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Clinical manifestations and diagnosis of aortic dissection

Clinical manifestations and diagnosis of aortic dissection

INTRODUCTION — Aortic dissection is a relatively uncommon, though catastrophic illness often presenting with severe chest pain and acute hemodynamic compromise. Early and accurate diagnosis and treatment are crucial for survival.

The clinical manifestations and diagnosis of aortic dissection will be reviewed here. The management of this disorder is discussed separately. (See "Management of aortic dissection".)

PATHOPHYSIOLOGY — The primary event in aortic dissection is a tear in the aortic intima. Degeneration of the aortic media, or cystic medial necrosis, is felt to be a prerequisite for the development of nontraumatic aortic dissection [1] . Blood passes into the aortic media through the tear, separating the intima from the surrounding media and/or adventitia, and creating a false lumen. It is uncertain whether the initiating event is a primary rupture of the intima with secondary dissection of the media, or hemorrhage within the media and subsequent rupture of the overlying intima.

Propagation of the dissection can occur both distal and proximal to the initial tear, involving branch vessels and the aortic valve and entering the pericardial space [2] . Such propagation is responsible for many of the associated clinical manifestations, including ischemia (coronary, cerebral, spinal, or visceral), aortic regurgitation, and cardiac tamponade (see below). In addition, multiple communications may form between the true lumen and the false lumen.

EPIDEMIOLOGY

Incidence — Data concerning the incidence of acute aortic dissection in the general population are limited; estimates range from 2.6 to 3.5 per 100,000 person-years [3-5] . Patients with acute aortic dissection tend to be 60 to 80 year-old men [1,6,7] . In a review of 464 patients from the International Registry of Acute Aortic Dissection (IRAD), 65 percent were men and the mean age was 63 years [7] . Women presenting with aortic dissection tend to be older than men (67 versus 60 years) [8] .

Predisposing factors — The most important predisposing factor for acute aortic dissection is systemic hypertension [1,6,7] .In the IRAD registry data, 72 percent had a history of hypertension [7] . In addition, 31 percent had a history of atherosclerosis. These factors are less important in young patients; in an IRAD analysis of patients under age 40, only 34 percent had a history of hypertension and only 1 percent had a history of atherosclerosis [9] .

Other predisposing factors, especially in younger patients, include:

  • Preexisting aortic aneurysm. In an IRAD review, 13 percent of patients had a known aortic aneurysm prior to dissection [9] . The ascending aorta was more often the site of origin of the dissection than the aortic arch or descending aorta. Such a history was more common in patients under age 40 (19 percent). (See "Clinical features and diagnosis of thoracic aortic aneurysm" and "Clinical evaluation of abdominal aortic aneurysm".)
  • Inflammatory diseases that cause a vasculitis (giant cell arteritis, Takayasu arteritis, rheumatoid arthritis, syphilitic aortitis). (See "Classification of and approach to the vasculitides in adults" and "Clinical manifestations of giant cell (temporal) arteritis", section on 'Aneurysms'.)
  • Disorders of collagen (eg, Marfan syndrome, Ehlers-Danlos syndrome, annuloaortic ectasia) (picture 1). Most patients with Marfan syndrome and aortic dissection have a family history of dissection. In an IRAD review, Marfan syndrome was present in 50 percent of those under age 40, compared to only 2 percent of older patients [9] . There may also be an association between Marfan syndrome and dissection in the third trimester of pregnancy [10] . (See "Genetics, clinical features, and diagnosis of Marfan syndrome and related disorders".)

    In addition to these disorders, it has been estimated that as many as 19 percent of patients with a thoracic aortic aneurysm/dissection have a positive family history; a number of mutations have been identified. (See "Clinical features and diagnosis of thoracic aortic aneurysm", section on 'Genetic factors'.)

