Chapter 22. Diseases of the Aorta

Anatomy of the Aorta

To have an understanding of aortic diseases, it is imperative to have a good knowledge base in anatomy, physiology, and function of the normal aorta. The aorta is a large conductance blood vessel, often called the greatest artery, normally reaching 0.7 m in length and 3.5 cm in diameter. It arises from the heart, arches anterocranially and then posterocaudally, and descends caudally, terminating as it bifurcates into the right and left common iliac arteries. The aorta is divided into six segments: (1) the aortic root, (2) the sinotubular junction, (3) the ascending aorta, (4) the aortic arch, (5) the isthmus and descending thoracic aorta, and (6) the abdominal aorta (Fig. 22–1).

The aortic root originates at the aortic valve annulus level and extends to the sinotubular junction. Normal diameter of the aortic root (at the widest proximal portion, known as bulbous) is up to 3.5 cm. The bulbous portion of the aorta is subdivided into the sinuses of Valsalva. The site where the bulbous portion of the aorta meets the narrower tubular-shaped aorta is termed sinotubular junction. Effacement of this junction suggests annuloaortic ectasia and often is seen in patients with Marfan syndrome. The ascending aorta follows the sinotubular junction and extends to the right brachiocephalic artery. The ascending aorta is an intrapericardial structure, measuring approximately 5 cm in length and up to 3 cm in diameter. The aortic arch is extrapericardial and courses leftward anterior to the trachea and continues posterior to the left of the trachea and esophagus and gives rise to all three great vessels: the innominate artery (brachiocephalic trunk), left common carotid artery, and left subclavian artery (Fig. 22–2). The normal diameter of the aortic arch is up to 2.9 cm. Arch branch vessel variants and right-sided arches can sometimes be seen. The isthmus is the narrower portion of the aorta (by approximately 3 mm) between the left subclavian artery and ligamentum arteriosus, a remnant of the ductus arteriosus. Blunt traumatic deceleration injury, resulting in transection to the aorta often occurs at this site. The descending thoracic aorta begins at the ligamentum arteriosus and continues to the level of the diaphragm. The descending thoracic aorta gives rise to intercostal, spinal, and bronchial arteries. The abdominal aorta starts at the hiatus of the diaphragm and courses retroperitoneal to its bifurcation. Major branch vessels include celiac arteries, superior and inferior mesenteric arteries, and middle suprarenal, renal, and testicular or ovarian arteries. The diameter of the suprarenal abdominal aorta is usually up to 2 cm. The infrarenal aorta diameter should never exceed that of the suprarenal aorta.

The aorta has a trilaminar wall comprised of tunica intima, tunica media, and tunica adventitia. The tunica intima consists of an endothelium, subendothelial tissue, and an internal elastic membrane. The tunica media is composed of elastic fibers arranged as circumferential lamellae, elastin, collagen, smooth muscle cells, and ground substance. Adventitia is a thin, outermost layer of the aorta consisting of collagen; vasa vasorum, which is the vascular supply to the aorta; and nervi vascularis, which is the network of primarily adrenergic fibers.

Physiology and Function of the Aorta

Elastin-to-collagen ratio in the tunica media progressively decreases from proximal aorta to distal aorta and to peripheral arteries. This architecture ensures accommodation of stroke volume (cushioning function) and storage of potential energy in systole (capacitor function) and release of kinetic energy in diastole, propelling blood forward (Windkessel phenomenon). In addition, this is responsible for progressive increase in pulse wave velocity of propagation from the proximal aorta to peripheral vessels. In the normal aorta, the pulse wave is transmitted down the aorta to its bifurcation and peripheral vasculature where it is reflected back toward the heart during diastole, contributing to diastolic pressure. These properties usually decline with aging or diseased aorta due to disruption of elastin and decreased elastin-to-collagen ratio. In the diseased aorta, the propagation wave is faster, which causes an earlier return of the reflected waves (during systole) and a summation with the systolic wave. This leads to self-perpetuation of hypertension. In addition, increased aortic stiffness has been associated with increased risk of myocardial infarction, stroke, congestive heart failure, and both cardiovascular and overall mortality.

Aortic pressure waveform can be distinguished by its four components: (1) the upslope, (2) the peak, (3) the dicrotic notch, also known as incisura, and (4) the pressure decay. The slope of the upslope and the peak are dependent on the elasticity of the vessel and the volume of blood entering the aorta. Dicrotic notch is only seen in the proximal aorta and results from the aortic valve closure. The pressure decay corresponds to diastolic pressure and is affected by peripheral resistance and reflected pressure waves (in the healthy aorta). There is a progressive decrease in mean aortic and diastolic pressures and an increase in systolic aortic pressure with increasing distance from the heart.

Imaging Considerations

Chest Radiography

The aortic "knob" may be seen in the superior mediastinum, on the left side, just lateral to spinal column on the chest roentgenogram. The aortic root and proximal ascending aorta are visible as they arise from the base of the heart on the lateral chest roentgenogram. Left anterior oblique projection provides the best view of the ascending aorta and the arch. Intimal calcifications and aneurysmal dilation may be seen.

Advantages of chest radiography include wide availability, no contrast exposure, low cost, minimal radiation exposure, and the ability to be performed quickly at the bedside. Disadvantages include low spatial resolution, two-dimensional (2D) static image, and relatively poor target acquisition for aortic diseases (Table 22–1).

Echocardiography and Ultrasonography

The aortic root, sinuses of Valsalva, sinotubular junction, and proximal ascending aorta are usually well seen on a parasternal long axis view by transthoracic echocardiography (TTE).1 However, mechanical and stented bioprosthetic aortic valves produce acoustic shadows and reverberations. Distal ascending aorta and the arch are less well visualized by TTE. Suprasternal notch and supraclavicular windows offer the best views of the aorta in long and short axes. TTE allows visualization of the retrocardiac aorta in short and long axis (with clockwise rotation and lateral angulation of the transducer) on parasternal long axis view. Lateral angulation of the image plane in the apical two-chamber view can also be used to image the descending thoracic aorta. A posterior chest wall approach is particularly useful for imaging of the descending aorta in patients with large left pleural effusion. In the subcostal window, a lateral angulation of the transducer in the inferior vena cava plane allows visualization of the proximal abdominal aorta.

Transesophageal echocardiography (TEE) at 30° to 45° and 120° to 140° rotation at mid to high esophageal level is particularly helpful and less artifactual in evaluation of the aortic root, sinuses of Valsalva, sinotubular junction, and proximal ascending aorta. Withdrawing the transducer in the esophagus in the esophageal long axis plane produces more cephalad images of the ascending aorta. Medial turning and inferior angulation of transesophageal transducer at the arch level provide superior detail of the aortic arch in the long axis plane. Transesophageal transducer maybe turned around (either direction) at 0° to obtain a short axis view of the descending aorta. Rotation of the image plane to 90° allows depiction of the aorta in the long axis view, and the left subclavian artery may be located in this plane.

Ultrasonography (US) is a useful modality for abdominal aorta examination. It is commonly used for aortic disease screening, sequential monitoring for the assessment of aortic size, and evaluation of branch vessel involvement.

Advantages of echocardiography and ultrasonography include superb temporal resolution (especially with M-mode imaging), three-dimensional (3D) imaging, ability to assess functional cardiac complications of aortic diseases, wide availability, no contrast or ionizing radiation exposure, relatively low cost, and ability to be performed quickly at the bedside. Disadvantages include suboptimal target acquisition, inability to image the entire aorta, and can be time consuming.

