Background
Aneurysmal degeneration can occur anywhere in the human aorta. By definition, an aneurysm is a localized or diffuse dilation of an artery with a diameter at least 50% greater than the normal size of the artery.
A blood vessel has 3 layers: the intima (inner layer made of endothelial cells), media (contains muscular elastic fibers), and adventitia (outer connective tissue). Aneurysms are either true or false. The wall of a true aneurysm involves all 3 layers, and the aneurysm is contained inside the endothelium. The wall of a false or pseudoaneurysm only involves the outer layer and is contained by the adventitia. An aortic dissection is formed by an intimal tear and is contained by the media; hence, it has a true lumen and a false lumen.
Most aortic aneurysms (AA) occur in the abdominal aorta; these are termed abdominal aortic aneurysms (AAA). Although most abdominal aortic aneurysms are asymptomatic at the time of diagnosis, the most common complication remains life-threatening rupture with hemorrhage.
Aneurysmal degeneration that occurs in the thoracic aorta is termed a thoracic aneurysm (TA). Aneurysms that coexist in both segments of the aorta (thoracic and abdominal) are termed thoracoabdominal aneurysms (TAA). Thoracic aneurysms and thoracoabdominal aneurysms are also at risk for rupture. A recent population-based study suggests an increasing prevalence of thoracic aortic aneurysms. Thoracic aortic aneurysms are subdivided into 3 groups depending on location: ascending aortic, aortic arch, and descending thoracic aneurysms or thoracoabdominal aneurysms. Aneurysms that involve the ascending aorta may extend as proximally as the aortic annulus and as distally as the innominate artery, whereas descending thoracic aneurysms begin beyond the left subclavian artery. Arch aneurysms are as the name implies.
Dissection is another condition that may affect the thoracic aorta. An intimal tear causes separation of the walls of the aorta. A false passage for blood develops between the layers of the aorta. This false lumen may extend into branches of the aorta in the chest or abdomen, causing malperfusion, ischemia, or occlusion with resultant complications. The dissection can also progress proximally, to involve the aortic sinus, aortic valve, and coronary arteries. Dissection can lead to aneurysmal change and early or late rupture. A chronic dissection is one that is diagnosed more than 2 weeks after the onset of symptoms. Dissection should not be termed dissecting aneurysm because it can occur with or without aneurysmal enlargement of the aorta.
The shape of an aortic aneurysm is either saccular or fusiform. A fusiform (or true) aneurysm has a uniform shape with a symmetrical dilatation that involves the entire circumference of the aortic wall. A saccular aneurysm is a localized outpouching of the aortic wall, and it is the shape of a pseudoaneurysm.
Treatment of abdominal aortic aneurysms, thoracoabdominal aneurysms, and thoracic aneurysms involves surgical repair in good-risk patients with aneurysms that have reached a size sufficient to warrant repair. Surgical repair may involve endovascular stent grafting (in suitable candidates) or traditional open surgical repair.
History of the Procedure
The development of treatment modalities for thoracic aneurysms followed successful treatment of abdominal aortic aneurysms. Estes' 1950 report revealed that the 3-y survival rate for patients with untreated abdominal aortic aneurysms was only 50%, with two thirds of deaths resulting from aneurysmal rupture.[1] Since then, increased attempts were made to devise methods of durable repair.
Most of these initial successful repairs involved the use of preserved aortic allografts, thus triggering the establishment of numerous aortic allograft banks. Simultaneously, Gross and colleagues successfully used allografts to treat complex thoracic aortic coarctations, including those with aneurysmal involvement.[2]
In 1951, Lam and Aram reported the resection of a descending thoracic aneurysm with allograft replacement.[3] Ascending aortic replacement required the development of cardiopulmonary bypass and was first performed in 1956 by Cooley and DeBakey.[4] They successfully replaced the ascending aorta with an aortic allograft. Successful replacement of the aortic arch, with its inherent risk of cerebral ischemia, was understandably more challenging and was not reported until 1957 by DeBakey et al.[5]
Although the use of aortic allografts as aortic replacement was widely accepted in the early 1950s, the search for synthetic substitutes was well underway. Dacron was introduced by DeBakey. By 1955, Deterling and Bhonslay believed that Dacron was the best material for aortic substitution.[6] Numerous types of intricately woven hemostatic grafts have since been developed and are now used much more extensively than their allograft counterparts. Such Dacron grafts are used to replace ascending, arch, thoracic, and thoracoabdominal aortic segments.
However, some patients required replacement of the aortic root, as well. Subsequently, combined operations that replaced the ascending aneurysm in conjunction with replacement of the aortic valve and reimplantation of the coronary arteries were performed by Bentall and De Bono in 1968, using a mechanical valve with a Dacron conduit.[7] Ross, in 1962, and Barratt-Boyes, in 1964, successfully implanted the aortic homograft in the orthotopic position.[8, 9] In 1985, Sievers reported the use of stentless porcine aortic roots.[10]
More recently, less invasive therapy for descending thoracic aortic aneurysm have been developed. Dake et al reported the first endovascular thoracic aortic repair in 1994.[11] In March 2005, the US Food and Drug Administration (FDA) approved the first thoracic aortic stent graft, the GORE TAG graft (W.L. Gore and Associates; Flagstaff, AZ).[12] Since 2005, 2 other devices have gained FDA approval: the Talent Thoracic endograft (Medtronic; Santa Rosa, CA) and the Cook TX2 endograft (Cook; Bloomington, IN). Several successive next-generation reiterations of all of these devices have also gained approval.
