Echocardiographic evaluation of the thoracic and proximal abdominal aorta

INTRODUCTION — Echocardiography enables qualitative and quantitative evaluation of the thoracic and proximal abdominal aorta. Transthoracic echocardiography (TTE) provides views of the proximal ascending aorta, aortic arch and portions of the descending aorta. However, transesophageal echocardiography (TEE) rather than TTE is indicated for comprehensive imaging of the aorta, especially in the emergency evaluation of aortic dissection or traumatic rupture of the aortic isthmus. (See "Clinical features and diagnosis of acute aortic dissection".)

Echocardiographic evaluation of the aorta for atherosclerotic plaque, sinus of Valsalva aneurysms, aortic dilation, and dissection will be reviewed here.

NORMAL AORTIC ROOT AND ASCENDING AORTA — The proximal ascending aorta attaches to the left ventricle at the annulus (hinge line of the aortic leaflets) and includes the aortic root (comprised of the three sinuses of Valsalva), the sinotubular junction, and the tubular ascending portion of the aorta. The aortic root is a direct continuation of the left ventricular outflow tract and is located right and posterior to the pulmonary infundibulum. The lower portion of the aortic root is connected to the muscular interventricular septum, the membranous septum, and to the mitral-aortic fibrous continuity (also known as the mitral-aortic intervalvular fibrosa).

Two-dimensional echocardiography — Transthoracic echocardiography (TTE) examination of the proximal ascending aorta is generally performed in the left parasternal long-axis view (image 1). Many sonographers limit their interrogation of the aorta to the proximal sinuses of Valsalva, missing the opportunity to more fully visualize the aorta. Moving up an intercostal interspace, moving the probe closer to the sternum, or tilting the probe cranially enables imaging of the more superior ascending aorta.

Right parasternal views, recorded with the patient in a right lateral decubitus position, may also be revealing [1]. This method is especially useful when the aorta dilates to the right of the sternum.

A cross sectional image of the aortic root is obtained in the parasternal short-axis view (figure 1). The suprasternal notch view visualizes the aortic arch.

Transesophageal echocardiography (TEE) provides more highly resolved images of the ascending aorta, aortic arch, and descending thoracic aorta than TTE, although a small portion of the distal ascending aorta and proximal arch cannot be seen due to interposition of the left mainstem bronchus and trachea.

All imaged portions of the aorta should be evaluated for the presence of plaque, dilation, and dissection (including intramural hematoma). Views used for measurement should be those that show the maximum diameter of the aortic root [1]. The aortic root at the level of the sinuses generally has the largest diameter (usually ≤3.7 cm), while the ascending aortic diameter at the sinotubular junction and above is slightly smaller (usually ≤3.5 cm). Age and body surface area adjusted normal values for the aortic root have been reported [2] and have been recommended for use as normative standards by the American Society of Echocardiography (ASE) [1]. Individual normal diameters vary by sex, age, ethnicity, and body size (figure 2 and figure 3) [3-5]. Integrating the data from various populations, any diameter approaching 4 cm must be viewed as very likely to be enlarged.

Evaluation of coronary arteries — The origins of the coronary artery are visible in both short- and long-axis views on adequate TTE or TEE images. The left main artery can often be followed to and beyond its bifurcation into the anterior descending artery and circumflex artery (image 2A-B); the right coronary artery can usually be followed for up to 3 cm from its origin. While echocardiography is not a practical method to detect luminal obstruction, careful gain manipulation makes it possible to detect larger bright densities along the course of the vessel that probably represent proximal calcification.

In adults, an anomalous origin of the coronary arteries is difficult to establish with an echocardiogram. The best clue to the presence of a single coronary artery, or one that ends in a coronary cameral fistula, is to observe the greatly enlarged coronary artery origin often associated with this abnormality. At times, these vessels dilate to the point that they may be confused with a sinus of Valsalva aneurysm. (See "Congenital and pediatric coronary artery abnormalities".)

Most cameral fistulae that terminate in one of the ventricles can be detected and localized by observing the abnormal color flow signal that marks the entrance of the fistula into the chamber. There are several reports from Japan of success in identifying the proximal aneurysms of Kawasaki's disease by echocardiography. (See "Cardiovascular sequelae of Kawasaki disease: Clinical features and evaluation".)

