You are in: eMedicine Specialties > Radiology > VASCULAR/INTERVENTIONAL Aorta, DissectionArticle Last Updated: Sep 5, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 13
  Author: Jackeline Gomez-Jorge, MD, Assistant Professor of Clinical Radiology, Department of Radiology, Vascular and Interventional Section, University of Miami/Jackson Memorial Medical Center Jackeline Gomez-Jorge is a member of the following medical societies: Alpha Omega Alpha, American College of Radiology, American Heart Association, and American Medical Association Editors: Glenn Krinsky, MD, Chief of Abdominal Imaging Section, Associate Professor, Department of Radiology, New York University School of Medicine; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; John D Newell, Jr, MD, FACR, FCCP, FASER, Co-Director of Thoracic Imaging, UCDHSC; Director of Lung Imaging Center, Professor of Radiology and Professor of Medicine, Department of Radiology, University of Colorado Health Sciences Center, National Jewish Medical and Research Center; Univ. Colorado Hospital; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Kyung J Cho, MD, FACR, William Martel Professor of Radiology, Interventional Radiology Fellowship Director, University of Michigan Health System Author and Editor Disclosure Synonyms and related keywords: aortic dissection, dissection of aorta, aortic intimal tear, pseudoaneurysm, aortic intramural hematoma, traumatic aortic dissection, AIH, aortic aneurysm INTRODUCTIONSection 2 of 13
  BackgroundClassic aortic dissection is a longitudinal split or partition in the media of the aorta. An intimal tear connects the media with the aortic lumen, and an exit tear creates a true and a false lumen. The smaller true lumen is lined by intima, and the false lumen is lined by media. Typically, flow in the false lumen is slower than in the true lumen, and the false lumen often becomes aneurysmal when subjected to systemic pressure. An acute aortic dissection is considered chronic at 2 weeks. The dissection usually stops at an aortic branch vessel or at the level of an atherosclerotic plaque.1, 2 Most classic aortic dissections begin at 3 distinct anatomic locations: the aortic root; 2 cm above the aortic root; and just distal to the left subclavian artery. Ascending aortic involvement may result in death from wall rupture, hemopericardium and tamponade, occlusion of the coronary ostia with myocardial infarction, or severe aortic insufficiency. Aortic intramural hematoma (AIH) is a more recently described entity in which no intimal flap is present. It results in a spontaneous medial hematoma that may be secondary to an infarction of the vasa vasorum of the adventitia. Aortic intramural hematoma accounts for approximately 25% of aortic dissections. Involvement of the ascending aorta, especially if the overall aortic diameter is greater than 5 cm, should be treated surgically to prevent rupture or progression to a classic dissection with intimal tear. Conservative management is indicated for AIH of the descending aorta.3, 4, 5, 6, 7, 8 PathophysiologyThe pathogenesis of aortic dissection is not well understood, but the following are associated risk factors9, 10:
Mortality/MorbidityThe mortality rate increases when diagnosis is delayed; this is especially the case for ascending aortic dissections. The mortality rate is 1% per hour for the first 48 hours. However, patients who survive long enough to be hospitalized and who are without significant comorbidities usually survive. Occlusion of aortic branch vessels from aortic dissection may result in stroke, renal failure, mesenteric ischemia, lower-extremity ischemia, and paraplegia (caused by obstruction of the spinal artery). Interestingly, aortic intramural hematoma is rarely associated with significant narrowing of aortic branch vessels. The incidence is approximately 2000 cases per year, but lack of reporting and the small number of autopsies performed lead to gross underestimations of the true incidence. Ischemic heart disease is roughly 1000 times more common than aortic dissection in patients who present to the ED with chest pain. However, once the diagnosis is suspected, it must be confirmed or refuted with an imaging study. RaceAortic dissection is more common in African-Americans, followed by whites. Untreated hypertension may account for the greater prevalence in African-Americans. Aortic dissection is least common in Asians. SexAortic dissections are more common in men than in women (ratio, 3:1) and in patients aged 35-85 years; the peak incidence occurs in those aged 50-65 years. Aortic intramural hematoma (AIH) usually occurs in slightly older patients. Clinical Details
Dissections of the aorta can be classified into types (see Image 1).
