Introduction Acute aortic dissection is an uncommon but

potentially catastrophic illness.

incidence of approximately 2.9/100,000/yr with at least 7000 cases per year in the United States.

Early mortality is as high as 1 percent per hour if untreated, but survival may be improved substantially by the timely institution of appropriate medical and/or surgical therapy.

Prompt clinical recognition and definitive diagnostic testing are essential in the management of patients with aortic dissection.

Pathology• Formation of a tear in the aortic intima that directly exposes an

underlying diseased medial layer to the driving force (or pulse pressure) of intraluminal blood .

• This blood penetrates the diseased medial layer and cleaves the media longitudinally, thereby dissecting the aortic wall.

• Driven by persistent intraluminal pressure, the dissection process extends a variable length along the aortic wall, typically antegrade (driven by the forward force of aortic blood flow) but sometimes retrograde from the site of the intimal tear.

• The blood-filled space between the dissected layers of the aortic wall becomes the false lumen.

• Shear forces may lead to further tears in the intimal flap (the inner portion of the dissected aortic wall) and produce exit sites or additional entry sites for blood flow into the false lumen.

• Distention of the false lumen with blood may cause the intimal flap to bow into the true lumen and thereby narrow its caliber and distort its shape.

Commonly Used Classification Systems to Describe Aortic Dissection

• Type (Site of Origin and Extent of Aortic Involvement)• DeBakey Type I• Originates in the ascending aorta, propagates at least to the aortic arch

and often beyond it distallyType IIOriginates in and is confined to the ascending aortaType IIIOriginates in the descending aorta and extends distally down the aorta or, rarely, retrograde into the aortic arch and ascending aorta

• StanfordType AAll • dissections involving the ascending aorta, regardless of the site of

originType BAll dissections not involving the ascending aorta• Descriptive• ProximalIncludes DeBakey types I and II or Stanford type

ADistalIncludes DeBakey type III or Stanford type B

• Alternatively, aortic dissection may begin with rupture of the vasa vasorum within the aortic media; that is, with the development of an intramural hematoma .

• Local hemorrhage then secondarily ruptures through the intimal layer and creates the intimal tear and aortic dissection.

• Because in autopsy series as many as 13 percent of aortic dissections do not have an identifiable intimal tear, at least in a minority of cases independent medial hemorrhage does appear to be the primary cause of dissection.

• One might argue that the lack of an intimal tear in these patients indicates they do not, in fact, have classic aortic dissection but rather have intramural hematoma of the aorta, a closely related condition

• CLASSIFICATION. • Most classification schemes for aortic dissection are based on the fact that the vast majority of aortic dissections

originate in one of two locations: (1) the ascending aorta, within several centimeters of the aortic valve; and (2) the descending aorta, just distal to the origin of the left subclavian artery at the site of the ligamentum arteriosum. Sixty-five percent of intimal tears occur in the ascending aorta, 20 percent in the descending aorta, 10 percent in the aortic arch, and 5 percent in the abdominal aorta.

• Three major classification systems are used to define the location and extent of aortic involvement, as defined in Table 56-1 and depicted in Figure 56-12 : (1) DeBakey types I, II, and III; (2) Stanford types A and B; and (3) the anatomical categories “proximal” and “distal.” All three schemes share the same basic principle of distinguishing aortic dissections with and without ascending aortic involvement for prognostic and therapeutic reasons; in general, surgery is indicated for dissections involving the ascending aorta, whereas medical management is reserved for dissections without ascending aortic involvement. Accordingly, because both DeBakey types I and II involve the ascending aorta, they are grouped together for simplicity in the Stanford (type A) and anatomical (proximal) classification systems, irrespective of the site of intimal tear. Less experienced clinicians will sometimes misclassify as type A those dissections that begin in the aortic arch and progress distally, but because the ascending aorta is not involved, such cases should, in fact, be classified as type B. Aortic dissections confined to the abdominal aorta, although quite uncommon, are best categorized as type B or distal dissections. Proximal or type A dissections occur in about two thirds of cases, with distal dissections composing the remaining one third.

•

• In addition to its location, aortic dissection is also classified according to its duration, defined as the length of time from symptom onset to medical evaluation. The mortality from dissection and its risk of progression decrease progressively over time, which makes therapeutic strategies for longstanding aortic dissections quite different from those seen acutely. A dissection present less than 2 weeks is defined as “acute,” whereas those present 2 weeks or more are defined as “chronic” because the mortality curve for untreated aortic dissections begins to level off at 75 to 80 percent at 2 weeks. At diagnosis, the large majority of aortic dissections are acute.

