20.2.1

Introduction

The term “acute aortic syndrome” (AAS) is often used to refer to a number of potentially life-threatening diseases, which can all present with acute chest pain [ ]. Traditionally, AAS encompasses three entities: classic aortic dissection (AD), intramural hematoma (IMH), and penetrating atherosclerotic ulcer (PAU). While these components of AAS have historically been considered separate entities, this view has been recently challenged. Improvements in CT imaging, including the introduction of ECG-triggered acquisitions, have increased the available imaging detail of the aortic wall leading to a better understanding of the underlying pathophysiological mechanism [ ]. It has also been demonstrated that the different AAS entities have both discriminating and overlapping features, which can dynamically evolve over time [ ]. This is especially true for classic aortic dissection and intramural hematoma, which can be considered two different presentations of the same pathologic process arising from the aortic media. Conversely, a PAU has a completely different etiology, as it is the consequence of an atherosclerotic degeneration of the intimal layer.

20.2.2

Clinical presentation

The clinical presentation of AAS is often non-specific, with a common denominator of severe chest pain. In acute aortic dissection this pain is often described as intense, acute, shearing/tearing, throbbing, or sometimes migratory. When the ascending aorta is involved, pain can be referred to the anterior chest, throat, neck, and jaw. When the abdominal aorta is also affected, patients can have back and abdominal pain. Furthermore, patients can develop ischemic symptoms due to stenosis and occlusion of side branches of the aorta by the dissection flap.

Given the myriad of potential clinical complaints, clinical features are as such unreliable to differentiate between the different etiologies of AAS and other important entities such as myocardial infarction and pulmonary embolism [ ]. Additional non-invasive imaging is therefore needed to establish the correct diagnosis and initiate the appropriate treatment.

20.2.3

Classification

Two classifications of aortic dissection exist: the DeBakey classification and the Stanford classification [ , ]. The DeBakey classification distinguishes three different types of dissection. Type I involves the ascending aorta, the arch, and a variable length of the thoracic and abdominal aorta. Type II is restricted to the ascending aorta. Type IIIA is confined to the descending thoracic aorta and type IIIB extends in the abdominal aorta to the bifurcation/iliac vessels.

The Stanford classification only distinguishes between two types: type A and B aortic dissection. Type-A is defined as involving the ascending aorta, regardless of further involvement of the aortic arch and descending aorta. A Stanford type-B dissection includes every dissection in which the ascending aorta is not involved.

Many centers routinely use the Stanford classification as the distinction between a dissection with its origin proximal or distal from the ostium of the left subclavian artery to quickly differentiate between different treatment strategies (in general urgent surgery for type-A dissection vs. strict hypertension control and follow-up for uncomplicated type-B dissections).

20.2.4

Classic aortic dissection—Intramural hematoma

An aortic dissection is caused by a primary intimal tear. While the exact etiology of this entry tear is often unknown, several predisposing conditions exist which can affect the integrity of the media ( Table 20.1 ). One of the most common risk factors is long-standing arterial hypertension, which increases shearing forces on and augments stiffness of the aortic wall. Other causes include connective tissue disorders like Marfan and Ehlers-Danlos syndrome, and traumatic aortic deceleration injuries.

Once an intimal tear has developed, this acquired entry point allows blood from the aorta to disseminate in the media layer and extend both proximally and distally with inward displacement of the intima ( Figure 20.1 ). The entry tear is typically located at the site of highest wall tension, most often a few centimeters above the aortic valve or at the attachment site of the ligamentum arteriosum just distally from the left subclavian artery origin ( Figure 20.1 ).

Contrast-enhanced CT-scan of a Stanford Type-B aortic dissection. The axial image (a) clearly shows a defect in the intima wall (arrow), allowing blood to flow from the original true lumen (T) into the vessel wall, as such, creating a second false (F) lumen. A second oblique sagittal reformatted image of the same patient (b) shows that the resulting dissection flap (arrow) has its origin just distally from the ostium of the unaffected left subclavian artery, a typical location for a classic Stanford type-B dissection.

