Atherosclerosis: Clinical Perspectives Through Imaging

By Allen J. Taylor

Atherosclerosis: Clinical Perspectives Through Imaging is aimed at practicing clinicians and relies on didactic tabular and bullet points and a host of fine imagery describing the common features of the imaging of atherosclerosis, enabling the reader to understand more about the advantages and limitations of each modality in investigating athersclosis.

Edited by and contributed to by a host of international experts in cardiac imaging, this book is a must read by all who image the heart.

• Language: English
• Publisher: Springer
• Release date: Nov 27, 2012
• ISBN: 9781447142881

Book preview

Part 1

Basics and Clinical Atherosclerosis

Allen J. Taylor and Todd C. Villines (eds.)Atherosclerosis: Clinical Perspectives Through Imaging201310.1007/978-1-4471-4288-1_1© Springer-Verlag London 2013

1. Insights into the Natural History of Atherosclerosis Progression

Masataka Nakano¹ , Jacob Stephen², Miranda C. A. Kramer³, Elena R. Ladich¹, Frank D. Kolodgie¹ and Renu Virmani¹

(1)

CVPath Institute, Inc., Gaithersburg, MD, USA

(2)

Department of Cardiology/Medicine, William Beaumont Army Medical Center, Uniformed Services University of the Health Sciences, El Paso, TX, USA

(3)

Department of Cardiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands

Abstract

Pathology of high risk atherosclerotic plaque provides the basis for understanding the imaging and treatment of atherosclerosis. The earliest vascular change described microscopically are adaptive intimal thickening and fatty streaks, whereas pathologic intimal thickening are the first of the progressive plaques subtypes. Fibroatheromas are characterized by an acellular necrotic core, accumulated cellular debris and cholesterol monohydrate, and a lack of extracellular matrix. The development of the necrotic core is believed to originate from apoptotic macrophages. Thinning of the fibrous cap leads to plaques vulnerable to rupture, or thin-cap fibroatheromas. Overlying thrombosis can arise from one of several mechanisms including ruptures, erosion, or calcified nodules. Calcium within atherosclerosis is a common imaging target which increases with lesion progression and is present in greatest frequent in healed plaque ruptures and fibrous plaques. Thin cap fibroatheromas most frequently contain speckled calcification but may show heavily calcified areas or an absence of calcification. which is not very useful in diagnosing these lesions by calcium-based imaging. Coronary lesions with thrombi in the absence of rupture (erosions) exclusively show stippled or no calcification. Rupture in the absence of calcification is rare. In contrast, diffuse calcification is almost always associated with healed ruptures.

Keywords

PathologyAtherosclerosisVulnerable plaqueThrombosisCalcification

Background

Atherosclerosis is a complex disease with a multi-factorial etiology related to inheritance, and traditional and nontraditional risk factors. Despite major medical advances in the treatment of atherosclerosis, approximately two thirds of patients remain refractory to statins, one of the most successful agents targeted for the prevention of myocardial infarction. The lack of a more substantial treatment effect underscores the limitations of lipid lowering monotherapy and emphasizes the complexity of the disease and requirement for multi-­targeting of other critical processes. Moreover, despite the rapid progress in newer and refined imaging modalities that can image atherosclerosis, the inability to completely characterize atherosclerotic lesions in individual patients presents another important issue. Therefore, advancing the field is contingent on a better understanding of the morphologic characteristics of high-risk plaques in living patients, who harbor the capability of producing symptomatic events. Insights into how critical elements influence lesions stability primarily involve:

macrophage foam cells

necrotic core size

fibrous cap thickness

neoangiogenesis/hemorrhage

Any advancement(s) towards designing therapies targeted at plaque stabilization will likely require the identification putative pharmacologic agents and clinical refinements to validate and monitor treatment effects, potentially with arterial imaging techniques. Finally, a better understanding of the temporal relationship between active and healing lesions is needed to recognize the natural changes in plaque composition caused by silent or symptomatic events.

While intensive research has lead to a few breakthroughs in preventative therapies such as lipid lowering, most mechanistic insights are yet to be translated into new treatments. This limitation partly exists since the precise causes(s) of lesion progression from asymptomatic stable fibroatheromas into high-risk plaques for rupture (thin cap fibroatheroma or vulnerable plaque) are incompletely understood.