M-mode echocardiography — The motion of the aortic root on the M-mode echocardiogram is an indicator of global left ventricular systolic and diastolic function [6-8]. Since aortic root motion reflects the events of atrial filling and emptying, it also provides information about left atrial function (image 3A-C).

During systole, the aortic root normally moves anteriorly over 7 mm and returns almost completely to its starting point immediately after the conclusion of ejection. The atrial or presystolic contribution to aortic root motion is normally minimal.

Abnormal aortic root motion on M-mode echocardiography — If the systolic excursion of the aortic root is decreased, stroke volume is probably reduced, an effect that is independent of the left ventricular ejection fraction. As an example, if the left ventricle is hypovolemic but contracts normally, the aortic root motion will be decreased. Aortic root motion will also be decreased if the ejection fraction is severely reduced and the ventricle is increased in size (image 4).

●Augmented root motion with full opening of the aortic valve suggests a high cardiac output. High output states are quite easy to recognize and their appreciation is helpful in clinical management. (See "Causes and pathophysiology of high-output heart failure".)

●Normal or augmented systolic motion of the aortic root in the face of reduced aortic leaflet separation suggests atrial filling out of proportion to aortic flow and is typical of mitral insufficiency.

●If the initial diastolic posterior motion of the aortic root is slowed, and the late diastolic posterior motion of the aorta is exaggerated with atrial systole, reduced LV compliance is suspected.

●Aortic root motion tends to be flat in restrictive diastolic states, reflecting the reduced cardiac output generally associated with restrictive cardiomyopathy.

AORTIC PLAQUE — Atherosclerotic plaque is visualized as a region of intimal thickening or protrusion. Plaque may be accompanied by focal calcifications, ulcerations, and/or superimposed thrombi. The presence of thoracic aortic plaque, even when visualized in the descending aorta, has been associated with an increased risk of ischemic stroke [9]. Aortic plaque may be a marker of vascular disease and other risk factors for cerebrovascular disease [10,11]. Care must be taken not to confuse anterior aortic wall thickening with the right coronary artery.

On TEE, the presence of thick, mobile, or ulcerated plaques is associated with increased risk of stroke [12,13].Aortic plaque thickness >4 mm in the ascending aorta or arch correlates with elevated risk of embolic stroke [14]. Our institution uses the following transesophageal echocardiography (TEE) grading scale for aortic intimal thickness: grade 0 = normal, grade 1 = mild intimal thickening, grade 2 = moderate intimal thickening less than 5 mm, grade 3 = protruding atheroma ≥5 mm thick, and grade 4 = mobile thrombi on atheroma. The 2010 European Association of Echocardiography recommends TEE as the imaging modality of choice in evaluating aorta atheroma [15]. The management of aortic plaques is discussed separately. (See "Thromboembolism from aortic plaque" and "Embolism from atherosclerotic plaque: Atheroembolism (cholesterol crystal embolism)".)

Calcification of the aortic valve, aortic root, and sinotubular junction is associated with reduced survival among individuals with coronary artery disease [16]. The presence of calcification along these sites may be a marker of increased vascular disease independent of other medical risk factors.

SINUS OF VALSALVA ANEURYSMS — Sinus of Valsalva aneurysms are occasionally seen on long- and short-axis views of the two-dimensional echocardiogram. However, quantitative criteria for sinus of Valsalva aneurysm are lacking. Since asymmetry of the sinuses is occasionally encountered in clinical practice, such a definition would be helpful. Lacking a published definition, we propose that a diameter from the widest portion of the asymmetric sinus to the opposing wall of greater than 4 cm in adults be adopted as a working definition.

The most common location is the right sinus of Valsalva, from which rupture may extend into the right ventricle or, less frequently, the right atrium or interventricular septum [17].

The next most likely location of the aneurysm is the noncoronary sinus, followed by the left sinus. Infrequently, the aneurysm ruptures into the left ventricle (mimicking aortic regurgitation) or into the left atrium. In a report of 86 patients undergoing sinus of Valsalva aneurysm repair, 44 percent had associated aortic regurgitation [17].

Contrast echocardiography is helpful in delineating the aneurysm and shunt arising from rupture [18]. However, color flow Doppler imaging is the technique of choice for identifying a ruptured sinus of Valsalva aneurysm.

