Acute Thoracic Aortic Dissection: Current Evaluation and Management
Part I: Pathophysiology, Risk Factors, and Clinical Presentation
Author: Gary Hals MD, PhD, Attending Physician, Department of Emergency Medicine, Palmetto Richland Memorial Hospital, Columbia, SC.
Peer Reviewer: Laurence J. Gavin, MD, Chief, Department of Emergency Medicine, Presbyterian Medical Center and Pennsylvania Hospital; Clinical Associate Professor, Department of Emergency Medicine, University of Pennsylvania Health System, Philadelphia, PA.
The nurse is ready to give thrombolytics to a middle-aged man to treat his acute inferior myocardial infarction (MI). As the emergency department (ED) physician gives the order, the portable chest x-ray returns—just in time. The film shows an abnormally wide mediastinum with apical capping and a left pleural effusion. The case of the straightforward MI has just taken a sudden turn, and use of thrombolytics in this patient would have been disastrous; he has an acute aortic dissection that is complicated by an acute MI.
Patients with acute dissections of the thoracic aorta can present in just such a fashion, and only by looking for this entity every time the diagnosis is in question will the astute emergency physician discover it in time. Acute aortic dissections can also present with a myriad of other, seemingly unrelated symptoms: acute ischemic stroke, mesenteric ischemia, acute pulmonary edema from sudden aortic insufficiency, acute paraplegia, and syncope. These varied symptoms are a result of arterial occlusion produced by the dissection, and are estimated to occur in 30-50% of cases of aortic dissection.1,2 Unfortunately, physicians in general do a poor job of diagnosis of aortic dissection. One study on data from the 1970s found that aortic dissection was suspected after initial clinical presentation in only 60% of patients.3 A more recent study shows no gain in the next 23 years; data from 1993 show essentially the same figure. Only 62% of clinicians initially suspected dissection in 236 patients with documented aortic dissections.4 Further, autopsy studies from the 1980s found that the correct antemortem diagnosis is made in between 11% and 35% of patients.5,6 These numbers have improved some in the past decade, with a recent study of 236 patients with dissection showing "only" 28% of patients were not correctly diagnosed before autopsy.4 Compared to other diseases, though, a correct diagnosis rate of 72% is still abysmal, especially considering the lethal nature of aortic dissection. Add to this several disturbing facts: 1) thoracic aortic dissection is the most common lethal disease affecting the aorta, and is 2-3 times as common as abdominal aortic aneurysm (AAA) rupture; 2) the absolute incidence by autopsy studies has risen two- to four-fold in the last 30 years; and 3) the mortality is as high as 1-2% per hour in untreated patients.1,7 Given these statistics, one can appreciate the need to be aware of this disease, how to recognize it in its many forms, and how to properly treat these patients in the ED.
The purpose of this article is to review the current literature concerning pathophysiology, risk factors, and diagnostic testing of these patients in the ED. Guidelines for optimal medical management, indications for emergent surgical intervention, and an update on new treatments being developed for acute thoracic dissection will also be discussed.
— The Editor
Introduction
"A man . . . was seized with a pain in the right arm and soon after of the left, . . . after these there appeared a tumor on the upper part of the sternum . . . He was ordered to think seriously and piously of his departure from this mortal life, which was very near at hand and inevitable."8 Morgagni provided the first known description of a patient with an aortic dissection, and was also the first to link the presentation with pathologic findings. Before discussing the incidence and classification of aortic dissections, it is important to clarify the term.
