| Warren J Manning, MD |
Mar 7, 2000 |
Aortic dissection is a relatively common, often catastrophic illness presenting with chest pain and acute hemodynamic compromise. Over 2000 new cases are reported each year in the United States. Early and accurate diagnosis and treatment are crucial for survival.
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, thereby 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, which is responsible for many of the associated symptoms (see below), can occur both distal and proximal to the initial tear. In addition, multiple communications may form between the true lumen (which is lined with vascular endothelium) and the false lumen (which has no endothelium).
Aortic dissections are classified under one of two anatomic systems. Under the DeBakey system, type I dissection involves both the ascending and descending thoracic aorta, type II is confined to the ascending aorta, and type III is confined to the descending aorta [2]. The Daily system classifies dissections involving the ascending aorta as type A, regardless of the site of the primary intimal tear, and all other dissections as type B [3]. The right lateral wall of the ascending aorta is the most common site of aortic dissection [1]. Proximal dissections are almost twice as common as distal dissections.
Predisposing factors ！ Patients with acute aortic dissection tend to be 60 to 80 year-old men with a long history of systemic hypertension [1,4]. Chronic or severe hypertension can lead to a number of changes that predispose to dissection including degeneration of the aortic media, and degeneration of collagen and elastic tissue with cystic changes.
Other predisposing factors, especially in younger patients, include disorders of collagen (Marfan's syndrome, Ehlers-Danlos syndrome, cystic medial necrosis), a bicuspid aortic valve, and aortic coarctation (show radiograph 1) [1].
Most patients with Marfan's syndrome and aortic dissection have a family history of dissection. There may also be an association between Marfan's syndrome and dissection in the third trimester of pregnancy [5]. (See "The Marfan syndrome").
Trauma very rarely causes a classic dissection, but can induce a localized tear in the region of the aortic isthmus (show radiograph 2).
More commonly, chest trauma from acute deceleration (as in a motor vehicle accident) results in aortic rupture [6]. (See "Transesophageal echocardiography in traumatic rupture of the aortic isthmus").
Intimal tear without hematoma ！ An uncommon variant of aortic dissection is characterized by stellate or linear intimal tear associated with exposure of the underlying aortic media or adventitial layers but without the progression and separation of the medial layers, resulting in extensive undermining of the intimal layers . 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, five percent had subtle aortic dissection which was not diagnosed preoperatively despite the use of
3
imaging techniques [7].
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 the absence of an intimal tear, is a variant of dissection which is increasingly being recognized with the use of noninvasive imaging (, , and show figure 2) [8,9,10].
The false channel is probably produced by a hemorrhage of the vaso vasorum into the aortic wall. The etiology is most often nontraumatic, the result of long-standing hypertension; a traffic accident (deceleration injury) is the most common cause for a traumatic hematoma.
The prognosis of this disorder when the ascending aorta or aortic arch are involved is similar to overt dissection and rapid intervention is indicated. In one review, 8 of 25 patients progressed to overt dissection, rupture, or cardiac tamponade within 24 to 72 hours [9]. However, when the descending thoracic aorta is involved, the outcome with surgery or medical therapy is identical and is better than than with an overt dissection [9,11].
CLINICAL MANIFESTATIONS ！ Patients who survive the initial tear 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) [4]. The pain can radiate anywhere in the thorax or abdomen [1]. (See "Differential diagnosis of chest pain"). Painless dissection has been reported, but is relatively uncommon. In one review of 464 patients, 73 percent of patients presented with chest pain which was abrupt in onset in 85 percent and was more often sharp than tearing [12].
Although a history of hypertension is common, hypertension at initial presentation is more common in those with a distal dissection (70 versus 36 percent for dissection of the ascending aorta) [12]. In addition to pain and usually preexisting hypertension, 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. Proximal propagation into the ascending aorta, for example, can induce one or more of the following [4]:
• Acute aortic insufficiency, leading to an diastolic decrescendo murmur and hypotension, may be seen in one-half to two-thirds of ascending dissections [12]. The murmur of aortic insufficiency related to aortic dissection (or aneurysm) most commonly is heard along the right sternal border, as compared with the left sternal border for aortic insufficiency due to primary aortic valve disease. With acute aortic insufficiency, 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-I").