Natural progression of atherosclerotic plaque in humans

In animal models of atherosclerosis, hyperlipidemia-induced macrophage infiltration of the intima, constitutes one of the earliest pathologic changes [1], which can be reversed if the dietary cholesterol intake is reduced and/or circulating cholesterol is decreased by pharmacologic means. As an endpoint, however, advanced animal lesions have limited resemblance to man since findings of luminal thrombi attributed to rupture are rare [2, 3]. Nonetheless, early atherosclerosis in humans is recognized in all populations irrespective of the presence of risk factors as early as the first decade [4].

Atherosclerotic lesions have been extensively studied at autopsy where specific plaque morphologies have been assigned to categories established by the American Heart Association consensus group, lead by Dr. Stary in the mid-1990s [5, 6]. Our laboratory subsequently modified this classification as the cause of coronary thrombosis is not exclusive to rupture as implied by the AHA, but alternatively includes erosion and eruptive nodular calcification. In addition, the thin-cap fibroatheroma, the assumed precursor lesion to rupture (vulnerable plaque), was also introduced since this definition is also missing in the AHA classification.

Atherosclerotic Plaque Morphologies

Non-progressive Atherosclerosis (Fig. 1.1a–f)

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Fig. 1.1

(a–f) Lesion morphologies consistent with non-progressive atherosclerosis. (a–c) Adaptive intimal thickening (AIT). Normal coronary vessel with a thin smooth muscle-rich neointima (b, α-SMC actin immunostaining, arrow). Note the absence of lesional macrophages (MACs, c). (d–f) Intimal xanthoma (IX) or so-called fatty streak. Serial sections of the same eccentric plaque show few α-actin positive SMCs (d), while CD68-positive macrophages are very prominent (f)

The earliest vascular change described microscopically is adaptive intimal thickening (AHA Type I), which is found in at least 30 % of neonates at birth. The next category represents fatty streaks (AHA Type II), which are characterized by non-raised lesions consisting of intimal macrophages with intra- and extracellular lipid deposits. These lesions tend to regress in certain locations, e.g., thoracic aorta and the mid right coronary artery [7].

Progressive Atherosclerosis (Fig. 1.2a–f)

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Fig. 1.2

(a–f) Lesion morphologies consistent with progressive atherosclerosis: (a–c) Movat pentachrome staining; (d–f) CD68 immunostaining = macrophages). (a) Pathologic intimal thickening (PIT) is characterized by a non-flow limiting smooth muscle cell-rich lesion with an acellular lipid pool (LP) containing proteoglycan. (b) Fibroatheroma (FA) represents a lesions with a relatively thick fibrous cap (FC) overlying an area of necrosis or necrotic core (NC). These lesions are also generally ­non-flow limiting. (c) Thin-cap fibroatheroma (TCFA), shows a relatively large necrotic core with a thin fibrous cap typically infiltrated by macrophages and T-lymphocytes. The TCFA or vulnerable plaque is a know precursor to rupture. (d–f) These show the varying distribution of macrophages in progressive plaques. Note in (d) (PIT) the macrophages (arrow) are located near the luminal surface outside the area of the lipid pool, which is a distinguishing feature of this plaque

Our laboratory recognizes pathologic intimal thickening, also known as the ­intermediate (AHA Type III) lesion, as the first of the ­progressive plaques [8, 9]. Lipid pools rich in proteoglycans (hyaluronan and versican) located near the medial wall, in areas that generally lack smooth muscle cells, define this lesion type. The luminal surface, however, is mostly rich in smooth muscle cells and often accompanied by infiltrating macrophage foam cells [10]. The precise origin of the lipid pool is debatable, although studies suggest that a loss of smooth muscle cells promoted by apoptosis may be involved as basement membranes remnants can be identified by periodic acid Schiff (PAS) staining. Another characteristic of these lesions is microcalcification, which is prominently seen with anionic stains such as the von Kossa’s stain [9].

The first of the advanced lesions are considered fibroatheromas (AHA Type IV), which are characterized by an acellular necrotic core, accumulated cellular debris and cholesterol monohydrate, and a lack of extracellular matrix [5, 8]. During the evolution towards a fibroatheromatous lesion an overlying layer of fibrous tissue (fibrous cap) becomes identifiably distinct from the circumscribed area of necrotic core. The fibrous cap has a critical role in harboring the contents of the necrotic core, and its integrity is one of the defining influences on plaque stability (AHA Type V) [5]. The development of the necrotic core is believed to originate from apoptotic macrophages.

