Atherosclerotic Plaque Rupture and Thrombosis

Core Idea

Acute coronary syndromes and strokes typically result from rupture of unstable atherosclerotic plaques, exposing prothrombotic lipid core and tissue factor to blood, triggering platelet aggregation and thrombin generation. Unstable (vulnerable) plaques have thin fibrous caps, large lipid cores, and abundant macrophages producing matrix metalloproteinases that weaken the cap. Plaque rupture is often triggered by hemodynamic stress (high shear), inflammation, or hemorrhage within the plaque. Subsequent thrombosis causes acute vessel occlusion and distal ischemia.

How It's Best Learned

Compare plaque morphology in stable angina (thick fibrotic cap, small lipid core) versus unstable angina/MI (thin cap, large lipid core). Understand how macrophage-derived foam cells in the lipid core secrete proteolytic enzymes destabilizing the cap.

Common Misconceptions

Atherosclerosis severity does not directly predict acute events; moderate plaques often rupture while severe concentric stenosis may be hemodynamically limiting but stable. Thrombotic occlusion in acute MI is not always complete; some restore flow (spontaneous thrombolysis) or collateral supply maintains viability.

Explainer

From your study of atherosclerosis pathophysiology, you know how plaques form: lipid-laden macrophages (foam cells) accumulate in the intima, a fibrous cap of smooth muscle cells and collagen forms over the lipid core, and the plaque grows to narrow the vessel lumen over decades. From thrombosis pathophysiology, you understand the coagulation cascade: vessel wall disruption exposes subendothelial tissue factor, activating the extrinsic pathway, and platelet adhesion amplifies clot formation. Plaque rupture is the event that connects these two processes — the moment a silent, years-long atherosclerotic lesion becomes an acute, life-threatening occlusion.

The critical distinction is between stable plaques and vulnerable (unstable) plaques. A stable plaque has a thick fibrous cap, a small lipid core, and few inflammatory cells. It may cause significant luminal narrowing — producing stable angina on exertion — but it is mechanically durable. A vulnerable plaque has the opposite architecture: a thin fibrous cap (often <65 µm), a large lipid-rich necrotic core, and abundant macrophages at the cap's shoulder regions. These macrophages are the biological weak point. Activated macrophages secrete matrix metalloproteinases (MMPs) — collagenases and gelatinases that degrade the fibrillar collagen giving the cap its tensile strength. As collagen is degraded faster than smooth muscle cells can replace it, the cap thins and weakens. The counterintuitive clinical reality is that a 40% stenotic vulnerable plaque poses a greater acute risk than a 70% stenotic stable plaque. Angiography reveals the stenosis but cannot detect the cap thickness or the inflammatory activity that determines rupture risk.

When a vulnerable plaque ruptures, its lipid-rich core is exposed to flowing blood. The core is laden with tissue factor — a potent activator of the extrinsic coagulation pathway — and the collapse of the physical barrier allows platelets to adhere to exposed collagen and subendothelial matrix proteins. Platelet activation triggers release of ADP and thromboxane A2, amplifying aggregation and recruiting additional platelets. Simultaneously, the coagulation cascade generates thrombin, which converts fibrinogen to fibrin and cross-links the growing clot. The result is an occlusive thrombus that forms within minutes, blocking blood flow downstream. The clinical syndrome depends on which artery is occluded and whether occlusion is complete: partial occlusion or spontaneous thrombolysis produces unstable angina or NSTEMI; complete occlusion sustained beyond 20 minutes causes STEMI with transmural infarction.

Plaque rupture is precipitated by mechanical and biological triggers. High shear stress at arterial branch points and bends concentrates hemodynamic force at the cap's shoulder — the thinnest and most macrophage-rich zone. Acute surges in sympathetic tone (physical exertion, emotional stress, cold exposure, early morning awakening) increase heart rate and blood pressure, increasing shear stress precisely when the vasomotor system is least protected. Intraplaque hemorrhage — from fragile new vessels growing into the lipid core — can rapidly expand plaque volume and tear the cap from inside. This explains the epidemiological pattern of MI clustering in the early morning hours and following acute psychological stress, patterns that seemed puzzling before the pathophysiology of plaque rupture was understood.