Acute Aortic Syndromes: Diagnosis and Management

Pathogenesis

Aortic dissection originates at the site of an intimal tear in more than 95% of patients. The resultant intimal tear exposes the media to pulsatile aortic flow, creating a second or “false” aortic lumen that then dissects in the outer layer of aortic media, propagating distally and, occasionally, proximally.

Ascending aortic dissections are almost twice as common as descending dissections. Some 50% to 65% of aortic intimal tears originate in the ascending aorta and approximately 20% to 30% of intimal tears originate in the vicinity of the left subclavian artery.

Once initiated, the dissection usually extends distally and, occasionally, proximally for a variable distance. As the dissecting process encounters branches of the aorta, it may pass around their origins, extend into their walls, or occlude them. Reentry of the dissection through a second, more distal intimal tear may occur, usually in one of the iliac arteries. External rupture of the dissecting process into the pericardial space is the most common cause of death in aortic dissection. Acute congestive heart failure, usually due to aortic regurgitation, is the second most common cause of death.

Predisposing Factors

Multiple risk factors for aortic dissection have been identified, the most common being advanced age, systemic hypertension, congenital abnormalities of the aortic valve, and heritable disorders of connective tissue.

Aortic dissection most frequently affects patients in the fifth to seventh decades of life (mean age, 63 years) and is more common in men (65.3% male). In patients younger than 40 years, the incidence between men and women is equal due to the occurrence of aortic dissection in women during pregnancy, with approximately 50% of all aortic dissections occurring during the third trimester of pregnancy.

Hypertension is present in 70% of type B dissections, but only in 25% to 35% of type A dissections. Hypertension may play a role in initiating the intramural hematoma along with having a direct weakening effect on the aortic media. The causative role of systemic hypertension is further supported by the finding that coarctation of the aorta predisposes to aortic dissection.

Other major risk factors for aortic dissection, especially proximal aortic dissection, are congenitally bicuspid or unicommissural aortic valves and the genetically mediated collagen disorders such as Marfan syndrome, Loeys Dietz syndrome, and Ehlers-Danlos syndrome. Additional predisposing factors for acute aortic dissection include preexisting aortic aneurysm and a positive family history (in as many as 19% of patients).

Iatrogenic aortic dissection is an uncommon but potentially serious complication of invasive angiographic procedures and cardiac surgery. Catheter-induced dissection can originate at any location; the majority are retrograde dissections and tend to decrease in size over time owing to thrombosis of the false lumen, whereas anterograde dissections tend to persist on follow-up. Nearly all iatrogenic aortic dissections can be treated medically with serial clinical examinations and noninvasive testing used to identify those in need of surgical therapy.

Cardiac surgical procedures complicated by aortic dissection include those that require cross-clamp or cannulation of the ascending aorta, such as aortic valve replacement and/or coronary artery bypass grafting. Dissection can arise at the site of ascending aortic cannulation, aortosaphenous vein anastomosis, aortic cross-clamp, or as a result of direct arterial injury. Dissection in association with these procedures usually occurs intraoperatively and is promptly diagnosed and treated, but chronic dissection in the postoperative period has been reported.

Aortic dissection has also been reported in association with inflammatory diseases (giant cell aortitis, Takayasu aortitis, rheumatoid arthritis, syphilitic aortitis, systemic lupus erythematosus, Noonan syndrome, Turner syndrome, fibromuscular dysplasia, annuloaortic ectasia, aortic coarctation, cocaine use, methamphetamine use, polycystic kidney disease, polyarteritis nodosa, trauma, and high-intensity weight lifting.

Classification

There are two main anatomic systems—the DeBakey and Daily (Stanford) systems—used to classify aortic dissection. The DeBakey system is based on the site of origin of the dissection and recognizes three types of dissection ( Fig. 28.1 ). For clinical purposes, because types I and II have a similar prognosis, the more widely used Stanford system classifies dissections that involve the ascending aorta as type A, and all other dissections as type B.

Dissections are categorized as acute if the diagnosis is made within 2 weeks of symptom onset and as chronic if more than 2 weeks have elapsed. The distinction is important owing to the fact that approximately 65% to 75% of patients with untreated aortic dissection die in the first 2 weeks after the onset of symptoms.