AORTIC DILATION — The 2011 ACC/AHA practice guidelines for echocardiography recommend echocardiography for evaluation of suspected dilation of the proximal aorta (movie 1) [19]. Transthoracic echocardiography (TTE) is recommended as the first choice for this indication with transesophageal echocardiography (TEE) used only if the TTE examination is incomplete or additional information is needed. Multimodality imaging guidelines from the American Society of Echocardiography recommend measuring aortic dimensions from leading edge to leading edge at end diastole, based on reference studies using this technique [20]. The echocardiography leading edge to leading edge measurement can correlate with computed tomography (CT) imaging inner-wall to inner-wall method [21]. There is correlation among the different measuring methods [22], and it is important to use the same technique when comparing serial studies. Two-dimensional speckle-tracking echocardiography has been shown to be useful in the evaluation of dilated aorta pathologies. Displacement and strain of aortic walls were lower in patients with more dilated ascending aortas [23].

The ACC/AHA guidelines also recommend echocardiography to evaluate aortic root dilation in Marfan syndrome or other connective tissue syndromes. In addition, the guidelines recommend TTE to examine first-degree relatives of patients with Marfan syndrome or other connective tissue disorders. The 2010 ACC/AHA guidelines for the diagnosis and management of patients with thoracic aortic disease recommend echocardiogram should be performed at the time of diagnosis of Marfan syndrome, six months thereafter to determine the rate of enlargement, and annually if stability of the aortic diameter is documented and less than 4.5 cm [24]. The diagnosis and management of the Marfan syndrome are discussed separately. (See "Genetics, clinical features, and diagnosis of Marfan syndrome and related disorders" and "Management of Marfan syndrome and related disorders".)

The 2014 ACC/AHA practice guidelines for valvular disease recommend measuring the diameters of the aortic root and ascending aorta by TTE for patients with a bicuspid aortic [25]. Magnetic resonance imaging (MRI) or CT is recommended if the aortic root or ascending aorta cannot be adequately measured by echocardiography. Yearly echocardiography, MRI, or CT is recommended for patients with bicuspid aortic valves and dilation of the aortic root or ascending aorta (diameter greater than 4.0 cm, with consideration of a lower threshold for patients of small stature). Issues related to bicuspid aortic valve disease are discussed separately. (See "Clinical manifestations and diagnosis of bicuspid aortic valve in adults" and "Bicuspid aortic valve: General management in adults" and "Natural history and management of chronic aortic regurgitation in adults" and "Bicuspid aortic valve: Management during pregnancy".)

Limited data are available to compare echocardiography and CT evaluation of thoracic aortic dilation and thoracic aneurysm. In a small prospective study of 44 patients with known ascending aortic aneurysm, TTE and CT measurements of aortic diameters correlated well [26]. Ectasia is defined as aortic dilation up to 50 percent greater than the normal reference diameter, and aneurysm is defined as greater than 50 percent dilation [24].

Causes of aortic root and ascending aortic dilation and aneurysm formation include hypertension (the most common cause), atherosclerosis, aortic dissection, aortic stenosis (post-stenotic dilation), bicuspid aortic valve (associated with aortic dilation even without significant stenosis), aortic regurgitation, and the Marfan syndrome and other causes of annuloaortic ectasia. Less common etiologies of aortic dilation include inflammatory causes, such as Takayasu arteritis and infectious causes, such as syphilis. In a 2017 community-based cohort study of African Americans, ascending aortic diameter was directly associated with an increased risk of cardiovascular events [27]. (See "Clinical manifestations and diagnosis of thoracic aortic aneurysm".)

Various disease processes are associated with different patterns of aortic dilation:

●Hypertension appears to have a minor impact on aortic root diameter at the level of the sinuses of Valsalva [28-30], but is associated with enlargement at the sinotubular junction and tubular ascending aorta [28].

●Congenital aortic stenosis is associated with more significant post-stenotic dilation than degenerative aortic stenosis with similar valve areas (image 5) [31]. In a 2018 study examining the interrelation between proximal aorta morphology and aortic stenosis progression, effacement of the sinotubular junction is independently associated with greater aortic stenosis progression, regardless of aortic valve phenotype or stenosis severity [32].

