posterior inferior cerebellar artery

**Anatomy / course:** PICA usually arises from the intracranial vertebral artery, just below the vertebrobasilar junction. It loops around the lateral medulla, passes near CN IX, X, and XI rootlets, and then supplies the inferior cerebellum. Its branches include the medial branch, which supplies the inferior vermis and the medial cerebellar hemisphere, and the lateral branch, which supplies the lateral cerebellar hemisphere. It also gives off the choroidal branch to the choroid plexus of the 4th ventricle. Anatomic variants are common. PICA may arise from the AICA in an AICA-PICA common trunk, or it may be hypoplastic. In some people the vertebral artery itself supplies the lateral medulla.

**Territory:** The lateral (dorsolateral) medulla, the inferior cerebellar vermis, the tonsils, and the posterior-inferior cerebellar hemispheres. The choroid plexus of the 4th ventricle is also in its territory.

**Lateral medullary (Wallenberg) syndrome:** This results from PICA or vertebral artery occlusion. Features, by structure: - Spinothalamic tract: contralateral loss of pain and temperature in the body. - Spinal trigeminal nucleus and tract: ipsilateral loss of pain and temperature in the face. - Nucleus ambiguus (CN IX, X): dysphagia, hoarseness, decreased gag reflex, and ipsilateral palate and vocal cord paralysis. - Vestibular nuclei: vertigo, nausea and vomiting, and nystagmus. - Inferior cerebellar peduncle and cerebellum: ipsilateral ataxia, dysmetria, and falling toward the side of the lesion. - Descending sympathetic fibers: ipsilateral Horner syndrome (ptosis, miosis, anhidrosis). - Mnemonic: "Don't pick a (PICA) horse (hoarseness) that can't eat (dysphagia)."

**Key points:** - Wallenberg syndrome spares the face from motor involvement. Weakness is absent because the corticospinal tract (the medullary pyramid) lies medially in the medulla and is supplied by the anterior spinal artery and vertebral branches. - Medial medullary syndrome involves the anterior spinal artery or vertebral artery. It causes contralateral hemiparesis, contralateral loss of proprioception and vibration, and ipsilateral tongue deviation from CN XII involvement. - Because the findings are crossed (ipsilateral face, contralateral body), they localize to the brainstem. - Cerebellar infarction in the PICA territory can cause edema and mass effect. This can compress the 4th ventricle, causing obstructive hydrocephalus, and can cause tonsillar herniation. It may require emergent suboccipital decompression. - Vertebral artery dissection (e.g., from neck manipulation or trauma) is an important cause in young patients. - The HINTS exam helps distinguish central vertigo (such as PICA stroke) from peripheral vertigo. Central signs are direction-changing nystagmus, a normal head impulse test, and skew deviation. - Comparison with other posterior circulation arteries: AICA occlusion causes lateral pontine syndrome, with ipsilateral facial paralysis and hearing loss (CN VII and VIII) in addition to the Wallenberg-like findings. SCA occlusion affects the superior cerebellum. Basilar artery occlusion can cause locked-in syndrome. - Cerebellar infarcts can present with isolated vertigo and gait ataxia and can be missed on early CT. MRI with DWI is more sensitive.