Aortic Dissection

Published on 26/03/2015 by admin

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199 Aortic Dissection

Aortic dissection involves the separation of the outer two-thirds of the aortic media by the introduction of pulsatile blood from a primary intimal tear. Aortic dissection can be a catastrophic medical condition. It most commonly occurs in hypertensive middle-aged men and requires rapid, optimal management to prevent serious morbidity or mortality. The variable extent of the proximal and distal extension along the aorta and its branches usually determines the seriousness of the condition. The blood dissecting within the aortic wall creates a false lumen that parallels the true aortic lumen. The term dissecting aneurysm is inaccurate because few acute aortic dissections are associated with an aortic aneurysm. Although most deaths occur early, aortic dissection in its chronic phase is responsible for a substantial proportion of thoracic aortic pathology and aortic rupture due to aneurysmal degeneration and enlargement of the false lumen.

The initial observation and description of an aortic dissection was made by Morgagni in the 18th century. This was followed by multiple anatomic and postmortem reports, including the description of the cause of death of King George II of England shortly before the American Revolution.1 In the early 1800s, Maunoir better defined the pathologic process and first used the term dissection to describe the pathology.2 Although many subsequent reports have described aortic dissection, premorbid diagnosis was not consistently possible until refinements in contrast aortography were made.3 Indeed, only in the past several decades has either medical or surgical management had a reasonable chance to alter the course of aortic dissection.

The first attempts to treat this condition surgically involved wrapping of the dissected aorta to prevent rupture4 or treatment of the complications of dissection without definitive repair. This usually resulted in a catastrophic outcome and death. DeBakey and colleagues pioneered the surgical treatment of aortic disease, including dissection, and first reported graft replacement of the dissected aorta as definitive treatment.5 Aortic graft interposition has become the cornerstone of modern surgical therapy.

The modern approach to acute aortic dissection involves initial control of blood pressure with vasodilator medications and decreasing the rate of change of aortic pressure with beta-blocker drugs, followed by surgical repair in appropriate cases. Improved surgical and anesthetic techniques and improved postoperative monitoring and management have dramatically improved the results of treatment of aortic dissection. Recently, stent grafting has emerged as a tool to treat malperfusion complications of distal dissections.

image Classification

An understanding and description of aortic dissection are critical for the optimal care of these patients. The first widely used classification system was developed by DeBakey and colleagues and consists of three categories: types I, II, and III.5,6 Type I involves dissection originating in the ascending aorta, which continues to course through the descending aorta. Type II involves a tear only in the ascending aorta, and type III involves a tear originating in the descending thoracic aorta, distal to the ligamentum arteriosum. Subsequently, Daily and associates at Stanford University developed a classification system involving only two groupings, now known as the Stanford system.7 In the Stanford classification system (Figure 199-1), type A dissections involve the ascending aorta, and type B involves the aorta distal to the innominate artery. There have been many other attempts to classify aortic dissection, but most have been abandoned. Despite the fact that different categories are used, the essential element of a classification system of aortic dissection is involvement of the ascending aorta, regardless of the location of the primary intimal tear and irrespective of the distal extent of the dissection process.8 This functional classification approach is consistent with the pathophysiology of aortic dissection, considering that involvement of the ascending aorta is the principal predictor of the biological behavior of the disease process, including the most common fatal complications—rupture with tamponade, congestive heart failure, and myocardial infarction. Moreover, functional classification simplifies diagnosis, because it is easier to accurately identify involvement of the ascending aorta than to determine the exact site of the primary intimal tear or the total extent of propagation of the dissection process.

The Stanford classification system facilitates the clinical decision-making process and definitive patient management. Patients presenting with acute Stanford type A dissections should be treated surgically in most cases, and individuals with Stanford type B dissections are generally treated medically, using surgical intervention or endovascular stentgraft placement only if major complications are present. Generally, aortic dissections are defined as acute if they are diagnosed within 14 days of the onset of presenting symptoms. When dissection is diagnosed more than 14 days after onset, it is classified as chronic. Chronic dissection usually occurs only if the initial diagnosis was incorrect or if the patient suffered mild symptoms and did not seek appropriate medical care.

