Pathology of the Vulnerable Plaque

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 23 ± 19 μm, with 95% of the caps measuring <65 μm (1). It has been observed that some plaques at other sites in the coronary tree resemble the rupture plaque but lack a luminal thrombus: these lesions have been designated as TCFA or vulnerable plaques (4). The term vulnerable plaqueshould 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.

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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).

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 2.5 ± 1.3 versus none in erosion and 0.05 ± 0.6 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.

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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;90:1385–91.)

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 mm², 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).

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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)High-power 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;22:1642–8. Figures 3B and 3C are reproduced with permission from Kolodgie FD, Burke AP, Wight TN, et al. Curr Opin Lipidol 2004;15:575–82.)

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 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 (15.7 ± 12.5% vs. 3.0 ± 2.7%, p = 0.05) and MPO-positive monocytes (12.2 ± 7.5% vs. 5.0 ± 2.7%, p = 0.006) and neutrophils (2.9 ± 3.4% vs. 0.36 ± 0.50%, 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 MPO-positive cells was greater in occlusive (5.5%) versus non-occlusive (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).

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.

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.

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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).

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 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 μg/dl vs. sudden death 2.7 μg/dl, p < 0.0001), 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 (3.0 ± 0.3) in patients with high CRP than in those with lower CRP (0.95 ± 0.22) (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 μm 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.