Is a patient with sudden onset headache, homonymous hemianopsia, and atrial fibrillation (AF) likely to have a clot in the Middle Cerebral Artery (MCA) or Posterior Cerebral Artery (PCA)?

Homonymous Hemianopsia with Sudden Headache and Atrial Fibrillation: Posterior Cerebral Artery Territory

This patient most likely has a clot in the Posterior Cerebral Artery (PCA), not the Middle Cerebral Artery (MCA). Homonymous hemianopsia is the defining localizing sign that points to the posterior visual pathway, specifically the occipital cortex supplied by the PCA.

Anatomic Localization

Homonymous hemianopsia localizes the lesion to the postchiasmal visual pathway, which includes the optic tract, lateral geniculate nucleus, optic radiations, or occipital cortex—all structures supplied by the PCA 1, 2.

• The PCA supplies the occipital lobe, which contains the primary visual cortex responsible for contralateral visual field processing 1, 2
• Isolated homonymous hemianopsia without other cortical signs (such as aphasia, neglect, or motor deficits) strongly suggests PCA territory infarction rather than MCA involvement 1
• MCA strokes typically present with motor and sensory deficits affecting the face and arm, with or without aphasia or neglect, but not isolated visual field defects 3

Cardioembolic Stroke Context

The presence of atrial fibrillation makes cardioembolic stroke the most likely mechanism, and emboli from AF can lodge in any cerebral vessel 4, 5.

• AF increases stroke risk through left atrial stasis and subsequent embolization to cerebral vessels 4
• While the American Heart Association notes that 55-77% of mycotic aneurysms in infective endocarditis occur in the MCA territory and 18% in the PCA territory, this patient's presentation lacks fever or other signs of endocarditis 3
• The sudden onset with headache and focal neurological deficit (hemianopsia) is consistent with acute embolic occlusion 5

Clinical Presentation Pattern

The combination of sudden headache with homonymous hemianopsia has specific diagnostic implications:

• Sudden severe headache with homonymous hemianopsia can indicate impending aneurysm rupture in the posterior circulation, though the American Heart Association specifically describes this pattern in the context of mycotic aneurysms affecting the MCA 3
• However, in the context of AF without signs of infection, acute ischemic stroke is far more likely than aneurysmal pathology 4, 5
• The American College of Radiology emphasizes that sudden onset headaches with neurological signs require immediate vascular imaging to distinguish ischemic from hemorrhagic causes 3

Critical Diagnostic Pitfall

Do not assume MCA involvement simply because the patient has AF-related stroke. The specific neurological deficit must guide anatomic localization:

• Isolated homonymous hemianopsia without motor, sensory, or language deficits points to PCA territory 1
• MCA strokes would be expected to produce contralateral hemiparesis, hemisensory loss, or cortical signs (aphasia if dominant hemisphere, neglect if non-dominant) 3
• Cerebral venous thrombosis can also present with headache and homonymous hemianopsia and should be considered if initial arterial imaging is unrevealing 2

Immediate Management Implications

• Urgent non-contrast CT head is essential to exclude hemorrhage before considering thrombolysis 3
• CT angiography or MR angiography should be performed to confirm PCA occlusion and assess for thrombolysis or thrombectomy candidacy 1
• Patients with isolated homonymous hemianopsia from posterior circulation stroke have been successfully treated with IV thrombolysis when imaging shows perfusion deficits 1
• For AF patients with acute ischemic stroke, anticoagulation timing is critical: initiating treatment 4-14 days after stroke onset (rather than earlier or later) is associated with reduced recurrence and bleeding risk 5