Over the past decade, advances in vascular imaging technology have led to the increased recognition of other conditions of the aorta, such as intramural hematoma and penetrating aortic ulcers, as distinct pathologic variants of classic aortic dissection.9,10 Both these entities are characterized by the lack of a classic intimal flap dividing the aortic lumen into true and false channels. Intramural hematoma can be precipitated by an atherosclerotic ulcer penetrating the aortic wall or can occur spontaneously without intimal disruption after rupture of the vasa vasorum. Intramural hematoma can involve the ascending aorta (type A) as well as the descending aorta (type B). Although it is possible, an intramural hematoma rarely evolves into an aortic dissection.11 Penetrating atherosclerotic ulcers occur most commonly in the descending thoracic aorta. Distinguishing intramural hematoma or penetrating aortic ulcer from aortic dissection is important because the prognosis and management of these lesions can differ.12,13

image Clinical Findings

Aortic dissection can occur in all age-groups, although the majority of cases are observed in men aged 50 to 80 years. Dissection in patients younger than 40 years is most commonly an acute type A dissection and often occurs in patients with Marfan syndrome or a similar connective tissue disorder. On rare occasions, women during the last trimester of pregnancy or during delivery present with acute aortic dissection, presumably due to hormone-induced weakness of the aortic connective tissue and the markedly increased intraaortic pressure that often occurs during delivery. There is a male predominance, with an estimated male-to-female ratio of approximately 2 : 1. The exact incidence of aortic dissection is difficult to ascertain because in many cases, the diagnosis is not made before death. Indeed, delayed recognition of acute aortic dissection is a frequent cause of malpractice suits. In one series, acute aortic dissection was found in 1% to 2% of autopsies.14 Recently it has been estimated that the incidence of acute aortic dissection in the United States might be as high as 10 to 20 or more cases per million population per year.15 Most aortic dissections (two-thirds) occur in the ascending aorta (Stanford type A) as opposed to the less frequent distal Stanford type B dissections. Most caregivers incorrectly believe that ruptured abdominal aortic aneurysms occur more commonly than aortic dissections; however, the former just tend to be diagnosed correctly more often than the latter.

Left untreated, most patients suffering an acute aortic dissection die, generally within the first 24 to 48 hours. Death may occur due to rupture of the dissected aorta into the pericardial space, leading to tamponade and cardiovascular collapse; proximal extension, leading to severe, acute aortic insufficiency and heart failure; or acute myocardial infarction if the dissection involves the ostia of the coronary arteries. It has been estimated that 40% of patients with dissection involving the ascending aorta die immediately or before reaching the hospital, and more than 67% die within the first 24 hours. In addition, mortality often results secondary to occlusion of major aortic branches supplying the cerebral or visceral circulation, causing massive stroke or visceral ischemia and severe metabolic acidosis. In contrast, among patients with Stanford type B dissections, 75% are alive 1 month after the onset of symptoms.

Patients with untreated acute type B dissection can expire from acute aortic rupture or from occlusion of one of several major aortic branches resulting in ischemic injury to vital abdominal organs. However, comparative studies have determined that in most cases of type B aortic dissection, survival is better with medical treatment alone (aggressive antihypertensive therapy) than with urgent surgical repair or aortic replacement. After acute aortic dissection, the false lumen remains patent in most cases, depending on the presence of distal reentry sites. When the false lumen remains patent, the aorta is prone to progressive expansion over time, necessitating the need for close long-term follow-up.

The most consistent clinical condition associated with aortic dissection is arterial hypertension. In patients with aortic dissection, the prevalence of arterial hypertension varies between 45% and 80%1619 and is highest in patients with acute type B dissection (Box 199-1). Hypertension may lead to smooth-muscle degeneration in the aortic wall, which may predispose to aortic dissection.

