What structure's damage is most consistent with sudden uncontrollable flinging movements of the right arm, likely due to hemiballismus?

Subthalamic Nucleus on the Left Side

The sudden uncontrollable flinging movements of the right arm are most consistent with damage to the subthalamic nucleus on the left side. This presentation is classic for hemiballismus, a dramatic hyperkinetic movement disorder that localizes to contralateral subthalamic pathology.

Anatomical Localization

• Hemiballismus is characterized by involuntary, violent, coarse, wide-amplitude flinging movements involving the ipsilateral arm and leg on one side of the body 1
• The movements are contralateral to the lesion, meaning right-sided movements indicate left-sided brain pathology 1, 2
• While classically attributed to subthalamic nucleus lesions, the subthalamic nucleus remains the primary structure associated with this movement disorder 1

Clinical Features Supporting This Diagnosis

• The sudden onset and uncontrollable flinging quality are pathognomonic for hemiballismus rather than other movement disorders 1
• These movements are typically large amplitude and ballistic in nature, distinguishing them from chorea or other dyskinesias 2
• The unilateral distribution (right arm only) confirms a focal contralateral lesion rather than bilateral basal ganglia pathology 1

Pathophysiology

• The subthalamic nucleus is part of the basal ganglia system involved in action selection and motor control 3
• Damage to this structure causes abnormal neuronal firing patterns in the internal segment of the globus pallidus, which is thought to be the mechanism underlying hemiballismus 1
• Stroke is the most common cause of subthalamic nucleus damage leading to hemiballismus, though nonketotic hyperglycemia and other etiologies can also be responsible 1, 2

Why Other Options Are Incorrect

• Putamen lesions typically cause dystonia, rigidity, or chorea rather than the ballistic flinging movements seen here 4
• Substantia nigra damage produces Parkinsonian features (resting tremor, bradykinesia, rigidity) rather than hyperkinetic ballistic movements 4
• Right-sided lesions would produce left-sided symptoms due to contralateral motor control 1

Clinical Implications

• The prognosis for hemiballismus is generally better than historically believed, with many patients experiencing spontaneous improvement over time 1, 2
• Treatment includes dopamine receptor blocking agents (haloperidol, risperidone) or catecholamine-depleting agents (tetrabenazine) for symptomatic control 2, 5
• Low-dose haloperidol has been successfully used in acute rehabilitation settings to reduce movement amplitude and frequency 5