[Elemental deficits underlying cerebellar ataxia]

Since classical studies by Babinski and Holmes, various symptoms have been described as cerebellar ataxia. Then, what are elemental factors underlying such symptoms? Here three pathophysiological characteristics are stressed. First, spatial incoordination in muscle activations occurs commonly in various limb movements and gaits, resulting in jerky and irregular movements. Second, deficits in anticipation lead to loss of accuracy and smoothness in limb movements, for most of movements are carried out at a high speed without visual feedback, and anticipatory and preprogrammed control by the cerebellum is necessary. Third, adaptation in reflexes is disordered. The gain of stretch reflexes during standing are exaggerated in patients with cerebellar ataxia compared with controls, causing large sways during walking. Among these elemental characteristics, dysfunction in anticipation and adaptation can be well explained by internal model theory, a hypothesis based on synaptic plasticity in the cerebellar circuits.