A 76-year-old woman with hypertension and coronary artery disease is brought to the emergency department after the sudden onset of right-sided weakness. Her pulse is 83/min and blood pressure is 156/90 mm Hg. Neurological examination shows right-sided facial drooping and complete paralysis of the right upper and lower extremities. Tongue position is normal and she is able to swallow liquids without difficulty. Knee and ankle deep tendon reflexes are exaggerated on the right. Sensation to vibration, position, and light touch is normal bilaterally. She is oriented to person, place, and time, and is able to speak normally. Occlusion of which of the following vessels is the most likely cause of this patient's current symptoms?

Contralateral lenticulostriate artery

Ipsilateral anterior cerebral artery

Contralateral middle cerebral artery

Ipsilateral posterior inferior cerebellar artery

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 68-year-old man comes to the physician because of double vision and unilateral right eye pain that began this morning. His vision improves when he covers either eye. He has hypertension, mild cognitive impairment, and type 2 diabetes mellitus. The patient has smoked two packs of cigarettes daily for 40 years. His current medications include lisinopril, donepezil, metformin, and insulin with meals. His temperature is 37°C (98.6°F), pulse is 85/minute, respirations are 12/minute, and blood pressure is 132/75 mm Hg. His right eye is abducted and depressed with slight intorsion. He can only minimally adduct the right eye. Visual acuity is 20/20 in both eyes. Extraocular movements of the left eye are normal. An MRI of the head shows no abnormalities. His fingerstick blood glucose concentration is 325 mg/dL. Further evaluation is most likely to show which of the following?

Positive swinging-flashlight test

Communicating arteries for USMLE Step 2 CK: High-Yield Anatomy Lessons

Anterior Communicating Artery - The Frontal Link

Function: A short vessel that connects the left and right Anterior Cerebral Arteries (ACAs), completing the anterior part of the Circle of Willis.
Location: Situated superior to the optic chiasm.

Circle of Willis arterial diagram with key arteries labeled

Clinical Significance:
- Most common site for saccular (berry) aneurysms.
- Rupture leads to subarachnoid hemorrhage (SAH).
- Compression from an aneurysm can cause:
  - Visual defects: Bitemporal hemianopia (due to pressure on the optic chiasm).
  - Frontal lobe syndrome: Personality changes, abulia (lack of initiative), and executive dysfunction.

⭐ High-Yield: Aneurysms of the AComm are the most frequent cause of subarachnoid hemorrhage in the Circle of Willis. Sudden, severe "thunderclap" headache is a classic presentation.

Posterior Communicating Artery - The Side Connector

Bridge Function: Connects the anterior circulation (Internal Carotid Artery) to the posterior circulation (Posterior Cerebral Artery).
Anatomical Course: Travels posteriorly from the ICA, passing superior to the oculomotor nerve (CN III), to anastomose with the PCA.
Key Branches: Gives rise to small thalamoperforating arteries.
- Supply: Thalamus (anterior and ventral nuclei), hypothalamus, optic chiasm, and mammillary bodies.
Clinical Pearl: Common site for berry aneurysms.

⭐ A classic exam scenario involves a PCom aneurysm compressing the ipsilateral oculomotor nerve (CN III). This leads to a fixed, dilated pupil (“blown pupil”), ptosis, and a “down and out” gaze due to unopposed CN IV and VI action.

Circle of Willis with key arteries labeled

Aneurysms & Strokes - When Links Break

Berry (saccular) aneurysms: Congenital weaknesses in arterial walls, predisposing to rupture. They are the most common cause of non-traumatic subarachnoid hemorrhage (SAH).
- Anterior Communicating Artery (AComm): Most common site. Rupture can cause SAH, while compression of the optic chiasm may lead to bitemporal hemianopia.
- Posterior Communicating Artery (PComm): Aneurysms at the junction with the internal carotid artery can compress the oculomotor nerve (CN III).

⭐ A PComm aneurysm classically presents with a painful CN III palsy: the eye is deviated "down and out," with ptosis and a fixed, dilated pupil (mydriasis) due to compression of superficial parasympathetic fibers.

Brain Aneurysm and Circle of Willis

High‑Yield Points - ⚡ Biggest Takeaways

The Anterior Communicating Artery (ACommA) is the most common site of saccular (berry) aneurysms.

The Posterior Communicating Artery (PCommA) is the second most common site.

Rupture of these aneurysms is a major cause of subarachnoid hemorrhage (SAH).

ACommA aneurysms can compress the optic chiasm, causing bitemporal hemianopia.

PCommA aneurysms can cause CN III palsy, with a fixed, dilated pupil and a “down and out” eye.

They provide critical collateral circulation between hemispheres and circulations.

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Communicating arteries