  • A bicuspid aortic valve. In an IRAD review, 9 percent of patients under age 40 had a bicuspid valve, compared to 1 percent of those over age 40 [9] . Aortic dissection in these patients always involves the ascending aorta, usually with severe loss of elastic fibers in the media [11] . The predisposition to dissection may reflect a congenital defect in the aortic wall, as enlargement of the aortic root and/or ascending aorta is frequently associated with bicuspid aortic valves, even those that function normally, independent of their function [12,13] . (See "Clinical manifestations and diagnosis of bicuspid aortic valve".)
  • Aortic coarctation. Aortic dissection occurs in patients with an aortic coarctation when surgery leaves behind abnormal paracoarctation aorta that has intrinsic medial faults, when balloon dilatation of native coarctation mechanically damages the inherently abnormal paracoarctation aorta, and in the inherently abnormal aortic root above a coexisting bicuspid aortic valve. (See "Clinical manifestations and diagnosis of coarctation of the aorta" and "Clinical manifestations and diagnosis of bicuspid aortic valve".)
  • Turner syndrome. Aortic dissection or rupture, often occurring with coarctation, is an increasingly recognized cause of death in women with Turner syndrome. In a survey of 237 patients, at least 15 (6.3 percent) had aortic dilation: all involved the ascending aorta; 12 had an associated risk factor such as hypertension or another cardiovascular malformation (eg, coarctation); and two had a dissection [14] . (See "Clinical manifestations and diagnosis of Turner syndrome (gonadal dysgenesis)".)
  • Coronary artery bypass graft surgery (CABG). Ascending aortic dissection is a rare complication of CABG, occurring with both conventional on-pump CABG and, perhaps more often, with minimally invasive OP CABG [15-18] . In a review from a single institution, ascending aortic dissection occurred in 1 of 2723 patients (0.04 percent) treated with conventional CABG and 3 of 308 undergoing OP CABG (1 percent) [17] . (See "Early noncardiac complications of coronary artery bypass graft surgery", section on 'Aortic dissection' and "Minimally invasive coronary artery bypass graft surgery: Clinical efficacy of beating heart surgery".)
  • Previous aortic valve replacement. In an IRAD review, 5 percent of all patients (but 12 percent of those under age 40) had such a history [9] .
  • Cardiac catheterization with or without coronary intervention was reported to cause 14 of 723 dissections (2 percent) in a report from the IRAD registry [19] .
  • Trauma rarely causes a classic dissection, but can induce a localized tear in the region of the aortic isthmus (picture 2). More commonly, chest trauma from acute deceleration (as in a motor vehicle accident) results in aortic rupture [20] .
  • High-intensity weight lifting or other strenuous resistance training may cause acute ascending aortic dissection, possibly mediated by significant transient elevations in blood pressure [21] . (See "Exercise in the treatment of hypertension", section on 'Exercise-induced hypertension'.)
  • Crack cocaine, which accounted for 37 percent of dissections in a report of a largely African American, inner city population [22] . The mean duration from last cocaine use to the onset of symptoms was 12 hours. The mechanism may be abrupt, transient hypertension due to catecholamine release. (See "Evaluation and management of the cardiovascular complications of cocaine abuse".)

TYPES OF AORTIC DISSECTION

Classification — Two different anatomic systems — the DeBakey and Daily (Stanford) systems — have been used to classify aortic dissection [2,23-25] . The Daily system is more widely used. It classifies dissections that involve the ascending aorta as type A, regardless of the site of the primary intimal tear, and all other dissections as type B. In comparison, the DeBakey system is based upon the site of origin with type 1 originating in the ascending aorta and propagating to at least the aortic arch, type 2 originating in and confined to the ascending aorta, and type 3 originating in the descending aorta and extending distally or proximally.

Ascending aortic dissections are almost twice as common as descending dissections. The right lateral wall of the ascending aorta is the most common site of aortic dissection [1] . In patients with an ascending aortic dissection, aortic arch involvement is seen in up to 30 percent [26] .

Isolated abdominal aortic dissection is reported sporadically and can be due to iatrogenic, spontaneous, or traumatic mechanisms [27] . The infrarenal abdominal aorta is more commonly involved than the suprarenal aorta. In one review of 52 reported cases, the entry site for spontaneous isolated abdominal aortic dissections (SIAAD) most commonly occurred between the renal arteries and inferior mesenteric artery [28] . A concomitant abdominal aortic aneurysm was identified in 40 percent of patients and indicated the need for repair. (See "Natural history and management of abdominal aortic aneurysm".)

