Aortic Aneurysm and Aortic Dissection
Matthew C. Bunte
Maran Thamilarasan
I. Introduction
A. Aorta.
The aorta is the principal conductance vessel in the body and is divided into the ascending, arch, descending thoracic, and abdominal components.
1.The ascending aorta includes the aortic root, which contains the sinuses of Valsalva. The left and right coronary arteries arise from the left and right coronary sinuses, respectively.
2.The aortic arch gives rise to the great vessels of the head and arms. These include the brachiocephalic (innominate), the left common carotid, and the left subclavian arteries.
3.The descending thoracic aorta provides the intercostal vessels as it courses through the posterior mediastinum. The vascular supply to the anterior spinal artery is included among these vessels.
4.The abdominal aorta begins just after the aorta crosses the diaphragm. It provides the splanchnic and renal arteries before bifurcating to become the common iliac arteries.
B. Histology.
The aorta comprises three layers: the intima, the media, and the adventitia.
1.The intima is the internal lining layer of the aorta and is easily damaged.
2.The media is the main structural layer of the aorta. It consists primarily of laminar layers of elastic tissue and smooth muscle in varying amounts. This structure allows for the high tensile strength and elasticity required to withstand the pressure changes of each heartbeat throughout the life of the individual.
3.The adventitia is the thin outer layer that anchors the aorta within the body, in addition to providing nourishment to the outer half of the wall through the vasa vasorum.
C. Physiology
1. The elasticity of the aortic wall allows it to distend under the pressure created during ventricular systole. In this way, the kinetic energy that was developed during ventricular systole is stored as potential energy in the distended aortic wall. Then, during ventricular diastole, the potential energy is converted back to kinetic energy by elastic recoil of the wall. Therefore, forward blood flow is maintained throughout the cardiac cycle.
2. The aorta aids in the control of systemic vascular resistance (SVR). Pressure receptors in the ascending aorta and aortic arch signal the vasomotor centers of the brain via the vagus nerve. When blood pressure is elevated, the reflex response is to lower heart rate and decrease SVR. The converse is true when blood pressure is decreased.
D. Acute aortic syndromes including aortic dissection, aortic intramural hema toma (IMH), and penetrating atherosclerotic ulcer (PAU) are life-threatening disorders that require prompt diagnosis and treatment.
E. The 2010 ACC/AHA/AATS/ACR/ASA/SCA/SCAI/SIR/STS/SVM Guidelines for the Diagnosis and Management of Patients with Thoracic Aortic Disease is the principal source of information discussed in this chapter. Where appropriate and unless otherwise indicated, class I guideline recommendations from this docu ment are embedded in italics within this chapter. The level of evidence following the corresponding recommendation is provided in parenthesis.
II. AORTIC DISSECTION
A. The International Registry of Acute Aortic Dissections (IRAD).
Since 1996, the IRAD has built a consortium of 24 referral centers in 12 countries dedicated to the study of acute aortic dissection that now includes over 1,600 patients. This research collaborative has provided considerable insight into and improvement in the management and outcomes of acute aortic dissection. More information about IRAD can be found at www.iradonline.org.
B. Etiology and pathology
1. Aortic dissection classically occurs when a tear in the intima results in separation of the intima from the media (90% of cases), forming a false lumen within the aortic wall. Less commonly, rupture of the vasa vasorum within the aortic wall may result in separation of the intima and media, thereby causing dissection. In either case, acute aortic dissection results from a pathologic weakening of the aortic wall due to medial necrosis, atherosclerosis, or inflammation. There are many risk factors for aortic dissection, although the most common is a history of systemic hypertension as evidenced in over 70% of cases. Younger patients suffering aortic dissection are more likely to have a genetic or morphologic risk factor, such as genetic syndromes associated with aortopathy, bicuspid aortic valve (BAV), or prior aortic surgery. The following list includes the most common conditions associated with aortic dissection:
a. Increased age and uncontrolled hypertension are the two most common risk factors.
b. Tobacco use, dyslipidemia, and cocaine use are important risk factors.
c. Genetic diseases, especially Marfan, Loeys-Dietz, and vascular-type Ehlers-Danlos syndromes, are associated with aortic aneurysm and dissection.
d. Familial thoracic aortic aneurysm (TAA) and dissection syndrome and other congenital anomalies associated with aortic aneurysm and dissection, includ ing BAV and Turner syndrome.
e. Inflammatory vasculitides, including Takayasu arteritis, Giant cell arteritis, and Behçet arteritis.
f. Infections involving the periaortic tissue, as seen in prosthetic aortic valve endocarditis.
g. Aortic trauma, particularly with deceleration and torsional injuries, although may occur with direct endoluminal trauma during arterial catheterization or with cardiothoracic surgery.
h. Pregnancy.