Terminology. There is often confusion when discussing thoracic dissection. Many physicians use the term "dissecting aortic aneurysm" or "dissecting aneurysm" when describing thoracic aortic dissection. The incorrect term dates back to 1819 when Laennec coined the term "anaeurysm disseguant" in reference to an aortic dissection, and misuse of the phrase continues today.9 It is an understandable mistake when one also considers that one of the most common complications following survival of an aortic dissection is aneurysm formation in the affected aorta. These aneurysms may lead to death by rupture in a similar fashion as do aneurysms not arising from a dissection. Actually, thoracic aortic aneurysms have more in common in AAA than thoracic dissection. When not originating as a complication of chronic dissection, they share similar risk factors, and the pathophysiology leading to aneurysmal dilatation is similar in both thoracic and abdominal areas. Dissection of a thoracic aortic aneurysm is a rare event, as most thoracic aneurysms produce symptoms by compression of surrounding structures or by acute rupture. Likewise, only 5% of AAAs will produce acute dissection.10 Thus, the term aortic dissection is favored when discussing this entity. Figure 1 illustrates the differences between a true aneurysm, dissection, and false aneurysm. False or pseudoaneurysms are typically seen as complications of aortic repair and appear at the aortic graft anastomosis. False aneurysms should not be confused with the false lumen of aortic dissection, seen as a hematoma in the aortic wall in Figure 1.
Incidence. Acute aortic dissection is neither common nor rare, with most articles quoting an incidence of 5-10 per million people per year, or 2000 new cases per year (compared to acute MI, with 600,000 cases per year).11 However, this figure is derived from a Danish autopsy study done in the 1960s on 6480 autopsies.12 In this same study, incidence of AAA rupture was 3.6 per million, and thoracic aneurysm rupture was 1.2 per million population per year. This illustrates that aortic dissection is actually several times more common than AAA rupture, and that the number of deaths per year from dissection actually exceeds that of AAA rupture.13 The misperception that AAA rupture is more common may stem from the fact that it is diagnosed correctly much more often, with up to 60% correct diagnoses of AAA rupture compared to as low as 11% in aortic dissection.5,14 More recent estimates suggest an incidence of 10-20 cases per million population annually, or up to 4500 cases yearly.15,16 The actual true incidence is likely to be much greater, as these figures do not include cases of aortic dissection that lead to sudden death out of the hospital, where acute MI is often assumed to be responsible and no autopsy is done to show otherwise. Indeed one small autopsy study found that acute dissections were the cause of 4.2% of sudden deaths in men over a three-year period.17 The apparent increase in incidence of aortic dissection may be a result of increasing age of the population as a whole. The prevalence of hypertension, one of the prime risk factors for aortic dissection, is also likely to be responsible. Indeed, a higher incidence of aortic dissection in African Americans is thought to be a result of a higher incidence of hypertension.18
Classification. Two primary types of classifications of aortic dissection have been in use: the DeBakey and Stanford classifications. Figure 2 depicts the different types of DeBakey dissections. These anatomic classifications were developed with diagnostic and therapeutic considerations in mind. The differences in the types of dissections stem from involvement or exclusion of the ascending aorta, regardless of the original site of the intimal tear. Prognostically, the origin of the dissection and the distal involvement are less important than the presence of ascending aorta involvement.19 The DeBakey system is the most widely used system, and was proposed in 1955.20 As can be seen from Figure 2, type I dissections involve the ascending aorta, aortic arch, and the descending aorta. Type II dissections also involves the ascending aorta, but does not extend beyond the left subclavian artery. Type III dissections involve only the descending aorta, with subtypes for extension limited to the diaphragm (IIIA) and for continuation beyond the diaphragm (IIIB). The Stanford system divides dissections into only two types: type A, involving the ascending aorta (with or without descending aorta involvement); and type B, involving only the descending aorta.