Variants — There are several variants of aortic dissection, including intimal tear without hematoma and intramural hematoma (figure 1) [8,23,29] .

Intimal tear without hematoma — Intimal tear without hematoma is an uncommon variant of aortic dissection that is characterized by a stellate or linear intimal tear associated with exposure of the underlying aortic media or adventitial layers. There is no progression or separation of the medial layers (figure 1) [29] .

Current imaging techniques may be inadequate for diagnosing this type of dissection because of its limited extent and the presence of only a minimal amount of blood in the dissected aortic wall. In one study of 181 patients who underwent repair of the ascending aorta or aortic arch, nine (5 percent) had a subtle aortic dissection that was not diagnosed preoperatively despite the use of three or more imaging techniques [29] . Ascending aortic dilation, which was often due to an eccentric aortic bulge, was present in all of these patients and most had significant aortic regurgitation. All of these patients survived after surgical correction.

Aortic intramural hematoma — Aortic intramural hematoma, characterized by blood in the wall of the aorta in the absence of an intimal tear, is another variant of aortic dissection that accounts for 5 to 13 percent of patients with symptoms consistent with an aortic dissection (figure 1). The false channel is probably produced by a rupture of the vaso vasorum into the media of the aortic wall. The clinical features and management of this disorder are discussed separately. (See "Aortic intramural hematoma".)

Penetrating atherosclerotic ulcer — Penetrating ulceration of an atherosclerotic plaque often complicates an aortic intramural hematoma and can also lead to aortic dissection or perforation [23] . Noninvasive imaging shows an ulcer-like projection into the hematoma, and some have suggested that penetrating atherosclerotic ulcers are almost always seen with a type B hematoma (31 of 34 cases in one series) [30] . (See "Aortic intramural hematoma", section on 'Penetrating atherosclerotic ulcer'.)

CLINICAL MANIFESTATIONS — Patients with an aortic dissection typically present with severe, sharp or "tearing" posterior chest or back pain (in dissection distal to the left subclavian) or anterior chest pain (in ascending aortic dissection) [6,7] . The pain can radiate anywhere in the thorax or abdomen [1,7] . It can occur alone or be associated with syncope, a cerebrovascular accident, myocardial infarction (MI), heart failure or other clinical symptoms or signs (table 1).

In the IRAD review, 73 percent of patients presented with chest pain that was typically abrupt in onset and was more often sharp than tearing [7] . Chest pain was significantly more common in patients with type A dissections (79 versus 63 percent in type B dissections), while both back pain (64 versus 47 percent) and abdominal pain (43 versus 22 percent) were significantly more common with type B dissections.

Painless dissection has been reported, but is relatively uncommon. In an analysis from the IRAD registry, of 977 patients, only 63 (6.4 percent) had no pain [31] . Patients with painless dissection were slightly older (mean age 67 versus 62 years) and more often had a type A dissection (75 versus 61 percent). A prior history of diabetes, aortic aneurysm, or cardiovascular surgery was more common in patients with painless dissection. Presenting symptoms of syncope, heart failure, or stroke were seen more often in this group. In-hospital mortality was significantly higher than for patients presenting with pain (33 versus 23 percent). In one study, up to ten percent of patients presented with neurologic symptoms, but without chest pain [32] .

Syncope during aortic dissection is associated with worse outcomes. In an analysis of 728 patients with acute aortic dissection, 96 (13 percent) had syncope [33] . Almost all had a proximal (Daily type A) dissection and, compared to the patients presenting with other symptoms, there was an increased incidence of cardiac tamponade and stroke, conditions that are more likely to produce syncope.

Although a history of hypertension is common, hypertension at initial presentation is more common in those with a distal (type B) dissection (70 versus 36 percent for dissection of the ascending aorta in the IRAD review) [7] . Other symptoms associated with aortic dissection are related to impaired blood flow to an organ or limb induced by the original dissection or by propagation of the dissection proximally or distally.

