| Y Joseph Woo, MD Joseph E Bavaria, MD Emile R Mohler III, MD |
Apr 27, 2000 |
A true aneurysm is currently defined as a localized dilatation of the aorta, 50 percent over the normal diameter, which includes all three layers of the vessel, intima, media, and adventitia [1]. Thoracic aortic aneurysms are less common than aneurysms of the abdominal aorta. (See "Clinical features and diagnosis of abdominal aortic aneurysm").
This card will review the classification, etiology, clinical features, and diagnosis of thoracic aortic aneurysm. The management and outcome of this disorder are discussed separately. (See "Management and outcome of thoracic aortic aneurysm").
CLASSIFICATION ！ There are two major types of aneurysm morphology: fusiform, which is uniform in shape with symmetrical dilatation that involves the entire circumference of the aortic wall; and saccular, which is more localized and appears as an outpouching of only a portion of the aortic wall. A pseudoaneurysm or false aneurysm is a collection of blood and connective tissue outside the aortic wall, usually the result of a rupture.
Aneurysms of the thoracic aorta can be classified into three general anatomic categories:
• Ascending aortic aneurysms arise anywhere from the aortic valve to the innominate artery.
• Aortic arch aneurysms include any thoracic aneurysm that involves the brachiocephalic vessels.
• Descending and thoracoabdominal aneurysms arise anywhere distal to the left subclavian artery. Thoracoabdominal aneurysms are further divided according to Crawford classification [2]:
III. Distal descending with abdominal aorta
IV. Primarily abdominal aorta
Beyond distinguishing thoracic aneurysms by anatomic position, the three broad categories further provide guidance for the operative approach to repair the aneurysm, intraoperative circulatory management, and end-organ injury risk stratification.
EPIDEMIOLOGY ！ The incidence of thoracic aortic aneurysm is estimated to be around six cases per 100,000 patient years [3].
• Thoracic aneurysms occur most commonly in the sixth and seventh decade of life.
• Males are affected approximately two to four times more commonly than females.
• Hypertension is an important risk factor, being present in over 60 percent of patients
• Up to 13 percent of patients diagnosed with an aortic aneurysm are found to have multiple aneurysms; approximately 20 to 25 percent of patients with a large thoracic aortic aneurysm also have an abdominal aortic aneurysm [4,5].
• There are specific etiologic genetic defects when thoracic aneurysms occur in association with connective tissue disorders such as the Marfan or Ehlers-Danlos syndromes. (See "The Marfan syndrome").
ETIOLOGY AND PATHOGENESIS ！ There are a number of etiologies for a thoracic aneurysm.
Association with atherosclerosis ！ The vast majority of thoracic aneurysms are associated with atherosclerosis and the risk factors for aneurysm formation are the same as those for atherosclerosis (eg, hypertension, hypercholesterolemia, smoking) [6]. However, it remains unclear whether atherosclerosis is actually responsible for aneurysm formation [6]. It seems likely that there is a multifactorial, systemic, nonatherosclerotic causal process, such as a defect in vascular structural proteins, with atherosclerosis occurring secondarily [7].
Most theories emphasize the primary role of breakdown of the extracellular matrix proteins elastin and collagen by proteases such as collagenase, elastase, various matrix metalloproteinases, and plasmin (formed from plasminogen by urokinase plasminogen activator and tissue type plasminogen activator) [8]. These proteolytic factors are derived from endothelial and smooth muscle cells and from inflammatory cells infiltrating the media and adventitia [8].
The combination of protein degradation and mechanical factors are thought to cause cystic medial necrosis, which has the appearance of smooth muscle cell necrosis and elastic fiber degeneration with cystic spaces in the media filled with mucoid material. These changes result in vessel dilatation and subsequent aneurysm formation and possible rupture.
The following observations in animal models are consistent with the importance of plasmin and metalloproteinases in aortic aneurysm formation:
• Blockade of plasmin formation by overexpression of plasminogen activator inhibitor-1 prevents the formation of aneurysms and rupture by inhibiting metalloproteinase activation [9].
