A pictorial review of acute aortic syndrome: discriminating and overlapping features as revealed by ECG-gated multidetector-row CT angiography
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The term "acute aortic syndrome" (AAS) encompasses a spectrum of life-threatening conditions characterized by acute aortic pain. AAS traditionally embraces three abnormalities including classic aortic dissection, intramural haematoma, and penetrating atherosclerotic ulcer. Although the underlying etiologies and conditions predisposing to AAS are diverse, the clinical features are indistinguishable.
Multidetector-row computed tomography (CT) with electrocardiographic gating (ECG-gated MDCT) has greatly improved imaging of acute thoracic aortic diseases by virtually eliminating pulsation artifacts transmitted from cardiac motion and reveals subtle aortic abnormalities, which have been difficult to recognize by conventional non-gated CT.
While these advances in imaging technology provide additional discriminating features of acute aortic diseases, they also reveal a range of overlapping features of these life-threatening conditions that not uncommonly are dynamic and evolving. These overlapping and transitional features may be a major source of misunderstanding, confusion, and controversy for diseases that cause AAS.
In this pictorial review, we describe the discriminating and typical imaging features as revealed by modern ECG-gated MDCT angiography. In addition to the discriminating features, recognition of the overlapping and transitional features in AAS will allow a more comprehensive understanding of their underlying pathophysiologic conditions and their natural history, and may improve therapeutic management.
• The superior visualization of ECG-gated CTA improves the diagnostic accuracy of acute aortic syndrome.
• ECG-gated CTA provides discriminating features of underlying pathophysiologic conditions of AAS.
• Also, recognition of the overlapping features in AAS will allow a more comprehensive understanding.
KeywordsAortic diseases Tomography X-ray computed Aneurysm Aortic dissection Aortic aneurysm
Acute aortic syndrome
The term ‘acute aortic syndrome’ (AAS) refers to a spectrum of acute, life-threatening conditions of aortic diseases characterized clinically by abrupt, intense chest and/or back pain . AAS traditionally embraces three aortic diseases: aortic dissection (AD), intramural haematoma (IMH) and penetrating atherosclerotic ulcer (PAU) [1, 2]. The differentiation of disease entities in AAS is essential for predicting the natural history and for initiating the appropriate treatment in a timely fashion [2, 3]. However, the presenting clinical signs and symptoms of AAS of any etiology are usually clinically indistinguishable . Although typical cases demonstrate characteristic imaging features of each disease, imaging findings may also overlap between different entities, especially when the process is dynamic and evolving. These transitional and overlapping features of AAS, both clinical and pertaining to imaging findings, have led to several misconceptions and controversies concerning the disease concept of AAS.
Two anatomical systems—the DeBarkey and Stanford classification—are used to classify aortic dissection [2, 10]. Recently, the Stanford system has been more widely used worldwide as it is simpler and directly connected with the therapeutic strategy. Stanford type A dissection, which involves the ascending aorta regardless of the site of the primary intimal tear, should be treated as a surgical emergency as these patients are at high risk for a life-threatening complication with a high mortality rate (1–2 % per hour early after symptom onset) [2, 10]. Patients with uncomplicated Stanford type B dissection, which was confined to the aortic arch and the descending aorta, are best treated with medical therapy; the in-hospital mortality for these patients was 10% [2, 10]. Radiological imaging plays an essential role in determination of the therapeutic strategy.
Discriminating and overlapping features of aortic dissection
Classic AD can be unambiguously differentiated from other forms of AAS by the presence of a double-barrel lumen, consisting of a true lumen and a false lumen, resulting from a primary intimal tear, which is usually clearly visualized with state-of-the-art CT technology: the false lumen is contained entirely within the aortic media and is separated from the true lumen by a dissection flap or ‘intimal flap.’ However, confusion with respect to the terminology arises when classic AD is associated with non-flowing blood or haemorrhagic content within the medial layer. This is not uncommon in situations when the primary intimal tear is more distal (e.g., in the descending aorta) than the most proximal extent of the medial separation (which may extend retrogradely into the ascending aorta), where stagnating blood or clots can be observed. In CT, stagnating blood in the false lumen is isodense to blood within the true lumen, whereas thrombosed blood is hyperdense relative to flowing blood in non-contrast CT. The extreme within the spectrum of separation of medial layers as a consequence of underlying ‘cystic media necrosis’ occurs in complete thrombosis of the false lumen, resulting in the imaging features of IMH, and in this context, IMH is considered a variant of classic AD. Another variant of AD—called limited intimal tear or discrete/subtle tear—is characterized by a complex primary intimal tear with only minimal separation between the medial layers at the edges of the lesion; this disease entity also has a pathophysiology and etiology overlapping with classic dissection [9, 13, 14]. These topics are elaborated below.
