Acute aortic dissection is considered the most common acute process involving the aorta that necessitates surgery, even more common than ruptured aortic aneurysm (1,2). Although dissection may develop within an aneurysm and rupture may occur as a complication of dissection, aneurysm and dissection are separate entities that must be clearly distinguished. Dissection of the aorta is classified as type A (proximal) if the ascending aorta is involved or type B (distal) if it occurs beyond the left subclavian artery (3). Separation of the layers of the aortic wall is characteristic and results in the development of a false lumen or channel. Blood enters the intima media space, and further propagation of the dissection may occur in an antegrade or retrograde manner or both. Communication between the true and false lumina may occur via one or more intimal tears. Intramural hematoma (IMH) without an intimal tear is now recognized as a distinct pathologic lesion that is believed to result from hemorrhage of the vasa vasorum and occurs more frequently in the distal aorta (4). IMH may be the initiating event in patients with cystic medial necrosis. Atherosclerotic plaque may ulcerate, which leads to intramural hemorrhage or frank dissection. Typically, penetrating aortic ulcer is a localized lesion of the descending thoracic and abdominal aorta (5).

Most patients with aortic dissection have a structural abnormality of the arterial wall, evidence of systemic hypertension, or both (2,6). Several predisposing factors are seen in aortic dissection (Table 30.1). A history of hypertension is documented in the majority of patients and is present in more than 70% of patients with distal dissection (7). Men are more commonly affected than women in a ratio of approximately 2:1, and the incidence of aortic dissection increases with advancing age. Abnormal connective tissue within the aortic wall predisposes patients with Marfan syndrome and other connective tissue disorders to aneurysm formation and dissection at a much younger age. Dissection should always be suspected in any patient with Marfan syndrome who has chest, back, or abdominal pain. Similarly, many patients with bicuspid aortic valve have abnormal connective tissue in the aortic wall. As many as one in five patients initially seen with acute dissection may have a history of prior or recent cardiac surgery (7). The dissection may result from shared risk factors (advanced age, hypertension, cigarette smoking, vascular disease) and trauma from surgical instrumentation or catheterization of the aorta. Dissection or disruption of the aorta secondary to chest trauma is most often located in the region of the left subclavian artery, where the aorta is relatively fixed by the ligamentum arteriosum (8). Acute aortic dissection has been reported to occur in rare instances during pregnancy, in certain inflammatory disorders, and as a result of cocaine and amphetamine use.

Because dissection is a dynamic process that may occur throughout the course of the aorta and because perfusion to any organ
system may be compromised, patients are first seen with a broad range of symptoms (Table 30.2). The characteristic feature of acute aortic dissection is the abrupt onset of severe pain, typically in the chest, back, or both. Often the patient can pinpoint the onset of symptoms with “freeze-frame” accuracy. Although a tearing or ripping sensation is an obvious clue, patients are more likely to describe the quality of the pain of aortic dissection as sharp in nature (7). Pain is most often located in the upper back, anterior chest, or upper abdomen.

Migration of pain, another suggestive symptom, appears to be less common than previously thought. Pain may not be part of the presenting symptoms at all, especially in patients with neurologic deficits secondary to stroke. Syncope is a less-common but welldescribed presenting symptom of aortic dissection. Most patients with aortic dissection and syncope have other neurologic or clinical findings, which are useful in suggesting the diagnosis. However, a small percentage of patients have syncope alone. This implies that aortic dissection should be considered in the differential diagnosis of unexplained syncope, especially in the population at increased risk: the elderly and those with a history of hypertension. Patients may appear with clinical features of shock if dissection has resulted in cardiac tamponade or blood loss. Cardiac failure may be the predominant clinical feature, especially in the presence of aortic regurgitation or myocardial ischemia. Arterial compromise may be caused by direct obstruction by the dissection flap, displacement of the true lumen by the false lumen, or thromboembolic occlusion. This may result in the clinical presentation of gastrointestinal, renal, limb, or spinal cord ischemia. In rare cases, extrinsic
compression from the aorta results in hoarseness, dysphagia, or superior vena cava syndrome. Often, pain resolves for a period shortly after the initial presentation, which creates a false sense of the patient’s stability. Recurrent pain may herald propagation or extension of dissection.

The physical examination findings are often unremarkable. Classic physical findings such as a pulse deficit or aortic regurgitation murmur are helpful clues but usually absent (7). A pulse deficit is recorded in fewer than 20% of all patients and may be transient if the dissection flap obstructs the arterial ostium intermittently. The murmur of aortic regurgitation is noted in fewer than half of patients with type A dissection (7). The murmur of acute aortic regurgitation may be faint, and other peripheral findings of chronic severe regurgitation, such as wide pulse pressure, are frequently absent. Pleural effusions may be present as a reactive phenomenon or as a result of hemorrhage into the pleural space.

Blood pressure at presentation is highly variable. Hypertension is more common in patients with type B (distal) dissection. Hypotension or shock suggests a serious complication such as rupture or pericardial tamponade and carries a poor prognosis. Presenting features of IMH and penetrating aortic ulcer appear to be similar to those of classic aortic dissection, and progression to classic dissection with intimal tear may occur (5,9).

Immediate confirmation of the diagnosis and urgent institution of appropriate therapy are essential (Table 20.3). In addition to being the most frequently encountered acute aortic pathologic process, dissection is important because serious complications develop rapidly. The chest radiograph has traditionally been considered helpful in the initial evaluation of suspected aortic dissection (10). Although a widened mediastinum and an abnormal aortic contour may be suggestive of the diagnosis, they are nonspecific and are absent in more than 10% of patients. With the widespread availability of safe, rapid, and accurate noninvasive imaging techniques, chest radiography should have a limited role in the evaluation of suspected dissection. A normal chest radiograph should not dissuade the clinician from further investigation. Differentiating the pain of aortic dissection from myocardial ischemia is a common clinical dilemma. The presence of a normal ECG in the setting of acute chest pain may lead clinicians away from a diagnosis of myocardial ischemia and toward one of dissection (11,12). However, dissection and myocardial ischemia may occur together, and the electrocardiogram at presentation most often shows nonspecific ST/T-wave abnormalities. Thus the electrocardiogram is frequently unhelpful in the differential diagnosis (7). Laboratory studies are nonspecific and generally unhelpful in the differential diagnosis of aortic dissection. The development of a biochemical assay involving the use of a monoclonal antibody to smooth muscle myosin has been reported to be a rapid and accurate marker in a small number of Japanese patients with acute dissection. Further investigation is under way to determine its role in a larger population (13).