The presence of impaired or absent blood flow to peripheral vessels is manifest as a pulse deficit, defined as a weak or absent carotid, brachial, or femoral pulse resulting from the intimal flap or compression by hematoma. A pulse deficit has been described in 19 to 30 percent of patients with an acute type A dissection [7,34]  compared to 9 to 21 percent with a type B dissection [7,35] . These patients have a higher rate of in-hospital complications and mortality than those without a pulse deficit [34] . Women are less likely to have a pulse deficit than men [8] .

Involvement of the ascending aorta — In addition to pain (chest more often than back or abdominal pain) [7] , a dissection that involves the ascending aorta can induce one or more of the following [2,6] :

  • Acute aortic insufficiency, leading to an diastolic decrescendo murmur, hypotension, or heart failure, in one-half to two-thirds of ascending dissections [7,36] . The murmur of aortic insufficiency related to aortic dissection is most commonly heard along the right sternal border, as compared with the left sternal border for aortic insufficiency due to primary aortic valve disease. The diastolic murmur may be quite short due to rapid ventricular filling and early equilibration of aortic and left ventricular diastolic pressures. (See "Auscultation of cardiac murmurs" and "Acute aortic regurgitation in adults".)
  • Acute myocardial ischemia or MI due to coronary occlusion. The right coronary artery is most commonly involved and, in infrequent cases, leads to complete heart block.
  • Cardiac tamponade and sudden death due to rupture of the aorta into the pericardial space. Tamponade occurs more often in women than in men [8] .
  • Hemothorax and exsanguination if the dissection extends through the adventitia, with hemorrhage into the pleural space.
  • A considerable variation (>20 mmHg) in systolic blood pressure between the arms.
  • Neurologic deficits, including stroke or decreased consciousness due to direct extension of the dissection into the carotid arteries or diminished carotid blood flow. Alterations of consciousness are more common in women than in men [8] .
  • Horner syndrome if there is compression of the superior cervical sympathetic ganglion.
  • Vocal cord paralysis and hoarseness due to compression of the left recurrent laryngeal nerve.

Elderly — There are some important differences between elderly and younger patients with dissections involving the ascending aorta. This was illustrated in a review from IRAD of 550 such patients, 32 percent of whom were ≥70 years of age [37] . The following differences were noted between the two groups:

  • Marfan syndrome was not seen in any elderly patient compared to 8.5 percent of the younger patients (mean age 55 years).
  • Elderly patients were significantly more likely to have atherosclerosis, prior aortic aneurysm, iatrogenic dissection, or intramural hematoma, and were significantly less likely to have the abrupt onset of pain (77 versus 89 percent), any pulse deficit (24 versus 33 percent), or a murmur of aortic regurgitation (29 versus 47 percent).
  • Elderly patients were significantly less likely to undergo surgery (64 versus 86 percent) and had a higher mortality with either surgery or medical therapy.

Involvement of the descending aorta — In addition to pain, a dissection that involves the descending aorta can lead to splanchnic ischemia, renal insufficiency, lower extremity ischemia, or focal neurologic deficits due to spinal artery involvement and spinal cord ischemia [2,35] .

In a review of 384 type B dissections from the IRAD registry, the following clinical manifestations on presentation were noted [35] :

  • Chest or back pain — 86 percent; an earlier IRAD series also noted abdominal pain in 43 percent [7]
  • Abrupt onset of pain — 89 percent
  • Migrating pain — 25 percent
  • Hypertension — 69 percent
  • Hypotension/shock — 3 percent
  • Pulse deficit — 21 percent
  • Spinal cord ischemia — 3 percent
  • Ischemic peripheral neuropathy — 2 percent

During the course of hospital management, hypotension/shock occurred in 12 percent, acute renal failure in 14 percent, mesenteric ischemia in 5 percent, limb ischemia in 7 percent, and coma or altered consciousness in 5 percent. Factors independently associated with in-hospital mortality included hypotension/shock, lack of chest pain on presentation, and branch vessel involvement.