This intramural blood flow and the consequent dissection of the aortic wall commonly lead to a double-barrel lumen consisting of two channels: a true and a false lumen. The false lumen is completely contained within the aortic media, and divided from the true lumen by a dissection flap consisting of intima and a portion of the media layer.

In a classic dissection , both an entry and exit site exists for the intramural flowing blood. Consequently, both true and false lumina contain flowing blood, with faster flow in the true lumen. As pressure builds in the false lumen, the cross-sectional diameter of the false lumen decreases and often becomes the smallest channel ( Figure 20.2 ).

Contrast-enhanced CT-scan of a Stanford type-A aortic dissection. The axial CT image shows a double-barreled aortic lumen, consisting of a small true lumen (T) and a larger false (F) lumen. Note the difference in contrast enhancement between these two channels, reflecting a different flow speed. The slower flow in the false lumen further contributes to the forming of thrombus material adjacent to the posterior aortic wall (*). Also, there is an overall increase in diameter of the affected aorta as a post-dissection aortic dilatation settles in secondary to decreased wall strength. Finally, a pericardial effusion can also be appreciated (arrow), a common finding in Stanford Type-A aortic dissections.

Conversely, an intramural hematoma is best understood as a dissection in which there is a clear intramural entry site, but no clear or adequate exit site for the blood back in the true lumen. As such, the false lumen contains thrombosed blood, and remains the smallest channel next to a larger true lumen ( Figure 20.3 ). It is important to realize that over time, an exit site can develop, allowing an intramural hematoma to transgress into a channel with flowing blood and reaching the configuration of a classic dissection ( Figure 20.4 ).

Contrast-enhanced CT-scan of an aortic dissection initially originating in the proximal descending aorta, but with retrograde extension into the ascending aorta. The axial image (a) shows a clear defect in the aortic wall (arrow), as entry site for a classic Stanford type-B aortic dissection with two channels of flowing blood: the true (T) and false (F) lumen. The sagittal reformatted image (b) shows retrograde extension of this dissection into the aortic arch and ascending aorta. However, in contrast to the initial rupture in the descending aorta, the dissection flap (arrows in b) does not have a re-entry site. As a consequence, in the ascending aorta the false lumen contains stagnant blood, with thrombus from the aortic sinus upwards, as such forming an intramural hematoma (IMH in b, c). An axial image at a mid-level of the ascending aorta (c) shows that the intramural hematoma (arrows) has a slight hyperdense appearance, and can be distinguished from a large pericardial effusion (asterisk). These images show an initially Stanford type-B dissection with a secondary retrograde acquired Stanford type-A dissection, in which both intramural hematoma and classic dissection are simultaneously present in the same patient.

Contrast-enhanced CT examinations of the same patient within a time span of a week, initially presenting with acute chest pain. The first CT examination (a, b) shows a Stanford type-B dissection starting just distally from the ostium of the left subclavian artery and ending just above the renal arteries. However, while the entry site is clearly visible (arrow in a), there is no identifiable exit site. As such, the false lumen if formed by an intramural hematoma consisting of mostly stagnant blood (asterisk in a, b). Six days later, the patient was re-evaluated on clinical grounds. The second CT examination (c, d) now shows multiple new defects in the dissection flap (asterisks in c, d). With the development of these multiple entry and exit sites, the false lumen contains now flowing blood. The now increased pressure in the false lumen pushes the dissection flap inwards (arrow in d), with true and false lumen now having similar dimensions.

20.2.5

Imaging techniques

Many imaging techniques can be used to visualize an aortic dissection, each with their specific strengths and weaknesses. An overview is given in Table 20.2 .

20.2.5.2

Ultrasound

Ultrasound techniques available for the detection of AAS include transthoracic and transoesophageal echocardiography (TTE/TOE).