Yet a third and not widely known classification system for aortic dissection has recently been described.21,22 This system primarily differentiates dissections based on their type of pathological lesion that initiates the dissection. As it provides no distinction between sparing or involvement of the ascending aorta, it gives no useful information on the prognosis or type of treatment needed. Therefore, it is not given as an alternative to the above two standard classification systems, but rather is described in order to illustrate the complexity of the disease process. Class 1 is the typical dissection, with intimal flap leading to blood flow in true and false lumens. Class 2 is an intramural hematoma arising without an intimal tear. Class 3 is a localized intimal tear that arises without classic dissection into the media and produces only a localized bulge in the aortic wall. Class 4 is a dissection that arises from a penetrating atherosclerotic ulcer leading to a localized hematoma. Class 5 is a dissection produced from trauma, including iatrogenic damage from cardiac catheterizations. An important point of the paper is that the class 3 type is not typically seen with any of the four usual tests used to diagnosis aortic dissection (transesophageal echo [TEE], magnetic resonance imaging [MRI], computed tomography [CT], or angiography). In the series of 181 patients with dissection, 5% had the class 3 type, and fully six of the nine patients had multiple diagnostic tests with negative results leading to discharge from the hospital. They were not correctly diagnosed until surgery was performed on subsequent visits. Further, this subtype of dissection may be just as deadly as the classic type, as three of the nine patients had evidence of cardiac tamponade at time of surgery.21 The authors suggest that the class 3 type of dissection may progress to become the typical class 1 dissection, and its true incidence may therefore be underreported in autopsy series. These data illustrate the complexity of this apparently straightforward disease process and suggest aortic dissections may be more elaborate than first thought.
Roughly 60% of aortic dissections involve the ascending aorta, and 30% are limited to the descending aorta.23 In only 10% of cases does the dissection begin in the transverse aorta and travel variable lengths of the ascending and descending thoracic aorta.24 Very seldom will the dissection be limited to the transverse arch or to the abdominal aorta. Patients with type A or B dissections differ in several features. Patients with type A dissections tend to be younger, and often have under-lying connective tissue disorders or congenital heart defects such as a bicuspid aortic valve. Some authors believe that atherosclerotic plaques act to limit the spread of the dissection and suggest that this is why younger patients tend to have their entire aorta involved more often. Others suggest that atherosclerotic plaques can give rise to dissections, but this is thought to occur infrequently.25 Patients with type B dissections are more often hypertensive, older, may have chronic pulmonary disease from tobacco smoking, and have generalized atherosclerotic disease.
In general, medical treatment is begun initially in any type of acute aortic dissections. The definitive treatment for type A dissections (ascending involvement) is emergency surgery, and type B dissections are either treated medically or with early surgical intervention for certain complications. This stems from the fact that involvement of the ascending aorta is associated with a much higher incidence of fatal complications, including intrapericardial rupture and subsequent tamponade, acute aortic regurgitation and left ventricular (LV) failure, free pleural rupture, and loss of cerebral or coronary blood flow. Acute type B dissections can have fatal complications as well, but they occur less often. These complications stem primarily from occlusion of major aortic branches to the kidneys and gastrointestinal (GI) tract, producing acute renal failure or mesenteric ischemia. Descending dissections also often produce acute limb ischemia, but this is typically not immediately life-threatening.
Finally, an aortic dissection is defined as acute if it has been present for less than two weeks and chronic if it has been present for longer than two weeks.26 Unfortunately, the chances of making it past the two-weeks mark undiagnosed are slim; more than 75% of patients will die if untreated by two weeks from symptom onset.7 Several large autopsy studies confirm these grim numbers. Left untreated, nearly 33% of patients will die in the first 24 hours, 50% by 48 hours, and up to 90% by one month from onset of dissection.27,28 Combining this knowledge with the low number of patients correctly diagnosed before their demise (as low as 11%, and one can begin to appreciate the difficult nature of diagnosis in the ED.5 A recent text on vascular surgery notes, "delays in diagnosis are due in part to the perceived rarity of acute aortic dissection and variable presenting signs and symptoms, which can mimic a plethora of other, more common acute medical or surgical illnesses."29 Thus, the purpose of this article is to minimize the number of patients with acute dissections that are "missed" in the ED by reviewing the risk factors, pathophysiology, clinical findings, and treatment options currently recommended in the literature.
