the contralateral leg alone. Therefore, the still intact contralateral ventral spinocerebellar tract only incompetely compensates for the ipsilateral loss of afferent input. Pontine lesions cause contralateral and not bilateral ataxia presumabely due to an interruption of the descending corticopontine projections that run in proximity to the corticospinal fibres up to the mid-pontine level. Moreover, lesions of the ipsilateral pontine nuclei projecting to the contralateral cerebellum may contribute to clinical hemiataxia. Already crossed pontocerebellar fibres emerging from the contralateral pons may be unaffected, because according to animal studies these projections run further dorsally through pontine areas that are usually spared by pontine ischemia.