• ETIOLOGY AND PATHOGENESIS. • Cystic medial degeneration, as described earlier, is the chief predisposing factor in aortic

dissection. Therefore, any disease process or other condition that undermines the integrity of the elastic or muscular components of the media predisposes the aorta to dissection. Cystic medial degeneration is an intrinsic feature of several hereditary defects of connective tissue, most notably Marfan and Ehlers-Danlos (see Chap. 8) syndromes, and is also common among patients with bicuspid aortic valve. In addition to their propensity for thoracic aortic aneurysms, patients with Marfan syndrome are indeed at high risk for aortic dissection, especially proximal dissection, at a relatively young age. In fact, Marfan syndrome accounts for 5 percent of all aortic dissections.[56]

• In the absence of Marfan syndrome, histologically classic cystic medial degeneration occurs in only a minority of cases of aortic dissection. Nevertheless, the degree of medial degeneration found in most other cases of aortic dissection still tends to be qualitatively and quantitatively much greater than expected as part of the aging process. Although the cause of such medial degeneration remains unclear, advanced age and hypertension appear to be two of the most important factors.

• The peak incidence of aortic dissection is in the sixth and seventh decades of life, with men affected twice as often as women. About three quarters of patients with aortic dissection have a history of hypertension. A bicuspid aortic valve is a well-established risk factor for proximal aortic dissection and occurs in 5 to 7 percent of aortic dissections. As is the case with ascending thoracic aortic aneurysms, the risk of aortic dissection appears to be independent of the severity of the bicuspid valve stenosis. Certain other congenital cardiovascular abnormalities predispose the aorta to dissection, including coarctation of the aorta and Turner syndrome. Rarely, aortic dissection complicates arteritis involving the aorta (see Chap. 84) , particularly giant cell arteritis. A number of reports describe aortic dissection in association with cocaine abuse, typically among young, black, and hypertensive men. However, cocaine abuse likely accounts for less than 1 percent of cases of aortic dissection and the mechanisms by which it causes dissection remain speculative.[57]

• An unexplained relation exists between pregnancy and aortic dissection (see Chap. 77) . About one-quarter of all aortic dissections in women younger than 40 years occur during pregnancy, typically in the third trimester and also occasionally in the early postpartum period. The increases in blood volume, cardiac output, and blood pressure seen in late pregnancy may contribute to the risk, although this explanation cannot account for postpartum occurrence. Women with Marfan syndrome and a dilated aortic root are at particular risk for acute aortic dissection during pregnancy, and in some cases, diagnosis of Marfan syndrome is first made when such women are evaluated for peripartum aortic dissection.

• Direct trauma to the aorta may also cause aortic dissection. Blunt trauma tends to cause localized tears, hematomas, or frank aortic transection (see Chap. 71) and only rarely causes classic aortic dissection. Iatrogenic trauma is associated with true aortic dissection and accounts for 5 percent of cases.[58] Both intraarterial catheterization and the insertion of intraaortic balloon pumps may induce aortic dissection, probably from direct trauma to the aortic intima. Cardiac surgery also entails a very small risk (0.12 to 0.16 percent) of acute aortic dissection. The majority of these dissections are discovered intraoperatively and are repaired at that time, although 20 percent are detected only after a delay. In addition, aortic dissection sometimes occurs late (months to years) after cardiac surgery; in fact, as many as 18 percent of those with acute aortic dissection have had prior cardiac surgery. Of cardiac surgical patients, those undergoing aortic valve replacement have the highest risk for aortic dissection as a late complication. The association with aortic valve surgery may occur because many such patients had surgery to replace dysfunctional bicuspid aortic valves. As discussed earlier, cystic medial degeneration often accompanies this condition and can predispose them to subsequent dissection. This association argues for an aggressive approach in replacing even a mildly dilated ascending aorta at the time of bicuspid aortic valve replacement, as discussed earlier

• Clinical Manifestations • SYMPTOMS. • Much of the data presented regarding the clinical manifestations of aortic dissection are

from older clinical series, as well as from a more recent series from the International Registry of Acute Aortic Dissection (IRAD), which studied 464 consecutive patients with acute aortic dissection from 12 international referral centers.[56] By far, the most common initial symptom of acute aortic dissection is pain, which is found in up to 96 percent of cases, whereas the large majority of those without pain are found to have chronic dissections. The pain is typically severe and of sudden onset and is as severe at its inception as it ever becomes, in contrast to the pain of myocardial infarction, which usually has a crescendo-like onset and is not as intense. In fact, the pain of aortic dissection may be all but unbearable in some instances and force the patient to writhe in agony, fall to the ground, or pace restlessly in an attempt to gain relief. Several features of the pain should arouse suspicion of aortic dissection. The quality of the pain as described by the patient is often morbidly appropriate to the actual event, with adjectives such as “tearing,” “ripping,” “sharp,” and “stabbing” frequently used in more than one half the cases. In fact, it is not uncommon to hear descriptors that are collectively almost diagnostic of aortic dissection, but quite unlike the symptoms of myocardial ischemia or infarction, such as someone “stabbed me in the chest with a knife” or “hit me in the back with an ax.”

