Hemiballismus and the subthalamic nucleus
Hemiballismus (flinging of one side of the body) is a rare movement disorder caused by damage to the subthalamic nucleus, a small nucleus just below the thalamus.
Remember
Sudden flinging movements of one side of the body, often after a small deep stroke, point to damage to the subthalamic nucleus on the opposite side.
Hemi means one side and ballismus means flinging, as if throwing a ball. The subthalamic nucleus (STN) is a small nucleus (a group of nerve cell bodies) lying just below the thalamus, and it belongs to the basal ganglia, a group of nuclei deep within the brain.
The STN normally excites (drives) the globus pallidus internus (GPi), the main output of the basal ganglia. The GPi restrains the thalamus, so it acts as a brake on the thalamus and, through it, on the motor cortex (the part of the cortex that sends the commands that move muscles).
When a small stroke destroys the STN, the GPi is no longer driven, so it restrains the thalamus less. The brake comes off: the thalamus and motor cortex become overactive, and sudden, violent, flinging movements follow. These movements are irregular and cannot be suppressed.
Because the basal ganglia circuits influence the opposite side of the body, the flinging appears on the side opposite to the damage (contralateral). Left-sided flinging therefore means the right STN is damaged. The classic cause is a lacunar stroke (a small stroke caused by blockage of a tiny artery deep in the brain), and the disorder is very rare. Damage to the putamen (another part of the basal ganglia) can occasionally cause flinging movements too, but the subthalamic nucleus is the classic and most direct site.
Key facts
- Cause
- damage to the subthalamic nucleus, often a lacunar (small deep) stroke.
- Movements
- sudden, violent, flinging, unilateral and not suppressible.
- Side
- opposite to the damage (contralateral).
- Normal circuit
- the subthalamic nucleus excites the globus pallidus internus, which restrains the thalamus.
- After damage
- the globus pallidus internus is excited less, so it restrains the thalamus less and the thalamus overdrives the motor cortex.
- Rarity
- a very rare movement disorder.
Exam traps
- Damage to the subthalamic nucleus on the same side as the flinging limbs does not fit, because hemiballismus follows damage on the opposite side: flinging of the right arm and leg means the left subthalamic nucleus is damaged.
- Damage to the globus pallidus internus on both sides would affect both sides of the body, so it does not explain flinging confined to one side, and the globus pallidus internus is also not the classic site of damage.
- Damage to one side of the cerebellum causes incoordination on the same side as the damage, not violent flinging.
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