2. Genetic syndromes associated with aortic aneurysm and dissection. Marfan, Ehlers-Danlos, and Loeys-Dietz syndromes are associated with an increased risk of aortic dissection. These patients require comprehensive aortic imaging at diagnosis and heightened surveillance to follow aortic diameter owing to the increased risk of complications related to aortic disease. Aortic imaging is recommended for first-degree relatives with TAA and/or dissection to identify those with asymptomatic disease (Level of Evidence: B).
a. Marfan syndrome. Marfan syndrome is a genetic disorder with high pen etrance and variable expression affecting connective tissue. Marfan syndrome is associated with mutations of the FBN1 gene, which encodes fibrillin-1, a large glycoprotein that contributes to the structure of the extracellular matrix and serves as a regulator of transforming growth factor-beta (TGF-β). The principal features of Marfan syndrome involve the cardiovascular, ocu lar, and skeletal systems, with patients at exceedingly high risk for aortic disease. In fact, nearly all patients with Marfan syndrome demonstrate some form of aortic disease during their lifetime.
b. Loeys-Dietz syndrome. An autosomal dominant disorder associated with a triad of arterial tortuosity and aneurysm, hypertelorism, and bifid uvula, Loeys-Dietz syndrome results from mutations in either TGF-β receptor type 1 or 2 (TGFBR1 or TGFBR2). Vascular disease among these patients is highly prevalent, with 98% demonstrating aortic root aneurysms, and por tends a grim prognosis. Early reports of Loeys-Dietz syndrome suggested a particularly aggressive disease process with arterial complications occur ring at a mean age of 26 years. However, subsequent data have revealed less aggressive phenotypes with later presentations, and a mean age of death closer to the fifth decade among less severe phenotypes. Repair of the aortic root is recommended at lesser aorta diameters (< 5.0 cm) due to the aggres sive nature of this condition.
c. Ehlers-Danlos syndrome, type IV (vascular form). The vascular form of Ehlers-Danlos syndrome is characterized by an autosomal dominant inheritance of the COL3A1 gene mutation that encodes type III procollagen. Clinical features include easy bruising and rupture of the uterus, intestines, and arteries. Median survival is 48 years and often no aneurysms are docu mented. Gravid women with this condition have a particularly poor progno sis during childbirth due to the high risk of arterial and uterine rupture.
3. Hereditary conditions and congenital anomalies such as BAV and coarctation of the aorta are also established risk factors for aortic dissection. Turner syndrome is associated with BAV (10% to 25%), aortic coarctation (8%), and dilatation of the ascending aorta. Although patients with Turner syndrome require screening for aortic disease at diagnosis, requirements of surveillance for aortic dilatation follow those of other patients with BAV. All patients with BAV should have both the aortic root and ascending aorta evaluated for evidence of aortic dilatation (Level of Evidence: B). First-degree relatives of patients with a BAV, premature onset of thoracic aortic disease with minimal risk factors, and/or a familial form of TAA or dissection should be evaluated for the presence of a BAV and asymptomatic thoracic aortic disease (Level of Evidence: C).
4. Vasculitides associated with large vessel inflammation and aortitis contribute to medial degeneration of the aortic wall and may increase the risk of aortic dissection. Examples of these inflammatory disorders include giant cell arteritis, Takayasu arteritis, syphilis, and Behçet disease.
5. Aortic dissection exhibits a strong association with pregnancy. Among cases of aortic dissection in women < 40 years of age, up to half may present during the third trimester or early in the postpartum period. Gravid women with Marfan syndrome and preexisting aortic root dilatation are at especially high risk for aortic dissection.
6. Direct aortic trauma is associated with aortic dissection. Blunt chest trauma, such as that occurring in a motor vehicle accident, may cause aortic transection or mural hematoma. Intravascular instrumentation as during arterial catheterization, insertion of an intraaortic balloon pump, or aortic cannulation, cross-clamping, and graft insertion may also serve as a source of intimal damage and dissection.
C. Epidemiology.
The incidence of aortic dissection has been estimated from 2 to 3.5 cases per 10,000 person-years, corresponding to 6,000 to 10,000 cases per year in the United States. The male-to-female ratio approaches 3:1, with the peak inci dence in the sixth and seventh decades of life. The mortality for untreated acute aortic dissection is largely determined by the location of the dissection, but overall mortality is approximately 1% per hour within the first 48 hours if surgery is not performed. Approximately 65% of dissections originate in the ascending aorta (just above the right or noncoronary sinus), 20% in the descending thoracic aorta, 10% in the aortic arch, and the remainder in the abdominal aorta.
D. Classification schemes
1. Anatomic classification schemes used to commonly describe aortic dissection include the DeBakey and Stanford systems (see Table 26.1 and Fig. 26.1 for a description of the DeBakey and Stanford classifications). Anatomic classification refers to the portion(s) of aorta involved. The Stanford classification will be used throughout this chapter.
