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Non-traumatic acute thoracic aortic syndromes (AAS) describe a spectrum of life-threatening aortic pathologies with significant implications on diagnosis, therapy and management. There is a common pathway for the various manifestations of AAS that eventually leads to a breakdown of the aortic intima and media. Improvements in biology and health policy and diffusion of technology into the community resulted in an associated decrease in mortality and morbidity related to aortic therapeutic interventions. Hybrid procedures, branched and fenestrated endografts, and percutaneous aortic valves have emerged as potent and viable alternatives to traditional surgeries. In this context, current state-of-the art multidetector CT (MDCT) is actually the gold standard in the emergency setting because of its intrinsic diagnostic value. Management of acute aortic disease has changed with the increasing realization that endovascular therapies may offer distinct advantages in these situations. This article provides a summary of AAS, focusing especially on the MDCT technique, typical and atypical findings and common pitfalls of AAS, as well as recent concepts regarding the subtypes of AAS, consisting of aortic dissection, intramural haematoma, penetrating atherosclerotic ulcer and unstable aortic aneurysm or contained aortic rupture. MDCT findings will be related to pathophysiology, timing and management options to achieve a definite and timely diagnostic and therapeutic definition. In the present article, we review the aetiology, pathophysiology, clinical presentation, outcomes and therapeutic approaches to acute aortic syndromes.
Acute aortic syndrome (AAS) is a medical emergency that requires an immediate and accurate diagnosis and treatment.1 The term AAS includes all of the following: aortic dissection (AD), intramural haematoma (IMH), penetrating atherosclerotic ulcer (PAU) and large unstable aortic aneurysm.2–7 All these conditions are associated with a series of signs and symptoms. The most common symptom is chest pain. The grave concern regarding AAS is the possibility of rupture; diagnosis and treatment are vital for patient morbidity and mortality.6 Because the symptoms are vague, they can mimic other acute diseases processes. The incidence of AAS is estimated at 2–6/100,000/year−1 with two-thirds of cases affecting males 63 years old and over.2–4 Multidetector CT (MDCT) is the modality of choice because it has a sensitivity of 100% and specificity of 98–99%.7–9 MDCT is also readily available in most emergency departments and can be performed urgently.
In 1760, Dr Frank Nicholls discovered on necropsy an AD in King George II, which resulted in his demise.10 In the majority of patients (90%), an intimal disruption is present that results in tracking of blood in a dissection plane within the media.6
While the exact mechanism of AD is unclear, it is believed to be most commonly secondary degeneration of the medial layer of the aortic wall (cystic media necrosis), which results in the loss of normal aortic wall compliance and elasticity. It can be accelerated by other conditions such as hypertension and genetic mutations, such as Marfan syndrome, that predispose to premature degeneration of collagen and elastin within the aorta, but can also occur in a normal aorta. Histologically, AD is characterized by an entry intimal tear or primary intimal tear, allowing blood to penetrate and disrupt the aortic media. This subsequently forms a false lumen (FL) parallel to the original aortic true lumen (TL), with the FL causing pressure greater than or equal to the true lumen (Figure 1).1,11 The typical tear is transverse and does not involve the entire circumference of the aorta.12 As the dissection flap is composed of intima and the inner two-thirds of media, intimomedial flap is a more appropriate term. The intima is displaced inward, along with any intimal calcifications.12,13 The intimomedial layer is cleaved both longitudinally and circumferentially for a variable distance. An entry tear can progress along the aortic lumen in either an antegrade or the less common retrograde direction. As the proportion of media involved in the intimomedial flap increases, the external wall of the FL becomes thinner. This increases the risk of aortic rupture.
The most common comorbidity for media layer degeneration and AD is severe arterial hypertension. The constant exposure of the aorta to high blood pressure causes medial disruption and degeneration of the aortic wall. Multiple risk factors are associated with weakening of the aortic media or constant exposure to hydraulic pressure (Table 1).2,3,14,15
The symptoms of AD are severe sudden onset of chest or back pain. It is described as a tearing or ripping pain of knife-like or sharp quality in older adults (i.e. sixth or seventh decade) with poorly controlled hypertension. There is a wide spectrum of pathological conditions associated with AD because of the variable extent of dissection along the blood vessel (Figure 2).6,11 The diagnosis of AD is challenging because there are no biomarkers; thus, imaging is necessary. In 2010, the American College of Cardiology/American Heart Association guidelines proposed a priori a risk assessment tool based on three groups of information—predisposing conditions, pain features and clinical examination. The proposed scoring system graded these factors from 0 (none) to 3.6 The “DISSECT” mnemonic takes several clinical and imaging key factors into account; thus, it can aid in treatment planning (medical, endovascular or open surgical repair) (Table 2).16,17
AD can be classified according to the extent of dissection (Stanford and DeBakey classifications), to the status of blood flow in the FL (communicating and non-communicating types) or to disease phase (acute, subacute and chronic phases). The preferred Stanford classification, proposed by Dailey et al,18 is based on the extent of intimomedial flap, rather than the location of the entry tear. The Stanford Type A dissection (more common in autopsy series) involves the ascending aorta, regardless of the site of origin and is a surgical emergency, whereas the Stanford Type B dissection (more common in radiological and surgical series) affects only the descending aorta and is typically treated with medical management.5,6,12 Accurate classification is important, as it drives decisions regarding surgical vs non-surgical management. The term “complicated” Type B AD means persistent or recurrent pain, uncontrolled hypertension despite full medication, early aortic expansion, organ malperfusion and signs of rupture (haemothorax, increasing periaortic and mediastinal haematoma) and is usually managed by stent-graft endovascular repair (TEVAR).19,20 Imaging and accurate reporting play an essential role in the determination of the therapeutic strategy (Table 3).
