Carotid artery occlusive disease most often produces transient ischemic attacks or stroke by which of the following mechanisms?

B. Embolization of atheromatous debris and/or clot with occlusion of intracranial branches of the carotid artery.

DISCUSSION: The collateral network to the brain is extensive. Collateral flow to an area supplied by a carotid artery is provided by the contralateral carotid artery and vertebrobasilar system around the circle of Willis, by the external carotid artery branches around the eye, and by direct intracerebral connections between the anterior cerebral arteries. For these reasons, low-flow cerebral symptoms are extremely rare, even when carotid lesions are present bilaterally. Approximately 70% of all cerebral symptoms produced by carotid artery occlusive disease are embolic in origin. The surface of an atherosclerotic lesion at the carotid bifurcation is thrombogenic and acts as a nidus for the accumulation of platelet-rich thrombi. Similarly, the interior of the plaque can degenerate and rupture into the lumen, embolizing its contents into the distal bed of the carotid circulation. The final event in the progression of an atherosclerotic carotid lesion is total thrombosis. Because there are no extracranial branches of the internal carotid artery, the thrombus propagates distally. If the propagation of the clot stops at the first major intracranial branch (the ophthalmic artery) and does not disturb collateral flow through it, no cerebral ischemic event may occur. When propagation continues into the intracranial branches, a massive stroke may occur. This mechanism accounts for a minority of strokes.