2. Dissections are further classified according to chronicity: acute (> 2 weeks from onset) or chronic (> 2 weeks from onset).
3. Anatomic involvement and chronicity of dissection influence the recommended treatment approach and indicate prognosis.
a. Type A dissection. Predictors of death are age 70 years or older, abnormal electrocardiogram (ECG), pulse deficit, acute renal failure, and the compos ite of hypotension, shock, or tamponade.
b. Type B dissection. Predictors of death are branch vessel involvement, absence of chest or back pain, and hypotension/shock. Continued patency of the false lumen predicts a worse outcome in type B aortic dissection. The highest sur vival benefit is among those with complete thrombosis of the false lumen.
TABLE 26.1 Aortic Dissection Classification Systems | ||||||||||||||
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 FIGURE 26.1 Anatomic appearance of three different aortic dissection classifications. |
E. Atypical variants of aortic dissection
1. IMH represents a focal hemorrhage of the aortic wall caused by rupture of the vasa vasorum within the aortic wall and may cause secondary dissection. The natural history of IMH is similar to that of classic aortic dissection. In fact, in 4% to 10% of dissections, an intimal tear is not found. Therefore, it is reasonable to treat IMH similar to de facto aortic dissection including surgery if located in the ascending aorta or aggressive medical therapy if in the descending aorta (Level of Evidence: C).
2. PAU is a focal defect in the endoluminal surface of the aortic wall produced by atherosclerotic erosion through the intima with ulceration into the media. Risk factors for PAU include older age, extensive atherosclerosis, and uncontrolled hypertension. PAU may manifest as subtle asymptomatic erosions noted incidentally on radiography to symptomatic IMH with eccentric or saccular aneurysms of the aorta. In either case, PAU may progress to aortic dissection or aortic perforation, although the natural history of this condition remains uncertain. Nevertheless, surgery is often recommended for patients exhibiting unstable symptoms or lesions involving the ascending aorta. Otherwise, medical management and frequent radiologic follow-up for signs of progression is recommended.
F. Clinical presentation
1. Signs and symptoms
a. Aortic dissection may present with a wide range of clinical manifestations. Proximal dissections are most commonly characterized by a sudden onset of chest pain (80%) that is severe in intensity and ripping, tearing, stabbing, or sharp in quality. Pain may radiate to the interscapular region of the back (47%) or abdomen (21%). Among descending aortic dissections, back pain (64%), chest pain (63%), and abdominal pain (43%) are most common. Typical symptoms are less common in the elderly.
b. Presenting clinical findings may include the murmur of severe aortic insufficiency (AI) (45%) associated with proximal aortic dissection and contributing to acute heart failure (5% to 6%), hypotension (14%) or shock (13%) associated with cardiac tamponade (5% to 10%), syncope (13%), myocardial infarction (MI) (7% to 19%) with retrograde dissection into the ostia of the coronary arteries, cardiovascular accident (CVA) (8%) with cephalad carotid extension, paraplegia (2%) with extension into the intercostal and spinal arteries, or cardiac arrest. Dissections involving the arterial supply to the limbs may contribute to pulse deficits (26%), acute limb ischemia (10%) with distal extension, and neuropathy.
G. Diagnostic testing
1. Evaluation. Figure 26.2 provides an algorithm to aid in diagnosis. Key characteristics important in defining the extent of aortic dissection and clinical management include ascending versus descending aortic involvement, site of the intimal tear, presence or absence of AI, presence of pericardial effusion and/or tamponade, coronary involvement, and involvement of visceral arterial supply. Computed tomography (CT), magnetic resonance imaging (MRI), transesophageal echocardiography (TEE), and invasive aortography are common imaging modalities useful in the diagnosis of acute aortic dissection. The relative advantages and disadvantages of the four modalities are outlined in Table 26.2. Selection of the specific imaging modality for identification or exclusion of aortic dissection should be based on clinical variables, local expertise, and clinical availability to facilitate rapid diagnosis (Level of Evidence: C).
2. An ECG should be performed in all patients with suspected aortic dissection (Level of Evidence: B). Most frequently, ECG is useful to exclude an acute coronary syndrome presenting atypically as symptoms of aortic dissection. Because dissectionrelated acute MI is infrequent, ST-segment elevation on ECG should be treated as an independent coronary event without delay for aortic imaging unless the patient is at high risk for aortic dissection (Level of Evidence: B).