It is noteworthy that isolated dissections that begin in the aortic arch but do not involve the ascending aorta do not fit neatly into these classifications. AIH is classified in a similar fashion. Preferred ExaminationPreferred examinations for aortic dissection include contrast-enhanced spiral CT transesophageal echocardiography (TEE) in the emergency setting and MRI for hemodynamically stable patients. TEE has an advantage over CT and MRI in its ability to evaluate the status of the aortic valve and the ostia of the coronary arteries. CT and MR angiography have largely replaced conventional diagnostic angiography in the assessment of aortic dissection.11, 12, 13, 14 Several factors determine the best modality for the initial evaluation and postoperative follow-up. These factors include the following: stability of the patient's condition, the patient's renal function, suspected postoperative complication, and the availability of each imaging modality. Maffei et al performed a randomized, controlled trial in which 44 patients (252 evaluations) were examined with TEE and CT.15 The authors concluded that both TEE and CT are atraumatic, safe, and accurate techniques for serial follow-up studies of patients treated for aortic dissection. Limitations of TechniquesThree noninvasive studies are associated with high specificity and sensitivity for aortic dissection. CT and MRI are associated with a sensitivity and a specificity of 94-100% and 95-100%, respectively. Transesophageal echocardiography (TEE) is less sensitive and specific than spiral CT or MR, and TEE is operator-dependent. In addition, because of tracheal interposition, there is a 2 cm "blind spot" for TEE just proximal to the innominate arteries. Also, TEE is contraindicated in approximately 1% of patients (eg, TEE is contraindicated in patients with esophageal varices). DIFFERENTIALSSection 3 of 13
Aortic Regurgitation Aortic Stenosis Myocardial Infarct, Acute Other Problems To Be ConsideredBack pain, mechanical RADIOGRAPHSection 4 of 13
FindingsFindings of aortic dissection on plain film images include the following:
All findings on plain images are nonspecific but may help in determining the need for further workup. False Positives/NegativesMediastinal fat commonly causes a widening of the mediastinum, which may lead to a false-positive diagnosis of aortic dissection. CT SCANSection 5 of 13
FindingsSince its introduction in the 1970s, CT has become a widely used technology, particularly in the ED. With the advent of spiral CT, studies may be performed in less time than before, with less patient discomfort, greater accuracy, and lower iodine load. Spiral CT permits patient translation and data acquisition simultaneously. A major advantage of this technology is in the evaluation of thoracic trauma, which enables the rapid diagnosis of thoracic injury. Multislice or multidetector CT may be used for faster imaging or to acquire thinner slices that may be reconstructed in multiple planes.16, 17, 18, 19, 20 A typical helical scanning protocol for aortic dissection includes the following parameters: 5-mm collimation, 1.5 pitch, and 7.5-mm imaging spacing. Multidetector CT may be performed with 1-2.5 mm collimation. Initial nonenhanced CT is used for the diagnosis of acute hemorrhage and aortic rupture. This is followed by helical CT performed approximately 25-30 seconds after the injection of contrast material. Nonionic contrast material (120-135 mL) is power injected via a peripheral intravenous site at a rate of 3-4 mL/s. Because cardiac output is quite variable in these sick patients, a test injection of contrast should be used to determine circulation time; alternatively, an automated bolus detection scheme may be used. One advantage of the test injection method is that one may visually differentiate the true and false lumen on the basis of contrast arrival time. Usually, scanning is performed from the thoracic inlet to the common femoral arteries. When a dissection is identified, repeat scanning may be performed to obtain delayed images of the false lumen and aortic branches. Multiplanar reformation images are obtained in sagittal, coronal, oblique sagittal, and curved projections generated with an independent workstation. The use of volume rendering may be helpful for planning surgery. Typical CT findings in acute dissection or intramural hematoma include the following:
