posterior communicating artery

**Anatomy** - Arises from the supraclinoid (communicating) segment of the internal carotid artery (ICA), just distal to the ophthalmic artery origin and proximal to the ICA bifurcation into the ACA and MCA. - Runs posteromedially to join the posterior cerebral artery (PCA), usually at the P1–P2 junction. The PCA is normally a branch of the basilar artery. - Together with the anterior communicating artery (AComm), it completes the Circle of Willis. The PComs connect the anterior (ICA) and posterior (vertebrobasilar) circulations on each side. - Lies close to the oculomotor nerve (CN III), which passes beneath/lateral to it as it runs in the interpeduncular cistern toward the cavernous sinus. - Gives off small perforating (thalamoperforating/anteromedial) branches supplying the hypothalamus, thalamus (anterior and ventral parts), and the posterior limb of the internal capsule.

**Function** - Provides collateral flow between the anterior and posterior circulations if one system is occluded or stenosed (e.g., ICA stenosis). - A "fetal-type PCA" is a common variant (about 20–30%). In it, the PCA is supplied mainly by the ICA via a large PCom, and the P1 segment is hypoplastic.

**Clinical significance** 1. *PCom aneurysm (berry/saccular aneurysm)* - The PCom–ICA junction is the 2nd most common site of berry aneurysms after the AComm (about 30–35% are AComm; PCom about 20–30%). The MCA bifurcation is third. - Presents with a CN III palsy from compression: - Ptosis - Eye "down and out" (unopposed lateral rectus and superior oblique) - Dilated, nonreactive pupil ("blown pupil"). Parasympathetic fibers run on the outside of CN III and are compressed first, so the pupil is involved early. - A painful CN III palsy with pupil involvement is a PCom aneurysm until proven otherwise. This is a neurosurgical emergency because the aneurysm may be expanding or about to rupture. - By contrast, diabetic (microvascular) CN III palsy is typically pupil-sparing, because the ischemia affects the central fibers. - Rupture causes subarachnoid hemorrhage (SAH): "worst headache of my life" (thunderclap), nuchal rigidity, and possibly loss of consciousness. Workup is a noncontrast CT head first, then LP for xanthochromia if CT is negative and suspicion remains. CTA or angiography is used to localize the aneurysm. Complications include vasospasm (days 3–14, prevented with nimodipine), rebleeding, and hydrocephalus. - Risk factors: ADPKD, Ehlers–Danlos, Marfan, coarctation of the aorta, hypertension, smoking, and family history. Anterior circulation aneurysms are the most common overall. 2. *Stroke and collateral flow* - A patent PCom can partially compensate for ICA or basilar occlusion. - With a fetal-type PCA, an ICA embolus or occlusion can cause a PCA-territory infarct (occipital lobe, contralateral homonymous hemianopia with macular sparing). 3. *Treatment of aneurysm*: surgical clipping or endovascular coiling.

**High-yield associations** - PCom aneurysm → CN III palsy - AComm aneurysm → most common berry aneurysm site; may cause bitemporal hemianopia/visual field defects - Berry aneurysm → SAH → "worst headache of life" - Pupil-involving CN III palsy → compressive lesion; pupil-sparing → microvascular (diabetes/HTN)

**Mnemonic**: "PCom pops CN III": a posterior communicating aneurysm compresses CN III, which gives a down-and-out eye with a blown pupil.