Periaortic hematoma — Periaortic hematoma, which is detected by imaging, may reflect slow oozing from the damaged acutely dissecting aorta at or near the site of dissection. It is thought to be a harbinger of impending rupture.

In a review of 971 patients with acute dissections from IRAD, 227 (23 percent) had a periaortic hematoma [38] . These patients had higher rates of shock, cardiac tamponade, and altered consciousness/coma and had a significantly higher mortality rate compared to those without a periaortic hematoma (33 versus 20 percent, adjusted odds ratio 1.71, 95% CI 1.15-2.54).

DIAGNOSIS — Aortic dissection is generally suspected from the history and physical examination. An analysis of 250 patients with acute chest and/or back pain (128 with a dissection) found that 96 percent of acute aortic dissections could be identified based upon some combination of the following three clinical features [39] :

  • Abrupt onset of thoracic or abdominal pain with a sharp, tearing and/or ripping character
  • Mediastinal and/or aortic widening on chest radiograph
  • A variation in pulse (absence of a proximal extremity or carotid pulse) and/or blood pressure (>20 mmHg difference between the right and left arm)

The incidence of a dissection related to the presence or absence of these three:

  • All three absent (4 percent of dissections): 7 percent
  • Pain: 31 percent
  • Presence of chest radiographic abnormalities: 39 percent
  • Variation in pulse or blood pressure differential: ≥83 percent
  • Any two out of three variables (77 percent of dissections): ≥83 percent

Despite the reported utility of this diagnostic strategy, additional imaging studies are obtained in almost all patients (98 percent in data from IRAD) because of the limited sensitivity of the chest radiograph, especially in type B dissections [7,35,40] .

Differential diagnosis — The signs and symptoms of an aortic dissection may suggest other etiologies. The differential diagnosis includes [41] :

  • Myocardial ischemia due to an acute coronary syndrome with or without ST segment elevation
  • Pericarditis
  • Pulmonary embolus
  • Aortic regurgitation without dissection
  • Aortic aneurysm without dissection
  • Musculoskeletal pain
  • Mediastinal tumors
  • Pleuritis
  • Cholecystitis
  • Atherosclerotic or cholesterol embolism
  • Peptic ulcer disease or perforating ulcer
  • Acute pancreatitis

Electrocardiogram — The nature and location of the chest pain and the absence of the ECG changes characteristic of ischemia usually allows an aortic dissection to be distinguished from angina pectoris or an MI as the cause of chest pain. However, the ECG alone is less helpful when dissection leads to coronary ischemia. In the IRAD review of 464 patients, the ECG was normal in 31 percent, showed nonspecific ST and T wave changes in 42 percent (commonly, LVH and strain patterns associated with hypertension), showed ischemic changes in 15 percent, and, among patients with an ascending aortic dissection, showed evidence of an acute MI in 5 percent [7] . Data from IRAD further suggest that involvement of a coronary artery in an aortic dissection may not manifest changes in the electrocardiogram [42] .

Imaging — Imaging studies to establish the diagnosis of aortic dissection are not performed until the patient is stabilized medically (table 2A-B). (See "Management of aortic dissection", section on 'Acute management'.)

There has been a shift from an invasive (aortography) to a noninvasive diagnostic strategy for evaluating suspected thoracic aortic dissections [23] . In the 2000 IRAD review, most patients had multiple imaging studies performed (mean of 1.83 per patient); the initial study was computed tomography (CT) in 61 percent, echocardiography in 33 percent, aortography in only 4 percent, and magnetic resonance imaging (MRI) in only 2 percent [7] . Recent experience suggests that CT is even more prevalent as the initial study of choice, especially due to its widespread availability in the emergency department setting.

Multiplane transesophageal echocardiography (TEE), chest CT, and MRI are thought to be superior to transthoracic echocardiography and aortography. However, as with any study, availability may be limited and the accuracy is dependent upon the performance and interpretation of the test by skilled individuals (table 3 and table 4).

