CHAPTER 32: ATHEROTHROMBOSIS: DISEASE BURDEN, ACTIVITY, AND VULNERABILITY

Atherosclerosis with thrombosis superimposed, atherothrombosis, is the main cause of coronary heart disease (CHD), peripheral artery disease, and large-artery stroke.^1,2,3 The development of thrombosis-prone atherosclerotic plaques, also known as high-risk or vulnerable plaques,⁴ in the coronaries and other arteries is the leading cause of death and severe disability, not only in affluent countries, but also worldwide.^5,6

Causal and modifiable risk factors for atherosclerotic cardiovascular disease are well known (eg, smoking, dyslipidemia, high blood pressure, diabetes) and account for most heart attacks in both sexes.⁷ However, for unknown reasons, the individual susceptibility to these risk factors varies greatly, and consequently, their predictive value is limited.^8,9 Most first heart attacks occur among people with average or only slightly elevated risk factor levels.^10,11,12 Recurrent events still occur despite lowering of these levels,^13,14 indicating that we need both better detection and better treatment of those who are destined for a heart attack. Better detection of at-risk individuals may be achieved by visualizing the diseased arterial wall rather than just assessing risk factors for getting the disease.^15,16,17 The prospect for atherosclerosis-based risk assessment will be discussed in the concluding section of this chapter.

Atherosclerosis is a chronic, lipid-driven inflammatory disease of the arterial wall leading to multifocal plaque development,^18,19,20 predominantly at sites characterized by low and oscillatory endothelial shear stress (bifurcations, inner wall of curvatures) and preexisting intimal thickenings.^21,22 The speed of disease progression varies greatly, but it usually takes decades to develop the advanced atherosclerotic lesions responsible for clinical disease. Plaques are very heterogeneous in size and composition, even plaques located next to each other and exposed to the same systemic risk factors (Fig. 32–1). Most plaques remain asymptomatic (subclinical disease), some become obstructive (stable angina), and a few, if any, become vulnerable and lead to atherothrombotic events such as a fatal heart attack or a disabling stroke.

Endothelial Cells

The endothelium, located at the interface between the blood and the arterial wall, plays a key role in the development of atherosclerosis.²² In lesion-prone areas, apoprotein B–containing lipoproteins extravasate into intima, where they are retained and modified (eg, oxidized) into cytotoxic, proinflammatory, chemotaxic, and proatherogenic molecules.²³ The overlying endothelium becomes activated and mediates transendothelial trafficking of leukocytes, especially monocytes but also T cells and a few mast cells, primarily via upregulated adhesion molecules.^18,19

Macrophages

Monocyte-derived macrophages play a key role in the initiation, progression, and destabilization of atherosclerotic plaques.^{18,19,20,24,25} Some macrophages perform beneficial scavenger functions, cleaning up intima for toxic lipid-rich waste products, and others are destructive and accelerate the development of a destabilizing necrotic core and a rupture-prone fibrous cap, as described below. Consequently, macrophage number and function within atherosclerotic plaques are obvious putative markers of disease activity. However, because atherosclerosis is a protracted and smoldering inflammatory disease in which the advanced and potentially symptomatic lesions are dominated by fibrosis, extracellular lipids, necrosis, and calcification (Fig. 32–2),²⁶ the macrophage density is usually low, and macrophages rarely occupy more than a small percentage of the plaque.²⁷ In fact, advanced atherosclerosis is a smoldering inflammatory disease without much inflammation.

Smooth Muscle Cells

When intimal damage occurs, vascular smooth muscle cells (SMCs) modulate into a synthetic phenotype and perform fibroblast-like functions. They migrate, proliferate, and synthesize extracellular matrix, including collagen, and thus confer stability to plaques.²⁸ Analogous to scar contraction, collagen-rich plaques tend to contract and aggravate luminal narrowing. If the vascular SMCs vanish, healing and repair are compromised and the macrophage-mediated destructive processes will prevail.