The extent of fibrous cap thinning along with underlying necrotic core defines the thin-cap fibroatheromas (vulnerable plaque) [11, 12] while more complicated plaques are represented by surface defects, and/or hematoma-hemorrhage, and/or thrombosis (AHA Type VI) [5, 8]. By histology, thin cap fibroatheromas are considered high-risk plaques with fibrous caps of thickness less than 65 μm, which are typically heavily infiltrated by macrophages and T-cells [13]. This measure of fibrous cap thickness is derived primarily from histologic sections of plaque ruptures where cap thickness at rupture sites was found to measure 23 ± 19 μm with a 95 % confidence interval of 64 μm [13]. Lesions identified as thin-cap fibroatheromas are considered precursors to rupture since they retain most features of rupture except that the fibrous cap is intact without a superimposed luminal thrombus.

The complications of hemorrhage, calcification, ulceration, and thrombosis in late stages of atherosclerosis are poorly understood. As recent as twenty-years-ago, the belief remained that acute coronary ­thrombosis was the sole cause of plaque rupture. Study of human coronary plaques at autopsy from sudden death victims have recently disproven this theory as our laboratory has shown three main causes of thrombosis to include plaque ruptures as the most frequent, followed by erosion, and, least frequent, eruptive nodular calcification [8].

Fatal Coronary Plaques (Fig. 1.3a–d)

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Fig. 1.3

(a–d) Lesion morphologies fatal plaques associated with sudden coronary death. (a) Plaque rupture shows a relatively larger necrotic core (NC) with a disrupted fibrous cap and luminal thrombus (Th), which is in communication with the NC. (b) An example of plaque erosion. The underlying eccentric plaque is consistent with pathologic intimal thickening as it contains a lipid pool (LP) in the absence of necrosis. There is a superimposed non-occlusive luminal thrombus (Th), which does not communicate with the underlying plaque. The luminal surface near the thrombus is rich is smooth muscle cells and proteoglycans. Erosions are more common in young women and smokers. (c) Shows eruptive nodular calcification, a minor but viable mechanisms of acute coronary thrombosis. (d) The relatively fibrotic plaque shows both matrix and nodular calcification (CN) where the latter are seen extending into the luminal space where there is a non-occlusive luminal thrombosis (Th). (a–c) Movat pentachrome staining; (d) Hematoxylin and eosin (H&E).

Plaque rupture is defined by fibrous cap disruption or fracture whereby the overlying thrombus is in continuity with the underlying necrotic core.

Plaque erosion is identified when serial sectioning through a thrombus fails to show communication with a necrotic core or deep intima; the endothelium is absent, and the thrombus is superimposed on a plaque substrate primarily composed of smooth muscle cells and proteoglycans.

Calcified nodule is characterized by eruptive, dense calcified bodies protruding into the luminal space, and represents the least frequent morphology associated with luminal thrombosis.

In approximately 50–60 % of sudden coronary deaths, the culprit fatal lesion exhibits an acute coronary thrombus whereas the remainder include stable coronary plaques with >75 % cross-sectional area luminal narrowing [14]. Moreover, greater than half of patients without acute coronary thrombi have healed myocardial infarcts, and, in 15–20 % of cases, there is no underlying myocardial pathology implicating a terminal arrhythmia [8].

The cause(s) of rupture are poorly understood although responsible factors involve matrix metalloproteinase expression MMPs [15], high shear regions [16], stress points, calcification and iron deposition within the fibrous cap [17]. Recent data are also beginning to unravel critical differences in gene expression between stable and unstable atherosclerotic plaques [18].

Imaging Atherosclerosis

To move the field forward in the area of lesion imaging, an understanding of the plaque microenvironment will likely provide key systemic and local signatures, which would be helpful in the recognition of the vulnerable plaque. Considering the necrotic core is a likely indicator of significant plaque progression and is a recognized feature of lesion vulnerability, it is particularly important to understand how necrotic cores form from the perspective of primary and secondary inflammation, cell death and removal of debris, and the host of other crucial factors that may be involved such as tissue disruption proteases [15] and hemorrhage [19]. Further classification of immune cells into different cellular states or subtypes may help provide further insight into their disparate functions since macrophages likely play a diverse role in disease progression beyond their primary involvement in lipid uptake.