Computed Tomography

The aortic root, sinotubular junction, and ascending aorta have significant motion throughout the cardiac cycle, and thus are prone to significant motion artifact with non–electrocardiography (ECG)-gated imaging modalities. ECG gating is recommended when imaging these proximal structures by either computed tomography (CT) or magnetic resonance angiography (MRA). The aortic arch, isthmus, and descending aorta are subject to little motion artifact.

CT allows various postprocessing displays and generates very detailed images of the arterial wall and lumen with high spatial resolution. Pacemaker wires in the superior vena cava (SVC) and brachiocephalic vein, mechanical aortic valves, pericardial structures, and contrast dye concentrated in SVC may produce artifacts when the aortic root, ascending aorta, and aortic arch are imaged with CT. The prevalence of motion artifact on a 1-second scanning time CT is reported to be as high as 57%.2 Multidetector CT (MDCT) technology significantly reduces respiratory artifacts and improves temporal resolution due to reduction of scanning time to 0.5 seconds. However, non–ECG-gated MDCT was reported to produce motion artifacts in 91.9% of cases.3 Fujioka et al4 reported a prevalence of diagnosis nonhampering motion artifact in 6.7% of cases with ECG-gated MDCT. In the absence of spinal prosthesis, CT of descending aorta produces very few artifacts.

Advantages of CT include 3D imaging, superb spatial resolution, excellent temporal resolution, very fast scanning time (<1 minute), high sensitivity and specificity for the assessment of aortic disease, postprocessing reconstruction, and wide availability. Disadvantages include ionizing radiation and contrast exposure.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) and MRA can accurately depict the aortic lumen and the wall with minimal artifacts. The artifacts that one can encounter, however, include signal dropout from metallic clips and mechanical valves; ghosting artifacts (encountered when images are acquired during systolic phases of the cardiac cycle); insufficient dose of contrast agent (results in low signal-to-noise ratio); wraparound artifact, usually in coronal plane (aliasing due to data sampled in a discrete rather than continuous fashion); and susceptibility artifacts from concentrated contrast agent in the nearby venous structures.

MRI and MRA offer the ability to assess the aorta in both a 2D and 3D format. The number of different pulse sequences can be used to assess function and anatomy. A typical evaluation for the aorta would begin with anatomic assessment with axial, sagittal, and coronal dark blood turbo spin-echo (TSE) pulse sequence covering the entire thoracic or abdominal aorta. The dark blood TSE sequence obtains all the data for the particular slice within one heartbeat and thus allows for multiple slices without requiring the patient to hold their breath. Once the anatomic imaging is performed, cine images with a gradient echo pulse sequence are performed to assess the function of the aorta as well as the aortic valve (AV) when necessary. This image is generally performed in an oblique plane by scouting off of the dark blood TSE images. The next step in the evaluation includes typically a contrast-enhanced angiogram, which is gated to the heart (this avoids motion artifact within the proximal ascending aorta). The angiogram is a breath-hold sequence, which typically takes approximately 15 to 20 seconds to complete. If necessary, when blood flow needs to be quantified (ie, coarctation of the aorta), velocity-encoded pulse sequence is performed to assess the velocity of blood through the aorta at the level in question. This is the standard exam for the assessment of the aorta and can be tailored for each specific disease. The TSE dark blood sequence is useful in assessing the aortic wall, aortic dissection (AD), and other anatomic structures/abnormalities. If higher resolution is needed, then a single slice breath-hold TSE could be performed. This is often necessary when one wants to assess the aortic wall as in intramural hematoma or plaque assessment. The cine imaging is useful for more functional aspects of the aorta as in coarctation, AV insufficiency, and aortic distensibility. The MRA allows for a very high-resolution image of the aorta and is excellent in AD assessment and the 3D assessment of aneurysms. The benefit of 3D imaging is that the true cross-sectional diameter of an aneurysm can be easily assessed via postprocessing computer software.

Advantages of MRI include excellent temporal and spatial resolution, the wide spectrum of pulse sequences allowing for a 3D comprehensive assessment of the aorta, excellent target acquisition, and no ionizing radiation or contrast exposure. Disadvantages include lack of access at some centers, longer scanning time (can be prohibitive in patients who are hemodynamically unstable), artifacts due to respiratory motion in patients who are unable to breath-hold), inability of patients with large body habitus to fit inside the scanner, contraindication in patients with metal implants (eg, pacemakers/defibrillators, cerebral aneurysm clips), and the possibility that some patients may experience claustrophobia.

Aortography

Aortography is an invasive test during which a multiholed catheter (usually pigtail) is passed percutaneously via radial, brachial, axillary, or femoral approach into the aorta; contrast material is injected via a power injector; and multiple radiographic views (or cine) are recorded. The ascending aortogram is often performed in a moderately steep, 30° to 60° left anterior oblique view. In the past, aortography was a first-line diagnostic test for imaging the aorta; presently, it is typically reserved for guiding aortic interventions.

Advantages of aortography include superb spatial resolution, excellent target acquisition, and good temporal resolution. Disadvantages include invasiveness, exposure to ionizing radiation and contrast, lack of access in some centers, production of 2D images, and time consuming to perform.

Aortic Root Variants and Anomalies

In approximately 1% of healthy individuals, the AV is bicuspid forming two sinuses, rather than the usual tricuspid valve with three sinuses. Quadricuspid AVs with four sinuses are a rare finding with incidence of up to 0.01%. Bicuspid AVs may be associated with various pathologies of the ascending aorta and may result in AD. Anomalies of coronary artery ostia are common. These include separate ostia for left anterior descending artery and left circumflex artery, one or more infundibular (conal) arteries arising from separate ostia, duplicated arteries, high take-off (above sinotubular junction), and ostium arising from opposite sinus or pulmonary trunk.

Aortic Arch Variants and Anomalies

The second most common pattern of the aortic arch is a common origin of innominate and left common carotid arteries, and a less common variant occurs when the left common carotid artery originates directly from the innominate artery. Both are commonly referred to as bovine-type arch. However, a true bovine aortic arch has no resemblance to any of these variations. In cattle, a single brachiocephalic trunk originates from the aortic arch and gives rise to both subclavian arteries and a bicarotid trunk, which then branches into right and left common carotid arteries. Other variants include internal mammary artery originating from the thyrocervical trunk and left vertebral artery coming off the aortic arch. Kommerell's diverticulum is a diverticulum of the proximal portion of an aberrant left subclavian artery (occurs with right-sided aortic arch) or right subclavian artery (occurs with left-sided aortic arch), potentially compressing the esophagus posteriorly. Double aortic arch is a common form of vascular ring in which the trachea and esophagus are encircled by the aortic arches (right arch tends to be larger). The most common form of right-sided aortic arch is associated with an aberrant origin of the left subclavian artery, diverticulum of Kommerell, and left-sided ligamentum arteriosum. The second most common right-sided aortic arch is associated with brachiocephalic vessels originating from the arch in mirror-image fashion with the left innominate artery. The other two types are rare. A cervical aortic arch is a rare entity, sometimes associated with other cervical vessel anomalies and commonly presenting as a pulsatile mass on either side of the neck or supraclavicular area. Aortic spindle is a circumferential bulge in the posterior aortic arch area just distal to the left subclavian artery. Pseudocoarctation develops from elongation of the aortic arch and kinking at the level of insertion of the ligamentum arteriosum without a pressure gradient across this region (Fig. 22–3). Other very rare anomalies include double dorsal aorta, subclavian artery as the first branch of the aortic arch, persistence in the fifth aortic arch, and pulmonary artery sling. The great anterior radiculomedullary artery (the artery of Adamkiewicz) is the dominant feeder of the spinal cord. Therefore, in patients with thoracoabdominal aortic aneurysms, identification of this artery is critical to minimize the risk of postoperative spinal cord ischemia. The artery of Adamkiewicz originates from a left intercostal or lumbar artery, but in up to 75% of individuals, it arises from T9 to T12 intercostal artery. Ductus diverticulum is a focal, smooth convexity in the ventromedial aspect of proximal descending aorta. The important clinical significance of this structure lies in distinguishing it from a traumatic aortic disruption.