●Symmetric dilation of the three sinuses is most commonly seen in patients with Marfan syndrome [33,34]. This dilation usually, but not always, terminates abruptly at the sinotubular junction and gives these roots a distinctive appearance unlike that of other causes of annuloaortic ectasia (image 6A-B). In addition to root dilation, patients with Marfan syndrome frequently have aortic regurgitation because aortic annulus dilation causes cusp malcoaptation (image 6A-B). Issues related to echocardiography in Marfan syndrome are discussed separately. (See "Genetics, clinical features, and diagnosis of Marfan syndrome and related disorders".)

Aortic dissection — TEE is an appropriate initial test to evaluate suspected aortic dissection (image 7) [19]. Choice among TEE, MRI or CT for initial noninvasive imaging of aortic dissection is governed by clinical considerations and availability. (See "Clinical features and diagnosis of acute aortic dissection".)

TEE imaging can help determine the potential for aortic valve-sparing operations [35]. The 0-degree high esophageal view is appropriate for diagnosing ascending aortic dissection. However, TEE evaluation of branch vessel involvement may be incomplete and additional imaging with other techniques may be required [36].

The role of TTE in suspected aortic dissection is primarily for diagnosis of cardiac complications of dissection, including aortic insufficiency, pericardial effusion/tamponade, and regional left ventricular systolic function. Advances in echocardiography have improved the sensitivity of TTE for aortic dissection to approximately 85 percent or more [20], although TTE remains less sensitive for detection of aortic dissection than TEE, CT, and MRI. Thus, absence of a dissection flap on TTE should not be used to exclude aortic dissection. In a study of 172 consecutive patients receiving operations for proximal aortic dissection, TTE identified intimal dissection flaps in 159 [37]. TTE may be able to visualize an undulating intimal of a dissection (image 8), but the normal brachiocephalic vein can often be seen adjacent and superior to the aortic arch in the suprasternal notch view (image 9), and this should not be mistaken for a dissection.

DESCENDING THORACIC AORTA AND AORTIC ARCH — The descending thoracic aorta can be seen posterior to the long- and short-axis parasternal views on transthoracic echocardiography (TTE) (image 10A-D). In the parasternal long-axis view, the descending aorta can be seen in cross-section at the posterior atrioventricular groove, situated outside the pericardium. In the parasternal short-axis plane, an oblique longitudinal section of the descending aorta can be seen.

Imaging can identify dilation or an aneurysm and may permit detection of dissection. The descending aorta is a useful landmark for distinguishing pleural and pericardial effusions, since the pericardium encloses the heart anterior to the descending aorta.

On TTE, the aortic arch is visualized in the suprasternal notch view. This view is recommended as a routine component of TTE examination, particularly in cases with bicuspid aortic valve which is frequently associated with coarctation of the aorta. In the suprasternal notch view, color and spectral Doppler interrogation of the proximal descending aorta may detect accelerated flow characteristic of coarctation. If forward velocity in the descending aorta by continuous-wave Doppler exceeds 2 m/sec, aortic coarctation should be suspected. If the run-off at the site of coarctation is delayed, severe coarctation must be considered, and alternate imaging modalities such as MRI can be helpful in confirming diagnosis and grading severity. (See "Clinical manifestations and diagnosis of coarctation of the aorta", section on 'Echocardiography'.)

Views of the descending thoracic aorta as it courses along the spine can be obtained in the apical views; posterior angulation often produces long-axis (in the two chamber view) and short-axis (in the four chamber view) images (image 10C). Although the aorta is too deep in the far field to be well resolved, the size of the aorta can generally be measured. A normal caliber aorta is evidence against dissection at that location.

3D echocardiographic imaging provides an intuitive overview of structures and their relation to each other as illustrated by an example of a mobile aortic mass (movie 2). However, 2D images (movie 3) provide additional clues to the accurate diagnosis of a vegetation, including thickening of the aortic wall consistent with inflammation, a fluid-filled collection around the aorta consistent with abscess, and the absence of calcification or shadowing that would have been more typical of atherosclerotic disease with adjacent thrombus.

When aortic dissection involves the thoracic aorta, especially if there is extravasation of blood around the aorta, the vessel can be imaged from the left paraspinal window. This strategy can be used to supply additional evidence about the state of the thoracic aorta. However, transesophageal echocardiography is the method of choice for detecting pathology of the thoracic aorta.