Risk Factors
Many disorders, conditions, genetic defects, and congenital anomalies are associated with increased risk of aortic dissection. (See Table 1.) Aortic dissections have been reported in patients ranging from 14 months to 100 years,7 but are most commonly seen in patients between the ages of 50 and 70 years.20 Though rarely seen in patients younger than 40 years, when aortic dissection occurs in younger patients is it associated with certain syndromes or conditions that increase its risk. Specifically, 50% of the dissections in women younger than age 40 are associated with pregnancy and usually occur in the third trimester.30 Marfan’s syndrome and Ehlers-Danos syndrome are two connective tissue disorders that predispose to aortic dissection through a weakened media. Congenital heart abnormalities (bicuspid valves, coarctation) can also increase risk in younger patients for aortic dissection. Two common illicit drugs (crack cocaine and methamphetamine) have also been shown to increase risk for dissection, presumably due to drug-induced hypertension.31,32 These risk factors may be additive (i.e., pregnancy and cocaine, hypertension and bicuspid valve), so when patients present with a known risk factor it is important to quiz and examine them to discover other potential risks as well. Unfortunately, there are also reports of acute dissection in apparently young healthy people without any identifiable risk factors.33 By far the most important risk factor for aortic dissection is hypertension. It is associated with 70-90% of cases.10 A higher incidence of hypertension may also explain the higher incidence seen in African Americans.18 As will be discussed in the next section, increased blood pressure appears to contribute to a worsening of medial degeneration of the aortic wall, thereby increasing risk for dissection.
| Table 1. Risk Factors for Aortic Dissection35,56 |
| • Increased age—peak incidence > 50 and < 70 years |
| • Male sex (3:1) |
| • History of hypertension (60-90% of patients) |
| • Connective tissue disorders (Marfan's, Ehlers-Danlos) |
| • Turner's syndrome |
| • Aortic coarctation |
| • Third trimester pregnancy |
| • Congenital bicuspid or unicuspid aortic valve |
| • Ebstein's anomaly |
| • Aortic valve stenosis |
| • Familial incidence |
| • Illicit drugs (cocaine and methamphetamine) |
| • Iatrogenic (surgery or cardiac catheterization) |
| • Trauma (though blunt trauma usually produces aortic rupture) |
Pathophysiology
An aortic dissection occurs when blood enters the media of the aorta, typically through an intimal tear, and splits the wall longitudinally. This creates a false lumen in the aorta wall that may propagate in both directions, and may rejoin the true lumen at some point distally. Consequently, there are usually three main features to an aortic dissection: an intimal tear, an abnormal media, and hemodynamic forces that help propagate the dissection. The next section reviews current knowledge on how these three features are related in the development of aortic dissection.
The wall of the aorta consists of three definite layers (see Figure 1): the adventitia or outer covering, the intima or inner lining, and the media. The media is composed of smooth muscle cells and elastic tissue. "Cystic medial necrosis" is an old term that was inaccurately used to describe the pathological process found in the aortic media of patients with aortic dissections. More recent pathologic studies have found smooth muscle degeneration34 with associated scarring and fibrosis. As these changes were neither cystic nor necrotic, the newer term, medial degeneration, is preferred. Further, medial degeneration is now thought to be a normal part of the aging process, although it is seen in a more advanced state in patients with hypertension. Thus, patients with dissection are felt to have "more of" the normal aging process rather than a distinctly different process.34 It is also distinctly different from the defect in elastic tissue found in patients with connective tissue disorders (fibrillin gene defect), although they both lead to a common end point of dissection though weakening of the media.
It is easy to see how this "normal wear" can occur in the aorta when one considers the Herculean task put before it. Given an average of 70 beats per minute, the human heart beats approximately 100,000 times per day, 37 million times per year, and 2.5 billion times in an average lifespan!36 With each beat, the heart sways on its tether of major vessels inside the chest. As movement of the heart is limited anteriorly by the sternum, and posteriorly by the spine, most of the sway of the heart occurs in a side-to-side fashion. As the aorta is fixed just distal to the left subclavian artery to the left side of the vertebral bodies, the oscillation produces a flexion of the ascending and proximal descending aorta. This repetitive mechanical stress may be responsible for a weakening of the intima and aid in medial degeneration with time.