It is important to recognize that atherosclerosis, a relatively slow progressive disease, may unpredictably become fatal. The episodic nature of plaque rupture indicates that sudden coronary occlusion is often preceded by a variable period of plaque instability and thrombus evolution prior to presentation with symptoms, where the silent ruptures may only present as death itself. Notwithstanding, the vulnerable plaque is a proximal disease, which is more likely to occur near branch points, as its occurrence is directed by biomechanical flow disturbances [16]. Moreover, only a fraction of vulnerable plaques will likely go on to rupture; therefore, we need to identify which unique feature is critical [20]. Finally, consistent with its recognition, establishment of lesion ­vulnerability based on the pathologic definition of fibrous cap thickness of <65-μm may not be sufficient to be recognized by the imager, where cap length and circumference may be more critical to identifying lesions prone to rupture.

Intraplaque Hemorrhage

Data from our laboratory provide evidence that repeated intraplaque hemorrhage is a contributing factor to necrotic core expansion since red blood cells are a rich source of free cholesterol, which is an important constituent of ruptured plaques [19, 21]. The expression of glycophorin-A (a protein exclusive to red blood cell membranes) within the necrotic cores of advanced coronary atheroma is strongly positive while its presence in plaques with early necrosis or pathological intimal thickening remains absence or low [21].

Intraplaque hemorrhage likely occurs from leaky vasa vasorum that infiltrate the plaque as the lesion thickness increases [22]. Moreover, macrophage cell death alone cannot account for the extent of free cholesterol monohydrate (free cholesterol, seen as needle shaped crystals) that is observed in necrotic cores with late necrosis, especially in thin cap fibroatheroma and ruptures [23].

Healed Plaque Rupture (Fig. 1.4a–c)

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Fig. 1.4

(a–c) Healed stable plaques with varying degrees and types of calcification. (a) Shows a healed plaque rupture with evidence of a repair site (*). The area bordering the necrotic core (NC) shows focal micro-calcification (arrows) represented by the area inclusive of the black box in the middle panel. (b) Shows a highly stenotic plaque with a relatively large necrotic core (NC) and healed repair site (*). A large circumscribed area of calcified matrix also includes the NC. (c) Is an example of nodular calcification, in the absence of luminal thrombosis, although a healed repair site (*) is also clearly visible. Multiple nodules of calcium (arrows) are seen in the deeper areas of the plaque. The top three images are stained by Movat Pentachrome while the remaining are with hematoxylin and eosin (H&E)

Morphologic studies of human coronary plaques suggest that lesion progression beyond 40–50 % cross-sectional-luminal narrowing occurs secondary to repeat rupture.

Ruptured lesions with healed repair sites, namely healed plaque ruptures (HPRs) as shown by Mann and Davies [24] and our laboratory [25], are easily detected microscopically by the identification of breaks in the fibrous cap with an overlying repair reaction consisting of smooth muscle cells surrounded by proteoglycans and/or a collagen depending on the phase of healing. Ruptured lesions in the early phase of healing are rich in proteoglycans, which eventually later are replaced by type I collagen [25].

The prevalence of silent (undiagnosed) plaque ruptures in the clinical population is unknown. Few angiographic studies have demonstrated plaque progression, and short-term studies have suggested that thrombosis is the likely cause. Davies et al. report the frequency of HPRs correlates with the extent of lumen narrowing [24]. The incidence of HPRs was 16 % in plaques with 0–20 % diameter stenosis, 19 % in lesions with 21–50 % stenosis, and 73 % in plaques with >50 % luminal narrowing. In our laboratory, 61 % of hearts from sudden coronary death victims show HRPs where the incidence is highest (80 %) in stable plaques, followed by acute plaque rupture (75 %) and only (9 %) in erosions [25]. Multiple healed ruptures with layering were a common finding in segments with acute and healed ruptures where, notably, extent of cross-sectional-luminal narrowing was dependent on the number of healed repair sites.

Healed plaque ruptures typically contain areas of calcification.

Coronary Calcification (Fig. 1.4a–c)

Calcification is an invariable component of the atherosclerotic plaque. Coronary calcification can be observed in early lesions of pathologic intimal thickening but is absent in fatty streaks [26].