It is particularly important to rapidly identify acute dissections involving the ascending aorta, which are considered surgical emergencies. In comparison, hemodynamically stable dissections confined to the descending aorta should be treated medically. (See "Management of aortic dissection".) Transthoracic echocardiography may be useful to identify proximal ascending aortic dissections, particularly with regard to coexistent aortic valve disruption/regurgitation and hemopericardium, although it is not sufficient to delineate the extent of the dissection, or any associated bleeding or complications of dissection.

Imaging studies can identify the presence of aortic dissection and the following associated features [2,43] :

  • Involvement of the ascending aorta
  • The extent of dissection and the sites of entry and reentry
  • Thrombus in the false lumen
  • Branch vessel or coronary artery involvement
  • Aortic insufficiency
  • Pericardial effusion

The use of imaging studies such as chest radiography, echocardiography, contrast chest CT, or cardiac MRI to distinguish aortic dissection from myocardial infarction was recommended in the 2004 ST-elevation myocardial infarction guidelines of the American College of Cardiology and the American Heart Association [44] . No changes to this approach were made in the 2007 focused update [45] . (See "Management of suspected acute coronary syndrome in the emergency department".)

Chest radiograph — Conventional chest radiographs may show widening of the aorta with aortic dissections [7,40] . The IRAD review of 464 patients found that mediastinal widening was present in 63 percent with type A dissections, while 11 percent of patients had no abnormality on chest radiography [7] . The comparable value in patients with type B dissections were 56 and 16 percent.

Radiographic evidence of a pleural effusion was found in 19 percent of dissections; this finding is more common in women than in men (26 versus 15 percent) [8] . Other findings, which are less specific for dissection but have been described, include widening of the aortic contour, displaced calcification, aortic kinking, and opacification of the aorticopulmonary window [40] .

Because of the limited sensitivity of the chest radiograph, especially in type B dissections, additional imaging studies are obtained in almost all patients (98 percent in data from IRAD) [7,35,40] .

Aortography — Invasive aortography involves the injection of contrast media into the aorta, thereby permitting identification of the site of dissection, the relationship between the dissection and the major branches of the aorta, and the communication site between the true and false lumen (picture 3) [43] . Coronary angiography and evaluation for aortic insufficiency can also performed during the same procedure.

As mentioned above, aortography has generally been replaced by noninvasive testing to establish the diagnosis of aortic dissection. It may also take longer to obtain in critically ill patients.

Aortography had, in a comparative review of 164 patients (82 had a dissection), a sensitivity of 88 percent and a specificity of 94 percent; the positive and negative predictive values were 96 and 84 percent, respectively [46] . Lower values for sensitivity (77 percent) and accuracy (87 percent) were noted in other reports [47,48] . False negative results may be obtained when there is simultaneous opacification of the true and false lumen so that the intimal flap between them is not visible, when thrombosis of the false lumen results in lack of opacification with contrast, or when there is an intramural hematoma with noncommunicating dissection [46,47,49,50] .

CT scan — The diagnosis of aortic dissection by CT scanning requires that, after injection of intravenous iodinated contrast, two distinct lumens with a visible intimal flap can be identified (picture 4). In two reports of 162 and 110 patients, the sensitivity of standard CT for the diagnosis of aortic dissection were 83 and 98 percent and the specificity was 87 and 100 percent [46,51] .

Advantages of CT include ready availability at most hospitals. even on an emergency basis and identification of intraluminal thrombus and pericardial effusion. Two disadvantages of standard CT are that the intimal flap is seen in less than 75 percent of cases and that the site of entry is rarely identified [52] . In addition, potentially nephrotoxic iodinated contrast is required, and there is no capability to assess for aortic insufficiency (see "Pathogenesis, clinical features, and diagnosis of contrast-induced nephropathy").

The accuracy of CT appears to be substantially improved with spiral (helical) CT (picture 5A-B) and perhaps with electron beam (ultrafast) or multidetector (multislice) CT (picture 6) [53-56] .

Spiral CT may be more accurate than MRI or TEE in the detection of aortic arch vessel involvement [53] . A potential limitation is a spiral CT artifact that can simulate aortic dissection in patients without the disease [57]  if performed without ECG gating [58] .