Both TOE and TTE are non-invasive, widely available, and relatively inexpensive imaging modalities. They can also be performed bedside in a critically ill patient.

TTE is a valid imaging choice to detect complications involving the aortic valve, for example, aortic regurgitation. However, a large part of the ascending aorta and aortic arch are often not visualized with TTE due to poor acoustic window. TOE is superior in depicting the ascending aorta and therefore useful in the further evaluation of a potential type-A dissection ( Figure 20.5 ). Also, it can simultaneously visualize complications such as aortic regurgitation (in a dissection involving the aortic sinus), pericardial effusion, coronary artery involvement, and left ventricular function. It is less suitable for evaluation of the distal ascending aorta and proximal aortic arch. TOE can also detect localized wall thickening in patients with intramural hematoma.

Parasternal long-axis transthoracic echocardiographic image demonstrating a Stanford type-A dissection with a hyperechogenic dissection flap (arrow). Ao: aorta, asterisk: aortic valve.

Nevertheless, both TTE and TOE are operator-dependent techniques, which are unable to visualize the complete thoracic aorta, and can produce false-positive results due to reverberation artifacts. As such, they are rarely relied upon as the sole diagnostic imaging modality in patients with suspected acute aortic syndrome and have a more complementary function.

20.2.5.3

Computed tomography

Computed tomography angiography (CTA) is the imaging technique of first choice in patients with acute aortic syndrome. CTA is the only imaging modality that can provide an accurate and complete assessment of the whole thoraco-abdominal aorta and side-branches in a very short examination time, with a reported sensitivity and specificity for aortic dissection of more than 95%. Furthermore, it can simultaneously evaluate all other non-vascular structures (such as the pulmonary parenchyma and visceral abdominal organs), thus allowing detection of an alternative etiology for the acute clinical presentation.

In practice, all CTA examinations are invariably performed with the use of intravenous contrast administration for optimal vascular enhancement and evaluation. However, an initially performed non-enhanced CT-scan can have incremental value in patients with suspected aortic dissection, as it can depict a hyperdense acute intramural hematoma ( Figure 20.6 ), that may not otherwise be seen. Our recommendation is to add an unenhanced examination to your institutional CT protocol for evaluation of patients with suspected aortic dissection.

CT examination of a type-A dissection presenting as an intramural hematoma. Axial (a) and sagittal (b) unenhanced CT images. The spontaneous hyperdense acute intramural hematoma (asterisks) is clearly seen on these unenhanced CT images. Note the typical semicircular crescent-like shape of the hematoma, and the extension into the aortic sinus (arrow in b).

Despite all the clear advantages of CTA over other imaging modalities, its use of an iodinated contrast agent and associated radiation exposure limits its application in patients with impaired renal function and known intolerance to iodinated dye. Radiation exposure is especially an important issue, as patients with a stable type-B dissection require follow-up imaging over years to monitor for the development of (among others) a post-dissection aneurysmal dilatation of the affected aortic segments. Consequently, and especially in young patients, many centers often use MRI for follow-up of stable patients with a type-B dissection. However, the recent introduction of iterative reconstruction techniques has made radiation dose less of a concern [ , ].

20.2.5.4

Magnetic resonance imaging

Both non-contrast (phase-contrast and time-of-flight techniques) and contrast-enhanced acquisitions are available to visualize the thoracic aorta with MRI. However, non-contrast techniques have significant drawbacks, including a longer examination time, a shorter anatomic coverage, and greater difficulty in differentiating between slow-flow effects and the presence of thrombus. Consequently, contrast-enhanced MR angiography (MRA) is the method of choice to evaluate the thoracic aorta for the presence and extent of dissection.