Patients with acute chest pain will almost invariably undergo chest radiography (CXR). This is of minimal utility in AAS (sensitivity of 64% and specificity of 86% for overt AD),21,22 apart from its ability to establish or exclude alternate diagnoses. A CXR may show non-specific findings of widening of the aortic contour and mediastinal shadow enlargement; other features may include medially displaced intimal calcification, aortic kinking or opacification of the aortopulmonary window (Figure 3).21–23 Most importantly, the CXR cannot be relied upon to definitively exclude acute aortic disease. Up to 20% of patients with AD will have a normal or near normal CXR.24
The diagnosis of AD by standard transthoracic echocardiography is based on detecting intimal flaps in the aorta. Transthoracic echocardiography is restricted in patients with abnormal chest wall configuration, narrow intercostal spaces and obesity, and these limitations are usually overcome by transoesophageal echocardiography (TOE). A major advantage of echocardiography is its portability, which is useful in patients who are unstable. The disadvantages of TOE are operator dependency, limited acoustic window, innate blind spot (i.e. the distal ascending aorta and proximal aortic arch owing to air in the trachea) and inability to visualize the entire aorta.4–6
A major advantage of MDCT over TOE is the ability to look beyond the aorta for alternative diagnoses such as pulmonary embolism.25 Because of its long acquisition time and inability to monitor patients who are acutely unwell in the MRI suite, MRI is mainly used for the follow-up of chronic AD.24,26 Therefore, the choice of imaging modality for evaluating AD, considering the excellent accuracy of all modalities, should adapt to local expertise and be individualized according to the specific clinical situations.
The advantage of MDCT is in the acquisition of isovolumetric three-dimensional information without loss of spatial resolution in a single breath-hold. The exact protocol for each patient will vary, based upon the vendor and patient body habitus. At our institution, the evaluation of an AD is performed with triphasic CT angiography (CTA), which consists of unenhanced, arterial and then venous phase injection.27,28 Pre-contrast low-dose CT scan is performed with thick collimation and its coverage is from the lung apex to the upper abdomen. Then, a bolus-tracked CT angiogram is performed from the lung apex to the groin using 60–120ml of 370 mglml-1 iodinated contrast material (CM) delivered at a rate between 3 and 6mls−1, according to patient body weight, to achieve a target opacification of the aorta of 250 HU (Table 4). Delayed (venous) scans 1–2min after injection are obtained selectively, to assess for late filling of an FL lumen and to clearly depict abdominal organ malperfusion in AD or contrast extravasation from aortic rupture. Contrast media are administered using automated pump injectors to facilitate multiphasic injections, with sequential administration of CM and normal saline, to enable homogeneous concentrated bolus of CM through the aorta.29 The required amount of CM for CTA may be better predicted by weight-based calculations (average iodine concentration accounting for a flow rate of 1.0–1.6gs−1) than fixed-dose estimation.28 Oral contrast is not necessary unless gas is identified within the endovascular or perivascular soft tissue, or if there is a suspected bronchial/oesophageal vascular fistula. The images, acquired from software-assisted centreline reconstructions, can be used to either generate reliable and reproducible measurements or carefully assess changes in the luminal diameter and contours.
Streak artefacts are generated by high-attenuation prior surgery material, high-contrast interfaces and cardiac motion. Several periaortic structures, such as origins of the aortic arch vessels, left brachiocephalic, superior intercostal and pulmonary veins, may be misinterpreted as double lumina or intimal flaps.29–32 To avoid diagnostic errors, a contrast-enhanced thoracic acquisition should be always obtained with electrocardiogram (ECG) gating to reduce motion artefacts, especially if there is concern of complications involving the aortic valve, aortic sinus, valve plane, aortic root or proximal ascending aorta (Figure 4).33 ECG gating synchronizes the CT scan and the cardiac cycle.