Hemorrhagic pleural and pericardial effusions and mediastinal hemorrhage may be seen. CT is helpful in postoperative follow-up. It may accurately depict associated complications, including the following:
Degree of ConfidenceThe sensitivity of CT for aortic dissection is 87-94%, and the specificity is 92-100%. False Positives/NegativesInadequate contrast opacification may lead to false-negative findings of aortic dissection. Aortic intramural hematoma may be misinterpreted as an aneurysm with thrombus or arteritis. Spiral CT artifacts include perivenous streaks and motion artifacts. The perivenous streaks are caused by beam hardening and motion resulting from transmitted pulsation in a vein that carries undiluted contrast medium into the heart. Some authors recommend injecting the contrast agent at a rate of 2 mL/s via a peripheral intravenous site in the right arm. Aortic motion artifact is produced by the aortic wall motion from the end of diastole to the end of systole. Typically, this artifact is seen in the left anterior and right posterior margins of the aortic circumference. In some patients, especially those with cystic medial necrosis, the intimal flap may be subtle. MRISection 6 of 13
FindingsMRI is an accurate tool for use in diagnosis of aortic dissection, but it may not be readily available in the acute setting. In addition, unstable patients with Swan-Ganz catheters should not undergo MRI.21, 22, 23, 24, 25, 26, 14 MRI findings of aortic dissection include the following:
Newer pulse sequences such as true fisp or Fiesta offer very fast cine imaging. Basic MRI sequences for evaluating aortic dissection include spin-echo T1-weighted or breath-hold double inversion recovery sequences, cardiac-gated gradient-echo sequences, and three-dimensional (3D) thin-section MR angiography with a bolus injection of a single or double dose of gadolinium-based contrast agent MRI findings of AIH include a crescent of blood surrounding but not compressing the aorta. The signal intensity of the crescent varies with age on T1-weighted imaging: it is isointense to muscle in the acute setting and is markedly hyperintense after 3-7 days. MRI is helpful in postoperative follow-up. It may accurately depict associated complications, including the following:
Degree of ConfidenceThe sensitivity and specificity of MRI for aortic dissection are both more than 90%. False Positives/NegativesPotential drawbacks of MRI include reported artifacts on cardiac-triggered thoracic spin-echo phase images. These may appear as an artifactual borderlike feature across the aorta (caused by helical flow in the aorta) that may be interpreted as a dissection. Other potential causes of misinterpretation include an atypical configuration of the intimal flap seen in short dissections and multiple false channels in cases in which the flaps are complex (see Images 7-8). Aortic anomalies also may cause confusion. False-positive findings seen in gadolinium-enhanced MRA include a central line or "maki" artifact. This occurs when the acquisition is performed too early as intra-aortic gadolinium concentration is rising. This artifact may be readily differentiated from an aortic dissection because it does not take a spiral course like a true intimal flap. Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. ULTRASOUNDSection 7 of 13
FindingsECG is helpful in the diagnosis of aortic dissections. It is particularly helpful in cases of ascending thoracic dissections, cardiac tamponade, and aortic regurgitation; transesophageal echocardiography (TEE) has a greater sensitivity and specificity than CT or MRI in detecting coronary arterial occlusion, aortic insufficiency, and cardiac tamponade. The sensitivity is 97-99%. The specificity is in the range of 97-100%.27, 28 According to a study by Mastrogiovanni, TEE was the favored study for the evaluation of aortic dissection.29 In their report of 54 patients, TEE findings confirmed the diagnostic dissection in all patients but one. TEE noted the site of the intimal tear; the extension of the dissection, pericardial effusion, aortic incompetence; and left ventricular function. Because of the high level of correspondence between the diagnosis made at TEE and the surgical anatomic findings, the authors favored the use of TEE—for many cases, as the sole diagnostic modality. False Positives/NegativesA tortuous aorta may result in a false-positive diagnosis of dissection. In cases involving a massive dilated ascending aorta (usually occurring as a result of cystic medial necrosis), it may be difficult to identify a small intimal flap. NUCLEAR MEDICINESection 8 of 13