Intimal tear without hematoma is an uncommon variant of aortic dissection characterized by a localized intimal tear exposing the underlying media or adventitial layers to pulsatile aortic flow. There is no progression or separation of the medial layers.

Clinical Features

Owing to variable involvement of the aorta and its branches by the dissecting process, the patient with acute aortic dissection may have clinical manifestations of ischemia of various organ systems, singly or in combination, and symptoms and signs of cardiac disease. The diverse presentations of aortic dissection can make diagnosis difficult, and misdiagnosis commonly occurs. Despite major advances in the noninvasive diagnosis of aortic dissection and in medical and surgical therapy, up to 55% of patients in reported series die without a correct antemortem diagnosis. For these reasons, a high index of clinical suspicion for acute aortic dissection in any likely setting is imperative.

The classic presentation of a patient with acute aortic dissection, occurring in more than 70% of patients, is the sudden onset of severe pain, usually beginning in the anterior chest, radiating to the back, and moving distally as the dissection progresses. Chest pain is significantly more common in patients with type A dissection (79% in type A vs. 63% in type B dissection). In contrast, both back pain and abdominal pain are significantly more common with type B aortic dissection over type A: 64% versus 47% for back pain and 43% versus 22% for abdominal pain. Although painless dissection occurs in 14% to 21% of patients, it is relatively uncommon. Patients with painless aortic dissection tend to be slightly older, have a prior history of diabetes, aortic aneurysm, or prior cardiac surgery, and more often have type A dissection.

Patients with acute aortic dissection often appear to be in shock, although hypertension is present in one-half to two-thirds of cases, especially with type B dissection. Hypotension is more common in type A dissection and may result from severe aortic regurgitation and/or rupture of the dissection into the pericardial space with resultant cardiac tamponade or, less commonly, the pleural space or mediastinum.

A cardiac murmur may be present, usually at the cardiac base, and may be systolic, diastolic, or both. A diastolic decrescendo murmur of aortic regurgitation indicates involvement of the ascending aorta and is heard in one-half to two-thirds of patients with type A aortic dissection. Congestive heart failure, when present in association with proximal aortic dissection, is most often due to severe aortic regurgitation, but cases of congestive heart failure due to rupture of the dissecting process into the right or left atrium or right ventricle have also been reported. Myocardial infarction, most commonly inferior infarction, occurs in 5% of patients and is due to compromise of either coronary ostium by a hematoma or intimal flap. Peripheral pulse deficits are noted in 19% to 30% of patients, more commonly with type A aortic dissection, and are associated with a higher rate of in-hospital complications and mortality. Pulse deficits may be transitory owing to oscillation of the intimal flap or distal reentry of the hematoma into the true lumen. Acute lower extremity ischemia, with or without chest pain, as a result of dissection extending into the iliac arteries occurs in 6% to 12% of patients and may provide an important clue to the diagnosis. Other cardiovascular findings include a difference in systolic blood pressure between the arms (>20 mm Hg), tachycardia, pericardial friction rub, arterial bruits, pulsus paradoxus, and cardiac tamponade.

Syncope occurs in 5% to 10% of patients with aortic dissection and is an important event, as it is associated with a worse prognosis. Syncope most commonly results from either rupture of the dissecting process into the pericardial space, producing cardiac tamponade or involvement of the brachiocephalic arteries. Less commonly, rupture occurs into the left pleural space, producing a left hemothorax. Neurologic deficits—including cerebrovascular accident, disturbances of consciousness, ischemic paraparesis, and ischemic peripheral neuropathy—may also occur.

Other less frequent findings that occur in association with acute aortic dissection include Horner syndrome, a pulsatile sternoclavicular joint, vocal cord paralysis, hemoptysis, superior vena cava syndrome, upper airway obstruction, hematemesis, pleural effusion, unilateral pulmonary edema, signs of mesenteric or renal infarction, fever, and deep venous thrombosis.

As a general rule, aortic dissection should always be considered in the differential diagnosis of a patient with unexplained syncope, stroke, congestive heart failure, acute arterial occlusion, or an abnormal aortic contour on chest radiography, even in the absence of chest pain.

