Poor coordination of movement, often due to cerebellar dysfunction (see CEREBELLUM) or a familial degenerative disorder of the sensory and motor pathways such as FREIDRICH’S ATAXIA. Ataxic movements are inaccurate in positioning, and/or defective in timing and patterning. By far the most common form of ataxia is caused by cerebellar dysfunction. The most common manifestation of this kind of ataxia is disordered finger-to-nose pointing, where an ataxic tremor (or INTENTION TREMOR) is seen during the movement, and tends to increase in magnitude as the target is approached. Additionally, hypermetric errors are often encountered—patients will overshoot target position. Ataxia is invariably worse when the patient is required to make rapid movements. Nevertheless cerebellar ataxia is also revealed in tasks which require manual or ocular tracking of a drifting target.
Ataxia of GAIT can be seen in patients with damage to cerebellar structures. The typical pattern in such patients is short steps from a broad base and a tendency to lean forward. Most models of ataxia suggest that a cerebellum plays a role in pre-programming the timing of agonist/ antagonist muscle activities before the movement is actually initiated. Abnormal patterns of braking may be consequent to disruption in the pre-programming phase, and lead to movement overshooting, undershooting, and additional abnormal programming of the corrective movements which the ataxic errors make necessary. Additionally, difficulties in processing proprioceptive feedback related to the ongoing movement may be a consequence of cerebellar dysfunction and could contribute to the presence and magnitude of ataxia. Other researchers have suggested that ataxic movements of cerebellar patients could be understood in terms of poor utilization of efference copy (feedforward) signals received from the MOTOR CORTEX. Poor guidance of a limb movement towards a target has been, rather inaccurately, named ‘OPTIC ATAXIA’ by Rezsö Balint (see BALINT’S SYNDROME). Optic ataxia is a consequence of bilateral or unilateral lesions of the PARIETAL LOBE.
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