Thoracic Aortic Disease

Chapter 11 Thoracic Aortic Disease





AORTIC ANATOMY AND SIZE


The sinus part of the aorta includes the three sinuses of Valsalva above the aortic leaflets. The aortic annulus is that part of the fibrous skeleton of the heart to which the aortic leaflets attach. The ascending aorta extends from the sinuses of Valsalva to the brachiocephalic artery. The sinotubular ridge is the junction between the sinuses of Valsalva and the tubular ascending aorta. Most aortic diseases do not cross the sinotubular ridge but involve either the sinuses below or the ascending aorta above. The major exception is the annuloaortic ectasia seen in Marfan syndrome. The aortic arch is the transverse segment from which the brachiocephalic, left carotid, and left subclavian arteries originate. The aortic isthmus is the segment between the left subclavian artery and the ductus arteriosus or ligamentum arteriosus. The descending aorta begins after the ductus and ends at the aortic hiatus of the diaphragm. Most of the ascending aorta is within the pericardium.


The size of the aorta is often critical for diagnosing aortic disease. Several measurements are useful in identifying the upper range of normal. On the frontal chest film the distance between the left border of the trachea and the lateral border of the aortic arch is always less than 4 cm in adults and usually less than 3 cm in those younger than 30 years of age. On an aortogram or tomographic scan of the ascending aorta, the normal diameter should be less than 4 cm (Table 11-1). Longitudinal enlargement is more difficult to quantitate but is manifest by tortuosity, occasional kinking or buckling, and displacement into the adjacent lung or mediastinum.


TABLE 11-1 Size of the normal adult thoracic aorta



















  Mean (cm) Upper Limit of Normal* (cm)
Aortic root 3.7 4.0
Ascending aorta 3.2 3.7
Descending aorta 2.5 2.8

* Two standard deviations above the mean.


From Aronberg DJ, Glazer HS, Madsen K, et al.: Normal thoracic aortic diameters by computed tomography, J Comput Assist Tomogr 8:247-250, 1984; Drexler M, Erbel R, Muller U, et al.: Measurement of intracardiac dimensions and structures in normal young adult subjects by transesophageal echocardiography, Am J Cardiol 65:1491-1496, 1990.


The diameter of the normal adult aorta has a wide range that gradually increases with age (Fig. 11-1).



In children less than 2 months old, the aortic isthmus—that portion from the left subclavian artery to the ligamentum arteriosum—is normally smaller than the adjacent descending aorta. This appearance looks like a preductal coarctation but is actually the normal development of the fetal aorta as it remolds following ductal closure. The increased blood flow in the fetus presumably enlarges the descending aorta adjacent to the ductus arteriosus. A normal isthmus may have a diameter equal to 40% of the ascending aorta, although most normal neonatal aortic arches are slightly larger than this.



AORTIC ANEURYSMS





Size and Types


There are a number of definitions of thoracic aneurysm, but most are keyed on the size at which there is potential for rupture (Fig. 11-2). An aortic diameter greater than 1.5 times normal is a commonly accepted definition of aneurysm. For practical purposes, aneurysms in the ascending aorta are greater than 5 cm, and those in the descending aorta are greater than 4 cm.



True aneurysms have all elements of the aorta incorporated into the wall of the aneurysm. The most common true aneurysm is a fusiform aneurysm that involves the entire circumference of the aorta. A saccular aneurysm is an eccentric dilatation that involves one side of the aorta (Fig. 11-3). For example, most infected aneurysms are saccular. It can occur in a short segment or can involve the entire aorta. A vast majority of asymmetric or saccular aneurysms fall under the category of false or pseudoaneurysms. False aneurysms have a perforation into the intima and media. An example is an aortic transection from trauma.



A radiologic description of the aneurysm includes the location and extent, the type, and in conjunction with the clinical history, the cause. Box 11-1 lists the common associations, but many aneurysms fall into several categories. Atherosclerotic aneurysms are usually fusiform and occur in the inferior parts of the aorta. Infected aneurysms may be either true or false. Syphilitic aneurysms can be either saccular or fusiform. The list is not inclusive but serves as a starting point for diagnosis and management.