Diagnosis

Although aortic dissection may be suspected from the initial history and physical examination, the correct clinical diagnosis is made in less than 50% of patients. The clinical diagnosis of aortic dissection can be improved upon by utilizing the aortic dissection detection risk score, a clinical tool incorporating high-risk conditions, high-risk pain features, and high-risk examination features that can be used to estimate the pretest probability of disease and rapidly identify high-risk patients, facilitating prompt evaluation and treatment. The aortic dissection detection risk score has a sensitivity of 95.7%.

Although routine blood tests are nonspecific in acute aortic dissection, a D-dimer less than 500 ng/mL is highly predictive for excluding aortic dissection in low-risk patients. An aortic dissection detection risk score less than or equal to 1 and D-dimer less than 500 ng/mL accurately ruled out acute aortic dissection with a low failure rate (sensitivity 98.7%).

The most common electrocardiographic abnormality in patients with aortic dissection is left ventricular hypertrophy from chronic systemic hypertension. Acute electrocardiographic changes occur in up to 55% of patients and include ST segment depression, T-wave changes, and ST segment elevation, in decreasing order of frequency. Acute ischemic changes can occur when one or both coronary ostia become obstructed, either by the intimal flap or from external compression by the dissecting hematoma. The electrocardiographic changes of acute pericarditis may be seen if there has been leakage of blood into the pericardial space. Heart block resulting from proximal extension of the hematoma into the area of the atrioventricular node has also been reported. The chief value of the electrocardiogram is in distinguishing aortic dissection from acute myocardial infarction, although the two conditions can coexist.

Chest radiography may be helpful in suggesting the diagnosis of aortic dissection, with abnormalities of the aortic silhouette being the most common finding. Additional findings include pleural effusion, mediastinal widening, displacement of intimal calcification greater than 6 mm inside the outer edge of the aortic shadow, and the radiographic findings of congestive heart failure ( Fig. 28.2 ). Nonetheless, it is important to remember that normal chest radiographic findings (present in 11% to 16% of patients) do not exclude the diagnosis of aortic dissection .

Confirmation of the diagnosis of aortic dissection requires cardiovascular imaging that demonstrates the dissection flap separating a false lumen from the true lumen. Currently available noninvasive imaging modalities that are accurate in the diagnosis of acute aortic dissection include multiplane TEE, CT angiography, and MR angiography.

Transthoracic echocardiography (TTE) can be very useful in some patients with suspected aortic dissection ( Video 28.1 ). When the findings of a dilated aortic root (end-diastolic diameter >42 mm), widening of the aortic walls (16 to 20 mm for the anterior wall and 10 to 13 mm for the posterior wall), and a linear undulating echo representing the intimal flap are present, the positive predictive value for TTE is 100%. Advantages of TTE are its portability; real-time diagnosis; ability to identify associated aortic regurgitation; and to assess left ventricular function, regional wall motion abnormalities, pericardial effusion, and cardiac tamponade. The diagnosis of cardiac tamponade in a patient suspected of having aortic dissection deserves special mention. Echocardiographically guided pericardiocentesis should be avoided in this setting, as the rapid withdrawal of pericardial fluid can result in a prompt improvement in left ventricular systolic function, left ventricular dP/dT, and systolic blood pressure, producing aortic rupture . The patient with aortic dissection complicated by cardiac tamponade should be taken emergently to surgery for institution of cardiopulmonary bypass followed by evacuation of blood from the pericardial space. Disadvantages of TTE in the diagnosis of aortic dissection include difficulty in adequately visualizing the descending thoracic aorta and suboptimal echocardiographic windows in patients with obesity or chronic obstructive pulmonary disease. Overall, the sensitivity and specificity of TTE are inferior to TEE, CT, and MRI in the diagnosis of aortic dissection. Intravenous contrast agents improve the diagnostic accuracy of TTE in patients with aortic dissection, achieving a sensitivity and specificity similar to TEE in type A dissection. It should be remembered that a negative TTE does not exclude aortic dissection.

