Tuberculomas
This is another form of tumor that can result in the brain as a result of a tuberculous meningitis infection. This type of tumor can also damage parts of the basal ganglia, sometimes resulting in hemiballismus. Demyelinating plaques
Demyelinating plaques attack the myelin sheaths on neurons. This decreases the conduction velocity of the neurons, making the signals received by the basal ganglia garbled and incomplete. This disorganized signal can also cause the chaotic movements characterized by hemiballismus. Complications from HIV infection
Patients with HIV often have complications that arise along with AIDS. Hypoglycemia due to pentamidine use in patients with AIDS has been known to cause hemiballismus. In some patients, hemiballismus has been the only visible symptom to alert the physician that the patients may have AIDS. It is typically a result of a secondary infection that occurs due to the compromised immune system and the most common infection causing hemiballismus is cerebral toxoplasmosis. Most of the lesions that result from this infection are found in the basal ganglia. As long as the diagnosis is not missed, this type of hemiballismus can be treated just as well as in patients without HIV. Anatomy
Basal ganglia
The basal ganglia are a collection of nuclei that connects to several other areas of the brain. Due to the diverse nuclei that they contain, the basal ganglia are involved in numerous functions, including motor control. It is within this structure that hemiballismus primarily occurs in the brain.
Subthalamic nucleus
This structure within the basal ganglia innervates other structures, including a very important connection to the internal globus pallidus. The subthalamic nucleus essentially provides the excitement needed to drive the globus pallidus. Injury to this area or its efferent or afferent connections can induce this disorder contralateral to the side of the lesion. The structure itself is a regulator of motor function and is also involved in associative and limbic functions. It was traditionally thought that the disorder was only caused by injury to the subthalamic nucleus, but later studies have shown that damage to other basal ganglia regions can also be responsible for causing this disorder. Hemiballismus caused by lesions in the subthalamic nucleus is more severe than other forms of the disorder. Globus pallidus
From recent studies, it is now thought that hemiballismus can be associated with a decreased output of the globus pallidus. This is because studies have shown that firing rates decrease from 70/s to 40/s. In addition to a decreased firing rate, degenerative neurological disorders that cause patients to exhibit hemiballistic movements show a marked decrease in the globus pallidus mass as well. Increases in activity in this area causes there to be an inhibition of the motor thalamus. This causes cortical activation and thus a movement inhibition. In the case of hemiballismus, the opposite occurs, leading to the characteristic large, irregular movements.