Chest Film Findings


The chest film is one of the first examinations obtained when thoracic aortic aneurysm, dissection, or transection is suspected, even though its sensitivity and specificity for these diagnoses are widely variable. The purpose of the chest film is to assess the size of the aorta and to identify a rupture. A valuable use of the chest film is to follow the mediastinal contours over a time interval to detect an increasing mediastinal width (Box 11-2). Tracheal or bronchial compression can be suspected when these structures are extrinsically deviated (Fig. 11-4). Compression of a pulmonary artery is recognized by unilateral pulmonary oligemia. For most of its thoracic course, the lesser curvature of the aorta is not visible on the chest film so the only signs of aortic enlargement are those of the greater curvature displacing adjacent structures. Other signs appear when the aorta has ruptured into the mediastinum. Unilateral pleural fluid or pericardial fluid usually indicates impending exsanguination or cardiac tamponade. Rupture into the mediastinum is initially constrained by the tissue in the mediastinum and pleural compartments. The chest film signs of aortic rupture are those of mediastinal hemorrhage (Box 11-3).






ACQUIRED DISEASES



Aortic Dissection


Although Morgagni recognized aortic dissection at necropsy over 200 years ago, even today it may be difficult to diagnose, and it certainly remains a therapeutic enigma. Most aortic dissections, or dissecting hematomas, at least in their early stages, are not aneurysms because they are not a localized enlargement of the aorta. The term dissecting aneurysm should be reserved for those cases in which the aorta (usually the false channel) is actually dilated. The clinical picture of a person with abrupt, severe chest pain associated with a loss of one or more peripheral pulses strongly suggests dissection; however, atypical onset of myocardial infarction, pulmonary or systemic emboli, musculoskeletal syndromes, and other chest pain syndromes may mimic the presence of aortic dissection, and therefore aortic imaging is mandatory for a definitive diagnosis. Similarly, a small percentage of dissections are “silent,” occurring without pain, and are discovered only from an abnormal chest film. In these cases, other types of thoracic aneurysm, penetrating aortic ulcer, or nonvascular mediastinal disease must be distinguished from aortic dissection.


Morphologically, separation of the media from the adventitia for a variable length along the aorta characterizes dissections. Most dissections have a tear in the intima, which allows a column of blood to advance and fill the false channel. A few dissections, however, have no tear in the true lumen and presumably have arisen from a hemorrhage in the vasa vasorum.


Almost all dissections arise in either the ascending aorta, approximately 1 cm above the sinotubular ridge or in the descending aorta at or just beyond the aortic isthmus. Spontaneous dissections that originate elsewhere in the abdominal aorta, coronary, renal, carotid, and other arterial beds are uncommon.


The intimal tear into the false channel usually is single but variations abound so that multiple entry and distal reentry tears are frequently observed by aortic imaging and at necropsy. Although most tears go distally, the aorta can dissect in a retrograde direction. If the dissection reaches the aortic root, it may rupture into the aortic root causing cardiac tamponade, occlude the right coronary artery, or create aortic regurgitation. Dissections in the ascending aorta usually follow the greater curvature of the aortic arch; the false channel forms anteriorly on the right side in the ascending aorta and then follows a spiral course to the posterior and left lateral portion of the descending thoracic aorta. Although its distal extent is quite variable, the false channel frequently proceeds on the left side to compromise the left renal artery and left iliac artery.




Complications


Aortic rupture into the pericardium, pleural space, or mediastinum can be suggested on the plain chest film by a large heart diameter, pleural fluid, and a wide mediastinum. This heralds the need for immediate pericardiocentesis and other cardiopulmonary supportive measures. A dissection can partially or completely occlude a branch of the aorta by compression of the true channel by the false channel or by adjacent compression from an intimal flap. Any artery arising from the aorta can be occluded, but the right coronary artery and the three arch vessels are commonly affected. Surgical treatment aims at preventing retrograde tear into the heart and pericardium by resecting the segment that contains the entry tear. An interposition graft is then inserted, collapsing the false channel distally. Dissections in the aortic root are treated with a composite graft with the prosthetic aortic valve sewn to the graft and the coronary arteries replanted into the sides of the graft.


When there is dissection of the ascending aorta, over half the patients have aortic regurgitation, which contributes to hemodynamic instability. Aortic regurgitation can be caused by an asymmetric tear that misaligns one leaflet below the other, by a tear that results in a flail leaflet, or by disruption of the annulus with subsequent lack of coaptation of the leaflets. Subsequent proximal dissection may extend into the walls of the heart, producing a fistula between the aorta and the atria or the right ventricle.




Plain Film Findings


The plain film findings of dissection are indirect but can suggest the need for further evaluation (Fig. 11-5). An abnormally wide mediastinum, separation of calcium from the wall of the aortic arch, a left apical pleural cap, pleural fluid, and displacement of the trachea and esophagus from the midline are important characteristics of a thoracic aortic abnormality. However, the chest film findings are insensitive for the detection of aortic dissection: Nearly one fifth of patients with dissection have normal chest films.