• Aneurysm rupture correlates with an increase in metalloproteinase (gelatinase A and B) levels; local overexpression of tissue inhibitor of matrix metalloproteinases, produced by retrovirally infected smooth muscle cells, can prevent aneurysmal degeneration and rupture (show figure 1) [10].
Dissecting aortic aneurysm ！ Dissecting aortic aneurysms often involve the ascending and thoracic aorta, respectively. (See "Clinical manifestations and diagnosis of aortic dissection").
Genetic factors ！ Familial associations with thoracic aneurysms have led researchers to propose a genetic predisposition to aneurysms formation, although specific gene defects are unknown in atherosclerotic aneurysms [11].
Marfan syndrome ！ The Marfan syndrome is associated with aortic root dilatation due to cystic medial degeneration prior to aneurysm formation. This disorder is due to mutations in the fibrillin gene [12]. (See "The Marfan syndrome").
Aortic root disease, which leads to the formation of aneurysmal dilatation, aortic regurgitation, and dissection, is the main cause of morbidity and mortality in the Marfan syndrome [13]. Dilatation of the aorta is found in 50 percent of children and will progress with time. In one study of 76 patients who had an average age of 30, the greatest progression of aortic dilatation occurred in the aortic root at 0.2 cm/year [14]. Echocardiography demonstrates that 60 to 80 percent of adult patients have dilatation of the aortic root (normal <35 mm), often with aortic regurgitation, that may involve other segments of the thoracic aorta, the abdominal aorta, or even the carotid and intracranial arteries [15].
Untreated Marfan syndrome is frequently associated with aortic dissection which begins just above the coronary ostia and extends the entire length of the aorta; it is a type I dissection in the DeBakey classification or a type A in the Dailey scheme. Approximately 10 percent of dissections begin distal to the left subclavian (type II or type B) but dissection is rarely limited to just the abdominal aorta. (See "Clinical manifestations and diagnosis of aortic dissection").
In a large necropsy series, Marfan syndrome accounted for 7 of 161 (4.3 percent) of aortic dissections [16]. Many patients with Marfan syndrome and aortic dissection have a family history of dissection. Pregnant women are at particular risk for aortic dissection, particularly those who already have aortic root dilatation [17,18]. The risk appears to be low in women with an aortic root diameter less than 40 mm [18].
There appears to be little correlation between the severity of the cardiovascular and the ocular or skeletal manifestations [15]. Furthermore, mutations in FBN1 have been identified in some patients with ascending thoracic aortic aneurysms who do not have Marfan syndrome [19]. (See "The Marfan syndrome").
Although dilated, the aorta in Marfan syndrome tends to be stiffer and less distensible than in controls; these changes, which can be detected by echocardiography or magnetic resonance imaging, increase with age [20,21]. However, there is only variable correlation between these parameters and aortic distension.
Ehlers-Danlos syndrome ！ The Ehlers-Danlos syndrome is a group of conditions due to defects in type III collagen which cause hyperelasticity and fragility of the skin and hypermobility of the joints. Mitral valve prolapse is often present. Although aortic root dilatation is uncommon, spontaneous rupture of large and medium sized arteries, usually without dissection, is the most serious cardiovascular complication.
Inflammatory/infectious disorders ！ A broad grouping of inflammatory and infectious disorders, classified under aortitis, can occasionally cause aortic aneurysm. These diseases include giant cell arteritis, syphilitic aortitis, mycotic aneurysm often due to bacterial endocarditis, giant cell arteritis, Takayasu's disease, rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, reactive arthritis, Wegener's granulomatosis, and Reiter's syndrome. (See appropriate cards)
The pathophysiologic process leading to aortic dilatation and aneurysm formation is thought to be due to intramural inflammation and degeneration either in response to spirochete infection as in syphilitic aortitis or as a manifestation of a systemic autoimmune process.
Thoracic aneurysm formation is a particular problem in patients with giant cell arteritis who are 17 times as likely as other subjects to develop this complication [22]. It is a prognostically important and usually late manifestation of the disease. It can be caused by chronic or late recrudescent aortitis resulting in elastin and collagen disruption. As an example, ascending aortic aneurysms with active aortitis were found in otherwise asymptomatic patients with giant cell arteritis who had been treated for four to eight years prior to aortic surgery [23]. Alternatively, dilatation may develop due to mechanical stress on an aortic wall that was weakened in the early active phase of the disease. As a result, it is recommended that yearly chest x-rays be performed for up to ten years to identify patients with thoracic aortic aneurysms prior to rupture. (See "Treatment of giant cell (temporal) arteritis").
