Pathology of the Vulnerable Plaque

Transcription

1 Journal of the American College of Cardiology Vol. 47, No. 8 Suppl C 2006 by the American College of Cardiology Foundation ISSN /06/$32.00 Published by Elsevier Inc. doi: /j.jacc Renu Virmani, MD,* Allen P. Burke, MD,* Andrew Farb, MD, Frank D. Kolodgie, PHD* Gaithersburg and Rockville, Maryland The majority of patients with acute coronary syndromes (ACS) present with unstable angina, acute myocardial infarction, and sudden coronary death. The most common cause of coronary thrombosis is plaque rupture followed by plaque erosion, whereas calcified nodule is infrequent. If advances in coronary disease are to occur, it is important to recognize the precursor lesion of ACS. Of the three types of coronary thrombosis, a precursor lesion for acute rupture has been postulated. The non-thrombosed lesion that most resembles the acute plaque rupture is the thin cap fibroatheroma (TCFA), which is characterized by a necrotic core with an overlying fibrous cap measuring 65 m, containing rare smooth muscle cells but numerous macrophages. Thin cap fibroatheromas are most frequently observed in patients dying with acute myocardial infarction and least common in plaque erosion. They are most frequently observed in proximal coronary arteries, followed by mid and distal major coronary arteries. Vessels demonstrating TCFA do not usually show severe narrowing but show positive remodeling. In TCFAs the necrotic core length is approximately 2 to 17 mm (mean 8 mm) and the underlying cross-sectional area narrowing in over 75% of cases is 75% (diameter stenosis 50%). The area of the necrotic core in at least 75% of cases is 3mm 2. These lesions have lesser degree of calcification than plaque ruptures. Thin cap fibroatheromas are common in patients with high total cholesterol (TC) and high TC/high-density lipoprotein cholesterol ratio, in women 50 years, and in those patients with elevated high levels of high sensitivity C-reactive protein. It has only recently been recognized that their identification in living patients might help reduce the incidence of sudden coronary death. (J Am Coll Cardiol 2006;47:C13 8) 2006 by the American College of Cardiology Foundation Patients with acute coronary syndromes (ACS) present with unstable angina, acute myocardial infarction, and sudden coronary death. Most of the ACS are thought to be the result of sudden luminal thrombosis (1 5). Luminal thrombosis occurs from three different pathologies: plaque rupture, erosion, and calcified nodules. Plaque rupture is defined as a lesion consisting of a necrotic core with an overlying thin ruptured fibrous cap that leads to luminal thrombosis because of contact of platelets with a highly thrombogenic necrotic core. Plaque erosion shows a luminal thrombus with an underlying base rich in proteoglycans and smooth muscle cells with minimal inflammation. Most erosion lesions are devoid of a necrotic core, but when present, the core does not communicate with the lumen because of a thick fibrous cap. The least common of all lesions is the calcified nodule. The calcified nodule shows an underlying calcified plate with superimposed bony nodules that result in discontinuity of the fibrous cap and is devoid of endothelial cells with overlying luminal thrombus. We have examined over 400 cases of sudden death that have been published in the last 15 years highlighting differences in type of thrombi; influence of race, age, and gender; and the role of risk factors on plaque morphologies (1 12). The frequency of thrombi in sudden death is 60% From the *CVPath, International Registry of Pathology, Gaithersburg, Maryland; and the U.S. Food and Drug Administration, CDRH-ODE-DCD-ICDB, Rockville, Maryland. The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or reflecting the views of the Department of the Army, the Department of the Air Force, or the Department of Defense. Dr. William A. Zoghbi acted as guest editor. Manuscript received June 16, 2005; revised manuscript received October 10, 2005, accepted October 24, with underlying etiology in 55% to 60% plaque rupture, in 30% to 35% plaque erosion, and in 2% to 7% calcified nodule. In the case of myocardial infarction, an autopsy study has shown that only 20% to 25% of acute myocardial infarcts occurring in hospitalized patients are due to plaque erosion (13). In both acute myocardial infarction and sudden death, plaque erosion occurs primarily in patients under the age of 50 years and represents the majority of acute coronary thrombi in premenopausal women. In women older than 50 years, 80% of coronary thrombi occur from plaque rupture; when they occur in women younger than 50 years, there is a strong association with hyperlipidemia. Plaque ruptures occur in men at all ages, but, as is the case with all types of thrombosis, their relative incidence in sudden coronary death decreases with advancing age. In 40% of sudden coronary death patients, no acute thrombi are observed; however, healed infarction and total occlusions are observed in the vast majority with only 15% dying of severe coronary narrowing in the presence or absence of cardiomegaly. PLAQUE RUPTURE AS THE BASIS OF ACS It has been postulated that thin cap fibroatheroma (TCFA), which resemble the plaque rupture in morphology, are the precursor lesion of plaque rupture. A necrotic core characterizes plaque rupture with an overlying thin-ruptured cap infiltrated by macrophages (Fig. 1). Smooth muscle cells within the cap are absent or few. The thickness of the fibrous cap near the rupture site measures m, with 95% of the caps measuring 65 m (1). It has been observed that some plaques at other sites in the coronary