Tomographic Imaging


Computed tomography with intravenous contrast, magnetic resonance imaging (MRI), and echocardiography all have excellent sensitivity and specificity for detecting aortic dissection but each has limitations specific to its technology. Optimal computed tomography (CT) imaging is with intravenous contrast on a spiral scanner (Figures 11-6, 11-7). Spiral CT requires a correctly timed bolus of contrast media and has streak artifacts from the pulsating aortic wall that can mimic the intimal flap. Multidetector CT scanners with three-dimensional postprocessing reconstructions can rival the accuracy of catheter angiography.





Magnetic Resonance Imaging


MRI does not require intravenous contrast and has more options to characterize the extent of the dissection. The imaging plane can be placed parallel to that of the aorta in addition to the coronal and axial slices. “White blood” gradient echo sequences and phase reconstruction techniques can help identify slowly flowing blood in the false channel. The major limitation of MRI is the resolution of the arch vessels, so that the distal extent of the dissection into small arteries is frequently not shown.


Because of slow blood flow, clotted false channels, and occasionally the twisted shape of the intimal flap, aortic dissection can be difficult to distinguish from other types of aneurysm and aortitis (Fig. 11-8). MRI is particularly useful in these situations because different pulse sequences and reconstruction techniques can be exploited to produce a distinction between flowing blood and static tissue. Spin echo sequences producing “black blood” images easily show the intimal flap when there is moderate flow in both channels. But in regions of slowly flowing blood, the signal in that region may be similar to tissue in the aortic wall and adjacent mediastinum. A number of techniques exist that can image slow velocity differences and separate nonmoving and clotted blood in the false channel from slow-moving blood. Even echocardiographic rephasing, the fortuitous occurrence of velocity compensation in the second echocardiogram, is recognized as a higher signal intensity in the second echocardiogram in regions of slowly flowing blood (Fig. 11-9). A caveat is that failure of the signal intensity on the second echocardiogram to be greater than the first echocardiographic image is often observed in regions of complex velocity changes such as those that occur in vortices and eddies around bends in the aorta.




One of the most sensitive ways to make the distinction between thrombus and slowly flowing blood is to reconstruct the original data as a phase image. All MRIs are generated as complex numbers, which are typically reconstructed as magnitude images. However, the same data can be displayed as a phase image, which then becomes a picture of the velocity of the tissue within each pixel. The phase image needs to be interpreted with the magnitude image to identify the area of concern where a thrombosed channel may be present. Changes in signal intensity, including alternating white to black phase breaks in the phase image, indicate flowing blood (Fig. 11-10).



Another strategy that can be quite helpful is to obtain a gradient echo cine study through the area that has questionable flow in the standard spin echo image. With velocity compensation, the gradient echo pulse sequence should be obtained at only one slice to avoid the inflow of partly saturated spins from a neighboring slice (Fig. 11-11).



Unlike the other techniques, transesophageal echocardiography (TEE) can be carried out at the bedside. The major advantage over the other techniques is the quantitation of aortic regurgitation with color-flow Doppler. Its disadvantage is that the aortic arch cannot be completely imaged with either the transthoracic or transesophageal technique.




Catheter Approach


The site of catheterization will depend on which pulses are present. If no extremity pulses are felt, a pulmonary angiogram with delayed follow-through may show the dissection. However, a general principle is that angiography should be performed with an injection as close to the abnormality as possible, and therefore a femoral, axillary, or brachial arterial approach is preferred. Because the left iliac artery is most frequently involved with a dissection, the preferred route is a percutaneous transfemoral approach from the right side.


The ultimate goal is to place the catheter in the true lumen of the aorta about 2 cm above the sinotubular ridge of the aortic root. The identification of the true channel cannot always be ascertained from catheter location, the size of the channel, or the speed of blood flow alone because the false channel may compress the true channel in unusual ways. Separation of a catheter from the greater curvature of the aortic arch by a centimeter or more confirms the presence of an abnormality. The false channel generally has less flow velocity than the true channel and therefore fills later. Another sign that the false channel is opacified is that there are no arteries originating from it.


