Multiterritory Bihemispheric Ischemic Stroke in the Setting of Unilateral Internal Carotid Artery Disease.

Accurate determination of stroke mechanism is essential for secondary stroke prevention. Bilateral, multiterritory ischemic infarcts are commonly attributed to a central embolic source, prompting evaluation for cardioembolism, paradoxical embolism, or hypercoagulable states. However, this diagnostic approach assumes conventional intracranial vascular anatomy despite the high prevalence of Circle of Willis variants. We report the case of a 66-year-old man who presented with acute bilateral cortical and subcortical ischemic infarcts involving the right middle cerebral artery (MCA), right anterior cerebral artery (ACA), and left ACA territories. Although transthoracic echocardiography identified a patent foramen ovale (PFO), digital subtraction angiography (DSA) demonstrated a hypoplastic left A1 segment with the left ACA supplied by the right internal carotid artery (ICA) through a patent anterior communicating artery (ACom). This vascular configuration provided a plausible pathway by which artery-to-artery emboli arising from the ulcerated right proximal cervical ICA plaque could reach both hemispheres. The collective clinical and imaging findings supported the right ICA plaque as the most likely culprit source, prompting carotid endarterectomy. This case highlights the importance of incorporating individualized cerebrovascular anatomy into stroke mechanism determination and secondary stroke prevention.