Abdominal aorta — Subcostal imaging of the proximal abdominal aorta is often included in the TTE examination [38]. The structure can be found to the left of the spine running parallel, but to the left of and deep to, the inferior vena cava (image 11). Differentiation of the aorta from the vena cava can be made by appreciating the systolic pulsations of the aorta, which are usually easy to recognize.

Using the subcostal approach, atheromatous irregularities and aneurysms of the proximal abdominal aorta are readily seen (image 12A-C). Since the descending aorta is closer to the transducer in this view than in other TTE views, the yield for intimal flaps of aortic dissection is higher from this window. In addition, comparing the smoothness of the inner layer of the aorta with the vena cava gives some indication of the degree of atheromatous change that is present in the aorta and, by inference, in the remainder of the vascular tree. Atheromatous change is typically appreciated as obvious irregularities along the usually smooth interior of the vessel.

Screening for abdominal aortic aneurysm with additional views of the abdominal aorta is feasible [38,39]. Among observational reports, the prevalence of finding an abdominal aorta diameter >3 cm was approximately 3 percent, with higher prevalence among those with coronary artery disease [39].

Although transesophageal echocardiography (TEE) is the preferred technique for evaluating the aorta, it does not image the aorta very far below the diaphragm [40]. Ideally, linear arrays should be used for more comprehensive evaluation of the abdominal aorta.

Transcatheter aortic valve implantation — Echocardiographic evaluation of the aorta is a critical component of multimodality imaging for transcatheter aortic valve replacement as discussed in detail separately. Complications such as aortic hematoma can be clearly visualized using TEE (movie 4). (See "Imaging for transcatheter aortic valve implantation".)

OTHER GREAT VESSELS — Transthoracic echocardiography (TTE) is useful in evaluating the other great vessels.

Pulmonary artery — In the parasternal long-axis view, imaging just superior to the left atrium usually demonstrates the left pulmonary artery as it crosses under the ascending aorta. Inspecting the bifurcation of the pulmonary artery in its long-axis (in the parasternal short-axis view) may reveal the relationship between the left pulmonary artery and the descending aorta (image 13A-B). Color Doppler near the pulmonary artery bifurcation in this view can detect the retrograde continuous flow characteristic of a patent ductus arteriosus.

Carotid arteries and subclavian vessels — The innominate, left common carotid, and left subclavian vessel origins can be imaged by TTE from the suprasternal notch. From the neck, the carotid and vertebral arteries can be studied effectively by trained vascular sonographers using dedicated linear array transducers. Skill in performing this examination is helpful when evaluating a patient for dissection of the aorta because detection of extension of the dissection into the carotid arteries has important clinical implications.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Aortic dissection and other acute aortic syndromes".)

SUMMARY AND RECOMMENDATIONS

●Transthoracic echocardiography (TTE) provides views of the proximal ascending aorta, aortic arch, and portions of the descending aorta. However, transesophageal echocardiography (TEE) is superior to TTE for comprehensive imaging of the aorta, especially in the emergency evaluation of aortic dissection or traumatic rupture of the aortic isthmus. (See "Clinical features and diagnosis of acute aortic dissection".)

●TEE provides more highly resolved images of the ascending aorta, aortic arch, and descending thoracic aorta than TTE, although a small portion of the distal ascending aorta and proximal arch cannot be seen by TEE due to interposition of the left mainstem bronchus and trachea. (See 'Two-dimensional echocardiography' above.)

●The presence of large, mobile, or ulcerated plaques in the thoracic aorta on TEE is associated with an increased risk of stroke. (See 'Aortic plaque' above.)

●Echocardiography is the primary modality for identification of sinus of Valsalva aneurysms and any associated shunt arising from rupture. (See 'Sinus of Valsalva aneurysms' above.)

●Echocardiography enables identification of aortic dilation and is indicated for monitoring of individuals at risk for progressive aortic dilation, particularly those with Marfan syndrome or a bicuspid aortic valve. (See 'Aortic dilation' above.)

●TEE is an appropriate initial test to evaluate suspected aortic dissection. Choice among TEE, MRI, or CT for initial noninvasive imaging of aortic dissection is governed by clinical considerations and availability. (See "Clinical features and diagnosis of acute aortic dissection".)