Although there has been much concern about the potential consequences of injecting into the false channel (such as possible extension of the dissection or aortic rupture), a more important criterion for a safe angiographic injection is the rapid washout of contrast material during the test injection. This assures a large-capacity reservoir for the contrast agent, allowing a safe injection. At times, even on the final angiogram, it is not possible to determine which is the true or which is the false channel; however, you can still perform an adequate and safe angiogram as long as a high-pressure injection is not made into a cul-de-sac. Either the true channel or false channel may be the main conduit, and therefore filming should extend about 20 seconds to see late filling. If only one channel appears to fill, either the false channel is clotted or it is a retrograde dissection with a distal entry point. Another injection, distal to the first, in the descending aorta with filming over the thorax should opacify the false channel if it is patent. Based on clinical presentation, an abdominal aortogram may be appropriate to search for complications involving the arteries to the gut, kidneys, and legs.



Signs of Dissection


The intimal flap, a lucency several millimeters thick outlined by contrast on both sides, is the hallmark of dissection. You can often identify the actual entry from one channel into another or into multiple channels and the subsequent flow of blood in either an antegrade or retrograde direction. This intimal tear may extend only a few centimeters (Fig. 11-12) or may extend the entire length of the aorta, even into peripheral vessels (Fig. 11-13). The leaflets of the aortic valve, particularly with annuloaortic ectasia or Marfan syndrome, may be effaced and appear as a lucency; the large aortic leaflets can be difficult to distinguish from a true intimal tear in the aortic root, particularly if large-film technique is used. Cine angiography usually resolves this problem. True tears may be further differentiated by their origin above the sinotubular ridge and extension backward into the valve.




An ulcerlike projection from the aorta may represent an early sign of dissection, although other types of aneurysms, including those caused by infection and penetrating atherosclerotic ulcer, may be associated with this finding. The base of the ulcer represents the defect in the intima leading to a thrombosed false channel (see Figure 11-13). In the descending aorta where side branches originate, the ulcer may be an occlusion or detachment of the intima from an intercostal artery.


The false channel may not opacify during angiography and, therefore, may present as a thick wall (usually >1 cm) along the greater curvature of the aorta (Fig. 11-14). An eccentric wall thickness greater than 1 cm is unusual in a clotted atherosclerotic or syphilitic aneurysm or aortitis. An unopacified channel may occasionally be seen if the injection was made proximal to the entry tear and the distal dissection has extended in a retrograde direction. Before the diagnosis of a thrombosed false channel is made, an injection should be made with the catheter at the level of the diaphragm to search for retrograde flow from a distant entry site. The thrombosed false channel has been called a “healed” dissection and is less liable to rupture late. Initially, the true channel is frequently smaller than the false channel, although either may enlarge to greater than the normal aortic width, then justifying the term dissecting aneurysm.



Aortic regurgitation may result from three mechanisms (Fig. 11-15):









In addition to occluding or transecting any vessels from the aorta, a dissection may compromise other mediastinal vessels. The expanding hematoma of the false channel may compress the right pulmonary artery. Similarly, it may both displace and compress the superior vena cava. On the left side of the mediastinum, because the false channel extends posterolaterally, the pulmonary veins are occasionally compressed. If this abnormality leads to reduced flow through the left lung, then a dissection may be confused with pulmonary embolism on a ventilationperfusion lung scan.




Annuloaortic Ectasia and Marfan Syndrome





Radiologic Techniques


Many of the clues to the diagnosis of Marfan syndrome are frequently visible on the chest film (Fig. 11-21). On the frontal film, the thoracic cage appears large and elongated with large-volume lungs. The heart may be shifted to the left from a narrow anteroposterior thoracic diameter. In normal young adults less than 20 years of age, the aorta should be inapparent. In contrast, aortic elongation and ectasia in this age group are common signs of Marfan syndrome. Cardiomegaly is usually nonspecific and may reflect only the pectus excavatum, but aortic regurgitation from annuloaortic ectasia and mitral regurgitation from prolapsing mitral leaflets are common conditions that pathologically enlarge the heart. On the lateral film, a pectus excavatum is frequently identified as well as a narrow thoracic diameter.



Marfan patients without symptoms are easily observed with serial MRI every 6 to 12 months. Surgical referral is usually undertaken if a previously stable aortic aneurysm begins to enlarge or if the aortic arch and descending aorta exceed a diameter of 5 cm. MRI allows detection of the onset of annuloaortic ectasia (Fig. 11-22) with dilatation of the aortic root and ascending aorta, and visualization of a dissection. Aortic regurgitation can be observed and quantified with velocity-encoded pulse sequences. Observations on the aortic root and quantification of aortic regurgitation can also be made by echocardiography.