Incidence, Etiology, and Pathophysiology

Acute AD is an uncommon but catastrophic aortic disease. AD may frequently be overlooked, with the diagnosis in a significant number of cases being established only postmortem. The estimated incidence is up to 3.5 per 100,000 per year, with at least 7000 cases per year in the United States.5 Although early mortality was estimated at 1% per hour for untreated cases, the survival may be dramatically improved with timely management. In 1996, the International Registry of Acute Aortic Dissection (IRAD) was established, with the mission to better understand the presentation, diagnosis, management, and outcomes of patients presenting with acute AD. To date, IRAD has enrolled more than 2000 patients from 26 institutions and enlightened the understanding of this very important disease.6

Among the risk factors of AD, the most common are hypertension, advanced age, vascular atherosclerosis, preexisting aortic aneurysms, iatrogenic during cardiac surgery and percutaneous procedures, pregnancy (usually peripartum period), bicuspid AV, direct blunt trauma, Marfan syndrome, Ehlers-Danlos syndrome, inflammatory diseases, and cocaine use.

AD usually begins with an intimal tear (spontaneous due to diseased aortic wall, known as cystic medial degeneration, or iatrogenic, as a complication of various percutaneous or surgical procedures), allowing luminal blood to penetrate the medial layer, dissecting it longitudinally and forming the false lumen. Typically, the dissection extends anterogradely to or beyond common iliac arteries. The false lumen is often lateral to the true lumen in the descending aorta; however, medial or spiral (barber pole) orientations are described. AD may also be initiated with the rupture of vasa vasorum, resulting in aortic hematoma. Occasionally, the hematoma may extend distally or rupture through the intima causing an intimal tear and dissection.

Classification

Several classifications of AD have been developed, two of which, Stanford and DeBakey, are based on the aortic segment involved (Fig. 22–4). Recently, the Svensson classification was proposed and is based on the underlying pathologic process (Table 22–2). Stanford and DeBakey classifications share the same principle of grouping AD based on the involvement of ascending aorta (location of the intimal flap, irrespective of site of origin or location of the intimal tear), which in turn dictates the management and predicts the overall prognosis. ADs involving the ascending aorta comprise approximately two-thirds of AD, and current recommendations are to treat surgically, whereas ADs not involving the ascending aorta seem to do better with medical therapy (barring specific complications). Because the duration of AD (the time from symptom onset to presentation) also predicts the associated morbidity and mortality, AD is also classified as acute (<2 weeks) and chronic (≥2 weeks).

Diagnostic Imaging

The imaging techniques for AD center on the use of CT, TEE, MRI, and aortography. The choice of diagnostic modality depends on patients' clinical status, equipment availability, and staff expertise. CT and TEE are the two most frequently used diagnostic tests, and the majority of the patients usually undergo two or more imaging tests (Fig. 22–5).

Computed Tomography

Spiral (helical) CT is the most frequently used imaging test for diagnosing acute AD around the world due to its accuracy and wide accessibility. CT provides a very detailed 3D depiction of the aorta and its branches. Earlier studies reported CT to be associated with insufficient sensitivity and specificity. However, a recent systematic review of the diagnostic accuracy of CT reported a mean sensitivity and specificity of 100% and 98%, respectively.7 When compared with TEE and MRI, CT was also better for ruling out AD in patients at low pretest probability.7 In addition to aiding in establishing the diagnosis of AD, CT assists in the management by providing invaluable information on the extent of the dissection, location of the intimal tear and flap, branch involvement, presence of an intramural hematoma, size of the true and false lumen, and presence of pericardial and/or pleural fluid (Fig. 22–6). Although motion artifacts of the aortic root and ascending aorta were rather common with older CT systems, contemporary 64-slice MDCT technology with ECG gating has significantly shortened scanning time, increased resolution, eliminated motion artifacts, and allowed simultaneous imaging of the aorta and coronary and pulmonary arteries.

Echocardiography

Echocardiography is well suited not only for the diagnosis of AD, but most importantly, for the evaluation of cardiac complications of acute AD such as presence and severity of aortic regurgitation, identification of pericardial effusion and tamponade, and evaluation of regional left ventricular wall motion abnormalities (in case of extension of a dissection into the coronary arteries, usually the right coronary artery). TTE has a sensitivity of 59% to 85% and a specificity of 63% to 96% for the diagnosis of AD.8 TTE evaluation includes the standard and high parasternal windows for imaging of the ascending aorta, suprasternal notch window for the aortic arch, parasternal and apical windows for the descending aorta, and subcostal window for proximal abdominal aorta (Figs. 22–7 and 22–8).

TEE is the second most commonly used imaging test in the diagnosis of acute AD. Anatomic proximity of the esophagus to the aorta makes it a far better technique than TTE, although proximal aortic arch is still difficult to image due to interference from the air-filled trachea and main stem bronchus (Fig. 22–9). The sensitivity of TEE for detecting AD is 98%, and the specificity is approximately 95%.7 Differential flow between true and false lumens can often be visualized (Fig. 22–10). TEE does not require intravenous contrast or radiation, and advantageously in hemodynamically unstable patients, it can be performed at the bedside in 10 to 15 minutes. However, TEE may be associated with bradycardia, aspiration, hypertension, hypotension, transient atrioventricular block, and rarely, esophageal perforation, especially in patients with esophageal disease (eg, strictures, varices).

The 2003 American College of Cardiology (ACC)/American Heart Association (AHA)/American Society of Echocardiography (ASE) practice guidelines for echocardiography give a class I recommendation for TEE use in patients with AD, aortic rupture, and follow-up of AD, especially after surgical repair when complication or progression is suspected (class IIa recommendation is given to follow-up of AD after surgical repair without suspicion of complication or progression). The practice guidelines give a class IIa recommendation for TTE use in AD and class IIb recommendation for aortic rupture.9

Briefly, the ACC/AHA/ASE classification of recommendations is as follows: class I—conditions for which there is evidence for and/or general agreement that a given procedure or treatment is beneficial, useful, and effective; class II—conditions for which there is conflicting evidence and/or a divergence of opinion about the usefulness/efficacy of a procedure or treatment; class IIa—weight of evidence/opinion is in favor of usefulness/efficacy; class IIb—usefulness/efficacy is less well established by evidence/opinion; and class III—conditions for which there is evidence and/or general agreement that a procedure/treatment is not useful/effective and in some cases may be harmful.