FindingsNuclear medicine has no role in the diagnostic evaluation of acute aortic dissection. In a stable patient with chronic dissection and severe renal insufficiency, a renal scan may demonstrate the presence or absence of renal perfusion (usually of the left kidney). ANGIOGRAPHYSection 9 of 13
FindingsAortography was the reference standard for the preoperative evaluation and diagnosis of aortic dissection. With the advent of transesophageal echocardiography (TEE), CT, and MRI, its role has become important only if nonsurgical interventional procedures are indicated. It is quite controversial whether coronary angiography should be performed before sternotomy in a stable patient with aortic dissection, inasmuch as concomitant coronary bypass grafting may be performed if diseased vessels are present. Diagnostic criteria include visualization of a lucent flap and delayed filling and washout of the false lumen. The expanding false lumen may compress the true lumen and cause it to become narrowed. A dual lumen aorta is noted when both the true and false lumens are opacified (see Image 9). Aortic intramural hematoma (AIH) is almost impossible to diagnose with aortography because no compression of the lumen exists. During aortography, overinjection of the false lumen should be avoided if it is entered during the procedure. The operator should be suspicious if he or she has difficulty advancing the guidewire into the aortic valve. Abdominal and pelvic aortography should be included in the diagnostic study to assess the level of the reentry site. Obstruction of the aortic branches may be noted (most commonly in the left renal artery, which is the site in approximately 25-30% of patients). Visceral and extremity ischemia may occur when the superior or inferior mesenteric arteries and the iliac arteries are compromised. Degree of ConfidenceAortographic findings are less sensitive than those of newer noninvasive techniques, especially for aortic intramural hematoma. False Positives/NegativesPitfalls of angiography include a lack of visualization of the false lumen because of thrombosis or inadequate opacification with contrast material. Streak artifacts secondary to aortic or cardiac motion or opacification of the sinus of Valsalva may be confused with thrombus. Pitfalls also include missing the diagnosis of an intramural hematoma (frequently associated with progression to frank dissection) and misdiagnosis when the false lumen is thrombosed. INTERVENTIONSection 10 of 13
Aortic fenestration is an alternative in treating the complications of aortic dissection, including renal and/or mesenteric ischemia in type B dissections. Surgical repair of type A or B dissection usually redirects blood to the true lumen and cures branch vessel compromise. However, because operative mortality is 35% in acute type B dissection, interventional techniques are a viable option. The mortality rate of patients with aortic dissection and renal or mesenteric ischemia is extremely high: 50-70% for patients with renal ischemia and up to 87% for patients with mesenteric ischemia. Aortic fenestration, with or without stenting of an underperfused vessel, appears to have a role in the treatment of static or dynamic obstruction of aortic branches.30, 31, 32, 33, 34, 35, 36 The goal of treatment is to create a tear in the dissection flap that separates the true and the false lumens. This procedure allows local blood flow across the flap and reperfuses the organs supplied by the true lumen or those that are underperfused. The following steps are required:
Some centers have begun treating patients who have type B dissections by placing a covered stent-graft across the entry tear. This approach yields the same physiologic results as surgical repair (ie, it redirects blood flow to the true lumen). However, the long-term results of this technique are not known. Often, multiple reentry tears are present; these may keep the false lumen patent and result in aneurysmal dilatation.8 Medical/Legal Pitfalls
FURTHER READINGSection 11 of 13
Aortic aneurysm and dissection. MULTIMEDIASection 12 of 13
REFERENCESSection 13 of 13
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