Hypertension also contributes to the wear that time produces on the aorta by increasing the forces involved. Coarctation produces upper extremity hypertension and is also associated with a bicuspid valve. Unicuspid or bicuspid aortic valves alter the laminar flow of blood in the aorta and may weaken a specific site on the wall by redirecting flow to that point. Cocaine and methamphetamine produce transient but significant hypertensive states that may trigger a weak point of the aorta to dissect which would otherwise be able to maintain integrity. Hemodynamic changes of pregnancy include increased stroke volume, total blood volume, and cardiac output adding further stress to the wall of the aorta over a rather short period of time. These changes are at their peak in the third trimester of pregnancy, when dissections tend to occur. Patients with connective tissue disorders are thought to have defects in structural proteins that produce accelerated medial degeneration. The best known example is Marfan’s syndrome, where a mutation of the fibrillin gene has been identified.37 Fibrillin is a structural protein that, along with elastin and collagen, act together to produce the aorta’s strength and elasticity. Thus, patients with Marfan’s syndrome have such weakened aortic media that even normal hemodynamic forces may produce dissection.
In the setting of a structurally weakened media, two theories are proposed concerning the trigger for aortic dissection. One proposes that an intimal tear occurs. This is likely a combined result of medial degeneration beneath it, increased hemodynamic stress, and repeated flexion from normal cardiac motion in the chest. Once access to the weakened media is gained, a column of blood can spread down (or up) the aorta. The other theory proposes that, over time, the same forces act to weaken the vasa vasorum which supply the media with blood. Once ruptured, the blood would again spread through the media. Autopsy data from the 1950s found that an intimal tear was not found in only 4% of cases .28 One more recent series found intimal tears in every case.38 Taken together, these autopsy data support that either the intimal tear is the initiating event or that when vasa vasorum rupture, they are highly likely to produce intimal tears.
Regardless of the true mechanism, once the blood enters the aortic wall it often advances in a spiral fashion distally. Propagation of the column of blood is dependent in part on the blood pressure and the steepness of the pulse wave, or dP/dT (rate of change in pressure/time). Higher blood pressure and more forceful and rapid contractions of the heart will accelerate the dissection. As will be discussed later, medical management of aortic dissection is aimed at decreasing these forces to slow its progress. As the dissection spreads down (or up) the aorta, blood flow to many of its branches can be decreased or completely blocked. Loss of these major blood vessels can occur from several mechanisms: blockage by intimal flaps, compression by blood in the false lumen, and "steal" of blood into the false lumen from the true vessel branch. As these vessels lose blood flow, one can see how many different organ systems, including the brain, spine, kidneys, intestines, and the heart itself, can experience acute ischemia as a result of the dissection. These complications are far from rare; up to 20% of patients with acute dissection will also have neurological symptoms, and overall, up to 50% of patients experience peripheral vascular complications.2,39 Compression of the entire aorta by blood in the false lumen can even occur at the diaphragmatic hiatus, leading to complete ischemia of the lower half of the body.40 The various branches of the aorta were frequently involved according to data from a series of 450 autopsies.28,41 The multitude of symptoms that can be produced by involvement of these different vessels is just one of the reasons that misdiagnosis and delay of diagnosis are so prevalent in patients with aortic dissection.