 FIGURE 26.2 Aortic dissection diagnostic/therapeutic algorithm. CCU, coronary care unit; CT, computed tomography; MRI, magnetic resonance imaging; TEE, transesophageal echocardiography; TTE, transthoracic echocardiography. |
TABLE 26.2 Comparison of Imaging Modalities in Aortic Dissection | ||||||||||||||||||||||||||||||||||||||||||||||||||
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3. Chest radiography may occasionally detect findings suggestive of dissection, although it is inadequately sensitive to definitively exclude the presence of acute aortic dissection. A negative chest x-ray should not delay definitive aortic imaging in patients determined to be high risk for aortic dissection by initial screening (class III recommendation, Level of Evidence: C).
H. Selected imaging modalities for diagnosis of acute aortic dissection
1. Computed tomography.
Contrast-enhanced, cardiac-gated multidetector CT is a widely available and the most commonly used imaging modality for the detection of aortic dissection, with excellent sensitivity and specificity approaching 100%. This modality has many advantages, including rapid scan and interpretation times. Disadvantages include iodinated contrast and radiation exposure.
2. MRI and magnetic resonance angiography (MRA).
Like CT, MRI provides multiplanar imaging of the thoracic aorta with high sensitivity and specificity that is very accurate for diagnosis of acute aortic disease. MRA offers unique gadoliniumenhanced and black blood imaging techniques to evaluate aortic anatomy and morphology that prove particularly useful in assessing the aortic wall. Advantages of MRI include the ability to identify anatomic variants, such as IMH or penetrating aortic ulcer, assess branch arterial involvement, and provide useful information on aortic valvular and left ventricular systolic function while avoiding exposure to iodinated contrast or radiation. MRI is well suited for chronic follow-up of aortic syndromes since ionizing radiation is not necessary. Use of MRI is limited by availability, prolonged acquisition time, and incompatibility with implanted ferromagnetic devices. MRI is not an appropriate test for patients that are hemodynamically unstable.
3. Transthoracic echocardiography (TTE) and TEE.
TTE allows for a rapid noninvasive evaluation, primarily of the proximal aorta with overall limited sensitivity and specificity. Visualization of the proximal aorta and other critical structures using TTE may be limited by factors that reduce image quality, such as emphysema, mechanical ventilation, and obesity. With an esophageal approach, TEE overcomes many of the challenges with improved sensitivity and specificity while offering a safe and rapid assessment of acute aortic disease. A major limitation of either TTE or TEE includes the appearance of ultrasound artifacts that may mimic a dissection flap, such as that of reverberation artifact.
4. Invasive aortography.
Aortography offers accurate information about the location of dissection, providing visualization of the false lumen or intimal flap, branch vessel involvement, and communication between true and false lumens. Invasive aortography is useful in evaluating PAU, as it is characterized by endovascular aortic contrast protruding into an atherosclerotic plaque. False negatives can occur with thrombosis of the false lumen, IMH, or equal filling of the false lumen. Disadvantages or aortography include a low sensitivity, risks associated with any invasive procedure, contrast administration, and availability of experienced operators to perform the study.
5. Recommendations for aortic imaging techniques to determine the presence and progression of thoracic aortic disease
a. Measurements of aortic diameter should be taken at reproducible anatomic landmarks, perpendicular to the axis of blood flow, and reported in a clear and consistent format (Level of Evidence: C).
b. For measurements taken by CT imaging or MRI, the external diameter should be measured perpendicular to the axis of blood flow. For aortic root measurements, the widest diameter, typically at the mid-sinus level, should be used (Level of Evidence: C).
c. For measurements taken by echocardiography, the internal diameter should be measured perpendicular to the axis of blood flow. For aortic root measurements, the widest diameter, typically at the mid-sinus level, should be used (Level of Evidence: C).
d. Abnormalities of aortic morphology should be recognized and reported separately even when aortic diameters are within normal limits (Level of Evidence: C).
e. The finding of aortic dissection, aneurysm, traumatic injury and/or aortic rupture should be immediately communicated to the referring physician (Level of Evidence: C).
f. Techniques to minimize episodic and cumulative radiation exposure should be utilized whenever possible (Level of Evidence: B).
I. Therapy.
Death in aortic dissection results from vascular compromise, tamponade, or aortic rupture. Management of proximal (type A) thoracic aortic dissection requires immediate surgical treatment to resect the entire aneurysmal aortic segment and the proximal extent of dissection (Level of Evidence: C). Surgery greatly improves outcomes and avoids the risks associated with progression of dissection. One- and three-year survival after surgery for type A dissection is excellent, with survival rates of 96.1% and 90.5%, respectively. Survival rates at 1 and 3 years are 88.6% and 68.7%, respectively, among those who do not receive surgery for type A dissection. Patients with distal (type B) thoracic and abdominal aortic dissections should be managed medically unless life-threatening complications, such as malperfusion syndromes, progression of dissection, aortic enlargement, or refractory hypertension, develop (Level of Evidence: B). Percutaneous endovascular aortic repair (EVAR)
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