2 C14 Virmani et al. JACC Vol. 47, No. 8 Suppl C April 18, 2006:C13 8 Abbreviations and Acronyms ACS acute coronary syndromes CRP C-reactive protein HDL high-density lipoprotein MPO myeloperoxidase TC total cholesterol TCFA thin cap fibroatheroma tree resemble the rupture plaque but lack a luminal thrombus: these lesions have been designated as TCFA or vulnerable plaques (4). The term vulnerable plaque should be reserved for plaques that resemble all three causes of luminal thrombosis, and these morphologies include TCFA, pathologic intimal thickening, thick cap fibroatheroma, and calcified plaque with luminal calcified nodules. The TCFAs differ from ruptured plaques (Table 1, Fig. 2), by having a smaller necrotic core (statistically different from ruptured plaques), less macrophage infiltration of the fibrous cap, and less calcification. We have quantitated, in cross sections of coronary arteries with various types of plaques, the size of the necrotic core, the proportion of the lesion composed of cholesterol clefts, the percent macrophage infiltration of the fibrous cap, the number of vasa vasorum within the atherosclerotic plaque, and the number of hemosiderin-laden macrophages (Table 2). The numbers of cholesterol clefts in the necrotic core, vasa vasorum, and hemosiderin-laden macrophages were significantly greater in the ruptured plaques than in erosion or stable plaques with 75% cross-sectional luminal narrowing. Significant differences between rupture and TCFAs were only seen for necrotic core size, macrophages, and hemosiderin infiltration. Plaque hemorrhages are more common at other sites of the coronary tree in cases with plaque ruptures than in hearts from patients dying with severe coronary disease without acute ruptures. The mean number of hemorrhages in the coronary tree of patients with plaque rupture was versus none in erosion and in stable plaques (4). Evidence of prior hemorrhage in TCFA, when analyzed by glycophorin A staining, is significantly greater than in early or late fibroatheromas and correlates with both the necrotic core size and extent of macrophage infiltration. LOCATION, LENGTH, AND PERCENT LUMINAL NARROWING OF THE TCFA In a detailed morphometric analysis of ruptured plaques, 80% of necrotic cores were larger than 1.0 mm 2, and in nearly 90%, the lipid core comprised 10% of the plaque area (Fig. 3). Furthermore, almost 65% of plaque ruptures had 25% area of the plaque occupied by the necrotic core. In contrast, nearly 75% of TCFA have 10% area of the plaque occupied by necrotic core. The mean cross-section area narrowing of the TCFA is 71%, and most have 10% to 25% of the cross sectional area occupied by a necrotic core. The length of the necrotic core in ruptures and TCFAs is similar, varying from 2 to 22.5 mm, with mean of 8 and 9 mm, respectively (Table 3)(6). In 38 hearts with severe coronary luminal narrowing, in which the coronary arteries had been serially cut from coronary ostium to intramyocardial location, the mean luminal narrowing was least in sections with thin cap atheroma (59.6%), intermediate for hemorrhage into a plaque (68.8%), and highest in plaque rupture (73.3%) or healed plaque rupture (72.8%) (6). Overall, approximately 75% of the arteries showed 75% cross-sectional luminal-narrowing, indicating that sites with 50% diameter stenosis are the most useful for the detection of vulnerable plaque. Over 50% of the TCFAs occur in the proximal portions of the major coronary arteries, left anterior, left circumflex and the right, and another one-third in the mid portion of these arteries, and the rest are distributed in distal segments (5). A similar distribution is found in ruptures and healed plaque ruptures. ROLE OF MONOCYTE INFILTRATION OF THE OCCLUSIVE THROMBUS The rupture of the fibrous cap allows platelets and inflammatory cells to come in contact with the thrombogenic substrate, the necrotic core. Before the report of Nemerson et al. (14), the core was thought to be the main source of the tissue factor. It is now believed that circulating monocytes, instead of plaque Figure 1. Coronary plaque rupture. (A) Low-power view of a circumferential coronary plaque with fibrous cap rupture. Note the large necrotic core with numerous cholesterol clefts. There is a focal disruption of a thin fibrous cap (arrow) with an occlusive luminal thrombus (Movat Pentachrome, 20). (B) High-power view of the rupture site showing fibrous cap disruption (arrows); the thrombus shows communication with the underlying necrotic core (Movat Pentachrome, 400).