Connective tissue disorders such as Marfan or Ehlers-Danlos syndromes are associated with an increased risk of aortic dissection. Although both these conditions are relatively rare, they are frequently associated with acute dissection. In fact, aortic rupture or dissection is a common cause of death in patients with Marfan syndrome or other connective tissue disorders.20 In addition, aortic dissection is more common in patients with Turner’s syndrome and inflammatory disorders of the aorta such as syphilis or giant cell arteritis. Severe aortic atherosclerosis has been associated with a slight increase in the incidence of aortic dissection, but if dissection occurs, its extent seems to be more limited.

The risk of perioperative and late postoperative dissection21 is also increased in patients with a bicuspid aortic valve or aortic coarctation, presumably due to impaired connective tissue integrity. Aortic dissection is a rare complication of cardiac catheterization and other percutaneous diagnostic and therapeutic interventional techniques involving manipulation of catheters inside the thoracic aorta. Unfortunately, most veteran cardiac surgeons have experienced cases of intraoperative aortic dissection due to a clamp injury of the ascending aorta or a dissection initiating at the arterial cannulation site, especially when femoral arterial cannulation is performed.

One of the cardiovascular complications of cocaine use is acute aortic dissection, and this diagnosis should be considered in drug abusers presenting with acute chest pain.22,23 Aortic dissection in this setting occurs as a result of sudden severe hypertension secondary to catecholamine release.

image Pathologic Findings

Most surgeons and pathologists believe the initiating event in aortic dissection is a tear in the intima of the aortic wall that allows blood to enter, leading to separation of the medial layer of the aorta. The primary intimal tear causes communication between the true aortic lumen and a new false lumen. Few aortic dissections lack an identifiable intimal tear. Indeed, most extensive aortic dissections have multiple reentry sites. Intramural hematoma due to rupture of an intramural vessel is another potential initiating event, although this cause of dissection is less frequent. Dissections usually propagate antegrade in a spiral manner but may also extend in a retrograde fashion. The rate of increase of aortic systolic pressure, the absolute blood pressure, and the integrity and strength of the aortic wall determine the rate and extent of progression of the dissection. Ironically, distal progression of the dissection may be limited by extensive atherosclerotic disease, because the layers of the aorta are more tightly fused. Younger patients presenting with acute dissection frequently have involvement of the entire thoracic and abdominal aorta. Reentry into the true lumen may allow decompression of the false lumen and may maintain perfusion to distal organs. This is the rationale behind surgical and percutaneous techniques of fenestration for treatment of malperfusion syndromes associated with aortic dissection.

Organ or limb ischemia or malperfusion may occur when the dissection process compromises blood flow to various aortic branches. Malperfusion usually occurs when flow is impaired due to vascular compression of the true lumen by the false lumen, extravascular compression of abdominal viscera or vessels, or occlusion of a branch artery by a dissection flap. The pattern of involvement of branches of the thoracic and abdominal aorta is variable and often leads to confusion regarding the correct diagnosis.

image Presentation

Severe chest pain of a sudden nature is the most common presenting symptom of aortic dissection. The pain is typically abrupt and severe at onset and is often described as “tearing” and “the worst pain I have ever experienced.” This is especially true for patients who have never experienced childbirth. With type A dissections, the pain tends to be in the anterior chest and similar to that observed with myocardial infarction. Type B dissections classically cause midscapular pain, although this can be quite variable; this variability may lead to an incorrect diagnosis. Migration of pain and constant pain suggest continued expansion or progression. Differentiating the chest pain of acute aortic dissection from that of other causes such as acute myocardial ischemia, esophageal reflux disease, pericarditis, chest trauma, or abdominal pathology is critical in the initial evaluation of these patients to allow prompt, correct management. Unlike the crescendo-type pain frequently associated with acute myocardial infarctions, aortic dissections present with abrupt, sharp, unrelenting severe pain. On rare occasion, acute dissection can be painless, although this presentation is uncommon and is more often the case in patients presenting with chronic dissection. A relative minority of patients with acute aortic dissection present with signs of cardiac and other organ system involvement.17-19,2426 Other clinical manifestations may include stroke, paraplegia, upper- or lower-extremity ischemia, and anuria or abdominal pain due to renal or mesenteric ischemia. These latter findings portend a grave prognosis.

image Diagnosis

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