CLINICAL PRESENTATION ！ Patients with thoracic aneurysms are often asymptomatic at the time of presentation [5]. However, depending upon aneurysm location, chest, back, flank, or abdominal pain can be a presenting symptom. Symptoms are usually attributed to compression or distortion of adjacent structures or vessels, a vascular consequence such as aortic regurgitation, or thromboembolic sequelae.
Ascending aneurysms can present with congestive heart failure due to aortic regurgitation from aortic root dilatation and annular distortion. (See "Pathophysiology and clinical features of acute aortic regurgitation"). They can also lead to local compression of a coronary artery, resulting in myocardial ischemia or infarction, while a sinus of Valsalva aneurysm can rupture into the right side of the heart, producing a continuous murmur and, in some cases, congestive heart failure. (See "Auscultation of cardiac murmurs-I").
Ascending and arch aneurysms can erode into the mediastinum. Such patients can present with one or more of the following: hoarseness due to compression of left vagus or left recurrent laryngeal nerve; hemidiaphragmatic paralysis due to compression of the phrenic nerve; wheezing, cough, hemoptysis, dyspnea, or pneumonitis if there is compression of the tracheobronchial tree; dysphagia due to esophageal compression; or the superior vena cava syndrome. Aneurysmal compression of other intrathoracic structures or erosion into adjacent bone may cause chest or back pain.
Aneurysmal compression of branch vessels or the occurrence of embolism to various peripheral arteries due to thrombus within the aneurysm can cause coronary, cerebral, renal, mesenteric, lower extremity and rarely, spinal cord ischemia and resultant symptoms.
The most serious complications of thoracic aortic aneurysm are leakage, which may cause pain, or rupture, most often into the left intrapleural space or intrapericardial space (show radiograph 1).
A descending thoracic aortic aneurysm can rupture into the adjacent esophagus, producing an aortoesophageal fistula and presenting with hematemesis. Rupture is often catastrophic, being associated with severe pain and hypotension or shock.
DIAGNOSIS ！ A variety of noninvasive and invasive methods are useful for the diagnosis and evaluation of a thoracic aortic aneurysm.
Chest x-ray ！ A common way in which asymptomatic aneurysms are detected is on routine chest radiography. The aneurysm produces a widening of the mediastinal silhouette, enlargement of the aortic knob, or displacement of the trachea from midline (show radiograph 2).
However, smaller aneurysms may not be apparent on the chest x -ray.
Echocardiography ！ Transthoracic echocardiography is of limited value in thoracic aortic disease because of nonconductance of the signal by lung air (show echocardiogram 1A-1B).
However, imaging within the mediastinum using transesophageal
echocardiography can be extremely useful, particularly when a coexistent
dissection is suspected. (See
"Clinical manifestations and diagnosis of aortic dissection").
Computed tomography ！ Computed tomography
(CT) with intravenous contrast is an accurate diagnostic tool in the
evaluation of thoracic aneurysmal disease [24].
The aneurysm size, proximal and distal extent of disease, presence of leakage,
and coincident pathology can be evaluated with CT scanning (show
radiograph 3).
(See
"Electron beam (ultrafast) computed tomography for the evaluation of
cardiac disease and function").
Contrast angiography ！ Contrast
angiography is the preferred method for evaluation since it provides sharper
resolution of luminal characteristics, and is the best method for evaluating
branch vessel pathology. This procedure is invasive with potential
nephrotoxicity from contrast medium, and is unable to discern extraluminal
aneurysmal size.
Magnetic resonance angiography ！ Magnetic
resonance angiography is the newest diagnostic modality that offers
noninvasive angiography with multiplanar image reconstruction and
visualization of extraluminal structures [25].
This imaging technique has the disadvantage of limited availability, increased
cost, and lower resolution than traditional contrast angiography. (See
"Clinical utility of cardiovascular magnetic resonance imaging").