The etiology of IMH remains controversial . Initially, IMH was considered as a distinct independent disease manifestation that arises from haemorrhage of the vasa vasorum of the aorta . Increasingly, many authors have reported an overlap between classic AD and IMH [19, 20] and considered that the haematoma results from microscopic tears in the aortic intima . Focal IMH has also been reported in association with PAU . This widely propagated notion has resulted in a ‘classification’ of AAS that is used almost exclusively in the literature, where IMH is categorized as a separate ‘disease manifestation’ along with AD and PAU . Adding to the confusion, the term IMH has been used inconsistently in the literature referring to a ‘disease’ equivalent, a variant of acute AD, and PAU.
Currently, it is accepted that IMH can be a variant or a precursor of AD, and many have regarded IMH as synonymous with a ‘thrombosed type’ or ‘non-communicating’ AD (Fig. 4) [15, 21] where separation of the medial layers still occurs, but this is filled with thrombus rather than flowing blood in what would otherwise be the false lumen of a classic dissection. On initial imaging, like at the follow-up imaging of IMH, small ‘ulcer-like projections’ (ULP)—defined as a localized blood-filled pouch protruding from the true lumen into the thrombosed false lumen of the aorta—can be observed . In this context, the finding is considered to represent the site of an intimal disruption and is therefore a possible indicator of the formation of a flow channel between the true and thrombosed false lumen and hence evolves into classic AD. The term ‘ulcer-like lesion,’ however, is non-specific and has been used in the literature to include also non-penetrating as well as chronic ulcers of the aortic wall. In order to avoid confusion, we advocate using the term ‘primary intimal tear (PIT)’ for an intimal disruption in patients with the dissection variant IMH rather than using ‘ULP.’ The superior visualization of ECG-gated CTA dramatically improves the sensitivity for detecting small communications between the true and false lumen (PIT) in IMH (Fig. 4), which seems to be the elusive link between classic AD and IMH [6, 22]. In fact, a recent autopsy series showed that only 4 % of IMHs have no visible intimal tear; indeed, at the time of surgery a tear is found in most patients .
Overlapping and transitional features of IMH
In addition to dissection variant IMH, a wide range of acute aortic lesions may demonstrate haemorrhagic content within the aortic wall to variable degrees, most importantly PAU, discussed below, but also iatrogenic ADs, traumatic injuries and rupturing aneurysms, etc. In this broad sense IMH can be regarded simply as a non-specific imaging finding, and in fact, cases that were previously categorized as IMH often included a spectrum of unrelated underlying aortic conditions. In order to avoid confusion and misunderstanding, we use the term ‘dissection variant IMH’ for a thrombosed AD that has no complete flow channel, but tiny communications between the true and false lumen commonly exist and clearly show differentiation from other aortic diseases with haemorrhagic content within the aortic wall.
Limited intimal tear
A limited intimal tear (also known as ‘incomplete tear’ or ‘subtle/discrete dissection’) is a rare variant of AD that is the least recognized disease manifestation of AAS [13, 14]. Pathologic descriptions of this type of lesion can be found as early as 1930 by Erheim , who used the term ‘nontraumatic laceration.’ Although the incidence of limited intimal tears is not well known and is likely underestimated because of general unfamiliarity with this dissection variant , the European Society of Cardiology included this lesion in its classification of dissection subtypes (subtle/discrete dissection, class 3) , which was reiterated in the guideline of the American College of Cardiology Foundation and the American Heart Association in 2010 . The limited intimal tear is a subtype of AD that is characterized pathologically by a stellate or linear tear through the intima and underlying superficial media that results in exposure of the deeper media or adventitial layers . The intimal tear extends to a variable degree within the aortic media without significant separation of the medial layers and does not result in a second flow channel, as seen in classic AD .
Discriminating and overlapping features of limited intimal tear
Previously, limited intimal tears might have been diagnosed as other aortic diseases in AAS or missed all together because of the difficulty in detecting this discrete finding with standard imaging modalities [13, 26, 27]. As limited intimal tear remains an unfamiliar entity, the focal bulge associated with this lesion may be incorrectly diagnosed as either an atherosclerotic aneurysm or pseudoaneurysm. As a limited intimal tear and classic AD share the same pathophysiologic process—a weakened medial layer of the aortic wall—these two dissection subtypes may, not unexpectedly, show overlapping features. We have experienced some cases where an initial focal intimal tear evolves into a classic AD locally to form true and false lumens (Fig. 6). Overlapping imaging of features of IMH can also occur as limited intimal tears may also demonstrate a small amount of haemorrhagic content surrounding the tear within the aortic media (Fig. 6) .
