A 62-year-old woman comes to the physician for decreased vision and worsening headaches since this morning. She has hypertension and hypercholesterolemia. Pulse is 119/min and irregular. Current medications include ramipril and atorvastatin. Ocular and funduscopic examination shows no abnormalities. The findings of visual field testing are shown. Which of the following is the most likely cause of this patient's symptoms?

Occlusion of the posterior cerebral artery

Occlusion of anterior cerebral artery

Occlusion of the anterior inferior cerebellar artery

Impaired perfusion of the retina

A 75-year-old man is brought to the emergency room after being found unresponsive in his home. His medical history is unknown. On physical examination he does not demonstrate any spontaneous movement of his extremities and is unable to respond to voice or painful stimuli. You notice that he is able blink and move his eyes in the vertical plane. Based on these physical exam findings, you expect that magnetic resonance angiogram will most likely reveal an occlusion in which of the following vessels?

Anterior inferior cerebellar artery

A 38-year-old woman with moyamoya disease undergoes cerebral angiography showing bilateral progressive stenosis of distal internal carotid arteries with extensive collateral formation. She has had multiple TIAs despite medical management. CT perfusion shows reduced flow in bilateral MCA territories. The neurosurgery team debates between direct bypass (STA-MCA) versus indirect revascularization (EDAS). Evaluate the optimal approach considering Circle of Willis pathophysiology.

Combined direct and indirect approaches optimize collateral development regardless of Circle anatomy

Indirect revascularization allows gradual angiogenesis; better for incomplete Circle variants

Circle of Willis anatomy is irrelevant; external to internal carotid bypass is contraindicated in moyamoya

Observation only; intact Circle of Willis through ACoA and PCoA provides adequate collaterals

Direct bypass provides immediate flow; superior for adults with completed Circle

A 70-year-old man undergoes elective clipping of an unruptured basilar tip aneurysm. Preoperative angiography shows bilateral fetal-type posterior cerebral arteries (PCAs arising from internal carotid arteries) with hypoplastic P1 segments. The aneurysm involves both posterior communicating artery origins. Synthesize an approach to surgical planning that optimally preserves cerebral perfusion.

Clip reconstruction technique preserving both PCoA origins with intraoperative flow assessment

Staged bilateral STA-PCA bypasses followed by aneurysm clipping with PCoA sacrifice

Standard clip placement sacrificing one PCoA; contralateral PCoA provides adequate flow

Bypass surgery from external carotid to PCA prior to aneurysm clipping

Endovascular coiling instead of clipping to preserve both PCoA vessels

Vertebrobasilar system for USMLE Step 1: High-Yield Anatomy Lessons

Vertebral Artery - The Twin Lifelines

Origin: Arise as the first major branches of the subclavian arteries.
Course: Ascend through the transverse foramina of the C6-C1 vertebrae before entering the skull through the foramen magnum.
Key Branches (before forming the basilar artery):
- Posterior Inferior Cerebellar Artery (PICA): Supplies the lateral medulla & inferior cerebellum.
- Anterior Spinal Artery (ASA): Supplies the anterior 2/3 of the spinal cord.

Vertebral artery segments and vertebrobasilar system

⭐ The vertebral arteries are asymmetric in ~75% of people, with the left artery usually being dominant.

Basilar Artery - Brainstem's Powerhouse

Formation: Arises from the union of the two vertebral arteries at the pontomedullary junction, ascending along the anterior aspect of the pons.
Key Branches: Supplies the brainstem and cerebellum. Main branches from inferior to superior are:
- Anterior Inferior Cerebellar Artery (AICA)
- Pontine Arteries (multiple small perforators)
- Superior Cerebellar Artery (SCA)
Termination: Bifurcates at the pontomesencephalic junction into the two Posterior Cerebral Arteries (PCAs).
Mnemonic (Branches): 📌 "A Pimp Sleeps" → AICA, Pontines, SCA.

⭐ Clinical Pearl: Acute, complete occlusion of the basilar artery results in 'locked-in syndrome'. Patients are conscious and aware but have quadriplegia and cannot speak (anarthria), with preserved vertical eye movements.

Circle of Willis and Vertebrobasilar System Diagram

Posterior Cerebral Artery - The Vision Vessel

Circle of Willis and Vertebrobasilar System

Origin: Arises as a terminal branch of the basilar artery.
Connection: Links to the internal carotid system via the posterior communicating artery (PComm), completing the Circle of Willis.
Segments & Supply:
- P1 (Pre-communicating): Supplies midbrain structures (e.g., crus cerebri, substantia nigra).
- P2 (Post-communicating): Gives rise to thalamoperforating arteries supplying the thalamus.
- P3/P4 (Cortical): Supply the visual cortex in the occipital lobe and the inferomedial temporal lobe.

⭐ Exam Favorite: PCA strokes classically cause contralateral homonymous hemianopia with macular sparing. The macula is often spared due to its dual blood supply from the middle cerebral artery (MCA).

Clinical Syndromes - When Flow Fails

Occlusion of specific arteries in the vertebrobasilar system leads to distinct neurological syndromes. Recognizing the artery-syndrome link is key for localization.

Artery	Syndrome	Key Features
PICA	Wallenberg (Lateral Medullary)	Dysphagia, hoarseness, ↓ pain/temp from ipsilateral face & contralateral body.
AICA	Lateral Pontine	Paralysis of face, ↓ lacrimation/salivation, ↓ taste from anterior ⅔ tongue.
Basilar	"Locked-in" Syndrome	Preserved consciousness, vertical eye movements, and blinking; quadriplegia.
PCA	Occipital Lobe Infarct	Contralateral hemianopia with macular sparing.

⭐ Wallenberg syndrome does not cause contralateral limb paralysis because the corticospinal tracts (pyramids) are spared in the lateral medulla.

High‑Yield Points - ⚡ Biggest Takeaways

The vertebrobasilar system is crucial for supplying the brainstem, cerebellum, and occipital lobes.

Vertebral arteries arise from the subclavian arteries and merge to form the single basilar artery.

PICA (Posterior Inferior Cerebellar Artery), the largest vertebral branch, is key in Wallenberg syndrome.

The basilar artery supplies the pons and cerebellum, terminating in the Posterior Cerebral Arteries (PCAs).

Complete basilar occlusion can cause devastating “locked-in” syndrome.

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