3 JACC Vol. 47, No. 8 Suppl C April 18, 2006:C13 8 Virmani et al. C15 Table 1. Morphologic Characteristics of Plaque Rupture and TCFA Plaque Type Necrotic Core, % Fibrous Cap Thickness, m Macrophages, % SMCs, % T-Lymphocytes Calcification Score Rupture (n 25) TCFA (n 15) p value ns ns Mean values represent SD. Reprinted, with permission, from Kolodgie et al. (5). SMC smooth muscle cell; TCFA thin cap fibroatheroma. Figure 2. Thin-cap fibroatheroma. (A) Low-power view of an eccentric coronary plaque showing a thin fibrous cap overlying a relatively large necrotic core; the vessel was injected with barium (Movat Pentachrome, 20). (B) Immunohistochemical staining reveals numerous CD68-positive macrophages within the fibrous cap (rose-red reaction product, 400). (C) Shows a cellular-rich thin fibrous cap with cholesterol clefts. (D) Staining for alpha-actin positive smooth muscle cells within the fibrous cap was virtually negative ( 400). (Reproduced with permission from Kolodgie FD, Virmani R, Burke AP, Farb A, et al. Pathologic assessment of the vulnerable human coronary plaque. Heart 2004;90: ) Table 2. Comparison of the Size of the Necrotic Core, Number of Cholesterol Clefts, Macrophage Infiltration, Number of Vasa Vasorum, and Hemosiderin-Laden Macrophages in Plaque Rupture, TCFAs, Erosion, and Stable Plaques Plaque Type Necrotic Core, % No. of Cholesterol Clefts, % Macrophage Infiltration, Fibrous Cap, % Mean Vasa Vasorum Mean Hemosiderin Laden Macrophages Rupture (n 25) 34 17* # 44 22** TCFA (n 15) ** Erosion (n 16) 14 14* Stable (n 19) # Reprinted, with permission, from Virmani et al. (4). *p p p p p p #p **p p p p p p TCFA thin cap fibroatheroma.