Clinical Presentation
When patients present to the ED with aortic dissection of sudden onset, severe chest pain is the most common complaint; it is present in more than 90% of cases.7 The pain rapidly reaches peak intensity and is described as "ripping" or "tearing," although the classic "tearing" pain was actually rare in one recent review.42 Pain may begin in the anterior chest or in the interscapular region, and often migrates down the back or from chest to back as the dissection progresses. In fact, migration of pain is highly specific for aortic dissection. One article reported fully 71% of patients had migration of pain with their dissections.3 Typically, dissections of the ascending aorta produce pain at different sites than with descending dissection. Ascending dissections produce pain in the anterior chest, and arch dissections cause pain in the neck and jaw. Dissections of the descending thoracic aorta give pain in the interscapular area, while abdominal aorta involvement causes pain in the abdomen or lower back. Surprisingly though, in one series aortic dissection was painless in 15% of 236 patients, leading the patient to present only with associated ischemic or neurologic complications.4,43 Although rare, there are also reports of patients presenting with chronic dissections manifested by symptoms of systemic illness including fever of unknown origin.44
A great variety of symptoms may accompany the event and may be the source of considerable confusion and delay in obtaining the true diagnosis. Figure 3 shows similar data but they are grouped by type of dissection (A or B), showing the relative frequency of complications based on the location of the dissection. These data are based on peripheral vascular complications from 168 patients, and it is important to note that 85 patients (50%) had a total of 120 individual complications. In other words, it is not unusual for the same patient to have multiple peripheral complications. The following section reviews these complications grouped by organ system.
Neurologic. As with any sudden, severe pain (i.e., ruptured ectopic) the patient may experience vaso-vagal symptoms of lightheadedness, diaphoresis, nausea, and vomiting. Extreme cases may lead to syncope. Nearly 10% of aortic dissections have associated syncope,3 but it may have more ominous origins such as hypotension from cardiac tamponade or cerebral ischemia. Figure 3 shows that 9% of patients with type A dissections presented with acute stroke. The usual mechanism of stroke is extracranial carotid obstruction leading to near total hemispheric ischemia. This causes the expected hemiplegia and aphasia if involving the dominant hemisphere. Unfortunately, being unable to communicate your chest pain to the treating physician can be catastrophic in the days of thrombolytic therapy for acute stroke. To make matters worse, there are also reports of painless dissections presenting with acute stroke or intermittent lower extremity paralysis.45,46 Granted that aortic dissection is far from the most common cause of cerebral infarcts, the fact remains that one should consider the possibility of dissection in patients before instituting thrombolysis. A recent report where a patient presenting with an acute stroke and was nearly given tPA before the aortic dissection was discovered by routine chest x-ray, prompted the authors to review the literature. They found no cases where tPA was given for acute stroke that was caused by aortic dissection.47 Cases of inadvertent thrombolysis given for acute MI caused by dissection have been reported though, and as expected, the majority of these patients have poor outcomes.48 Routine chest films should be taken to help rule out the diagnosis, but as will be mentioned in the discussion of diagnostic tests, up to 20% of chest films will be non-diagnostic in acute aortic dissection.49 Spinal cord ischemia from blockage of the spinal artery of Adamkiewicz will produce acute paraplegia, and is also reported with dissection. Abrupt onset of Horner’s syndrome (unilateral ptosis, miosis, and anhydrosis) is also reported, resulting from compression of the sympathetic chain by an enlarged aorta.
Cardiovascular. Aortic dissection can result in hemodynamic instability from a variety of causes. With type A dissections, involvement of the aortic valve is more common. This can cause acute aortic regurgitation, and one series reported some degree of valvular regurgitation in 60% of patients.50 Although most physicians can recognize the features of chronic aortic insufficiency (wide pulse pressure, etc), acute aortic insufficiency can present in a different manner. The pulse pressure in acute cases is not wide, the diastolic murmur is short and soft, and the left ventricle is not dilated as it has not had time to remodel. The sudden increased load on the left ventricle can produce acute congestive heart failure in some patients as well.