Aortography is usually reserved for urgent clinical situations in which noninvasive imaging was inconclusive. Some surgeons request coronary angiography to evaluate whether a dissection extends near or into the coronary arteries. Occasionally, aortography can identify an entry site of a dissection that is not apparent on other methods (Fig. 11-23).




Sinus of Valsalva Aneurysms



Etiology


Dilatation of one or all of the sinuses of Valsalva may be associated with abnormalities in the aortic valve or the aorta. These aneurysms may be classified radiologically as discrete (localized to the sinuses) or annuloaortic (involving both the aortic root and the ascending aorta). The classic type is annuloaortic ectasia with a pear-shaped configuration of the aortic root and equal dilatation of all sinuses.


An outline of sinus of Valsalva aneurysms is presented in Box 11-4. Discrete aneurysms that involve a single sinus are usually congenital (Fig. 11-24), although rarely dilatation of two or all three sinuses may also be congenital. These are generally less than 4 cm in diameter and involve mainly the right sinus. The tissue in the aortic annulus adjacent to the leaflet histologically has sparse fibroelastic elements and grossly may have fenestrations through the cusp. A sinus of Valsalva aneurysm can develop as a consequence of a ventricular septal defect. One of the ways a ventricular septal defect can close spontaneously is to form fibrous tissue around its edges. As the membranous ventricular septal defect becomes smaller, the adjacent leaflet of the aortic valve is pulled inferiorly into the defect. The clinical consequence of the developing leaflet prolapse is that the left-to-right shunt through the ventricular septal defect is transformed to that of aortic regurgitation.




Acquired discrete aneurysms usually involve all three sinuses if they are a consequence of a generalized inflammatory process, for example, syphilis or an immune complex aortitis. Aortic root abscesses are actually false aneurysms since they erode through the aorta into cardiac or mediastinal tissue.


Because the sinuses of Valsalva lie completely within the cardiac silhouette (Fig. 11-25), the discrete type of aneurysm is not visible on the plain chest film. If the ascending aorta is also dilated, the right side of the mediastinum will have the characteristic convexity of the aorta as it extends into the adjacent lung.





Complications


Aortic regurgitation is the main complication of progressive dilatation of the aortic annulus and the resultant lack of coaptation of the leaflets. Any type of sinus of Valsalva aneurysm can rupture into an adjacent structure. The onset is abrupt with severe aortic regurgitation or a torrential left-to-right shunt. Most sinus aneurysms rupture into the right sinus; they perforate anteriorly into the right ventricular outflow tract, dissect into the ventricular septum, or perforate posteriorly in the right atrium. Aneurysms of the noncoronary sinus rupture into the right atrium. Rupture of the left sinus into the left atrial appendage is extremely rare. When an aneurysm ruptures, aortography shows contrast medium entering the cardiac chamber and opacifying downstream structures on subsequent films (Fig. 11-26). Both a left ventriculogram and an aortogram may be necessary to distinguish a ventricular septal defect with aortic regurgitation from a ruptured sinus of Valsalva aneurysm. The contrast in the right ventricle from an aortogram could have passed into the left ventricle from aortic regurgitation and then across a ventricular septal defect, or it could have flowed directly from the aorta through the rupture into the right ventricle.



Because an aneurysm of the right sinus of Valsalva can compress and distort adjacent structures, significant hemodynamic complications can occur as the aneurysm dilates. Right coronary artery compression, superior vena cava obstruction, right ventricular outflow obstruction, and endocarditis can produce dramatic clinical events.



Aortitis


A number of clinical syndromes have vasculitis that involves the aorta. Most of these diseases are either associated with or caused by immune complexes deposited in the vessel wall. Intimal proliferation and fibrosis, degeneration of the elastic fibers, round cell infiltration, and occasionally giant cells usually allow a specific histologic diagnosis. The gross changes, except in Takayasu disease, are far less specific, regrettably so because these are the features seen on angiography and cross-sectional imaging. Aortitis produces aneurysms in many portions of the aorta and its related branches. They are usually fusiform but occasionally saccular. Takayasu disease is the only aortitis that produces stenoses in the thoracic aorta. Significant stenoses in the aortic arch are well known in the acquired disease: aortic dissection, false aneurysms from laceration of the aortic arch after a motor vehicle accident, infected aortic aneurysms with abscess formation, Behçet’s disease, and rarely atherosclerotic and syphilitic disease.


Dilatation of the ascending aorta is one of the earliest signs of aortitis, but it is not specific for aortitis because systemic hypertension and aortic valve disease also widen the aorta.


Dec 26, 2015 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Thoracic Aortic Disease
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