MRI — Although less commonly used [7] , MRI is a highly accurate noninvasive technique for evaluating the thoracic aorta in patients with suspected dissection. The presence of a double lumen with a visible intimal flap is the diagnostic criterion for aortic dissection (picture 7). Additional suggestive findings include widening of the aorta with a thickened wall and thrombosis of the false lumen.

In a prospective trial of 110 patients with suspected aortic dissection, the sensitivity and specificity of MRI were both 98 percent, with an 85 percent sensitivity for identification of the site of entry [51] .

MRI is safe in adequately monitored patients with aortic dissection, and MR contrast agents have a more favorable safety profile than iodinated contrast agents. Other advantages include the ability to assess branch vessels, although it may be less sensitive than spiral CT [53] , and to assess for aortic insufficiency.

The main disadvantages of MRI are inconvenience (patients are required to remain motionless with relatively limited access for more than 30 minutes) and limited applicability (MRI cannot be performed in patients with claustrophobia, pacemakers, or certain types of aneurysm clips or metallic ocular/auricular implants). MRI is also not readily available on an emergency basis at many institutions, and there are concerns about patient monitoring and relative patient inaccessibility during prolonged scanning.

Gadolinium administration for contrast-enhanced MRI in patients with moderate to severe kidney disease (particularly dialysis patients) has been associated with the often severe syndrome of nephrogenic systemic fibrosis. It is recommended that gadolinium-based imaging be avoided in such patients. This issue, including the definition of patients at risk, is discussed separately. (See "Nephrogenic systemic fibrosis/nephrogenic fibrosing dermopathy in advanced renal failure".)

TTE — Although transthoracic (surface) echocardiography (TTE) is the cornerstone of noninvasive imaging of the heart, it has limited utility for evaluation of the thoracic aorta for dissection (figure 2). The primary problem with TTE is its inability to adequately visualize the distal ascending, transverse, and descending aorta in a substantial majority of patients (figure 3A-B). Furthermore, although an undulating intimal flap may be seen in the proximal aorta in some patients, the sensitivity and specificity of TTE are inferior to those with CT, MRI, and TEE [51,59,60] . As a result, TTE is most useful for the assessment of cardiac complications of dissection, including aortic insufficiency, pericardial effusion/tamponade, and regional left ventricular systolic function.

TEE — Advantages of multiplane TEE for the detection of aortic dissection include the close proximity of the esophagus to the thoracic aorta and the absence of an intervening lung or chest wall. Although it requires esophageal intubation, TEE is a portable procedure, which is easily performed in the emergency department and yields a diagnosis within minutes from the start of the procedure. It may be particularly useful in patients too unstable for MRI [51] , but it often requires procedural sedation which may have untoward hemodynamic effects in unstable patients. TEE requires the availability of experienced operators (both physicians and technicians) to assure accurate results. As such, it is often not attainable on a "stat" basis in many centers.

The following findings may be seen on TEE in patients with aortic dissection [43,51,61] :

  • Intimal dissection flaps can be identified with high spatial resolution (figure 4 and movie 1 and movie 2 and movie 3). The use of M-mode echocardiography may improve diagnostic accuracy by demonstrating a lack of relation between movement of the intimal flap and the aortic wall [62] .
  • The true and false lumens can be identified. They may not be distinguishable without color Doppler imaging or identification of the proximal border of the dissection. However, in some cases, the false lumen can be seen to surround the true lumen (figure 4 and movie 1 and movie 2). Color Doppler permits clear identification of flow within and between the true and false lumens (figure 5A-B and movie 4). The presence of flow does not absolutely distinguish the true lumen from the false lumen The true lumen has an endothelial lining and is contiguous with the aortic valve.
  • Thrombosis in the false lumen, pericardial effusion, concomitant aortic regurgitation, and the proximal coronary arteries can be readily visualized.
  • The 135º long axis view from TEE can define the severity and mechanism of aortic regurgitation that complicates acute type A dissections [36] . Patients with an intrinsically normal valve who have aortic regurgitation due to a correctable aortic lesion (incomplete leaflet closure, leaflet prolapse, or dissection flap prolapse) can undergo aortic valve repair (movie 5). In contrast, unrepairable abnormalities (eg, Marfan syndrome, bicuspid valve, aortitis) require valve replacement. (See "Management of aortic dissection".)