Although usually non-specific changes are present, the electrocardiogram (ECG) is abnormal in most patients with aortic dissection. Varying degrees of heart block can be produced by propagation of the dissection into the ventricular septum with subsequent compression of the atrioventricular (AV) node. Acute MIs are seen from blockage of coronary vessels by intimal flaps, and ECG change consistent with MI or ischemia have been reported in 10-40% of cases.51 Unfortunately when present, the MI will likely be diagnosed first, as this can be done with a single ECG, while uncovering the dissection may take a more extensive and purposeful search. As mentioned previously, thrombolytics will not repair the cause of the MI from a dissection and are likely to greatly increase mortality. With the great impetus to meet the 30-minute "door to drug" time, the dissection can easily be overlooked. One recent article addressing this issue concluded that continuing to use standard criteria for administration of thrombolytics in suspected MI (ST segment elevation or new left bundle block) is the recommended approach.48 They do however, point out that the possibility of aortic dissection precipitating MI needs to always be considered in screening patients for contraindications to thrombolytics and that delaying their use for screening studies (TEE or CT) to rule out dissection should be a rare occurrence. Considering that in a review of the literature they found a 71% mortality associated with the use of thrombolysis inadvertently given for dissections presenting as acute MI,48 spending an additional 15-30 minutes to rule out dissection when it is suspected certainly seems justified.
When patients present with hypotension and tachycardia and aortic dissection is in question, one must evaluate the patient for pericardial involvement and subsequent tamponade. This occurs when the dissection spreads proximally to open the pericardial space to aortic blood flow. The diagnosis of tamponade is best confirmed by bedside ultrasound, where a pericardial effusion and diastolic compression of the right ventricle will be noted. The only effective ED treatment for symptomatic tamponade is urgent pericardiocentesis. Hypotension can also be produced secondary to hypovolemia from actual aortic rupture—usually a fatal event. The hypotension can be artifactual though, as a condition of pseudohypotension can be associated with acute dissection.
Pulmonary. Dissections can produce hemoptysis if the aorta ruptures into the tracheobronchial tree. Compression of the trachea or a bronchus by an expanding hematoma can produce wheezing—a potentially dangerous complaint to treat. Beta-agonists would not relieve the symptoms, and the tachycardia produced would only exacerbate the dissection. Sudden hoarseness can be caused by compression of the recurrent laryngeal nerves, and may intensify the patient’s respiratory distress. Aortic rupture into the chest cavity will produce a pleural effusion. New onset effusions in patients with chest pain (especially with hypotension) should prod the physician into consideration of aortic dissection as the unifying diagnosis.52 Often these effusions can be large and decreased breath sounds can easily heard on exam.
Gastrointestinal. Hematemasis can result if the aorta ruptures into the adjacent esophagus. More commonly, dysphagia is produced by compression of the esophagus by an expanding hematoma. Compromised mesenteric perfusion is relatively rare in aortic dissection.53 Autopsy data confirm figures at the higher end with a level of 10%, but the important point is the effect on mortality.28 Accompanied mesenteric ischemia was correlated with a high death rate of 88% in several series.40,54 Interestingly, the incidence was not different between type A and B dissections in the Stanford series, but was significantly different in data shown in Figure 3.1 The degree of ischemia can vary from asymptomatic angiographic evidence of visceral ischemia to complete infarction. A difficulty entity to diagnosis itself, intestinal ischemia should be suspected with abdominal pain out of proportion to physical findings, and when diagnosed, aortic dissection should be ruled out as a cause.
Renal. Loss of renal perfusion is another devastating complication in several ways. First, the incidence is reported from 10% to as high 60% making renal involvement not a rare event.1,55 Second, impaired renal blood flow may produce refractory hypertension from renin release—a real problem as the initial therapy for both types of dissection is medical reduction of blood pressure and contractility. Third, the patient may be asymptomatic for renal involvement or may complain of back pain and have hematuria/oliguria—complaints and findings that are hardly specific. True diagnosis can only be made on angiography. Fourth, and most critical, is that the only way to correct the problem is through urgent endovascular repair (balloon dilatation, stenting, or fenestration). The clock starts ticking as soon as the blood flow is reduced, and renal function will only be restored if circulation can be reestablished in 2-3 hours. Given the difficulty in simply making the diagnosis of aortic dissection and beginning ED therapy for the dissection, this time-table hardly seems attainable.