The extent of calcification increases with lesion progression and is present in greatest frequent in healed plaque ruptures and fibrous plaques with nodular calcification.

In terms of risk, calcification correlates with advancing age with and without pre-existing coronary artery disease. Although, calcification is observed in over 80 % of sections with rupture, its severity is greater in culprit stable fibrocalcific plaques with >75 % luminal narrowing.

Coronary calcification is more frequent in men while women lag behind in earlier third to sixth decades but show an equivalent extent of calcification by the seventh and eigth decades [27].

Diabetics have been shown to have greater calcification than non-diabetics [28].

Calcification as assessed by electron beam tomography is reported to be predictive of future coronary events in both symptomatic and asymptomatic population. It has also been shown to be an acceptable marker for plaque burden; however, absolute calcium scores do not indicate plaques that are unstable [29, 30].

Calcification may exist in the form of (micro) calcification, at the organelle and/or cellular level, but has a tendency to extend and arranged as nodules, which are mostly observed at the edge of the necrotic core. In advanced plaques, calcium is also present as sheets integrated with fibrotic tissue consisting of collagen and smooth muscle cells, which also includes the necrotic core. Postmortem radiographs of coronary arteries show four different patterns of calcification: speckled, focal nodular, multifocal, and diffuse [31].

The least common form of calcium is nodular calcification existing as small fragments of calcium separated usually by fibrin. Nodular calcification typically exists in tortuous arteries of older individuals where sometimes the nodules may show ossification with ­intervening marrow formation.

Thin cap fibroatheromas most frequently contain speckled calcification but may show heavily calcified areas or an absence of calcification, which is not very useful in diagnosing these lesions by calcium-based imaging. Coronary lesions with thrombi in the absence of rupture (erosions) exclusively show stippled or no calcification. Although speckled calcification is also common in ruptures, multifocal or diffuse calcification is generally present; rupture in the absence of calcification is rare. In contrast, diffuse calcification is almost always associated with healed ruptures.

In fibrotic plaques without a significant necrotic cores, smooth muscle cells, and collagen may calcify as plates or solid masses, with little macrophage infiltration or indication of repeat ruptures. Similarly, multiple healed ruptures are accompanied by larger areas of irregular calcium deposits, possibly initiated by a series of intraplaque hemorrhages and organization. Nonetheless, calcified plaques are resistant to undergoing changes in size in response to systemic interventions targeting atherosclerotic risk factors [32].

Ethnic and Gender Differences in Atherothrombosis

Most of the data available on atherothrombotic risk and progression have been derived from single regional studies focusing on a single subtype of patient. How the progression of atherothrombosis varies between gender and different ethnicities has been difficult to determine:

The Reduction of Atherothrombosis for Continued Health (REACH) Registry is a multicenter, multinational analysis of individuals with established atherothrombosis [33, 34]. Follow up information at 1-year in a stable outpatient setting was analyzed. The following results can be broadly applied to gender and ethnic differences in atherothrombotic rates:

Stable patients with established atherosclerotic disease or with at least three atherothrombotic risk factors were observed for 1 year. Patients with abdominal aortic aneurysms (AAA) were compared to ­non-AAA patients. Male gender and white ethnicity were independently related to the diagnosis of AAA.

These AAA patients had increased rates of cardiovascular deaths and atherothrombotic events at 1-year.

The reported overall adjusted mortality across all geographic regions was consistent. Eastern Europe had the highest all-cause mortality and cardiovascular death rates. Japan had the lowest overall and cardiac mortality but had higher rates of nonfatal stroke and MI when compared to Western Europe, North America, and Australia.

Differences exist in risk factor contribution, outcomes, and management of atherothrombotic disease in women [35]:

The risk of coronary disease is three to seven fold greater in women with diabetes when compared to men with diabetes who only have a two to four fold risk of coronary disease.

More women than men die during the initial 1 year after a recognized MI. Women are more likely than men to have a second MI within 6 year.

Primary prevention trials have shown that aspirin significantly reduces strokes in women whereas men benefit more from a reduction in MI rates.

The thin cap fibroatheroma (TCFA) is the most common atherothrombotic lesion in men of all ages and women >50 year dying from acute coronary syndrome.