4 C16 Virmani et al. JACC Vol. 47, No. 8 Suppl C April 18, 2006:C13 8 macrophages, supply tissue factor that trigger and propagate acute thrombi overlying unstable coronary atherosclerotic plaques. We have shown that monocyte infiltration of the thrombus correlates with the presence of an occlusive thrombus (7). Monocytes and neutrophils were identified in the fibrous cap by myeloperoxidase (MPO) staining. In clots, occlusive thrombi have greater density of CD68-positive macrophage ( % vs %, p 0.05) and MPO-positive monocytes ( % vs %, p 0.006) and neutrophils ( % vs %, p 0.03) than in non-occlusive thrombi. Similarly, the length of the thrombus showed a positive correlation with the intra-clot density of macrophages (p 0.004) and MPO positive cells (p 0.04). In the disrupted fibrous cap the density of MPOpositive cells was greater in occlusive (5.5%) versus nonocclusive (0.9%); this association was similar for neutrophils (0.7% vs. 0.4%) but not for total CD68-positive macrophages (13% vs. 20%) (7). The precise role of MPO in triggering acute coronary thrombosis is unclear. In addition to providing a pro-oxidant milieu and increasing oxidized low-density lipoprotein cholesterol, there is evidence that macrophage MPO might be responsible for the disruption of the fibrous cap by production hypochlorous acid (8). Figure 3. Coronary plaque erosion. (A) Shows a low-power view of a coronary artery obstructed by a luminal thrombus (Th) with no established communication with the deep underlying plaque consistent with plaque erosion (Movat Pentachrome, 20). The plaque substrate shows a large lipid pool (Lp) with superficial smooth muscle cells and proteoglycans (bluish-green stain). (B) Eroded lesions with a necrotic core (Nc). There is a non-occlusive luminal thrombus with partial organization. (C) Highpower view of the plaque/thrombus (Th) interface in the lesion shown in panel A, showing an absence of endothelium and a substrate rich in smooth muscle cells and proteoglycan matrix ( 400). (Figure 3A is reproduced form Kolodgie FD, Burke AP, Farb A, et al. Arherioscler Throm Vasc Biol 2002;22: Figures 3B and 3C are reproduced with permission from Kolodgie FD, Burke AP, Wight TN, et al. Curr Opin Lipidol 2004;15: ) PLAQUE EROSION AS THE BASIS OF ACS Plaque erosion is defined as an acute thrombus in direct contact with the intima, in an area of absent endothelium (Fig. 3). The intimal plaque underlying plaque erosion is rich in smooth muscle cells and proteoglycan matrix (9). We speculate that coronary vasospasm might be involved in the pathophysiology of erosion. This hypothesis is based on the observation that there is lack of endothelium and the media in these segments is intact and is thicker than at sites of plaque rupture (15). There are usually few or absent macrophages and lymphocytes in plaque erosions. The lesions tend to be eccentric and are infrequently calcified. The underlying plaque in erosions consists of pathologic intimal thickening or fibrous cap atheroma. The most frequent location for both erosion and rupture is the proximal left anterior descending artery (66%) followed by the right (18%) and the left circumflex (14%). Single (56%) vessel disease is twice as frequent as double vessel (26%) disease. Plaque erosions tend to embolize more frequently than plaque rupture (74% vs. 40%, respectively) (10). Plaque erosion accounts for 20% of all sudden deaths or 40% of coronary thrombi in patients dying suddenly with coronary artery atherosclerosis (1,3,4). The risk factors for erosion are poorly understood and are different from those of rupture. Consistently, plaque erosion is associated with smoking, especially in women. On average, patients are younger than those with plaque rupture, and there is less severe narrowing at sites of thrombosis. Plaque erosion accounts for over 80% of thrombi occurring in women 50 years of age.

5 JACC Vol. 47, No. 8 Suppl C April 18, 2006:C13 8 Virmani et al. C17 Table 3. Approximate Sizes of Necrotic Core in Fibroatheroma, Thin Cap Atheroma, and Acute Plaque Rupture Dimension Fibrous Cap Atheroma (n 17) Plaque Type Thin Cap Fibroatheroma (n 10) Acute Plaque Rupture (n 15) Mean length, mm (range) 6 (1 18) 8 (2 17) 9 (2.5 22) Necrotic core area, mm Necrotic core, % Reprinted, with permission, from Burke et al. (11). CALCIFIED NODULE AS THE BASIS OF ACS The least frequent lesion of thrombosis shows a plaque that is heavily calcified consisting of calcified plates and surrounding area of fibrosis in the presence or absence of a necrotic core (Fig. 4). The luminal region of the plaque shows presence of breaks in the calcified plate, bone formation, and interspersed fibrin with a disrupted