Mechanisms of plaque vulnerability and rupture.

Rupture of atherosclerotic plaque has been identified as the proximate event in the majority of cases of acute ischemic syndromes. Plaque rupture exposes thrombogenic components of the plaque, activating the clotting cascade and promoting thrombus formation. Future culprit lesions are difficult to identify, however, and angiographic assessment of stenosis severity is prone to underestimation. Compared with plaques that cause severe luminal stenosis, vulnerable plaques may cause relatively minor stenosis, although they account for more cases of rupture and thrombosis. Such unstable, vulnerable plaques may be associated with outward remodeling of the vessel. Because severely stenotic plaques are more likely to stimulate collateral circulation to the post-stenotic segment, plaque rupture and thrombosis at such sites may be clinically silent. Characteristic histomorphologic features of vulnerable plaques include a high lipid content, increased numbers of inflammatory cells, and extensive adventitial and intimal neovascularity. The fibrous cap of an atherosclerotic plaque may become thin and rupture as a result of the depletion of matrix components through the activation of enzymes, such as matrix-degrading proteinases and cystine and aspartate proteases, and through the reduction in the number of smooth muscle cells. Activated T cells may also inhibit matrix synthesis through the production of interferon-gamma. A number of triggers of plaque rupture have been identified. Also, some thrombi may occur without rupture of the fibrous cap. Reducing the lipid component and inflammation in atherosclerotic plaques may help reduce the risk of plaque rupture. This may account for the clinical benefit of risk-factor reduction gained from changes in lifestyle and from drug therapy.