Magnetic Resonance Imaging

MRI and MRA have a high mean sensitivity and specificity (98%) and yield the highest value for confirming AD in patients with high pretest probability for AD.7 Despite this, MRI is much less commonly used in the diagnosis of acute AD (Fig. 22–11) because of its limited availability, time delay, incompatibility with implantable devices (MRI-safe permanent pacemakers are becoming more available) and surgical clips, limited monitoring during the study, and claustrophobia.

However, MRI has certain advantages. It is not associated with radiation exposure, has a high sensitivity for type A and type B acute AD, is useful in detecting features of intramural hematoma and penetrating aortic ulcer, and is considered the test of choice for follow-up evaluations of patients with surgically and/or medically managed aortic disease (Fig. 22–12). In addition, the safety profile of gadolinium is more favorable than that of the iodinated contrast medium used in CT, even though rare cases of nephrogenic systemic fibrosis in patients with severe renal insufficiency have been documented with gadolinium.10

Aortography

Retrograde aortography previously was considered the gold standard for the diagnosis of acute AD. It has largely been replaced by other widely available, noninvasive, and more accurate imaging modalities. The disadvantages of this procedure include the small risk of contrast-induced nephropathy, the risk of perforation by canalizing the false lumen, radiation exposure, and the time delay associated with the patient transfer and preparation. However, the positive aspects of aortography include its ability to assess the presence and severity of aortic regurgitation and major branch vessel and coronary artery involvement (Fig. 22–13A).

Chest Radiography

Although the chest radiogram is often abnormal in patients with AD, it is not an adequate test to either confirm or exclude the diagnosis of AD. The most common features include widening of the aortic silhouette and superior mediastinum (Fig. 22–13B). Intimal calcification ≥1 mm medial to the outer border of the aortic shadow, tracheal shift, pleural effusion (left to right), and new findings in the aortic and mediastinal silhouettes compared with a prior radiogram can also be seen. However, up to 10% to 15% of patients with acute AD have normal findings on chest radiography.11

Coronary Angiography

The role of routine coronary angiography (preoperative or intraoperative) in the setting of acute type A AD to assess the presence of dissection extension into the coronary arteries or presence of coronary artery disease (CAD) (that may require surgical intervention) is not clear. Although, some reports suggest preoperative coronary angiography in all stable patients, the majority of experts recommend against it, unless there is a known history of CAD, prior coronary artery bypass graft, or evidence of ischemia on ECG.

Management Strategies

No randomized trials have been conducted to date evaluating management strategies for AD. Therefore, all proposed management strategies are based primarily on registries, case reports, systematic reviews, and experts' opinion. The only society-based consensus or guidelines were published in 2001 by the European Society of Cardiology.12

Incidence, Risk Factors, and Pathophysiology

Aortic intramural hematoma (IMH) was first described in 1920. At present, the etiology and pathogenesis remain debatable. Traditionally, IMH was described as a hemorrhage occurring within the medial layer of the aortic wall in the setting of intact intima due to spontaneous rupture of the aortic vasa vasorum. Another, less established, explanation is an atherosclerotic ulcer penetrating into the internal elastic lamina, leading to IMH formation. The prevalence of IMH in patients with suspected AD is up to 30%.12 The risk factors associated with IMH are similar to those seen with AD, and it is more commonly identified in the descending aorta (type B) than in the ascending aorta (type A).

Diagnosis

Noninvasive imaging modalities (CT, MRI, and TEE) are critical in IMH diagnosis. Conventional aortography images the aortic lumen, rather than the aortic wall and, therefore, should not be used to diagnose or exclude IMH. In CT, demonstration of continuous, usually crescentic, high attenuation areas along the aortic wall without an intimal flap is characteristic before contrast injection; these areas fail to be enhanced after injection of contrast medium13 (Fig. 22–14). Acute IMH on MRI gives a high signal on T2-weighted images and isodense signal on T1-weighted images, whereas subacute IMH produces high signal both on T1- and T2-weighted images. A typical crescentic wall thickening, which also does not enhance after the injection of contrast material, is seen on MRI. MRI is an excellent modality in detecting recurrent bleeds in the aorta wall and therefore is recommended in subacute and chronic phases. TEE is another excellent imaging modality because it allows direct observation of the aortic intima, and demonstration of flow communication is feasible with the Doppler technique.13 According to experts' recommendations, regional circumferential or crescentic aortic wall thickness ≥7 mm (normal <3 mm) has been suggested as diagnostic of IMH. Sometimes a large echo-free space is associated with contrast enhancement in CT, possibly due to "intimal microtear," which is too small to be detected by the Doppler flow measurement. In addition to being the most operator-dependent method, TEE is of limited utility in the setting of heavily calcified aortic wall or plaques and when the identification of penetrating ulcers is required.

Atherosclerosis is one of the most common causes of mortality and morbidity in the United States and worldwide. It is a systemic disease and affects all arterial vasculature, including the aorta. The principle clinical syndromes associated with aortic atherosclerosis are aortic aneurysms, IMH aortic atheromatous plaques, atheroembolization, thrombosis and thromboembolization, and aortic penetrating atherosclerotic ulcers (PAUs).

Pathophysiology

The process of atherosclerosis usually begins early in life and involves a series of interrelated events. These include lipid abnormalities (elevated low-density lipoprotein and triglycerides and decreased high-density lipoprotein), platelet activation and thrombosis (aggregation of platelets is triggered by plaque rupture and facilitated by tissue factor and von Willebrand factor), vascular endothelial regulation and dysfunction (primarily by releasing nitric oxide, a potent vasodilator, and endothelin and angiotensin II, which antagonize the actions of nitric oxide), inflammatory process (mediated by macrophages and T cells, releasing various cytokines), oxidant stress (production of reactive oxygen species or radicals), vascular smooth muscle cell proliferation (positive and then negative remodeling), altered matrix metabolism (mainly secretion of matrix metalloproteinase), vasa vasorum, and genetic and acquired risk factors (eg, cigarette smoking, diabetes mellitus, hypertension). Atherosclerosis affects the abdominal aorta to the greatest extent, possibly due to local hemodynamic stresses, thin aortic wall, and the lack of vasa vasorum (damage or decrease in vasa vasorum is associated with plaque formation) (Fig. 22–15).

Aortic Atheromatous Plaques and Atheroembolization

An aortic atheromatous plaque, known as aortic atheroma, is suspected to be among the leading causes of embolic stroke. Because emboli propagation is flow dependent, plaques in the ascending aorta are usually associated with stroke, whereas those in the descending aorta are associated with mesenteric, renal, or peripheral ischemia and infarction. Aortic atheroma is characterized by irregular intimal thickening, usually protruding into the aortic lumen. Its thickness, plaque burden, and complexity (mobile debris, overlying thrombus, and ulcerations) have been shown to be associated with stroke. Other predisposing factors to atheroembolization are absence of calcifications, pedunculation, iatrogenic instrumentation, and aortocoronary bypass. Commonly, plaques ≥4 mm in thickness are classified as complex and are strongly associated with the risk of ischemic stroke.14 Another classification based on TEE was proposed, where grade I is normal intima, grade II is minimal intimal thickening, grade III is raised irregular plaque <5 mm in thickness, and grade IV is complex protruding plaque with ≥5 mm thickness, ulceration, or calcific density.15

TEE is the imaging modality of choice because in addition to measuring plaque thickness, ulceration, and calcifications, it can sometimes predict the risk of embolization by visualizing mobile thrombi (Fig. 22–16). MRI with MRA is the second best imaging modality and can also visualize various plaque components, including the fibrous cap, which may be helpful in assessing plaque stability. The major advantage over TEE includes its ability to visualize the distal ascending aorta. MRI is frequently used for chronic monitoring of aortic plaques. The availability, cost, and contraindication in patients with implantable devices, however, remain problems. MDCT is widely available, noninvasive, and less costly and can be rapidly performed. However, lower sensitivity for noncalcified plaques and exposure to radiation and iodinated contrast limit its use for plaque imaging. Although not used routinely in clinical practice, fluorine 18 (18F) fluorodeoxyglucose (FDG) positron emission tomography (PET) may emerge as an important imaging modality for noninvasive identification of plaques with high embolic risk, (vulnerable plaques) by assessing inflammation within the plaque.