Extremity Ischemia. As mentioned above, pseudohypotension can also be produced by aortic dissection. In this situation, the blood pressure in the upper extremities is low or absent while the central arterial pressure is actually normal or even elevated. This occurs when intimal flaps or compression of the subclavian arteries cuts off their blood flow. Fortunately, this does not often involve both arms simultaneously and checking blood pressure in each arm should show a discrepancy. The true danger of a falsely low blood pressure reading is that treatment to raise the patient’s blood pressure is the exact opposite of what needs to be done to slow progress of the dissection. Therefore, hypotension in a patient with known dissection needs to be rapidly verified and cautiously treated; this is not the time to slam in fluids and bolus the patient with pressors.
Pulse deficits (diminished or absent pulses) occur frequently in dissections—up to 50% of cases (see Figure 3). It is important to verify pulses in all extremities when aortic dissection is suspected and imperative to recheck them with time. The pulse deficits can be transient as the intimal flaps are quite mobile. According to a recent series, patients with type A dissections are much more likely to have pulse deficits. Of 168 total cases of dissection, pulse changes were found in 64% of patients with type A dissections and in only 22% with type B disease.1 The most common arteries involved were right arm (30%) or right leg (25%), followed by left leg (17%) and left arm (12%). Differences in carotid pulses were rare (7%) but present and should also be examined. Duplication of pulses (palpable pulse without ventricular contraction) can also occur as blood can travel at different rates through true and false lumens. This can provide another clue to the presence of dissection. Finally, patients may present with acute limb ischemia as their chief complaint. It is not surprising that restoration of blood flow to the limb may become the focus of treatment before the complaints of chest or back pain are fully addressed, or that the patient may have no pain at all other than the affected extremity. Without considering the possibility of a unifying life-threat, patients have been taken to the operating room for emergent revascularization only to find no thrombus or embolus in the femoral artery—termed the "peek and shriek phenomenon."29 Thus aortic dissection needs to be considered in any patient presenting to the ED with acute limb ischemia.
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Physician CME Questions
1. When discussing pain in acute aortic dissection, which is false?
A. All patients with aortic dissection have pain.
B. Classic pain is ripping or tearing with sudden onset.
C. Pain typically migrates location with propagation of the dissection.
D. Migration of chest pain is a helpful clinical sign of dissection.
2. Which is true concerning peripheral complications of acute aortic dissection ?
A. Peripheral complications with aortic dissection are rare.
B. Neurological symptoms may be transient simulating TIAs.
C. Intestinal ischemia in aortic dissection is not associated with high mortality.
D. Pulse deficits are rare in aortic dissection.
3. Concerning classification of acute aortic dissections, which is true?
A. Type A involves descending aorta.
B. Type B involves ascending aorta.
C. Type A involves ascending aorta, and may involve descending as well.
D. Type B involves descending aorta, and may involve ascending as well.
4. Thoracic aortic dissection is how many times more common than abdominal aortic aneurysm rupture?
A. As common
B. Less common
C. Four times as common
D. 2-3 times as common
E. None of above
5. Which of the following is considered a prime risk factor for thoracic aortic aneurysm rupture?
A. Peptic ulcer disease
B. Chlamydia
C. Hypertension
D. Mycoplasma
E. None of the above
6. What percentage of thoracic aortic dissections involve the ascending aorta?
A. About 20%
B. About 30%
C. About 40%
D. About 50%
E. About 60%
7. What percentage of thoracic aortic dissections involve the descending aorta?
A. About 20%
B. About 30%
C. About 40%
D. About 50%
E. About 60%
8. Which of the following are risk factors for aortic dissection?
A. Increased age
B. Male sex
C. Turner’s syndrome
D. Aortic valve stenosis
E. All of the above
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