Multiplane TEE has a sensitivity of 99% and a specificity of 98% in the diagnosis of aortic dissection. TEE is portable, minimally invasive, and can accurately determine the type and extent of dissection safely in an emergent setting, allowing rapid triage of patients to either surgical or medical therapy ( Video 28.2 ). Color flow Doppler imaging significantly improves the sensitivity of TEE by allowing visualization of the intimal flap, dissection entry site, the true and false lumens, presence of thrombus, mechanism and degree of aortic regurgitation, and the proximal coronary arteries (see Fig. 28.2 ). In addition, multiplane TEE provides a comprehensive assessment of left ventricular systolic function, regional wall motion, pericardial effusion, and cardiac tamponade. The overall sensitivity of TEE is comparable with CT, MR, and aortography in the diagnosis of aortic dissection. Current multiplane TEE probes have largely overcome impediments in the ascending aorta, although artifacts in this region continue to be a diagnostic challenge.

CT with intravenous iodinated contrast enhancement is an accurate noninvasive screening test in patients with suspected aortic dissection. Advantages of CT include ready availability at most hospitals and improved accuracy with spiral (helical) CT and electron beam (ultrafast) or multidetector (multislice) CT. CT can reliably demonstrate the intimal flap, pericardial and pleural effusion, associated mediastinal hemorrhage, and involvement of the aortic arch vessels and branches of the abdominal aorta, as well as coronary artery disease ( Fig. 28.3 ). Disadvantages of CT include the need for iodinated contrast exposure and nonportability, limiting its use in patients with significant renal insufficiency and in hemodynamically unstable patients, respectively. In addition, the site of entry is rarely identified.

Although less commonly used, MRI is a highly accurate noninvasive technique in the evaluation of patients with suspected aortic dissection. MRI is superior to TEE and CT in detecting arch vessel involvement and in identifying the anastomosis in patients managed with surgical therapy and may facilitate comparison of serial studies. Gated spin-echo MRI accurately demonstrates the entry site and intimal flap and may be the optimal method for demonstrating thrombus formation and entry site location within all segments of the aorta ( Fig. 28.4 ). The ability to obtain oblique and longitudinal planes of a section makes MRI especially valuable in demonstrating dissection without intimal tear. Disadvantages of MRI include cost, examination time, reduced availability, nonportability, and standard contraindications to MRI.

Of the definitive noninvasive imaging modalities in suspected acute aortic dissection (TEE, CT, MRI), a systematic review of the diagnostic accuracy of these imaging techniques has demonstrated a pooled sensitivity (98% to 100%) and specificity (95% to 98%) that is comparable between the three imaging techniques. Therefore the choice of test depends upon which imaging modality is readily available at a particular institution and the hemodynamic stability of the patient.

Aortography, the traditional definitive diagnostic method in aortic dissection, is able to localize the site of origin of the dissection and delineate the extent of the dissection and circulation to vital organs. Diagnostic aortographic features include opacification of the false lumen, deformity of the true lumen by the false lumen, dilation of the aorta, narrowing or occlusion of branches of the aorta, and the presence of an intimal flap ( Fig. 28.5 ). Disadvantages of aortography include nonportability, invasive technique, exposure to ionizing radiation, the use of intravenous iodinated contrast agents, and an inherent delay in diagnosis. False-negative aortogram results can occur if there is simultaneous and equal opacification of the true and false lumina or if the false channel is very faintly opacified. For these reasons, aortography has generally been replaced by noninvasive imaging tests in the diagnosis of acute aortic dissection. However, for patients in whom the suspicion for ascending aortic dissection is very strong but noninvasive imaging is unavailable or inconclusive, digital subtraction aortography should be performed. Intravascular ultrasound, in combination with standard aortographic technique, greatly improves the accuracy of aortography, can be performed rapidly and safely, and could serve as an accessory diagnostic procedure in selected patients with suspected aortic dissection.

In view of the increased early mortality of untreated acute aortic dissection, the screening test chosen depends on which test is most readily available at a particular institution and the patient's hemodynamic status. Noninvasive diagnosis of acute aortic dissection by TEE, CT, or MR, if readily available, is preferred as it avoids the risks and delays inherent in invasive angiography.