• Acute myocardial ischemia or infarction due to coronary occlusion. The right coronary artery is most commonly involved.
• Cardiac tamponade and sudden death due to rupture of the aorta into the pericardial space.
• Hemothorax and exsanguination if the dissection extends through the adventitia, with hemorrhage into the pleural space.
• A considerable variation (>30 mmHg) in 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.
• 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.
Descending aortic dissections can lead to manifestations such as splanchnic ischemia, renal insufficiency, lower extremity ischemia or pulse deficits, or focal neurologic deficits due to spinal artery involvement and spinal cord ischemia.
DIAGNOSIS ！ Aortic dissection is generally first suspected from the history and physical examination. The differential diagnosis may include myocardial ischemia, pericarditis, or pulmonary embolus. The nature and location of the chest pain and the absence of the characteristic electrocardiographic changes usually allows an aortic dissection to be distinguished from angina pectoris or a myocardial infarction. The ECG alone is less helpful when dissection leads to coronary ischemia. As an example, in a review of 464 patients, the ECG most often showed only nonspecific abnormalities, while it was normal in 31 percent of patients; 15 percent had ischemic changes, while only 4 percent with an ascending aortic dissection had evidence of an acute myocardial infarction [12].
Routine blood tests are generally nondiagnostic in aortic dissection, although the lactic dehydrogenase level may be elevated due to hemolysis of blood in the false lumen. It is possible, however, that newer tests might prove to be more useful. A preliminary report evaluated the utility of an immunoassay for the serum concentration of smooth muscle myosin heavy chain in patients suspected of having an aortic dissection [13]. Significant elevations (>10 ng/mL) were noted within the first 24 hours after the onset of the dissection. Elevated levels within the first 12 hours were highly sensitive (90 percent) and specific (97 percent) for dissection. Further data are needed to confirm the utility of this test.
Conventional chest radiographs may show widening of the aorta with ascending aortic dissections. As an example, a review of 464 patients found that mediastinal widening was present in 63 percent with an ascending aortic dissection; 12 percent of patients had no abnormality on chest radiography [12]. In addition, radiographic evidence of a pleural effusion (hemothorax) may, in the absence of heart failure, be another manifestation of aortic dissection.
In the past decade, there has been a dramatic shift from an invasive (aortography) to a noninvasive diagnostic strategy for evaluating suspected thoracic aortic dissection. As an example a review of 464 patients found that most patients had multiple imaging studies performed (mean of 1.83 per patient); however aortography was the initial imaging technique in only four percent of patients while the initial study was computed tomography or echocardiography in 61 and 33 percent, respectively [12]. The radiologic tests can identify the following [14]:
• The presence of aortic dissection
• 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
It is particularly important to 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")
Aortography ！ Retrograde aortography has been the definitive technique for diagnosing aortic dissection. This invasive technique is safe when performed by experienced individuals and has a sensitivity of 88 percent and a specificity of 94 percent [14]. It involves the injection of contrast media into the aorta, thereby permitting the 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 (show radiograph 3).
Coronary angiography as well as evaluation for aortic insufficiency can also performed during the same procedure. 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 [15] or when thrombosis of the false lumen results in lack of opacification with contrast [15,16].
Noninvasive methods ！ Over the past decade computed tomography (CT), magnetic resonance imaging (MRI) and transesophageal echocardiography (TEE) have all been recognized as effective noninvasive imaging techniques for aortic dissection. MRI and multiplane TEE are thought to be superior but, as with any imaging technique, the accuracy is dependent upon the performance and interpretation of the test by skilled individuals.
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 be identified (show figure 3).
The sensitivity and specificity of standard CT for the diagnosis of aortic dissection was, in one study, 94 percent and 87 percent, respectively [17]. Two major 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 [17,18]. In addition, intravenous contrast is required, and there is no capability to assess for aortic insufficiency or to evaluate the coronary arteries.
Advantages of CT are ready availability at most hospitals. even on an emergency basis, and identification of intraluminal thrombus and pericardial effusions. Furthermore, the accuracy of this technique appears to be substantially improved with spiral (helical) CT (show radiograph 4A-4B)
and perhaps with electron beam (ultrafast) CT [19].