• Another important characteristic of the pain of aortic dissection is its tendency to migrate from its point of origin to other sites, generally following the path of the dissection as it extends through the aorta. However, such migratory pain is described in as few as 17 percent of cases. The location of pain may be quite helpful in suggesting the location of the aortic dissection because localized symptoms tend to reflect involvement of the underlying aorta. Spittell and colleagues found that when the location of chest pain was anterior only (or if the most severe pain was anterior), more than 90 percent of patients had involvement of the ascending aorta. Conversely, when the chest pain was interscapular only (or when the most severe pain was interscapular), more than 90 percent of patients had involvement of the descending thoracic aorta (i.e., DeBakey type I or III). The presence of any pain in the neck, throat, jaw, or face strongly predicted involvement of the ascending aorta, whereas pain anywhere in the back, abdomen, or lower extremities strongly predicted involvement of the descending aorta.[59] In rare cases, the presenting pain is only pleuritic in nature caused by acute pericarditis that results from hemorrhage into the pericardial space from the dissected ascending aorta. In such cases, the underlying diagnosis may be overlooked if one does not search for other symptoms or signs that might suggest the presence of aortic dissection.

• Less common symptoms at initial evaluation, occurring with or without associated chest pain, include congestive heart failure (7 percent), syncope (13 percent), cerebrovascular accident (6 percent), ischemic peripheral neuropathy, paraplegia, and cardiac arrest or sudden death. The presence of acute congestive heart failure in this setting is almost invariably caused by severe aortic regurgitation induced by a proximal aortic dissection (discussed later). Patients with syncope have a higher rate of mortality than those without syncope and are more likely to have cardiac tamponade or stroke. However, when the complications of cardiac tamponade and stroke are excluded, syncope alone does not increase mortality.[60] On occasion, a patient presents with acute chest pain, and the initial imaging study reveals hemopericardium yet fails to demonstrate an aortic dissection. In such a scenario, unless another diagnosis, such as tumor metastatic to the pericardium, is evident, one must still suspect the presence of acute aortic dissection (or contained aortic rupture). Ideally, such a patient would be taken presumptively to the operating room or, at the very least, would immediately undergo additional imaging with other modalities to confirm the diagnosis.[61]

• PHYSICAL FINDINGS. • Although extremely variable, findings on physical examination generally reflect the location of aortic dissection and

the extent of associated cardiovascular involvement. In some cases, physical findings alone may be sufficient to suggest the diagnosis, whereas in other cases, such pertinent physical findings may be subtle or absent, even in the presence of extensive aortic dissection. Hypertension is seen in 70 percent of patients with distal aortic dissection but in only 36 percent with proximal dissection. Hypotension, on the other hand, occurs much more commonly among those with proximal than those with distal aortic dissection (25 and 4 percent, respectively). True hypotension is usually the result of cardiac tamponade, acute severe aortic regurgitation, intrapleural rupture, or intraperitoneal rupture. Dissection involving the brachiocephalic vessels may result in pseudohypotension, an inaccurate measurement of blood pressure caused by compromise or occlusion of the brachial arteries.

• The physical findings most typically associated with aortic dissection—pulse deficits, the murmur of aortic regurgitation, and neurological manifestations—are more characteristic of proximal than of distal dissection. Reduced or absent pulses in patients with acute chest pain strongly suggests the presence of aortic dissection. Such pulse abnormalities are present in about 30 percent of proximal aortic dissections and occur throughout the arterial tree, but occur in only 15 percent of distal dissections, where they usually involve the femoral or left subclavian artery. Impaired pulses, and similarly, visceral ischemia, result from extension of the dissection flap into a branch artery with compression of the true lumen by the false channel ( Fig. 56-13 ), which diminishes blood flow in the aortic true lumen because of narrowing or obliteration by the distended false lumen (occurring most commonly in the descending or abdominal aorta); impaired pulses may also result from proximal obstruction of flow caused by a mobile portion of the intimal flap overlying the branch vessel's orifice. Whichever the cause, the pulse deficits in aortic dissection may be transient, secondary to decompression of the false lumen by distal reentry into the true lumen or secondary to movement of the intimal flap away from the occluded orifice.