MRA has similar advantages over echocardiography as CT, including a comparable high sensitivity and specificity (> 95%) to diagnose aortic dissection, a large anatomic coverage, and the evaluation of the relation of the dissection flap with aortic side branches. Additionally, given the lack of radiation exposure, multiple acquisitions are possible to visualize the dynamics of contrast enhancement of the true and false lumen with no compromise to the patient’s safety ( Figure 20.7 ). The use of Gadolinium-chelates as a contrast agent makes MRA also the technique of choice in patients with impaired renal function. It is also a good choice for repeated follow-up of patients with a known and stable (type-B) dissection to monitor for the presence and evolution of a post-dissection aortic aneurysm. Finally, it has increased sensitivity compared with CT to detect an intramural hematoma, providing as such additional value to CT in equivocal cases.

Follow-up of a Stanford type-B aortic dissection in a 52-year-old man. Sagittal reformatted T1-weighted images after intravenous contrast administration, acquired in arterial (a) and venous (b) phase. These MR angiography images demonstrate the presence of a double-barreled aortic lumen, consisting of a true (T) and false (F) lumen. In the first-pass arterial images (a) the true lumen is nicely enhanced with almost no opacification of the false lumen. The image acquired 60 s (b) later now shows the delayed enhancement of the false lumen, compatible with slower flow. In this contrast phase, the contrast in the true lumen has almost completely washed out. This example shows the dynamics on contrast-enhancement of the different lumina within an aortic dissection.

However, it is a technically more complex study with a longer examination time, requiring a higher level of patient cooperation compared with CT. Therefore, it is less suitable for critically ill patients. Also, while the anatomic coverage acquired during a single study is generally large enough to cover the thoracic and supra-renal aorta, contrary to CT, most MR systems cannot cover the whole aorta and iliac arteries in a single examination. These disadvantages make MRA rarely the first imaging choice in a patient with a suspected aortic dissection.

20.2.6

Structured reporting

Communication of the imaging findings to the surgeon and clinician is facilitated when using a logically structured report. A summary of the most important points to evaluate can be found in Table 20.3 .

Generally, echocardiography will be used to evaluate the presence and extent of a dissection in the aortic root and ascending aorta. It also provides excellent assessment of the functional cardiac repercussions in general, and more specifically at the level of the aortic valve.

A more complete and accurate evaluation of an aortic dissection can be delivered with contrast-enhanced CT or MR. Here, the presence and extent of the dissection must be accurately evaluated, as well as the involvement of the supra-aortic and splanchnic side branches.

Finally, the possibility of treatment using an endovascular approach in type-B dissection can be evaluated, discussing entry and exit sites and potential sealing locations.

20.3.1

Introduction

A broad variety of diseases can lead to symptomatic inflammatory changes in the arterial wall. In clinical practice, the distinction is made between large vessel vasculitis (i.e., giant cell arteritis and Takayasu arteritis), and medium vessel and small vessel vasculitis [ ]. In this chapter, we will focus on thoracic aortitis, which can have different infectious and non-infectious causes ( Table 20.4 ).

20.3.2

Clinical presentation

The initial clinical presentation of aortitis is often characterized by non-specific symptoms like persistent fever without a clear origin, malaise, night sweating, and arthralgia. In patients with a late clinical presentation symptoms related to complications (stroke, myocardial infarction, heart failure, aortic rupture, and ischemia distal to a stenosis) can often be observed [ ]. In patients with giant cell arteritis, a minority (15%) show involvement of extracranial vessels. In this subgroup of patients, amaurosis fugax may be the presenting symptom if the ophthalmic artery is involved. In contrast, Takayasu arteritis develops characteristically in the thoracic aorta with involvement of the subclavian arteries. It can extend into the abdominal aorta/iliac and femoral vessels and cause hypertension due to occlusion/stenosis of the renal arteries ( Table 20.5 ).

20.3.3

Imaging techniques

Several imaging modalities are used and suitable for evaluation of patients with (suspected) thoracic aortitis. The advantages and disadvantages of the different techniques are listed in Table 20.6 .

Table 20.6

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