Thrombosis and Thromboembolization

The aorta has been reported to be the source of thromboembolism in a minority of cases. Most commonly, thrombus formation and thromboembolism are seen in the setting of an atherosclerotic aorta with an atheroma protruding into the lumen (Fig. 22–17). The risk may even be higher with concomitant hereditary (eg, antiphospholipid antibody syndrome, hyperhomocysteinemia, prothrombin G20210A mutation, antithrombin deficiency, protein S or C deficiency) or acquired (eg, malignancy, pregnancy, recent surgery, trauma, immobility, estrogen use) hypercoagulable states, as well as with vasculitis or smoking. Rarely, aortic thrombus formation may occur spontaneously in the nonatherosclerotic aorta. Generally, diagnostic and management strategies for aortic thrombosis and thromboembolism are similar to those of atheromatous plaques.

Aortic Penetrating Atherosclerotic Ulcers

PAU, first recognized by Shennan in 1934 and further defined by Stanson in 1986, is an ulceration of an atherosclerotic lesion of the aorta that disrupts and penetrates the internal elastic lamina of the aortic wall. Such ulcerations typically affect the descending thoracic aorta of elderly patients with advanced atherosclerosis and may precipitate a variable amount of hematoma formation. The hematoma formed by a PAU usually remains localized but infrequently may lead to formation of frank IMH, pseudoaneurysm, AD, or rupture. Rarely, PAU may arise in the ascending aorta or the arch. Table 22–3 outlines the main characteristics of PAU, IMH, and AD.

The best estimate of incidence of PAU in patients presenting with signs and symptoms of acute AD comes from the IRAD registry and is reported to be between 2.3% and 11%. However, because a proportion of PAUs progress to IMH and AD prior to patient presentation and imaging, these numbers may be falsely low.

The clinical diagnosis of PAU is not always straightforward. Although its presentation may sometimes resemble that of acute AD, often there is a paucity of specific symptoms and signs. Previously, aortography has been used as a standard for PAU diagnosis. However, this was largely replaced by noninvasive imaging. CT and MRA accurately depict the lesion as a focal, contrast-filled, pouch-like aortic protrusion in the absence of a dissection flap or false lumen, usually surrounded by marked atherosclerosis, calcifications, and small hematoma (Figs. 22–18 and 22–19). TEE usually reveals a crater-like ulcer with jagged edges in the presence of significant atheroma, thrombus, or even dissection (Fig. 22–20).

The term aortic aneurysm is used to describe a pathologic dilation (>4 cm or >1.5 times larger than normal) of the aorta. Morphologically, aneurysms are described as fusiform, a symmetrical dilation involving the full circumference of the aorta, or saccular, a localized outpouching of only a portion of the aortic wall. Pseudoaneurysms or false aneurysms are better described as a collection of blood outside the aortic wall (Fig. 22–21). In addition to morphology, aortic aneurysms are classified according to their location, size, and etiology.

Thoracic Aortic Aneurysms

Epidemiology and Classification

The true incidence of thoracic aortic aneurysms (TAAs) is not completely known due to underdetection but is estimated to be 6 to 10 cases per 100,000 patient-years. TAAs most commonly occur in the sixth and seventh decade of life and are more common in men than women (3:1). Hypertension is a contributing factor and is found in a majority of patients.

TAAs involve the ascending and descending thoracic aorta and are much less prevalent than abdominal aortic aneurysms (AAAs). TAAs can be classified into four general categories: ascending aortic aneurysm (60%), descending aortic aneurysm (40%), aortic arch aneurysm (10%), and thoracoabdominal aneurysm (10%). A categorization of ascending aortic aneurysm into supracoronary type, Marfanoid type (also termed annuloaortic ectasia), and tubular type was proposed.16 Supracoronary type is the most common type and is associated with a normal-size aortic annulus as well as a normal aorta between the annulus and the coronary orifices. The Marfanoid type has a "flask-like" shape and is associated with dilated aortic annulus and proximal ascending aorta. The tubular type has a moderate, uniform dilation of the entire ascending aorta (Fig. 22–22). Aneurysms of the sinuses of the Valsalva are rarely seen. These aneurysms may be single (right sinus aneurysms being the most common type) or multiple and may fistulize into an adjacent chamber (Fig. 22–23). Thoracoabdominal aortic aneurysms contain elements of both thoracic and abdominal aneurysms and constitute only a minority of all aneurysms. Crawford classification (type I to IV), based on the extent of involvement, is used for their categorization.17

Pathophysiology and Risk Factors

TAAs most often result from cystic medial degeneration or necrosis that leads to the weakening of the aortic wall. Congenital and acquired causes of cystic medial necrosis are manifold. Aneurysms of the aortic root are typically associated with Marfan syndrome and bicuspid AVs. Congenital causes of ascending aortic aneurysms include Marfan syndrome, Turner syndrome, Ehlers-Danlos syndrome, bicuspid AVs, osteogenesis imperfecta, and polycystic kidney disease. Acquired causes of ascending aortic aneurysms include atherosclerosis, hypertension, various rheumatologic processes (Takayasu arteritis, giant-cell arteritis, rheumatoid arthritis, and systemic lupus erythematosus), trauma, and infections (syphilitic or mycotic). The majority of aneurysms involving the descending thoracic aorta are atherosclerotic in etiology. Infrequently, in the setting of diffuse atherosclerosis, these may extend to the ascending aorta or the arch. Although some of the descending aortic aneurysms may rarely compress mediastinal structures, most of them are asymptomatic.

Imaging

Chest Radiography

Asymptomatic aneurysms of the ascending aorta and the arch are sometimes incidentally detected on the routine chest radiography. The most common radiographic features include the finding of mediastinal widening, enlarged or prominent aortic knob, displaced intimal calcifications, or midline shift of the trachea. However, all of these are not very specific and cannot distinguish an aneurysm from the normal variant or tortuous aorta.

Echocardiography

TTE has an important role in imaging of the aortic root and proximal ascending aorta, and in the current 2003 ACC/AHA practice guidelines, it is given a class I recommendation for imaging of aortic aneurysms, particularly aortic root, and aortic root dilation in Marfan or other connective tissue syndromes.9 However, its use is limited in imaging the arch and the descending aorta. Therefore, TEE is preferred in the imaging of the thoracic aorta (except the upper ascending aorta and the arch due to the artifacts created by a column of air in the trachea) and also carries a class I recommendation9 (Fig. 22–24). The 2008 ACC/AHA guidelines on valvular heart disease recommend an initial assessment of the aortic root and ascending aorta diameters with TTE in patients with known bicuspid AVs.18 Importantly, both TTE and TEE are commonly used for diagnosis of complications such as aortic insufficiency. Given that TEE is semi-invasive, CT and MRI are usually preferred for the diagnosis or chronic follow-up in clinically stable patients.

