A 58-year-old man presents with acute onset right-sided facial droop, right ptosis, and left-sided hemiparesis. MRI brain shows an acute infarct in the midbrain. Which is the most common site of vascular occlusion in midbrain stroke?

## Vascular Supply of the Midbrain **Key Point:** The midbrain is supplied by penetrating branches of the posterior cerebral artery (PCA), which are the most common source of midbrain infarction. ### Anatomical Basis The midbrain receives its blood supply from: 1. **Penetrating branches of PCA** — supply the medial midbrain (including oculomotor nerve fascicles and cerebral peduncle) 2. **Superior cerebellar artery (SCA) perforators** — supply the lateral and dorsal midbrain 3. **Basilar artery perforators** — small contribution to ventral midbrain ### Clinical Correlation The patient's presentation of **Weber syndrome** (ipsilateral CN III palsy + contralateral hemiparesis) localizes to the ventromedial midbrain, which is classically supplied by PCA penetrating branches. Occlusion of these vessels is the most common cause of midbrain stroke. **High-Yield:** PCA perforator occlusion accounts for >70% of midbrain infarcts. The PCA penetrating branches arise from the proximal P1 segment and supply the medial midbrain tegmentum. ### Differential Vascular Territories | Artery | Territory | Clinical Syndrome | | --- | --- | --- | | **PCA perforators** | Medial midbrain (CN III, cerebral peduncle) | Weber syndrome | | **SCA perforators** | Lateral midbrain (spinothalamic, trigeminal) | Nothnagel syndrome | | **Basilar perforators** | Ventral midbrain | Rare; usually combined with pontine infarction | **Clinical Pearl:** Weber syndrome (ipsilateral oculomotor palsy + contralateral hemiparesis) is the hallmark of medial midbrain infarction from PCA perforator occlusion.