Cell Replication and Death

During atherogenesis, some cells proliferate and others die by apoptosis or necrosis. The rate of cell proliferation is low and predominantly confined to inflammatory cells.²⁹ Apoptosis is also most common in inflammatory cells, but its rate is more difficult to determine because of defective clearance of dead cells in advanced lesions.^30,31,32 Death of macrophage foam cells and SMCs are believed to play important pathogenic roles in the formation of a necrotic core and thinning of the overlying fibrous cap, respectively.^25,30,31,32

Circulating Progenitor Cells

Subsets of mononuclear cells in the blood, termed endothelial progenitor cells and smooth muscle progenitor cells, form colonies in cell culture with endothelial or SMC-like features, respectively.^33,34 These circulating cells, assumed to originate solely or mainly from the bone marrow, have been claimed to home and differentiate into bona fide endothelial and smooth muscle cells in atherosclerotic lesions and thus promote healing and repair.^35,36 However, more recent data question this theory. First, the putative endothelial progenitor cells identified in clinical studies have been shown to give rise to colonies composed of inflammatory and immune cells, rather than endothelial cells, in vitro.^37,38 Second, new mouse studies using improved techniques to track endothelial and SMC origin in atherosclerotic plaques have found that both cell types are derived from local sources with rare, if any, contributions from the blood.^39,40,41 Thus, rather than differentiating into endothelial or smooth muscle cells, the so-called progenitor cells in the blood might influence the progression of atherosclerosis by paracrine signaling, as appears to be the case for neovascularization (angiogenesis).⁴²

The great majority of symptomatic coronary thrombi (~75%) are caused by plaque rupture.²⁰ Plaque rupture with mural thrombosis (with or without plaque hemorrhage) is also a common cause of episodic but asymptomatic progression to severe stenosis.^43,44 The remaining thrombi are caused by less well-defined mechanisms, of which so-called plaque erosion is the most common type.²⁰ Plaque rupture is a more frequent cause of coronary thrombosis in men (~80%) than in women (~60%) but, except for sex and menopause, no other risk factors have consistently been connected with a particular mechanism of thrombosis.²⁰ By inference, there are two major types of vulnerable plaques, rupture-prone and erosion-prone, that are presumed to look like the corresponding thrombosed plaques, just without rupture and thrombosis.⁴⁵

Plaque Rupture

The prototype of a presumed rupture-prone plaque contains a large and soft lipid-rich necrotic core covered by a thin and inflamed fibrous cap (Fig. 32–3).^25,45 Associated features include big plaque size, expansive positive remodeling mitigating luminal obstruction (mild stenosis by angiography), neovascularization (angiogenesis), plaque hemorrhage, adventitial inflammation, and a spotty pattern of calcifications (Fig. 32–4). Although the macrophage density in ruptured caps is high,^25,45 whole-plaque macrophage density rarely exceeds a small percent because ruptured caps are tiny (Fig. 32–5).

Plaque Erosion

Vulnerable plaques of the erosion-prone type are heterogeneous and defined only by their fate (thrombosis, mostly mural) (Fig. 32–6).^20,45 The surface endothelium is missing, but whether it vanished before or after thrombosis remains unknown. No distinct morphologic features have been identified but, in general, eroded plaques with thrombosis are scarcely calcified, rarely associated with expansive remodeling, and only sparsely inflamed.^20,45 So, irrespective of plaque type, it is a misconception that vulnerable plaques are heavily inflamed.

Location and Natural History

Vulnerable plaques, plaque rupture, and thrombosed plaques tend to cluster in hot spots within the proximal segments of the major coronary arteries,^46,47,48 and rarely more than one or a few such lesions exist simultaneously.^49,50 The natural history of vulnerable plaques such as speed of development, lifetime (persistency), and fate, is, however, unknown.

Plaque rupture requires the presence of a lipid-rich necrotic core covered by a thin fibrous cap. The size of the necrotic core and the thickness of the fibrous cap appear to be the two major structural determinants of vulnerability (Fig. 32–7).

Necrotic Core

During atherogenesis, atherogenic lipoproteins are retained within intima, modified, and accumulate, predominantly deeply in the abluminal part of intima.^51,52 Some of these pools of lipids seem to attract macrophages that secrete proteolytic enzymes and engulf lipid until they die, leaving behind a soft and destabilizing lipid-rich cavity containing cholesterol crystals and devoid of supporting collagen and cells, the necrotic core.^20,45 Such a plaque is called an atheroma or fibroatheroma.^53,54 Later on, extravasation of erythrocytes into the necrotic core may expand it (see Neovascularization and Plaque Hemorrhage).