CT and MRI

Contrast-enhanced CT and MRA can very accurately depict the size and location of TAA, as well as branch vessel or mediastinal structure involvement (Fig. 22–25). Recent advances, such as 3D reconstruction, allow accurate measurement of the aortic diameter in cases of very tortuous aorta when axial images alone are insufficient (axial images often overestimate the aortic diameter because it transects the aorta off axis). Aneurysms involving the aortic root are much better imaged with MRI than with CT. The 2008 ACC/AHA valvular heart disease practice guidelines give a class I recommendation for CT or MRI use in patients with bicuspid AVs when morphology of the aortic root or ascending aorta cannot be assessed accurately with echocardiography.18 Serial annual evaluations with echocardiography, CT, or MRI are also recommended in patients with bicuspid AV and >4.0-cm aortic root or ascending aorta diameter (lower value for patients of small stature).18

Aortography

Retrograde aortography had long been the preferred method for aneurysm evaluation. It provides very accurate information regarding the aortic anatomy and branch vessel involvement. However, it is being increasingly replaced by the noninvasive imaging modalities described earlier due to its inability to discern extraluminal aneurysmal size and the procedure-associated risks.

Abdominal Aortic Aneurysms

Epidemiology and Classification

Similar to TAA, the true prevalence of AAA is not completely known due to various diagnostic criteria (usually when anteroposterior diameter of the aorta is ≥3.0 cm), various imaging modalities used, and underdetection. However, it is known that AAA is 5 to 10 times more common in men than women and that its incidence rises dramatically in the sixth and seventh decades. Generally, the prevalence of AAA of 2.9 to 4.9 cm in diameter ranges from 1.3% for men age 45 to 54 years to up to 12.5% for men age 75 to 84 years. Comparable prevalence figures for women are 0% and 5.2%, respectively.19 AAA is the thirteenth leading cause of death and third leading cause of sudden death among men over the age of 60 in the United States, which translates into approximately 15,000 deaths annually. Some estimate that the actual rate may be as high as 30,000 deaths per year. AAAs are much more prevalent than TAAs. Most are fusiform, and the minority are saccular. AAAs are classified as suprarenal aneurysms, affecting the aortic segment containing the superior mesenteric and celiac arteries; juxtarenal aneurysms, arising distal to the renal arteries but in very close proximity to them; pararenal aneurysms, involving the origin of one or both renal arteries; and infrarenal aneurysms, occurring below the renal arteries (95% of cases).

Pathophysiology and Risk Factors

Elastin and collagen comprise the aortic extracellular matrix, which plays a critical role in maintaining the aortic shape and integrity. Degradation of these important proteins may weaken the aortic wall and lead to formation of the aneurysm. Although, there is still some controversy regarding the factors degrading elastin and collagen, an inflammatory process appears to be the most accepted. Inflammatory cells (in particular macrophages and T cells) and endothelial and smooth muscle cells produce proteolytic enzymes, such as matrix metalloproteinases, plasmin, and cathepsin S and K, which degrade the extracellular matrix. The early experimental reports of inhibition of matrix metalloproteinases with hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors and tetracyclines in animal models are promising.

Among the factors that may contribute to the aortic wall inflammation, smoking appears to be the most important one, followed by sex, family history (first-degree relative) of AAA, age, hypertension, and atherosclerosis. The risk of AAA in heavy smokers is six to seven times that of nonsmokers. AAA >4.0 cm occurs up to 10 times more frequently in men compared with women. Women, however, may be at higher risk of rupture. Approximately 15% to 28% of patients with AAA have first-degree relatives affected with aneurysms.

Imaging

Ultrasonography

US is an accurate and inexpensive modality used for screening and surveillance of infrarenal aneurysms. It has an excellent diagnostic sensitivity and specificity of 92% to 99% and 100%, respectively, and can image an aneurysm in the transverse and longitudinal planes. However, US is less reliable for imaging of juxtarenal or suprarenal aneurysms. Because of this limitation, it is usually insufficient alone for preoperative evaluation of patients undergoing surgical repair. The 2005 ACC/AHA guidelines give a class I recommendation for AAA screening with a physical exam and US in men ≥60 years of age who are either the siblings or offspring of patients with AAA and class IIa recommendation (grade B* recommendation from the US Preventive Services Task Force20) for AAA screening with a physical exam or one-time US in men who are 65 to 75 years of age who have ever smoked.19

CT and MRA

CT and MRA are highly accurate imaging modalities in assessing the size and location of the aneurysm, as well as the extent of disease, presence of mural thrombus and calcification, adjacent fluid, and iliofemoral and renal artery involvement. These modalities are preferred over US for suprarenal aneurysm imaging. However, CT tends to overestimate the size of the aneurysm by as much as 0.94 ± 0.69 cm.21 In addition, intramural calcifications may result in less accurate evaluation of the peripheral arteries with CT, so either retrograde arteriography or MRA may be required. Presently, there is no consensus to indicate the superiority of MRA versus CT.

Aortography

Although aortography is an excellent modality for assessment of the size and location of AAA, as well as branch vessel involvement, it is presently used only in selected cases due to its invasiveness, cost, limited availability, and exposure to intravenous contrast and radiation. Because contrast nonenhancing mural thrombus is nearly always associated with an AAA, aortography may also underestimate the true size of the aneurysm.

*Grade B: The USPSTF [US Preventive Services Task Force] recommends that clinicians provide [the service] to eligible patients. The USPSTF found at least fair evidence that [the service] improves important health outcomes and concludes that benefits outweigh harms.

The proportion of individuals with congenital vascular diseases surviving into adulthood is rapidly rising. Therefore, familiarity with these diseases is essential. This section will focus on the major congenital aortic malformations, such as coarctation and atresia of the aorta, bicuspid valve–associated aortopathy, and Marfan syndrome–associated aortopathy, which account for a significant proportion of congenital vascular malformations.

Coarctation and Atresia of the Aorta

Incidence, Etiology, and Pathophysiology

Coarctation of the aorta was described pathologically at autopsy by Morgagni in 1760 and was clinically recognized in the early 1900s. It is usually a discrete narrowing (rarely involves a long segment) of the thoracic aorta just distal to the ligamentum arteriosum (juxtaductal) resulting in at least a 20-mm Hg gradient across the coarctation (Fig. 22–26A). Less frequently, coarctation occurs proximal to the left subclavian artery. Previous categorization as preductal (infantile) and postductal (adult) is misleading and should not be used. It is a common malformation responsible for up to 8% of all cardiovascular congenital defects and occurs two to five times more frequently in males than females. Simple coarctation is the most common form detected de novo in adults and is not associated with other intracardiac lesions. Complex coarctation has a frequent association with bicuspid AV (50%-80% of cases), ventricular septal defect, patent ductus arteriosus, subvalvular aortic stenosis, parachute mitral stenosis, and circle of Willis cerebral artery aneurysm (berry aneurysm; 10% of cases). The etiology of aortic coarctation is not known. The reduction of antegrade intrauterine blood flow causing underdevelopment of the aortic arch (flow theory), constriction of ductal tissue extending into the thoracic aorta (ductal theory), and a primary defect of the aortic wall are the proposed theories for the development of congenital aortic coarctation. Acquired causes include inflammatory processes, such as Takayasu arteritis, and severe atherosclerosis.