Mechanisms of compromised perfusion of branch arteries due to aortic dissection. A, The branch artery still originates from the true lumen, but the true lumen (T) is markedly compressed by the false lumen (F)

throughout the cardiac cycle, resulting in low pressure and reduced flow within the true lumen and its branches. B, The intimal flap of the aortic dissection extends into the ostium of a branch artery,

potentially narrowing or obstructing it.

• Aortic regurgitation is an important feature of proximal aortic dissection, with the murmur of aortic regurgitation detected in one-third of cases. When present in patients with distal dissection, aortic regurgitation generally antedates the dissection and may be the result of preexisting dilation of the aortic root from the underlying aortic pathological condition, such as cystic medial degeneration. The murmur of aortic regurgitation may wax and wane, the intensity varying directly with the height of the arterial blood pressure. Depending on the severity of the regurgitation, other peripheral signs of aortic regurgitation may be present, such as collapsing pulses and a wide pulse pressure. In some cases, however, congestive heart failure secondary to severe acute aortic regurgitation may occur with little or no murmur and no peripheral signs of aortic runoff.

• The acute aortic regurgitation associated with proximal aortic dissection, which occurs in one half to two thirds of such cases, may result from any of several mechanisms ( Fig. 56-14 ). First, the dissection may dilate the aortic root, thereby widening the sinotubular junction from which the aortic leaflets hang so that the leaflets are unable to coapt properly in diastole (incomplete closure). Second, the dissection may extend into the aortic root and detach one or more aortic leaflets from their commissural attachments at the sinotubular junction, thereby resulting in diastolic leaflet prolapse. Not infrequently, both incomplete closure and leaflet prolapse are present at the same time. Finally, in the setting of an extensive or circumferential intimal tear the unsupported intimal flap may prolapse into the left ventricular outflow tract, occasionally appearing as frank intimal intussusception, and produce severe aortic regurgitation.

Mechanisms of aortic regurgitation in proximal aortic dissection. A, Normal aortic valve anatomy, with the leaflets suspended (dotted lines) from the sinotubular junction. B, A type A dissection dilates the

ascending aorta, which in turn widens the sinotubular junction from which the aortic leaflets hang so that the leaflets are unable to coapt properly in diastole (incomplete closure). Aortic regurgitation (arrow)

results. C, A type A dissection extends into the aortic root and detaches an aortic leaflet from its commissural attachment to the sinotubular junction. Diastolic leaflet prolapse results. D, In the setting of an extensive or circumferential intimal tear, the unsupported intimal flap may prolapse across the aortic

valve and into the left ventricular outflow tract and prevent normal leaflet coaptation.

• Neurological manifestations occur in as many as 6 to 19 percent of all aortic dissections and accompany proximal dissection more frequently. Cerebrovascular accidents may occur in 3 to 6 percent when the innominate or left common carotid arteries are directly involved. Less often, patients may have altered consciousness or even coma. When spinal artery perfusion is compromised, ischemic spinal cord damage may produce paraparesis or paraplegia.

• In a small minority, about 1 to 2 percent of cases, a proximal dissection flap may involve the ostium of a coronary artery and cause acute myocardial infarction. Because most proximal dissections arise above the right sinus of Valsalva, retrograde extension into the aortic root more often affects the right coronary artery than the left, which explains why these myocardial infarc-tions tend to be inferior in location. Unfortunately, when secondary myocardial infarction does occur, its symptoms may complicate the clinical picture by obscuring symptoms of the primary aortic dissection. Most worrisome is the possibility that in the setting of electrocardiographic evidence of myocardial infarction, the underlying aortic dissection may go unrecognized. Moreover, the consequences of such a misdiagnosis in the era of thrombolytic therapy can be catastrophic, with an early mortality rate of 71 percent (many from cardiac tamponade) among patients with aortic dissection treated with thrombolysis. It remains essential that when evaluating patients with acute myocardial infarction, particularly inferior infarctions, one carefully considers the possibility of an underlying aortic dissection before thrombolytic or anticoagulant therapy is instituted. Although some physicians feel reassured that performing a chest radiograph before the institution of thrombolysis is adequate to exclude the diagnosis of dissection, studies have shown it is not sufficient.

• Extension of aortic dissection into the abdominal aorta can cause other vascular complications. Compromise of one or both renal arteries occurs in about 5 to 8 percent and can lead to renal ischemia or frank infarction and, eventually, severe hypertension and acute renal failure. Mesenteric ischemia and infarction—occasional and potentially lethal complications of abdominal dissection—occur in 3 to 5 percent of cases. In addition, aortic dissection may extend into the iliac arteries and cause diminished femoral pulses (12 percent) and acute lower extremity ischemia. If in such cases the associated chest pain is minimal or absent, the absent pulse and ischemic peripheral neuropathy may be mistaken for a peripheral embolic event.