Fibrous Cap

The fibrocellular part of the plaque located between the necrotic core and the lumen is called the fibrous cap. It is extremely thin in coronary plaque rupture.^25,45 Assessed by microscopic examination postmortem, ruptured caps were usually less than 65 μm thick.⁵⁴ Assessed by optical coherence tomography in vivo, the mean thickness was only 49 μm.⁵⁵ If the fibrous cap is thin, the plaque is called a thin-cap fibroatheroma (TCFA).^54,56 In TCFA, the necrotic core occupies approximately 23% of plaque area.⁵⁶ Thin fibrous caps are usually heavily inflamed (macrophage density ~14%), particularly those that have ruptured (macrophage density ~26%),⁵⁶ but because they are thin, their ability to accommodate macrophages is limited (see Fig. 32–5). Apoptosis is common at the site of fibrous cap rupture, usually confined to macrophages because the vascular SMCs already have vanished when rupture occurs.^31,56 With their ability to synthesize extracellular matrix, including collagen, SMC apoptosis is associated with impaired healing and repair, increasing the risk of plaque rupture.

Cap Inflammation

Atherosclerosis is an innate inflammatory disease in which smoldering inflammatory activity is not confined to just a few atherosclerotic lesions but is present, more or less, in all such lesions throughout the body.^18,19 In contrast, vulnerable plaques are relatively rare,⁴⁹ and inflammation may play a causal role in plaque rupture only if located within a thin fibrous cap (ie, the microstructure of the plaque needs to be permissive for rupture). Thus, although plaque inflammation may be useful as a marker of disease activity,⁴⁵ it is probably not useful as a stand-alone marker for plaque vulnerability (see Figs. 32–5 and 32–7).

Plaque Size and Expansive Remodeling

During atherogenesis, the artery tends to remodel in such a way that the luminal obstruction caused by some plaques is attenuated (expansive remodeling) and by others accentuated (constrictive remodeling). Although vulnerable plaques of the rupture-prone type (TCFA) usually are big, they often appear nonobstructive by angiography because of local expansive remodeling and extension of the disease to the adjacent reference segments judged to be normal by angiography.^25,45,57,58 In contrast, plaques responsible for stable angina usually are smaller but nevertheless are often associated with more severe luminal narrowing by angiography because of concomitant constrictive remodeling.⁵⁹ The reasons for the different modes of remodeling remain to be defined, but recent clinical observations indicate that diabetes is accompanied by inadequate compensatory remodeling.⁶⁰

Neovascularization and Plaque Hemorrhage

Angiogenesis and inflammation often coexist at the base of advanced plaques.⁶¹ The new microvessels rarely originate from the lumen but usually originate from vasa vasorum in adventitia.⁶² They lack supporting cells and are fragile and leaky, giving rise to local extravasation of plasma proteins and erythrocytes.^63,64,65 Such intraplaque bleeds are common⁶⁶ and may expand the necrotic core, causing rapid progression of the lesion.⁶⁷ Another common source of plaque hemorrhage is extravasation of blood through a ruptured fibrous cap.⁵⁰ Neovascularization and inflammation are dynamic processes that disappear with plaque regression induced by cholesterol-lowering in animals.^68,69

Spotty Calcification

Focal calcifications in atherosclerotic plaques are very common and increase with age.⁷⁰ At autopsy, the amount of coronary artery calcium (CAC) correlates poorly with luminal narrowing but strongly with plaque burden.⁷¹ Apoptotic cells, extracellular matrix, and necrotic cores may calcify; healed ruptured plaques are often heavily calcified; and microcalcifications have been described in the fibrous cap.^72,73 The pathogenesis and clinical significance of these different forms of calcifications are, however, poorly understood. Nonatherosclerotic calcifications of coronary arteries are exceedingly rare, except in chronic renal failure, where Mönckeberg medial calcific sclerosis may be seen.⁷⁴

Atherosclerosis is a generalized, multifocal arterial disease. However, compared with other arteries, the coronary arteries are in general the most susceptible to atherosclerosis and its thrombotic complications.^75,76 Therefore, if atherosclerosis is present in noncoronary arteries, the coronary arteries will usually also be diseased, irrespective of symptoms.⁷⁷ The CHD risk equivalent concept introduced in the previous prevention guidelines included symptomatic disease in noncoronary arteries, carotid stenosis of more than 50%, ankle-brachial blood pressure index less than 0.9, and abdominal aortic aneurysm.⁷⁸ Asymptomatic carotid bruit is also associated with high CHD risk.^3,79

