Ventral midbrain lesion with contralateral hemiparesis and ipsilateral CN III palsy corresponds to which syndrome?

• A. Weber's Syndrome
• B. Locked-In Syndrome
• C. Thalamic Syndrome
• D. Top of Basilar Syndrome

Answer: (A)

This pattern points to anterior midbrain (Weber’s) syndrome. A lesion in the ventral midbrain affects the cerebral peduncle where the motor corticospinal tract runs, producing contralateral hemiparesis. It also damages the fibers of the oculomotor nerve as they pass through the ventral midbrain, causing ipsilateral CN III palsy with the typical “down-and-out” eye, ptosis, and often a dilated pupil due to parasympathetic fiber involvement. The combination of contralateral motor weakness and ipsilateral oculomotor palsy is classic for Weber’s syndrome, usually from occlusion of the paramedian branches of the basilar/posterior cerebral circulation. Other syndromes involve different locations (for example, a ventral pons lesion causes Locked-In syndrome; thalamic or top-of-basilar lesions produce distinct sensory, gaze, or consciousness problems).