• Additional clinical manifestations of aortic dissection include the presence of small pleural effusions, seen more commonly on the left side. The effusion typically arises secondary to an inflammatory reaction around the involved aorta, but in some cases larger effusions may result from hemothorax caused by a transient rupture or leak from a descending dissection. Several rarely encountered clinical manifestations of aortic dissection include hoarseness, upper airway obstruction, rupture into the tracheobronchial tree with hemoptysis, dysphagia, hematemesis from rupture into the esophagus, superior vena cava syndrome, pulsating neck masses, Horner syndrome, and unexplained fever. Other rare findings associated with the presence of a continuous murmur include rupture of the aortic dissection into the right atrium, into the right ventricle, or into the left atrium with secondary congestive heart failure. A variety of conditions can mimic aortic dissection, including myocardial infarction or ischemia, pericarditis, pulmonary embolism, acute aortic regurgitation without dissection, nondissecting thoracic or abdominal aortic aneurysms, or mediastinal tumors.

• Because of the variable extent of aortic, branch vessel, and cardiac involvement occurring with aortic dissection, the signs and symptoms associated with the condition occur sporadically. Consequently, the presence or absence of aortic dissection cannot be diagnosed accurately in most cases on the basis of symptoms and clinical findings alone. In one series, of all aortic dissections (without a known diagnosis), the initial clinical diagnosis was aortic dissection in only 62 percent, and the other 38 percent of patients were initially thought to have myocardial ischemia, congestive heart failure, nondissecting aneurysms of the thoracic or abdominal aorta, symptomatic aortic stenosis, pulmonary embolism, and so on. Among this 38 percent in whom aortic dissection went undiagnosed at initial evaluation, nearly two thirds of patients had their aortic dissection detected incidentally while undergoing a diagnostic procedure for other clinical questions, and in nearly one third, the aortic dissection remained undiagnosed until necropsy. Given the clinical challenge that detection of aortic dissection presents, physicians should remain vigilant for any risk factors, symptoms, and signs consistent with aortic dissection if a timely diagnosis is to be made.

• LABORATORY FINDINGS. • Chest radiography is included in the discussion of clinical

manifestations of aortic dissection rather than the discussion of diagnostic techniques because an abnormal incidental finding on a routine chest radiograph may first raise clinical suspicion of aortic dissection ( Fig. 56-15 ). Moreover, although chest radiography may help support a diagnosis of suspected aortic dissection, the findings are nonspecific and rarely diagnostic. The results of chest radiography therefore add to the other available clinical data used in deciding whether suspicion of aortic dissection warrants proceeding to a more definitive diagnostic study.

Chest radiograph of a patient with aortic dissection. A, The patient's baseline study from 3 years prior to admission, with a normal-appearing aorta. B, The chest radiograph on admission, which is remarkable for the interval enlargement of the aortic knob (arrow). The patient was found to have a proximal aortic dissection

• The most common abnormality seen on a chest radiograph in cases of aortic dissection is widening of the aortic silhouette, which appears in 81 to 90 percent of cases. Less often, nonspecific widening of the superior mediastinum is seen. If calcification of the aortic knob is present, separation of the intimal calcification from the outer aortic soft tissue border by more than 1.0 cm—the “calcium sign”—is suggestive, although not diagnostic, of aortic dissection. Comparison of the current chest radiograph with a previous study may reveal acute changes in the aortic or mediastinal silhouettes that would otherwise have gone unrecognized. Pleural effusions are common, typically occur on the left side, and are more often associated with dissection involving the descending aorta. Although the majority of patients with aortic dissection have one or more of these radiographic abnormalities, the remainder, up to 12 percent, have chest radiographs that appear unremarkable. Therefore, a normal chest radiograph can never exclude the presence of aortic dissection.

• Electrocardiographic findings in patients with aortic dissection are nonspecific. One third of electrocardiograms show changes consistent with left ventricular hypertrophy, whereas another one third are normal. Nevertheless, obtaining an electrocardiogram is diagnostically important for two reasons: (1) in cases of aortic dissection, nonspecific chest pain and the absence of ischemic ST segment and T wave abnormalities on electrocardiogram may argue against the diagnosis of myocardial ischemia and thereby prompt consideration of other chest pain syndromes, including aortic dissection, and (2) in patients with proximal dissection, the electrocardiogram may reveal acute myocardial infarction when the dissection flap has involved a coronary artery.

