[Standing ataxia in cerebellar lesions. Differential diagnosis and pathophysiology of disorders of postural control].

Several forms of postural ataxia can be distinguished in cerebellar lesions by quantitative analysis of stance, a paleocerebellar, a vestibulocerebellar and a neocerebellar syndrome, for each of which a different underlying pathomechanism was found. The pathognomonic anterior-posterior 3 Hz postural tremor in the paleocerebellar syndrome is caused by delayed and enhanced long latency reflexes, which result in an oscillation of the posture control system. In the beginning disease this oscillation can be provoked by rapid tilt or electrical stimulation of the tibial nerves. The oscillation frequency correlates inversely with the severity of the disease. In contrast to that, postural ataxia in the vestibulocerebellar syndrome is caused by a loss of intersegmental reflex patterns and a loss of the systems set value. Patients with the neocerebellar syndrome show remarkably less instability. Disturbances are often only provoked by voluntary postural movements as in tracking experiments, which suggests a defect in the interfacing of postural and voluntary motor activity. These cerebellar atactic syndromes may be separated from several forms of postural ataxias of extracerebellar origin.