Aortic atresia, complete interruption of the aorta, is usually lethal unless it is treated surgically within the first month of life and can be classified based on the segment involved: type A, interruption of the aortic arch distal to the left subclavian artery; type B, between the left subclavian artery and the left carotid artery (most common); or type C, between the innominate artery and the left carotid artery.22

Aortic pseudocoarctation is a rare congenital anomaly of an excessive elongated aorta, resulting in its kinking and buckling. Pseudocoarctation is not associated with an aortic gradient (see Fig. 22-3).

Imaging

Chest Radiography

Prominent ascending aortic shadow, absence of the proximal descending aortic arch shadow, a "3 sign" (indentation at the coarctation site), and notching on the underside of the ribs (from dilated collateral vessels) may be seen on anteroposterior chest radiogram (Fig. 22–26B).

Echocardiography

According to 2008 ACC/AHA guidelines, TTE, including suprasternal notch acoustic windows, is useful for initial imaging and hemodynamic evaluation in suspected aortic coarctation (class I).23 Discrete area of narrowing with increased turbulence and velocities in the proximal descending aorta, characteristic forward diastolic flow, and decreased pulsatility and absence of early diastolic flow reversal in the abdominal aorta can be documented with color flow imaging and continuous wave spectral Doppler examination (Fig. 22–27). Abnormal flow in the dilated and tortuous collateral vessels may also be detected. TEE is rarely needed because suprasternal images from TTE usually provide excellent evaluation of the involved aorta.

CT and MRA

According to 2008 ACC/AHA guidelines, every patient with coarctation (repaired or not) should have at least one cardiovascular MRI or CT scan for complete evaluation of the thoracic aorta and intracranial vessels (class I).23 CT angiography and gadolinium-enhanced MRA are excellent techniques for the evaluation of anatomic details, precise location and severity of coarctation, and the status of a ductus arteriosus and extent of collateralization. The velocity-encoded phase-contrast magnetic resonance protocol allows accurate assessment of a gradient across the coarctation.

Aortography

Although CT and MRI are the preferred methods of imaging, diagnostic aortography is an excellent technique to delineate the location and extent of a coarctation, as well as the associated vascular malformations, and should be performed when noninvasive imaging modalities are inconclusive or unavailable or if catheter-based intervention is to be performed. Anterograde catheterization via radial approach maybe preferred over the retrograde catheterization via femoral artery due to absent or markedly diminished femoral pulse.

Bicuspid AV–Associated Aortopathy

Incidence, Etiology, and Pathophysiology

Bicuspid AV is the most common congenital cardiac malformation, occurring in up to 2% of the general population, with a male-to-female ratio of ≥3:1. It has an autosomal dominant with incomplete penetrance inheritance and is strongly associated with Turner syndrome and coarctation of aorta. Individuals with bicuspid valve (including those with normal AV function) have significantly a larger aortic root and ascending aorta than those with tricuspid AV, which translates into nine-fold higher incidence of AD. The prevalence of bicuspid AV–associated ascending aortic aneurysm was estimated to be approximately 7.5% to 59% at the annulus, 16% to 78% at the sinus of Valsalva, 15% to 79% at the sinotubular junction, and 35% to 68% at the proximal ascending aorta.24 This process of aortic dilation is due to aortopathy and is independent of valvular and hemodynamic abnormalities.

Imaging

For patients with known bicuspid AV, the 2008 ACC/AHA valvular heart disease guidelines recommend an initial TTE to assess the diameters of the aortic root and ascending aorta (class I).18 In individuals with large body habitus, TEE may offer an incremental yield. CT or MRI is only indicated when the aortic morphology cannot be assessed accurately by echocardiography (class I).18 Furthermore, if the root diameter is >4.0 cm, an annual echocardiogram, MRI, or CT should be performed (class I).18 Three-dimensional MRA and CT are the most accurate for aortic size measurement, whereas steady-state free precession MRI pulse sequence is commonly used for AV functional assessment (eg, visualization of an eccentric systolic jet and assessment of leaflet number) (Fig. 22–28). The flow velocity pulse sequence by MRI is also a very useful tool to assess the functional bicuspid valves by assessing the flow of blood through the opening of the AV. The appearance of a functionally bicuspid AV has an elliptical or "fish mouth" appearance rather than a triangular appearance with the functional tricuspid valve. At times, it can be a challenge to distinguish a tricuspid valve from a bicuspid valve with a prominent raphe. In diastole, there may not be any difference between these two valves, but it becomes evident during systole when the raphe does not open on either short axis TTE/TEE or on cine/flow velocity short axis MRI. If the raphe is partially fused, then classification can be quite challenging. In addition, on the long axis view by echocardiography and cine MRI, the valve will often have a doming appearance. In the case of severely calcified bicuspid AV, chest radiography may reveal increased perivalvular radiopacity.

Marfan Syndrome

Incidence, Etiology, and Pathophysiology

Marfan syndrome is a connective tissue disease affecting 1 in 3000 individuals and is autosomal dominant with spontaneous mutation observed in up to 30% of subjects. It is a systemic disease affecting central nervous, cardiovascular, pulmonary, and musculoskeletal systems. Cardiovascular abnormalities in Marfan syndrome include aortic root (sinuses of Valsalva) and ascending aorta dilation (70% of cases), mitral valve prolapse with mitral regurgitation (up to 80% of cases), TAAs and AAAs, AD (90% are type A), and IMH. In IRAD, 5% of patients with acute AD had Marfan syndrome (with an incidence of 50% among those who were <40 years of age, compared with only 2% among older patients). Various mutations involving fibrillin-1 (FBN1) (most commonly) and transforming growth factor-β (TGF-β) genes are typically responsible for the phenotype of Marfan syndrome. These mutations result in fragmentation of the elastic lamellae, cystic medial necrosis, and loss of smooth muscle cells within the aortic wall.

Imaging

The current strategy to screen for Marfan syndrome–associated aortopathy is echocardiographic assessment of the aorta, in particular the aortic root and ascending aorta. TTE is recommended as a first-line screening test for thoracic aortic disease in the affected individuals and their first-degree relatives (class I).9 Annual routine reimaging of the aorta is also reasonable. In addition, echocardiography and MRI can detect increased aortic wall stiffness and decreased distensibility, which are correlated with higher pulse wave and blood velocity in the aorta. If there is a suggestion or confirmation of aortic dilation, then 3D MRA or CT angiography should be performed for more complete assessment.

Traumatic aortic injury (TAI) is an infrequent complication of blunt chest trauma. However, it is responsible for 16% of injury-related deaths and is the second most common cause of fatality in motor vehicular accidents. Up to 80% of individuals die at the accident site.25 Aortic disruption may be complete (transection), a catastrophic event that almost always results in instantaneous death, or partial incomplete disruption of media and/or adventitia that contain the rupture.