• Currently, there are no reliable biomarkers that are diagnostic of aortic dissection, although a number or markers are under investigation. However, recent studies have shown that a markedly elevated D-dimer level may indicate acute aortic dissection. In a series comparing 94 consecutive patients with aortic dissection and 94 controls, a d-dimer of >400 ng/ml had a sensitivity of 99 percent and a specificity of 34 percent. Moreover, D-dimer levels correlated with the anatomical extent of the dissection and with in-hospital mortality.[62] This suggests that D-dimer levels may be useful as a screening test in the emergency department, with elevated levels prompting at least clinical consideration, if not diagnostic investigation, of possible aortic dissection.

• Diagnostic Techniques • Once suspected on clinical grounds, it is essential to confirm the diagnosis of aortic

dissection both promptly and accurately. The modalities currently available for this purpose include aortography, contrast-enhanced CT, MRI, and TTE or TEE. Each modality has certain advantages and disadvantages with respect to diagnostic accuracy, speed, convenience, risk, and cost, but none is appropriate in all situations.

• When comparing the four imaging modalities, one must begin by considering what diagnostic information is needed. First and foremost, the study must confirm or refute the diagnosis of aortic dissection. Second, it must determine whether the dissection involves the ascending aorta (i.e., proximal or type A) or is confined to the descending aorta or arch (i.e., distal or type B). Third, if possible, it should identify a number of the anatomical features of the dissection, including its extent, the sites of entry and reentry, the presence of thrombus in the false lumen, branch vessel involvement by the dissection, the presence and severity of aortic regurgitation, the presence or absence of pericardial effusion, and any coronary artery involvement by the intimal flap. Unfortunately, no single imaging modality provides all of this anatomical detail. The choice of diagnostic modalities should, therefore, be guided by the clinical scenario and by targeting information that will best assist in patient management.

• Aortography. • Retrograde aortography was the first accurate

diagnostic technique for evaluating suspected aortic dissection. The diagnosis of aortic dissection is based on direct angiographic signs, including visualization of two lumina or an intimal flap (considered diagnostic), as in Figure 56-16 , or on indirect signs (considered suggestive), such as deformity of the aortic lumen, thickening of the aortic walls, branch vessel abnormalities, and aortic regurgitation.

Aortogram in the left oblique view demonstrating proximal aortic dissection and its associated cardiovascular complications. The true lumen (T) and false lumen (F) are

separated by the intimal flap (I), which is faintly visible as a radiolucent line following the contour of the pigtail catheter. The true lumen is better opacified than the false lumen, and

two planes of the intimal flap can be distinguished (arrows). The branch vessels are opacified, along with marked narrowing of the right carotid artery (CA), which suggests that

its lumen is compromised by the dissection

• Aortography had long been considered the diagnostic standard for the evaluation of aortic dissection because for several decades it was the only accurate method of diagnosing aortic dissection antemortem, although its true sensitivity could not be defined. However, the more recent introduction of alternative diagnostic modalities has shown that aortography is not as sensitive as previously thought. Prospective studies have found that for the diagnosis of aortic dissection, the sensitivity of aortography is 88 percent and falls to only 77 percent when the definition of aortic dissection includes intramural hematoma with noncommunicating dissection. The specificity of aortography is 94 percent. False-negative aortograms occur because of thrombosis of the false lumen, equal and simultaneous opacification of both the true and false lumina, or the presence of an intramural hematoma.

• Important advantages of aortography include its ability to delineate the extent of the aortic dissection, including branch vessel involvement ( Fig. 56-17 ). It is also useful in detecting some of the major complications of aortic dissection, such as the presence of aortic regurgitation, and is often useful in revealing patency of the coronary arteries. In addition to the limited sensitivity of aortography, other disadvantages are the inherent risks of the invasive procedure, the risks associated with the use of contrast material, and the time needed to complete the study, both in assembling an angiography team and the long duration of the procedure. Finally, aortography requires that potentially unstable patients travel to the angiography suite.

Digital subtraction angiogram of the abdominal aorta, in a patient with a distal thoracic aortic dissection, to assess the status of renal perfusion. This study confirmed the presence of an intimal flap extending

down into the left common iliac artery. The celiac axis, superior mesenteric artery, and right renal artery are widely patent and fill from the true lumen. The left renal artery fills from the false lumen, with the

intimal flap involving the ostium of the artery and impairing distal flow. As a consequence, there is minimal contrast excretion by the left kidney compared to the right.