Similar to coronary atherosclerosis, plaque rupture is by far the most common cause of symptomatic thrombosis in carotid atherosclerosis,^80,81 but the thrombus is more often nonocclusive and prone to embolize, often prolonged because of slow-healing plaque ulceration.⁸² Intraplaque hemorrhage is common. Carotid plaques, including those that rupture, are rarely heavily inflamed.^83,84 The critical thickness for a rupture-prone cap is greater for carotid plaques than for the much smaller coronary plaques, but a relatively thin cap is most critical for rupture in both arteries.^82,85 In one study, the mean fibrous cap thickness in carotid plaque rupture was found to be nearly three times greater than in coronary plaque rupture (72 ± 15 μm vs 23 ± 17 μm; mean ± standard deviation [SD]), and the macrophage density within ruptured caps was lower (13.5% ± 10.9% vs 26% ± 20%).⁸²

The individual susceptibility to risk factors and atherosclerosis varies greatly, explaining the well-known limitations of risk factors as prognostic tools.^8,9,10,11 However, an alternative and potentially much more rewarding approach to finding those at highest risk for atherothrombotic events is at hand. It is not based only on risk factors for getting atherosclerosis, but it also incorporates direct and indirect evidence for already having diseased arteries without knowing it (subclinical atherosclerosis), capturing the overall impact of all risk and susceptibility factors combined, known as well as unknown.⁸⁶ Subclinical atherosclerosis can be detected by imaging, and its severity and extent (burden), progression rate (activity), and thrombosis-risk (vulnerability) can be assessed.^{15,16,17,45,86}

Burden of Atherosclerosis

The number and severity of stenoses determined by coronary angiography are signs of atherosclerosis with diagnostic, therapeutic, and prognostic implications.^87,88,89 However, most acute coronary events originate from angiographically nonobstructive plaques, probably because they are much more numerous.^{25,45,90,91,92,93} An irregular lumen and/or filling defects indicating plaque disruption and/or thrombosis are associated with worse outcomes.⁹⁴ In contrast to coronary luminography, CAC detected by computed tomography (CT) imaging reveals the diseased arterial wall directly and correlates strongly with plaque burden.^71,95 The total amount of CAC (usually expressed as the Agatston score) is a strong predictor of coronary events and provides prognostic information beyond that provided by traditional risk factor scoring.⁹⁶

Contrast-enhanced CT angiography not only visualizes the lumen but also the arterial wall. Because the strong relationship between the CAC score and coronary events is mediated predominantly by coexisting noncalcified or less calcified vulnerable plaques, total or noncalcified plaque burden detected by CT angiography may prove to be an even better marker of risk than the CAC score.^58,97

Intravascular ultrasound (IVUS) can detect and localize plaque as well as quantitate plaque burden, but it requires selective catheterization and motorized pullback in the arteries of interest. Serial examinations of well-defined coronary segments have been used to monitor the speed of plaque progression (or regression) over time in patients with established CHD.⁹⁸

Disease Activity

Imaging inflammation in atherosclerosis serves two different purposes: detecting vulnerable patients (systemic activity) or vulnerable plaques of the rupture-prone type (focal activity). Because atherosclerosis is an innate inflammatory disease, inflammatory activity is not confined to just a few atherosclerotic lesions but is present, more or less, in all such lesions throughout the body.^18,19 In contrast, vulnerable plaques are relatively rare,⁴⁹ and inflammation plays a causal role in plaque rupture only if the microstructure of the plaque is permissive for rupture (TCFA) (see Fig. 32–5), as described in Plaque Vulnerability.

With its high sensitivity, positron emission tomography (PET) imaging using fluorine-18-fluorodeoxyglucose (FDG) has evolved as a promising method for the detection of arterial inflammation when combined with higher-resolution CT or magnetic resonance imaging to help localize the signal to the arterial wall.^99,100,101 With adequate suppression of myocardial FDG uptake using a low-carbohydrate, high-fat diet rather than a fasting protocol, coronary FDG uptake was recently identified in a large proportion of cancer patients undergoing invasive coronary angiography because of suspected or manifest ischemic heart disease.¹⁰²

In carotid atherosclerosis, proof-of-concept studies, focusing on symptomatic or stenotic carotid plaques, have documented a positive correlation between FDG uptake and macrophage density.^103,104 Other studies have revealed widespread FDG uptake in asymptomatic plaques and arterial segments unlikely to harbor vulnerable plaques, and FDG uptakes among different arteries in the same person correlate strongly.^99,100 In asymptomatic persons undergoing ultrasound screening for carotid artery disease, as many as 30% of ultrasound-defined atherosclerotic arteries and 10% of normal arteries accumulated FDG,¹⁰⁵ supporting the current understanding that FDG-defined inflammation may be used to assess systemic disease activity unrelated to plaque vulnerability. Arterial inflammation defined by FDG-PET imaging appears to be dynamic and modifiable by a healthier lifestyle and short-term statin treatment.^106,107,108 In a population of stable cancer patients, extensive arterial FDG accumulation was associated with recent and/or near-term cardiovascular events,¹⁰⁹ supporting the “vulnerable patient” concept.¹¹⁰ Contrast-enhanced magnetic resonance imaging is also able to detect plaque inflammation, but its sensitivity is much lower than that of nuclear imaging.¹¹¹