Penetrating atherosclerotic ulcer (PAU)
PAU is defined as an atherosclerotic ulceration that penetrates from the pathologically thickened intima through the internal elastic lamina into the media of the aortic wall . PAU is a manifestation of advanced atherosclerosis and therefore a manifestation of a diseased intima (and not the media, as in AD) . The lesion may penetrate even beyond the media and extend through to the adventitia, producing a periaortic pseudoaneurysm and even transmural aortic rupture .
It is important to appreciate the fundamental distinction in the underlying pathological processes between PAU and AD . PAU occurs in the setting of severely and extensively diseased intima in patients with advanced atherosclerosis. Clinically, the typical profile of a patient with PAU is an elderly individual with multiple risk factors for atherosclerosis and often already documented manifestation of atherosclerotic disease, such as coronary aortic disease, cardiovascular disease, peripheral arterial disease or abdominal aortic aneurysm . Patients with AD, on the other hand, are relatively younger and besides hypertension may not have any other risk factors for atherosclerosis [10, 12, 30].
Differentiation between PAU and a dissection variant IMH is clinically important as the prognosis and management differ depending on the underlying disease entity. PAUs arise in segments of the aorta where atherosclerotic changes are more common, and therefore over 90 % of PAUs are localized in the descending thoracic aorta . Nevertheless, irrespective of their location, PAUs tend to have a bad prognosis—even when disease is limited to the descending aorta—with a higher incidence of aortic rupture compared with aortic dissection. For this reason, some authors advocate a more aggressive approach with endovascular treatment for acutely symptomatic PAUs in the descending aorta, as an open surgical approach is generally prohibitively risky in these patients with multiple comorbidities . This is in contrast to acute type B dissection variant IMH or AD, which tends to be more stable and respond well to aggressive medical management. The less common PAU in the ascending aorta should also be managed surgically, like any acute type A aortic lesion.
Discriminating and overlapping features of PAU
Nevertheless, clinical and imaging features of AD and PAU can overlap, especially in older patients, who often have two co-existing pathologic processes (i.e., severe atheroslceorsis, as well as ‘cystic media necrosis,’ which is also seen as part of normal aging). Elderly patients with many comorbidities may possess risk factors that indicate a predisposition to both atherosclerosis and AD simultaneously. In such patients, the differentiation between a limited intimal tear in AD and small atherosclerotic ulcer is sometimes difficult or impossible. In any case, the presence of acute haemorrhage in the aortic wall represents an aortic emergency irrespective of etiology.
Impending rupture of aortic aneurysms
Thoracic aortic aneurysms (TAA) are often clinically asymptomatic and gradually increase in size over time. Although relatively uncommon, TAAs may rapidly increase in size, with increased risk of rupture (Fig. 9). The risk of rupture is closely associated with aneurysm size: for aneurysms greater than 6 cm in diameter, the risk of rupture is significantly higher . Elective repair is indicated to prevent rupture. Rupturing TAAs may present as AAS with acute chest pain . Again, the clinical presentation is indistinguishable from AD or PAU . As such, although not included in the original classification of AAS by Vilacosta , we consider rupturing thoracic aortic aneurysms as one of the entities of AAS.
DeBakey originally used the term ‘dissecting aneurysm’ in 1965 for classic AD , which semantically put ‘aneurysm’ and ‘dissection’ into one disease category. While our understanding of AAS has evolved over the last 50 years, some confusion with respect to those two pathologic conditions still remains. A true aortic aneurysm is defined as dilatation of the aorta that contains all layers of the aortic wall and usually involves the entire circumference . Pathologically, an aneurysm is characterized by deterioration of the aortic wall with loss of elastin and smooth muscle cells . Reed et al. reported the clear etiological difference between degenerative aneurysm and dissection . The most striking difference was the age-specific incidence rates: aortic aneurysm increased steadily with 50 years of age, whereas AD peaked around 65 years and decreased thereafter . Degenerative aneurysms are also considered as a manifestation of atherosclerosis.
Atherosclerotic aneurysm vs. aneurysmal dilatation of various aortic pathologies
The conceptual classification of culprit aortic lesions of AAS into pathologies involving either a diseased media (medial degeneration) or a diseased intima (advanced atherosclerosis) avoids some of the confusing terminology regarding AAS. In addition to the discriminating features of each aortic disease, recognition of the overlapping and transitional features in the dynamic and evolving process of AAS will allow a more comprehensive understanding of their underlying pathophysiologic conditions and their natural history, and may improve strategies for therapeutic management. Understanding these features will provide the radiologist with helpful clues facilitating image assessment, allowing further prediction of the natural disease course as well as permitting appropriate management recommendations.
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