A 68-year-old man suddenly develops violent, flailing, involuntary movements of his left upper and lower limbs without weakness or sensory loss. MRI shows a small haemorrhage in the right diencephalon. The structure involved is:

• A Substantia nigra, pars compacta
• B Ventrolateral nucleus of thalamus
• C Globus pallidus, external segment
• D Subthalamic nucleus ✓

Explanation

Contralateral hemiballismus classically follows a lesion of the subthalamic nucleus, most often a small haemorrhage from the posterior communicating or posterior cerebral artery perforators. Loss of subthalamic excitation of the globus pallidus interna disinhibits the thalamus, producing large amplitude proximal choreiform movements. Substantia nigra lesions produce parkinsonism rather than ballism, and pure thalamic lesions give sensory loss.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.