surface fibrous cap and an overlying thrombus. There is often fibrin present in between the bony spicules along with osteoblasts and osteoclasts and inflammatory cells (4). It is more common in older male individuals than women. We believe that these lesions are commoner in the carotid arteries than the coronary and might be related to the frequent occurrence of plaque hemorrhage. CORONARY CALCIFICATION Coronary calcification correlates highly with plaque burden, but its effect on plaque instability is less evident. The earliest calcification in coronary lesions occurs in apoptotic smooth muscle cells, which form membrane-bound vesicles that actively calcify. With coalescence of microscopic calcium deposits, large granules and plates of calcium form that can be visualized by standard imaging techniques. Calcification of coronary arteries increases with aging of the population, and women show a 10-year lag compared with men, with equalization by the 8th decade (11). In a series of sudden death cases, over 50% of TCFA showed a lack of calcification or only speckled calcification on postmortem radiographs of coronary arteries (12). In the remaining lesions, calcification was almost equally divided into fragmented or diffuse, suggesting a large variation in the degree of calcification within the TCFA. In contrast, 65% of acute ruptures show speckled calcification, with the remainder showing fragmented or diffuse. Plaque erosion is almost devoid of calcification or, when present, there is only speckled calcification. Calcified nodules are lesion with the greatest amount of calcification relative to plaque area with even bone formation. This type of lesion, however, only rarely triggers thrombosis and tends to occur in the right or left anterior descending coronary artery of older individuals (4). It has been reported that calcification is greater in sudden coronary death victims than in those dying with acute myocardial infarction or unstable angina in arteries with 76% to 100% cross-sectional luminal narrowing (16,17). In our experience, however, calcification is dependent on the age of the patient in sudden coronary death victims; radiographic coronary calcification is present in 46% of men and women under the age of 40 years, 79% of men and women ages 50 to 60 years, and 100% of those older than 60 years (11). For women, the degree of calcification shows a 10-year lag compared with that of men, with equalization by the eighth decade (11). CORRELATION OF RISK FACTORS WITH ACS PATHOLOGY We have reported our findings relating coronary plaque morphology and risk factors in sudden coronary death. Thin-cap Figure 4. Calcified nodule. (A) Low-power view coronary artery showing a heavily calcified eccentric plaque with eruptive calcified nodules (Movat Pentachrome, 20). (B) Higher-power view of the plaque surface of the lesion in A, showing eruptive nodules with accumulated fibrin ( 400).

6 C18 Virmani et al. JACC Vol. 47, No. 8 Suppl C April 18, 2006:C13 8 fibroatheromas are a frequent finding in men dying suddenly with coronary thrombosis and are most frequent in patients with high total cholesterol (TC) and TC/high-density lipoprotein (HDL) cholesterol ratio ( 210 mg/dl and TC/HDL cholesterol ratio 5) (1). The incidence of TCFA in women is most frequent in women 50 years and also in those with TC 210 mg/dl (2). Smoking shows a positive correlation with presence of thrombosis in sudden coronary death and more so in women patients with plaque erosion as compared with rupture (2). Plaques of premenopausal women demonstrate relatively little necrotic core and calcification compared with post-menopausal women and men, which might be because of the relatively high rate of plaque erosion in young women (2). Another risk factor that has been reported to predict the development of ACS is C-reactive protein (CRP) (lower limit of normal is 3 mg/ml). The increased relative risk of sudden cardiac death associated with CRP is seen only in those in the highest quartile, who were at a 2.78-fold increased risk of sudden cardiac death (95% confidence interval 1.35 to 5.72) compared with men in the lowest quartile (18). We have shown that the median CRP was significantly higher in sudden coronary death victims dying of plaque rupture, erosion, or stable plaque than control subjects dying of noncoronary conditions (control CRP 1.4 g/dl vs. sudden death 2.7 g/dl, p ), and by multivariate analysis, log-transformed CRP levels were associated with plaque burden (p 0.03), independent of age, gender, smoking, and BMI (19). Immunohistochemically, CRP was localized to necrotic core and macrophages and was strongest in patients with high CRP as compared