Specific findings for aortic intramural hematoma on TEE include regional thickening of the aortic wall of more than 7 mm in a crescentic (primarily if nontraumatic) or circular shape (primarily if traumatic) and/or evidence of intramural accumulation of blood [63] . (See "Aortic intramural hematoma".)

The sensitivity, specificity, and accuracy of TEE for the identification of thoracic aortic dissection have been extensively studied, but much of the data are based upon monoplane TEE studies. In three large series, the sensitivity of TEE for the diagnosis of aortic dissection was 97 to 99 percent [46,51,62] . However, the specificity of TEE alone has been as low as 77 to 85 percent [51,62] . The lower specificity is primarily due to false-positive findings in the ascending aorta; these artifacts can be detected by the addition of M-mode imaging, resulting in an increase in specificity to almost 100 percent [62] .

One deficiency of monoplane TEE is its inability to visualize the upper portion of the ascending aorta due to the interposed trachea (between the aorta and esophagus). Biplane and multiplane TEE circumvent this deficiency by permitting the observation of the ascending aorta in multiple imaging planes [64,65] . In one study of 112 patients, for example, biplane or multiplane TEE was highly sensitive and specific for the detection of aortic dissection (98 and 95 percent, respectively) [65] .

It is not known if multiplane TEE is significantly superior to biplane TEE for aortic dissection. However, the flexibility of multiplane imaging for situations in which the aortic anatomy may be distorted makes it preferable.

Recommendations — Selection of a diagnostic test for suspected aortic dissection requires consideration both of the information required and of the access to and experience with the imaging modality at the institution. Thoracic MRI, thoracic CT, and multiplane TEE are the preferred methods for evaluating suspected aortic dissection, if available. Our recommendations for the use of different imaging modalities in acute and chronic aortic dissection are generally in agreement with guidelines published by a task force of the European Society of Cardiology in 2001 [41] .

  • We generally perform multiplane TEE at the bedside or in the Emergency Department for patients who present with acute chest pain and/or are clinically unstable (table 3). Hemodynamically unstable patients with a very strong suspicion of dissection can be emergently brought to the operating room and undergo TEE after induction of anesthesia as the chest is being prepared.
  • MRI is preferred in patients with chronic chest pain and in those who are hemodynamically stable, or are seen for follow-up of a chronic dissection (table 4).
  • CT scan with contrast is reserved for situations in which both TEE and MRI are unavailable or contraindicated. As such, it is often indicated as an initial screening study in patients with suspected aortic dissection, especially in the emergency department setting where TEE and MRI are less available, especially after hours. If CT is equivocal, or further delineation of the dissection is needed, TEE or MRI are indicated.
  • Aortography is used when ascending aortic dissection is strongly suspected, but noninvasive tests are unavailable or inconclusive.
  • Coronary angiography is generally safe in stable patients, although the delay to surgical invention for ascending dissections should be minimized. Retrospective data suggest no in-hospital benefit to coronary angiography [66] .

    At our institution, coronary angiography is generally attempted in all patients with a prior history or angina or MI, patients older than 60 years of age, and patients with multiple risk factors for coronary disease.

Blood tests — Routine blood tests are generally nondiagnostic in aortic dissection. The serum lactate dehydrogenase concentration may be elevated due to hemolysis of blood in the false lumen, but this is a nonspecific finding.

Newer tests have been developed, which might prove to be more useful. A rapid 30-minute immunoassay for the serum concentration of smooth muscle myosin heavy chain has been evaluated in patients suspected of having an aortic dissection [67,68] . The sensitivity and specificity of this assay in the first three hours were similar and possibly superior to those of TTE, conventional CT, and aortography, but were lower than those of TEE, helical CT, or MRI. The utility of this test needs further evaluation.

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