Plaque Vulnerability

Inflammation is not common in plaque erosion and not enough to make a vulnerable plaque of the most common type, the TCFA, where the microstructure of the plaque needs to be permissive for plaque rupture (see Figs. 32–3, 32–4, 32–5).^25,45 No distinct morphologic feature characterizes the erosion-prone type of vulnerable plaque (see Fig. 32–6).⁴ Consequently, useful targets for imaging of erosion-prone plaques remain elusive. In contrast, the TCFA has a distinct microstructure, including a large necrotic core covered by a thin and inflamed fibrous cap, and other characteristic plaque features are common, such as a big-size, expansive remodeling, neovascularization (angiogenesis), and a spotty pattern of calcification (see Fig. 32–7).^25,45

Although inflammation most likely plays a causal role in plaque rupture, it does not necessarily imply that inflammation also is a useful target for the detection of plaques assumed to be rupture-prone, such as TCFA. A unique feature of TCFA is the location of inflammation within the plaque rather than the overall severity of plaque inflammation. In coronary TCFA, the macrophage density within the thin and disrupted fibrous cap is high (~14%),⁵⁶ but the whole-plaque macrophage density is only 2.0% ± 1.9% (mean ± SD) versus 1.1% ± 1.5% in “stable” fibroatheromas.²⁷ These low values and the substantial overlap, indicated by the large SDs, probably preclude the use of plaque inflammation as a useful standalone marker of vulnerability (see Fig. 32–5).

Coronary CT angiography may not only visualize the lumen and detect obstructive and nonobstructive plaques, it may also quantify calcified and noncalcified plaque burden. Furthermore, coronary CT angiography may also provide additional prognostic information by detection of higher-risk plaques characterized by large plaque volume, low CT attenuation, napkin-ring sign, expansive remodeling, and spotty calcification.^112,113

Many catheter-based technologies have been developed, or are under development, for the assessment of coronary atherosclerosis and vulnerable plaques, including conventional grayscale IVUS, virtual histology IVUS and other ultrasound-based tissue characterization modalities, optical coherence tomography, angioscopy, near-infrared spectroscopy, intracoronary magnetic resonance imaging, thermography, and vascular profiling.^{114,115,116,117} Because vulnerable coronary plaques of the rupture-prone type (TCFA) are relatively large, not numerous, and often cluster proximally in the major coronary arteries, their detection in patients undergoing percutaneous coronary interventions might be feasible.⁴⁹

Although vulnerable (thrombosis-prone) coronary plaques must exist, the critical question is whether timely identification and targeted treatment of such lesions are possible and can reduce the risk for a future atherothrombotic event. Despite major advancements in imaging technology that allow visualization of plaque features assumed to confer vulnerability, the clinical benefit of searching for vulnerable plaques remains to be documented.¹¹⁸ Considering the multifocal nature of atherosclerosis and the complex interaction between plaque size, plaque composition, local flow disturbances, and the actual susceptibility to thrombosis, trying to predict the fate and the clinical impact of a particular plaque may be futile.^118,119 Atherosclerosis is a systemic disease calling for a systemic approach, focusing on cardiovascular risk factors and the overall burden of disease to ensure appropriate identification and treatment of those who are at highest risk, the so-called vulnerable patients.^110,120

Thrombosis-prone atherosclerotic plaques, also known as high-risk or vulnerable plaques, are the main cause of death and severe disability worldwide. Although causal risk factors are known and constitute important therapeutic targets, their usefulness in risk assessment and finding people at risk for an atherothrombotic event is limited. Most heart attacks and strokes occur in people who are not identified as being at particular high risk by conventional risk factor scoring. Atherothrombotic events are, however, always preceded by a long preclinical phase in which subclinical atherosclerosis evolves and can be detected by noninvasive imaging, offering unique opportunities for timely and individualized preventive care. The utility of such an imaging-based approach in the primary prevention of atherothrombotic events is being evaluated in the Progression of Early Subclinical Atherosclerosis (PESA) study¹⁵ and the BioImage study.¹⁶