A prospective gating is performed only during a desired phase of cardiac cycle, which is during left ventricle diastole, because the proximal aorta has less motion. In a retrospective scan, the CT scanner is on for the entire cardiac cycle, allowing increased scope for correction of artefacts from dysrhythmias or motion, but comes at the cost of increased radiation exposure.
Since retrospective ECG gating is associated with a significant increase in radiation dose, various dose reduction techniques may be used, such as prospective ECG triggering, ECG-based tube current modulation, automatic exposure control, lower peak kilovoltage and iterative reconstruction algorithms.33 Various post-processing techniques such as multiplanar reformation, maximum intensity projection and volume rendering help facilitate understanding of complex aortic pathology and to expedite communication with the surgeons and the attending physicians.
Although typical AAS 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, both clinical and pertaining to imaging findings, are sometimes definable as atypical and have led to misconceptions and controversies concerning the disease concept of AAS.34–37
The pre-contrast phase is needed to evaluate for displaced intimal calcifications which are suggestive of AAS, IMH and high-density blood in the pericardium, pleural space or mediastinum, indicating aortic rupture (Figure 5).
Typical MDCT findings of AD are direct visualization of media–intima entrance primary tear from TL to FL, as a distinct intimomedial flap defect. The TL is typically smaller and more intensely opacified than the FL in the early angiographic phase owing to higher pressure and faster mixing with blood. The FL is crescent shaped, with acute angles (beak sign=an acute angle between the dissection flap and the outer wall of the FL; the space formed by the acute angle could be filled with a high-attenuation material, contrast-enhanced blood, or a low-attenuation material in chronic dissections, haematoma) between the detached intima and the aortic wall (Figure 6).1,23,25,34–37
Contrast enhancement between the arterial and venous phase is required to differentiate between the TL and partially thrombosed FL. In some cases, media–intima separation is not complete, and cobwebs or tendrils of the media layer (cobweb sign) persist between the intima and media, generally over short segments of the dissection. The TL is identified by tracing back or forth from an uninvolved portion of the aorta; this is not easy, if the aortic root is involved proximally or the dissection extends into the iliac vessels.38
The intimomedial rupture sign is also helpful to distinguish the TL from FL. This sign refers to the discontinued ends of the intimomedial flap at the site of the entry tear that point towards FL (Figure 7).39 It indicates the direction of blood flow through entry tear from TL to FL. However, the direction of blood flow through the entry tear is bidirectional or reversed, depending on the cardiac phase. Intraluminal thrombus is more frequently encountered in the FL (46%) rather than TL (6%) because of a slow flow in the acute setting.36,38
The most common locations of entry tear and maximum hydraulic stress are:
Curved multiplanar reformatted images show entry and re-entry tears more intuitively (Figure 8). The differentiation between TL and FL is difficult, particularly in cases with involvement of the aortic root and especially in those with circumferential dissection involving the root.39 The celiac, superior mesenteric and right renal arteries typically emanate from the TL, and the left renal artery arises from the FL, but variations can occur.25 Depending on the circumference and pressures involved, dilation of the false channel may diminish the TL diameter (compared with that of the FL) until TL thrombosis and collapse; this can cause malperfusion of visceral or peripheral arteries (Figure 9). According to Laplace's law, a large FL is more likely to be associated with aortic rupture than a small one.37
On MDCT, limited intimal tear is described either as an eccentric one-sided bulge or as a minor contour abnormality of the aortic wall that may be the only imaging finding of this lesion. Asymmetrical bulges are sometimes accompanied by haemorrhagic content within the aortic wall on unenhanced MDCT imaging and linear filling defects from subtle undermined edges on MDCT angiography (Figure 13).47
On cross-sectional imaging, impaired perfusion of end organs can be due to four mechanisms (Figure 14):
Depending on the size of the entry tear in the artery, ischaemic organ changes may occur. If the entry tear is small, ischaemia of the organ will be due to hypoperfusion by either the TL or FL, inducing compression of the TL by pressure into the FL. Conversely, a large entry tear in the flap will not lead to vascular compression. Furthermore, the perfusion disturbance can be intermittent if caused by a dissection flap prolapse, or persistent in cases of obliteration of the organ arterial supply by FL expansion.49,50
A large entry tear (>10mm) is a strong predictor of poor mortality and surgical or TEVAR intervention.53 Patients with clear overall TL compression have a higher risk for rapid FL enlargement and further aortic complications. Major predictors of complications after acute phase during follow-up are secondary dilatation of the aorta (a descending aorta diameter >45mm or annual growth >5mm) and a persistently patent FL.
Aortic IMH is defined as a haematoma within the media of the aortic wall due to a spontaneous rupture of the vasa vasorum. In the absence of an FL, an intimal tear can develop. This concept is outdated, and modern imaging technology can often detect small communications between the aortic lumen and the haemorrhage within the wall.54
Current opinion about IMH is that it can be a variant or a precursor of AD with a small intimal defect and thrombosed FL without re-entry tear.11 This type was also defined as “thrombosed-type acute AD”, where the separation of the aortic wall layers is filled with thrombus rather than free-flowing blood in what would otherwise be the FL of a classic dissection.