• Computed Tomography. • In contrast-enhanced CT scanning, aortic dissection is

diagnosed by the presence of two distinct aortic lumina, either visibly separated by an intimal flap ( Fig. 56-18 ) or distinguished by a differential rate of contrast opacification. Spiral (helical) CT scanning, which is now used routinely, permits three-dimensional display of the aorta and its branches ( Fig. 56-19 ) and has improved the accuracy of CT in diagnosing aortic dissection, as well as in defining anatomical features. Several series have found that spiral CT scanning has both a sensitivity and specificity for acute aortic dissection of 96 to 100 percent (see Chap. 18) .

Computed tomography (CT) for diagnosing aortic dissection. Shown is a contrast-enhanced spiral CT scan of the chest at the level of the right pulmonary artery showing an intimal flap (I) in both the ascending and descending thoracic aorta separating the two lumina in a type B

aortic dissection

Computed tomography (CT) for diagnosing aortic dissection. Shown is a contrast-enhanced spiral CT scan of the chest at the level of the pulmonary artery showing an intimal flap (I) in

the descending thoracic aorta separating the two lumina in a type B aortic dissection.

• Computed tomography scanning has the advantage that, unlike aortography, it is noninvasive, although it does require the use of an intravenous contrast agent. Most hospitals are equipped with a readily accessible CT scanner available on an emergency basis. CT is also helpful in identifying the presence of thrombus in the false lumen and in detecting pericardial effusion. The use of CT angiography (three-dimensional reconstruction of axial CT data) permits assessment of branch vessel compromise in both the thoracic and abdominal segments.

• Magnetic Resonance Imaging. • MRI has particular appeal for diagnosing aortic dissection because it is entirely noninvasive and

does not require the use of intravenous contrast material or ionizing radiation. Furthermore, MRI produces high-quality images in the transverse, sagittal, and coronal planes, as well as in a left anterior oblique view that displays the entire thoracic aorta in one plane (see Chap. 17) . The availability of these multiple views facilitates the diagnosis of aortic dissection and determination of its extent and in many cases reveals the presence of branch vessel involvement.

• Magnetic resonance imaging has both a sensitivity and a specificity of approximately 98 percent. Furthermore, use of the cine-MRI technique in a subset of these patients showed 85 percent sensitivity for detecting aortic regurgitation. Intravenous administration of gadolinium yields a magnetic resonance angiogram, which defines the patency of aortic branch vessels. Still, MRI does have a number of disadvantages. It is contraindicated in patients with pacemakers or implantable defibrillators and certain types of vascular clips. MRI provides only limited images of branch vessels (unless gadolinium is used) and does not consistently identify the presence of aortic regurgitation. In most hospitals, magnetic resonance scanners are not readily available on an emergency basis. Many patients with aortic dissection are hemodynamically unstable, often intubated or receiving intravenous antihypertensive medications with arterial pressure monitoring, but magnetic resonance scanners limit the presence of many monitoring and support devices in the imaging suite and also limit patient accessibility during the lengthy study. Understandably, concern for the safety of unstable patients has led many physicians to conclude that the use of MRI is relatively contraindicated for unstable patients.

• Echocardiography. • Echocardiography is well suited for the evaluation of patients with

suspected aortic dissection because it is readily available in most hospitals, it is noninvasive and quick to perform, and the full examination can be completed at the bedside. The echocardiographic finding considered diagnostic of an aortic dissection is the presence of an undulating intimal flap within the aortic lumen that separates the true and false channels. Reverberations and other artifacts can cause linear echodensities within the aortic lumen that mimic aortic dissection. To distinguish an intimal flap definitively from such artifacts, the flap should be identified in more than one view, it should have motion independent of that of the aortic walls or other cardiac structures, and a differential in color Doppler flow patterns should be noted between the two lumina. In cases in which the false lumen is thrombosed, displacement of intimal calcification or thickening of the aortic wall may suggest aortic dissection.

• TRANSTHORACIC ECHOCARDIOGRAPHY.

• Transthoracic echocardiography has a sensitivity of 59 to 85 percent and a specificity of 63 to 96 percent for the diagnosis of aortic dissection. Such poor sensitivity significantly limits the general usefulness of this technique. Furthermore, image quality is often adversely affected by obesity, emphysema, mechanical ventilation, or small intercostal spaces.

• TRANSESOPHAGEAL ECHOCARDIOGRAPHY. • The proximity of the esophagus to the aorta enables TEE to overcome

many of the limitations of transthoracic imaging and permits the use of higher frequency ultrasonography, which provides better anatomical detail (Figs. 56-20 and 56-21 [20] [21]). The examination is generally performed at the bedside with the patient under sedation or light general anesthesia and typically requires 10 to 15 minutes to complete. The procedure does not require arterial access nor intravenous contrast or ionizing radiation. Relative contraindications include known esophageal disease such as strictures or tumors. The incidence of important side effects (such as hypertension, bradycardia, bronchospasm, or rarely, esophageal perforation) is much less than 1 percent. One important disadvantage of TEE is its limited ability to visualize the distal ascending aorta and proximal arch because of interposition of the air-filled trachea and main stem bronchus.

