A 58-year-old man presents with acute onset of vertigo, nystagmus, and ipsilateral limb ataxia following a sudden occipital headache. MRI shows an acute infarct in the territory of the posterior inferior cerebellar artery (PICA). Which of the following is the most common site of infarction within the cerebellum?

## Cerebellar Vascular Supply and PICA Territory **Key Point:** The posterior inferior cerebellar artery (PICA) is the most common artery to cause cerebellar infarction, accounting for approximately 50–60% of all cerebellar strokes. PICA supplies the inferior vermis, lateral hemispheres, and the dorsolateral medulla. ### Cerebellar Arterial Territories | Artery | Primary Territory | Clinical Syndrome | |--------|-------------------|-------------------| | PICA | Inferior vermis, lateral hemisphere, choroid plexus | Ipsilateral ataxia, vertigo, nystagmus | | AICA | Flocculus, nodulus, ventral pons | Vertigo, nausea, facial paresis | | SCA | Superior vermis, dentate nucleus, anterior lobe | Dysarthria, limb ataxia, tremor | **High-Yield:** PICA infarction is the most frequent cause of acute cerebellar stroke in clinical practice. The classic presentation includes ipsilateral cerebellar signs (ataxia, dysmetria) combined with ipsilateral Horner syndrome and contralateral spinothalamic loss (lateral medullary syndrome or Wallenberg syndrome when the medulla is involved). **Clinical Pearl:** The inferior vermis is critical for balance and midline coordination; its involvement produces truncal ataxia and vertigo. The lateral hemisphere infarction causes ipsilateral limb ataxia and dysmetria. **Mnemonic:** **PICA-AICA-SCA** — remember the rostro-caudal distribution: - **P**osterior **I**nferior → **I**nferior vermis and lateral hemisphere - **A**nterior **I**nferior → **F**locculus, nodulus (vestibular cerebellum) - **S**uperior → **S**uperior vermis, dentate nucleus (motor cerebellum) ### Why Inferior Vermis and Ipsilateral Hemisphere? PICA is the terminal branch of the vertebral artery and has the longest course within the cerebellum. It naturally perfuses the inferior and posterolateral cerebellar regions. Infarction in this territory produces the classic triad of vertigo, nystagmus, and ipsilateral ataxia seen in this patient. [cite:Robbins 10e Ch 28]