Pathophysiology

TAI usually follows an abrupt deceleration (most often) during automobile crashes, most commonly in a head-on impact. It has been proposed that most blunt aortic injuries involve one or more of the following forces. The isthmus is located in the proximal descending aorta, tethered by the ligamentum arteriosum in a relatively immobile position and, therefore, is subjected to the most stretching, torsional, and inertial forces with sudden deceleration or acceleration. A majority (95%) of TAI cases occur at this site. Other hypotheses involve a "water-hammer" effect, an instantaneous elevation in aortic blood pressure against a simultaneously occluded aorta, and the "osseous pinch," an entrapment of the aorta between the anterior chest wall and the vertebral column. Minimal aortic injuries affect only the intima of the vessel wall and occur approximately 10% of the time. A penetrating injury to thoracic aorta may also result from a fractured rib or vertebral body. The minority (5%) of TAIs involve the ascending and subdiaphragmatic aorta.

Imaging

Widened aortic silhouette and superior mediastinum may occasionally be seen on the chest radiogram. However, a significant proportion of patients with blunt traumatic injury have normal mediastinal shadow on chest radiography. Because CT is widely available, can be performed in a timely fashion, and has very high sensitivity for detecting TAI, it has become the imaging test of choice. A normal CT essentially rules out TAI. CT findings associated with blunt aortic injury frequently include traumatic pseudoaneurysm, contained rupture, intimal flap, periaortic hematoma, a focal bulge, abnormal aortic contour, and sudden change in aortic caliber (Fig. 22–29). In addition, CT has a crucial role in the diagnosis of minimal aortic injuries. It was reported that 50% of minimal aortic injuries identified by helical CT are missed by conventional aortography. Aortography has been considered as a gold standard for TAI identification for decades. However, given its invasiveness, limited availability, and the time delay associated with its performance, it has largely been replaced by CT. Frequently, retrograde aortography is performed preoperatively for better aortic contour and branch vessel involvement delineation in patients with already known TAI (Fig. 22–30). Echocardiography (particularly TEE) can also be used to visualize traumatic disruption of the aorta. MRA is an alternative methodology; however, in the acute setting, CT angiography is a better choice due to the faster imaging time.

Primary malignancies of the aorta are extremely rare. Sarcomas represent the majority of cases, with malignant fibrous histiocytoma being the most common subtype. Other less common subtypes include malignant endothelioma, angiosarcoma, and leiomyosarcoma. Except for leiomyosarcoma, which tends to grow outward into the adjacent structures, the majority of tumors involve the intimal layer and grow into the aortic lumen.

Clinical symptoms of malignant aortic tumors include locally occlusive aortic disease, peripheral emboli, or mesenteric emboli.26 Unlike retrograde aortography, echocardiography, MRI, and CT allow imaging of the aortic wall and, therefore, are preferred modalities for the evaluation of aortic tumors. In addition, CT and MRI are capable of surveying the whole body for the presence of metastatic disease. Given the rarity of the aortic tumors, the diagnosis, unfortunately, is frequently established only with surgical exploration or necropsy. Because distant metastases to bone, kidneys, liver, the adrenal glands, and lungs are common, oncologic approaches are frequently only palliative, and the overall prognosis remains very poor.

Aortitis may have infectious (bacterial and syphilitic) or noninfectious (systemic inflammatory disorders) etiology. Infections of the aorta are most commonly caused by direct seeding of the aortic wall (frequently diseased or traumatized) by circulating bacteria. Less common etiologies include a septic embolus from bacterial endocarditis, contiguous spread of infection from adjacent structures, impaired immunity, and infection of the vasa vasorum. Systemic rheumatologic diseases, such as Takayasu disease, giant-cell aortitis, rheumatoid arthritis, and systemic lupus erythematosus, can often affect the aorta to various degrees.

Imaging

18F-FDG PET is an excellent modality to define increased aortic wall metabolism due to inflammation. Given a very high sensitivity of PET, it may emerge as a gold standard for arteritis diagnosis, as well as assessment of response to therapy. MRI/MRA has also been used for detection of aortic wall thickening and branch vessel occlusion. The morphologic pulse sequence with black blood TSE is used to assess the aortic wall thickness. Different weighting with T1, T2, or proton density, combined with fat saturation, and contrast enhancement can help delineate aortitis from intramural hematoma and mural thrombus. MRA is also used to assess the luminal consequences of stenosis and, at times, dilation. CT and echocardiography are also reasonable initial diagnostic tests and may uncover luminal narrowing, aneurysms, and wall thickening.

Infectious Aortitis

Diseased aortic wall segments (eg, atherosclerotic aneurysm, healed dissection, iatrogenic trauma, repair site) are prone for bacterial infections. Staphylococcus aureus and Salmonella species are the two most common pathogens responsible for infectious or mycotic aneurysms. In addition to adhering to atheromatous aortic segments, Salmonella infrequently may affect normal endothelium. Mycobacterium tuberculosis, Streptococcus pneumoniae, and Treponema pallidum once accounted for a significant number of cases but are now rare with the advent of antibiotics. Because T. pallidum invades the aortic wall via vaso vasorum, it primarily causes inflammation and aneurysm formation in the ascending aorta and the arch (abdominal aorta has minimal amount of vaso vasorum). Other organisms that were reported to rarely cause mycotic aneurysms are fungi, Pseudomonas, Klebsiella, Bacteroides fragilis, and Neisseria gonorrhoeae.

The diagnosis of infectious aneurysm may be suspected on imaging studies. However, isolation of the organism by microbiologic (blood cultures) or serologic (VDRL or FTA-ABS) investigations is often important. Therapeutic options include parenteral antibiotic therapy alone or often in combination with surgical exploration or endovascular therapy.

Noninfectious Aortitis

Takayasu Disease

Takayasu disease, Takayasu arteritis, or pulseless disease is a chronic large-vessel vasculitis that primarily affects the aorta and its main branches, causing arterial stenoses (more common) and aneurysms. In 90% of cases, it affects women in their second and third decades of life. Although Asians have the highest prevalence of the disease (approximately 150 new cases are diagnosed annually in Japan), cases have been reported in all races and ethnicities. The incidence of Takayasu disease is estimated at approximately 2.6 cases per 1 million persons in the United States. The initial lesions commonly occur in the left subclavian artery. As Takayasu arteritis progresses, the great vessels, including the coronary, pulmonary, iliac, and visceral arteries, may be affected (Fig. 22–31). Peripheral arterial pulses are commonly diminished and often asymmetrical. Systemic hypertension is common due to stiffening and scarring of the aorta (secondary to chronic inflammation) and the involvement of renal arteries. Other findings include aortic insufficiency (due to aortic root dilation), congestive heart failure, acute coronary syndromes, or sudden cardiac death (in cases of coronary involvement).

Giant-Cell Arteritis

Giant-cell arteritis (GCA) is also known to cause sterile inflammation of medium- and large-sized vessels. In the United States, it has a prevalence of approximately 18 per 100,000 cases among individuals older than 50 years of age. Women are slightly more commonly affected than men. The most common clinical features of GCA include severe headache (60%-90%) and temporal artery tenderness (40%-70%). Unlike Takayasu arteritis, GCA causes clinically relevant aortitis associated with aortic aneurysms only in a minority of affected individuals (15%-20%). However, necropsy studies have suggested that these numbers might underestimate the true prevalence of GCA.

Other Noninfectious Aortitides

A rapidly progressive aortic valvulitis and less often aortitis and aortic aneurysms have been described in association with rheumatoid arthritis. Aortitis with associated aortic insufficiency can sometimes be seen in patients with systemic lupus erythematosus. Aortic root disease with associated valvular regurgitation has been found in a significant number of patients with ankylosing spondylitis as well. Rarely, aortitis may be observed with Behcet syndrome, Crohn disease, and sarcoidosis.