A circular or crescent-shaped thickening of >5mm of the aortic wall in the absence of detectable blood flow suggests an IMH. Owing to the similarities with AD, IMHs are also classified according to the Stanford classification.55 IMH consists of 5–15% of AAS.
A subintimal hyperdense (60±15 HU) crescent is the most common and important finding in pre-contrast images.7 The aortic lumen is patent: no intimal flap or aortic wall enhancement can be seen. IMHs have a smooth lumen–wall interface, with visualization of the subintimal semi-circular or curvilinear calcifications (Figure 15). The absence of obvious communication between the TL and FL explains the absence of flow on colour Doppler flow and the lack of enhancement on CT or MRI.
In acute IMH, imaging should always include a thorough attempt to localize a primary (micro) entry tear, which is very often present and directs the course of treatment, especially when considering TEVAR.56 One way to differentiate IMH from a thrombosed FL of AD is that IMH maintains a constant circumferential relationship with the aortic wall, whereas an AD tends to spiral longitudinally (Figure 16).
Details required from imaging are localization and extent of aortic wall thickening, coexistence of atheromatous disease (calcium shift) and the presence of small intimal tears.5 Patients should be closely followed up in the first 30 days.
The findings of a dissection-variant IMH are as follows:
In PAU (estimated incidence between 2.3% and 7.6%), an atherosclerotic plaque ulcerates and disrupts the internal elastic lamina. This suggests a diseased intima and occurs more commonly in the descending thoracic aorta (>90%) of elderly individuals with multiple risk factors for atherosclerosis and associated comorbidities of atherosclerotic disease.5,6,11,63 There is currently no clear cut-off for PAU diameter (depth) or neck diameter that warrants treatment; in one publication, a depth of >20mm or a neck >10mm was associated with higher complication rates.64,65
On MDCT, localized ulceration penetrating through the aortic intima is the characteristic finding. There is also an outpouching of the outer aortic contour, which differentiates uncomplicated atheromatous ulcer from PAU. A PAU is diagnosed by demonstrating a focal contrast-filled outpouching of the aortic wall with jagged edges usually in the presence of extensive aortic atheroma; unenhanced MDCT frequently (about 80%) shows high-density haematoma surrounding the ulceration (Figure 19).7,25 Invasive treatments such as surgery and stent grafting are indicated in acute unstable or symptomatic cases (rapid expansion of the aortic diameter, persistent or recurrent pain, development of pseudoaneurysm, pericardial effusion, bloody pleural effusion or distal embolization), but because they occur mainly in patients with severe comorbidities, course observation including periodical evaluation using imaging techniques is recommended in asymptomatic or chronic cases.66,67
Unstable thoracic AA is a part of AAS, and if it is symptomatic, it is clinically indistinguishable from AD, IMH or PAU.9,34 A thoracic AA is unstable, if it shows rapid enlargement and/or signs of impending rupture:
The detection of these findings advocates urgent treatment (early surgery or TEVAR). Patients with contained rupture should be managed with permissive hypotension to prevent a free rupture and keep the patients stable until treatment.69
Advances in MDCT hardware (wide detector panel, gantry rotation speed reduction and new detector composition and structure) and software post-processing evolution (automatic CAD of vessel stenosis, three-dimensional modelling, calcification subtraction and enhancing) are opening a new era, which will improve patient safety (reduction of dose and CM), reduce cost and improve patient care in this group of patients.74–81 With better understanding of predisposition and genetic risk, acute AD might soon become predictable and, to some degree, preventable by new biomarkers. The description of any given dissection will be individualized by addressing specific features rather than using simple classification systems of the past. The ongoing advancement of endovascular techniques (fenestrated and branched stent grafts) to the most challenging segment of the aorta (ascending and aortic arch) will revolutionize the treatment of aortic disease.
AAS comprises interrelated emergent aortic conditions with a similar and overlapping clinical presentation. Recent advances in imaging and therapeutic techniques have further emphasized the importance of early diagnosis of AAS, which continues to be crucial to survival. Immediate diagnostic imaging (TOE and MDCT) plays a pivotal role in management in the emergency setting. To minimize diagnostic error, the radiologist should be familiar with the spectrum of clinical presentations for AAS, the selection and optimization of imaging techniques, as well as with the key concepts behind the common and uncommon imaging features encountered. Technological, biological and therapeutic advancements have already led to an important new clinical paradigm, a multidisciplinary team is necessary to provide optimal outcomes for these patients.
The authors thank Dr Refky Nicola, Rochester, New York, for his precious comments and English language corrections.