with those with low CRP. In addition, mean number of thin-cap atheromas was most frequent ( ) in patients with high CRP than in those with lower CRP ( ) (19). CONCLUSIONS The TCFA has been postulated to be the precursor lesion of plaque rupture and is most frequently observed in patients dying with acute plaque rupture and least frequent in plaque erosion. It usually occurs with lesions showing 50% diameter stenosis and is mostly observed in the proximal left anterior descending, left circumflex, and right coronary arteries, followed by mid and is least frequent in distal coronary arteries. Thin-cap fibroatheroma lesion differ from plaque ruptures in that they have smaller necrotic core, less macrophage infiltration of the thin-fibrous cap that is 65 m in thickness and are less calcified. Risk factors include high TC, low HDLs, a high TC/HDL ratio, and a high high-sensitivity CRP level; however, the direct relationship between TCFA and plaque rupture needs to be proven in prospective randomized clinical trials once we have modalities to recognize the lesion by invasive or non-invasive means. Reprint requests and correspondence: Dr. Renu Virmani, CVPath, International Registry of Pathology, 19 Firstfield Road, Gaithersburg, Maryland rvirmani@cvpath.org. REFERENCES 1. Burke AP, Farb A, Malcom GT, et al. Coronary risk factors and plaque morphology in men with coronary disease who died suddenly. N Engl J Med 1997;336: Burke AP, Farb A, Malcom GT, et al. Effect of risk factors on the mechanism of acute thrombosis and sudden coronary death in women. Circulation 1998;97: Farb A, Tang AL, Burke AP, et al. Sudden coronary death. Frequency of active coronary lesions, inactive coronary lesions, and myocardial infarction. Circulation 1995;92: Virmani R, Kolodgie FD, Burke AP, Farb A, Schwartz SM. Lessons from sudden coronary death: a comprehensive morphological classification scheme for atherosclerotic lesions. Arterioscler Thromb Vasc Biol 2000;20: Kolodgie FD, Burke AP, Farb A, et al. The thin-cap fibroatheroma: a type of vulnerable plaque: the major precursor lesion to acute coronary syndromes. Curr Opin Cardiol 2001;16: Virmani R, Burke AP, Kolodgie FD, Farb A. Vulnerable plaque: the pathology of unstable coronary lesions. J Interv Cardiol 2002;15: Burke AP, Kolodgie FD, Farb A, Weber D, Virmani R. Role of circulating myeloperoxidase positive monocytes and neutrophils in occlusive coronary thrombi. J Am Coll Cardiol 2002;39:256A. 8. Sugiyama S, Okada Y, Sukhova GK, et al. Macrophage myeloperoxidase regulation by granulocyte macrophage colony-stimulating factor in human atherosclerosis and implications in acute coronary syndromes. Am J Pathol 2001;158: Farb A, Burke AP, Tang AL, et al. Coronary plaque erosion without rupture into a lipid core. A frequent cause of coronary thrombosis in sudden coronary death. Circulation 1996;93: Farb A, Burke AP, Kolodgie FD, et al. Platelet-rich intramyocardial thromboemboli are frequent in acute coronary thrombosis, especially plaque erosions. Circulation 2000;102:II Burke AP, Virmani R, Galis Z, Haudenschild CC, Muller JE. 34th Bethesda Conference: task force 2 what is the pathologic basis for new atherosclerosis imaging techniques? J Am Coll Cardiol 2003;41: Burke AP, Weber DK, Kolodgie FD, et al. Pathophysiology of calcium deposition in coronary arteries. Herz 2001;26: Arbustini E, Dal Bello B, Morbini P, et al. Plaque erosion is a major substrate for coronary thrombosis in acute myocardial infarction. Heart 1999;82: Nemerson Y. A simple experiment and a weakening paradigm: the contribution of blood to propensity for thrombus formation. Arterioscler Thromb Vasc Biol 2002;22: Hao H, Gabbiani G, Camenzind E, Bacchetta M, Virmani R, Bochaton-Piallatt ML. Phenotypic modulation of intima and medial smooth muscle cells in fatal cases of coronary artery lesions. Arterioscler Thromb Vasc Biol 2006;26: Kragel AH, Gertz SD, Roberts WC. Morphologic comparison of frequency and types of acute lesions in the major epicardial coronary arteries in unstable angina pectoris, sudden coronary death and acute myocardial infarction. J Am Coll Cardiol 1991;18: Kragel AH, Reddy SG, Wittes JT, Roberts WC. Morphometric analysis of the composition of coronary arterial plaques in isolated unstable angina pectoris with pain at rest. Am J Cardiol 1990;66: Albert CM, Ma J, Rifai N, Stampfer MJ, Ridker PM. Prospective study of C-reactive protein, homocysteine, and plasma lipid levels as predictors of sudden cardiac death. Circulation 2002;105: Burke AP, Tracy RP, Kolodgie F, et al. Elevated C-reactive protein values and atherosclerosis in sudden coronary death: association with different pathologies. Circulation 2002;105: