Hemiballismus is a neurological sign, a movement disorder, characterised by unilateral wild, large amplitude flinging movements of the arm and leg, normally causing falls and preventing postural maintenance.
It is caused by a lesion or infarction in the contralateral subthalamic nucleus or its connections, usually in patients with a history of hypertension or diabetes, or following TB meningitis. The subthalamic nucleus normally regulates the globus pallidus by exciting the GPi, which in turn normally inhibits the ventral anterior nucleus, ventral lateral nucleus and lateral dorsal nucleus of the thalamus (see thalamus). The reduced discharge in both causes disinhibition of the thalamus and consequent involuntary stimulation of the motor cortex. Altered dopaminergic feedback mechanisms may also be involved.
Chemical treatment is via dopamine blockade, or via surgical lesions to the globus pallidus, lenticular fasciculus or VL nucleus of thalamus. The prognosis is rather favourable both vitally and functionally
A demonstration of Hemiballismus is seen in you tube: http://www.youtube.com/watch?v=BsBFxhdGl0Q