(See
"Electron beam (ultrafast) computed tomography for the evaluation of
cardiac disease and function").
MR imaging ！ MRI is currently recognized
as the most 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 (show
radiograph 5).
Additional suggestive findings include widening of the aorta with a
thickened wall and thrombosis of the false lumen [20,21].
A large prospective trial demonstrated that the sensitivity and specificity of
MRI in aortic dissection were both 98 percent, with an 85 percent sensitivity
for identification of the site of entry [21].
The sensitivity for pericardial effusion has been reported to be 100 percent [20].
MRI is safe in adequately monitored patients with aortic dissection, and no
contrast material is needed. Other advantages include the ability to assess
branch vessels and to assess for aortic insufficiency. The main disadvantages
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.
Transthoracic echocardiography ！ Although
transthoracic (surface) echocardiography has become widely recognized as the
cornerstone of noninvasive imaging of the heart, it has limited utility for
evaluation of the thoracic aorta for dissection (show
echocardiogram 1,
show
echocardiogram 2A-2B).
The primary problem is its inability to adequately visualize the distal
ascending, transverse, and descending aorta in a substantial majority of
patients. Furthermore, although an undulating intimal flap may be seen in the
proximal aorta in some patients, the sensitivity and specificity of
transthoracic echocardiography are inferior to those with CT, MRI, and TEE [22,23,24].
This test is therefore most useful in the assessment of cardiac complications
of dissection, including aortic insufficiency, pericardial effusion/tamponade,
and regional left ventricular systolic function.
Transesophageal echocardiography ！ TEE, in
contrast to transthoracic echocardiography, is one of the premier imaging
techniques for dissection of the thoracic aorta. It has the advantages of
close proximity of the esophagus to the thoracic aorta and of 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 room
and yields a diagnosis in under five minutes. (See
"Transesophageal echocardiography: Technology; complications;
indications; and normal views").
The following findings may be seen on TEE in patients with aortic dissection [14,21,25]:
• Intimal dissection flaps can be identified with
high spatial resolution (show
echocardiogram 3
and ). The use of M-mode echocardiography may improve diagnostic
accuracy by demonstrating lack of relation between the intimal flap movement
and the aortic wall [26].
• 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 (show
echocardiogram 5).
• Thrombosis in the false lumen, pericardial
effusion, concomitant aortic valve insufficiency, and the proximal coronary
arteries can be readily visualized.
• Color Doppler permits clear identification of
flow within and between the true and false lumens (show
echocardiogram 6A-6B).
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.
The echocardiographic findings for a nontraumatic aortic intramural hematoma
include a crescentic aortic wall thickening with a thrombus-like echo
appearance and in most, but not all patients, an echo-free space within the
hematoma with slow or absent flow on color flow Doppler [10].
A traumatic hematoma usually shows a circular wall thickening.
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. One study, for example, found a
sensitivity of 98 percent but a specificity of only 77 percent with monoplane
TEE [21].
A large European cooperative study evaluated the efficacy of combined imaging
in 164 patients, one-half of whom had aortic dissection [14].
Combined transthoracic echocardiography and monoplane TEE had a sensitivity of
99 percent and a specificity of 98 percent.
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 [27,28].
In one study of 112 patients, for example, biplane or multiplane TEE was found
to be highly sensitive and specific for the detection of aortic dissection (98
and 95 percent, respectively) and of transmural hematoma (90 and 99 percent,
respectively) [28].
It is currently 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 your institution. Noninvasive MRI and multiplane TEE are the
preferred methods for evaluating suspected aortic dissection, if available [13].
• We generally perform multiplane TEE at the
bedside or in the Emergency Department for patients who present with acute
chest pain and/or for patients who are clinically unstable.
• 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.
• CT scan with contrast is reserved for
situations in which both TEE and MRI are unavailable or contraindicated.
• Aortography is used when a diagnosis of
ascending aortic dissection is strongly suspected, but other 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. At our institution, coronary angiography is
generally attempted in all patients with a prior history or angina or
myocardial infarction, patients older than 60 years of age, and patients with
multiple risk factors for coronary disease. The ACC/AHA Committee on Coronary
Angiography has published recommendations for the use of coronary angiography
in these patients [29].