Cross-sectional transesophageal echocardiogram of the descending thoracic aorta demonstrating aortic dissection. The aorta is dilated.

Evident is an intimal flap (I) dividing the true lumen (T) anteriorly and the false lumen (F) posteriorly. The true lumen fills during systole and is therefore seen bowing slightly into the false lumen in this systolic image.

Transesophageal echocardiogram of the proximal ascending aorta in long-axis view in a patient with proximal aortic dissection. A, The left atrium (LA) is closest to the transducer. The aortic valve (AV) is seen on the left in this view, with the ascending aorta extending to the right. Within the proximal aorta is

an intimal flap (I) that originates just at the level of the sinotubular junction above the right sinus of Valsalva. The true lumen (T) and the false lumen (F) are separated by the intimal flap. B, The addition of

color flow Doppler in the same view confirms the presence of two distinct lumina. The true lumen (T) fills completely with brisk blood flow (bright blue color), simultaneously, minimal retrograde flow (dark

orange) is seen in the false lumen (F).

• The results of large prospective studies have demonstrated that the sensitivity of TEE for aortic dissection is 98 to 99 percent, whereas the sensitivity for detecting an intimal tear ( Fig. 56-22 ) is 73 percent. TEE detects both aortic regurgitation and pericardial effusion in 100 percent of cases. The specificity of TEE for the diagnosis of aortic dissection is less well defined but is likely in the range of 94 to 97 percent. Among patients with suspected aortic dissection, the diagnosis is excluded in as many as two thirds of cases, which yields a group of patients with a chest pain syndrome of unknown origin. Among patients determined not to have aortic dissection, TEE identifies alternative cardiovascular diagnoses (e.g., other aortic abnormalities or evidence of acute myocardial infarction or ischemia) in 66 to 73 percent.

Cross-sectional transesophageal echocardiogram of a descending aortic dissection demonstrating a site of intimal tear. Blood flow (in orange) is evident in the true lumen (T)

during systole, while a narrow jet of high-velocity blood (in blue) crosses into the false lumen (F) through a tear in the intimal flap (I).

• Selecting an Imaging Modality • Each of the four imaging modalities has particular advantages and disadvantages. In selecting among

them, one must consider the accuracy as well as the safety and availability of each test. Given their extremely high sensitivity and specificity and ability to provide three-dimensional images, CT angiography and MRA are considered the current standards for evaluating aortic dissection. The four imaging modalities described earlier differ in their ability to detect complications associated with dissection, so the specific diagnostic information sought by the treating physician and/or surgeon should have a bearing on the procedure chosen.

• Both the accessibility of imaging studies and the time required to complete them are key considerations, given the high rate of early mortality associated with unoperated proximal aortic dissection. Aortography can be performed only rarely on an emergency basis, because it often requires assembly of an angiography team and is subject to the risks associated with an invasive procedure and use of a contrast agent. MRI is also generally unavailable on an emergency basis and poses the risk of limited patient monitoring and accessibility during the lengthy procedure. CT scanning is more readily available in most emergency departments and is quickly completed. TEE is also readily available in most larger centers and can be completed quickly at the bedside, which makes it ideal for evaluating unstable patients. Among the centers in the IRAD, CT was used most often (63 percent) as the imaging study of first choice, with TEE performed first about half as often (32 percent).[63] Aortography and MRI were rarely used as the initial imaging modality (4 percent and 1 percent, respectively).

• In a setting in which all these imaging modalities are available, CT should be considered first in the evaluation of suspected aortic dissection in light of its accuracy, safety, speed, and convenience. When CT identifies a type A aortic dissection, the patient may be taken directly to the operating room, where TEE can then be performed to assess the anatomy and competence of the aortic valve without unduly delaying surgery. However, in cases of suspected aortic dissection in which aortic valve disease is suspected or the patient is unstable, TEE may be the initial procedure of choice.

• Despite its relative disadvantages, aortography still plays an important role when clear definition of the anatomy of the branch vessels is essential for management. Aortography should also be considered when a definitive diagnosis is not made by one or more of the other imaging modalities.

• In the final analysis, each institution must determine its own best diagnostic approach to the evaluation of suspected aortic dissection and base it on available human and material resources and the speed with which they can be mobilized. The level of skill and experience of those who carry out each diagnostic procedure should also enter into the choice of diagnostic modality.