Disclosure
Background: Classic 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. 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. Pathophysiology: The pathogenesis of the disease is not well understood but the following are associated risk factors:
Mortality/Morbidity: The mortality rate increases with a delay in diagnosis, especially 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 without significant comorbid disease 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 grossly underestimate 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 raised it must be confirmed or refuted with an imaging study. Race: Aortic dissection is more common in African-Americans, followed by whites. Untreated hypertension may be the etiology for the greater prevalence in this population. It is least common in Asians. Sex: Aortic dissections are more common in men than in women (ratio, 3:1) and in patients aged 35-85 years, with a peak in those aged 50-65 years. AIH usually occurs in a slightly older patient population Clinical Details:
Dissections of the aorta can be classified into types (see Image 1).
Note 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 Examination: Preferred examinations include contrast-enhanced spiral CT transesophageal echocardiography (TEE) in the emergency setting and MRI for hemodynamically stable patients. The ability of TEE to evaluate the status of the aortic valve and the ostia of the coronary arteries is an advantage over CT and MRI. CT and MR angiography has largely replaced conventional diagnostic angiography in the assessment of aortic dissection. Several factors determine the best modality for the initial evaluation as well as postoperative follow-up. These factors include the following: stability of the patient's condition, patient's renal function, suspected postoperative complication, and availability of each imaging modality. In another study, Maffei et al in 1996 performed a randomized controlled trial in which 44 patients (252 evaluations) were examined with TEE and CT. The authors concluded that both TEE and CT are atraumatic, safe, and accurate techniques for serial follow-up studies in patients treated for aortic dissection. Limitations of Techniques: Three noninvasive studies are associated with high specificity and sensitivity. CT and MRI are associated with high sensitivity and specificity of 94-100% and 95-100%, respectively. TEE is less sensitive and specific than spiral CT or MR, and TEE is operator-dependent. In addition, due to tracheal interposition, there is a 2 cm "blind spot" for TEE just proximal to the innominate arteries. Also, approximately 1% of patients have a contraindication to TEE (eg, esophageal varices).
Aortic Regurgitation
Back pain, mechanical
Findings: Findings 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/Negatives: Mediastinal fat can commonly cause a widened mediastinum and a false-positive diagnosis of aortic dissection |
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Findings: Since its introduction in the 1970s, CT has become a widely used technology, particularly in the ED. With the advent of spiral CT, the studies can 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 can be used for faster imaging or to acquire thinner slices that can be reconstructed in multiple planes 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 can 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, use a test injection of contrast to determine circulation time or an automated bolus detection scheme. One advantage of the test injection method is that one may visually differentiate the true and false lumen based on contrast arrival time. Usually, scanning is performed from the thoracic inlet to the common femoral arteries. When a dissection is identified, repeat scanning can 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 can 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 also helpful in postoperative follow-up. It can accurately depict associated complications, including the following:
Degree of Confidence: The sensitivity of CT is 87-94%, and the specificity is 92-100%. False Positives/Negatives: Inadequate contrast opacification can lead to false-negative findings. Aortic intramural hematoma can 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 due to 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.
Findings: MRI is an accurate tool for use in diagnosis but it may not be readily available in the acute setting. In addition, unstable patients with Swan-Ganz catheters should not be studied with MR. 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 to evaluate for 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 markedly hyperintense after 3-7 days. MRI is also helpful in postoperative follow-up. It can accurately depict associated complications, including the following:
Degree of Confidence: The sensitivity and specificity are both more than 90%. False Positives/Negatives: Potential drawbacks of MRI include reported artifacts on cardiac-triggered thoracic spin-echo phase images. These can appear as an artifactual borderlike feature across the aorta (due to helical flow in the aorta) that can 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 where the flaps are complex (see Images 7-8). Aortic anomalies also can cause confusion. False positives seen in gadolinium-enhanced MRA include a central line or "maki" artifact. This occurs when the acquisition is performed too early as intraortic gadolinium concentration is rising. This artifact can be readily differentiated from an aortic dissection as 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 recently 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. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.
Findings: ECG is helpful in the diagnosis of aortic dissections. It is particularly helpful with ascending thoracic dissections, cardiac tamponade, and aortic regurgitation where TEE has a greater sensitivity and specificity than CT or MR in detecting coronary arterial occlusion, aortic insufficiency, and cardiac tamponade. The sensitivity is 97-99%. The specificity is in the range of 97-100%. TEE was the favored study for the evaluation of aortic dissection according to a study by Mastrogiovanni et al from Salerno, Italy. In their report of 54 patients, TEE findings confirmed the diagnostic dissection in all patients but one. The site of the intimal tear; the extension of the dissection, pericardial effusion, aortic incompetence; and left ventricular function was noted. Because of the high level of correspondence between the diagnosis made at TEE and the surgical anatomic findings, the authors favor the use of TEE, often as the sole diagnostic modality. False Positives/Negatives: A tortuous aorta may result in a false-positive diagnosis of dissection. In massive dilated ascending aortas (usually due to cystic medial necrosis) it may be difficult to identify a small intimal flap.
Findings: Nuclear 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 the left kidney).
Findings: Aortography was the reference standard for the preoperative evaluation and diagnosis of aortic dissection. With the advent of 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 prior to sternotomy in a stable patient with aortic dissection, as concomitant coronary bypass grafting can 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). AIH is almost impossible to diagnose with aortography as 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 in approximately 25-30% of patients). Visceral and extremity ischemia can occur when the superior or inferior mesenteric arteries and the iliac arteries are compromised. Degree of Confidence: Aortographic findings are less sensitive than those of newer noninvasive techniques, especially for aortic intramural hematoma. False Positives/Negatives: Pitfalls 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 can 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.
Intervention: 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% in renal ischemia and as high as 87% in mesenteric ischemia. Aortic fenestration, with or without stenting an underperfused vessel, appears to have a role in the treatment of static or dynamic obstruction of aortic branches. 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:
Recently, some centers treat patients with type B dissections by placing a covered stent-graft across the entry tear. This results in the same physiology as surgical repair (ie, it redirects blood flow to the true lumen). However, the long-term results of this technique are not known as often multiple reentry tears are present, which may keep the false lumen patent and result in aneurysmal dilatation. Medical/Legal Pitfalls:
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