Given the relative acceptance of the indications for thoracic endografts as an alternative to open procedures in the treatment of uncomplicated diseases of the descending thoracic aorta, experienced users of the devices now use them "off-label" in increasingly more complex indications, including use via "hybrid-procedures" in the ascending aorta and aortic arch. However, little long-term data are available at this time to support use in this fashion.
Problem
Aneurysms are usually defined as a localized dilation of an arterial segment greater that 50% its normal diameter. Most aortic aneurysms occur in the infrarenal segment (95%). The average size for an infrarenal aorta is 2 cm; therefore, abdominal aortic aneurysms are usually defined by diameters greater than 3 cm.
The normal size for the thoracic and thoracoabdominal aorta is larger than that of the infrarenal aorta, and aneurysmal degeneration in these areas is defined accordingly. The average diameter of the mid-descending thoracic aorta is 26-28 mm, compared with 20-23 mm at the level of the celiac axis.
Epidemiology
Frequency
Although findings from autopsy series vary widely, the prevalence of aortic aneurysms probably exceeds 3-4% in individuals older than 65 years.
Death from aneurysmal rupture is one of the 15 leading causes of death in most series. The estimated incidence of thoracic aortic aneurysms is 6 cases per 100,000 person-years. In addition, the overall prevalence of aortic aneurysms has increased significantly in the last 30 years. This is partly due to an increase in diagnosis based on the widespread use of imaging techniques. However, the prevalence of fatal and nonfatal rupture has also increased, suggesting a true increase in prevalence. Population-based studies suggest an incidence of acute aortic dissection of 3.5 per 100,000 persons; an incidence of thoracic aortic rupture of 3.5 per 100,000 persons; and an incidence of abdominal aortic rupture of 9 per 100,000 persons. An aging population probably plays a significant role.
Etiology
Aneurysmal degeneration occurs more commonly in the aging population. Aging results in changes in collagen and elastin, which lead to weakening of the aortic wall and aneurysmal dilation. According to the law of Laplace, luminal dilation results in increased wall tension and the vicious cycle of progressive dilation and greater wall stress. Pathologic sequelae of the aging aorta include elastic fiber fragmentation and cystic medial necrosis. Arteriosclerotic (degenerative) disease is the most common cause of thoracic aneurysms.
A previous aortic dissection with a persistent false channel may produce aneurysmal dilation; such aneurysms are the second most common type. False aneurysms are more common in the descending aorta and arise from the extravasation of blood into a tenuous pocket contained by the aortic adventitia. Because of increasing wall stress, false aneurysms tend to enlarge over time.
Authorities strongly agree that genetics play a role in the formation of aortic aneurysms. Of first-degree relatives of patients with aortic aneurysms, 15% have an aneurysm. This appears especially true in first-degree relatives of female patients with aortic aneurysms. Thus, inherited disorders of connective tissue appear to contribute to the formation of aortic aneurysms.
Marfan syndrome is a potentially lethal connective-tissue disease characterized by skeletal, heart valve, and ocular abnormalities. Individuals with this disease are at risk for aneurysmal degeneration, especially in the thoracic aorta. Marfan syndrome is an autosomal dominant genetic condition that results in abnormal fibrillin, a structural protein found in the human aorta. Patients with Marfan syndrome may develop annuloaortic ectasia of the sinuses of Valsalva, commonly associated with aortic valvular insufficiency and aneurysmal dilation of the ascending aorta.
Type IV Ehlers-Danlos syndrome results in a deficiency in the production of type III collagen, and individuals with this disease may develop aneurysms in any portion of the aorta. Imbalances in the synthesis and degradation of structural proteins of the aorta have also been discovered, which may be inherited or spontaneous mutations.
Atherosclerosis may play a role. Whether atherosclerosis contributes to the formation of an aneurysm or whether they occur concomitantly is not established. Other causes of aortic aneurysms are infection (ie, bacterial [mycotic or syphilitic]), arteritis (ie, giant cell, Takayasu, Kawasaki, Behçet), and trauma. Aortitis due to granulomatous disease is rare, but it can lead to the formation of aortic and, on occasion, pulmonary artery aneurysms. Aortitis caused by syphilis may cause destruction of the aortic media followed by aneurysmal dilation.
Traumatic dissection is a result of shearing from deceleration injury due to high speed motor vehicle accidents (MVA) or a fall from heights. The dissection occurs at a point of fixation, usually at the aortic isthmus (ie, at the ligamentum arteriosum, distal to the origin of the left subclavian artery), the ascending aorta, the aortic root, and the diaphragmatic hiatus.
The true etiology of aortic aneurysms is probably multifactorial, and the condition occurs in individuals with multiple risk factors. Risk factors include smoking, chronic obstructive pulmonary disease (COPD), hypertension, atherosclerosis, male gender, older age, high BMI, bicuspid or unicuspid aortic valves, genetic disorders, and family history. Aortic aneurysms are more common in men than in women and are more common in persons with COPD than in those without lung disease.
Pathophysiology
The occurrence and expansion of an aneurysm in a given segment of the arterial tree probably involves local hemodynamic factors and factors intrinsic to the arterial segment itself.
The medial layer of the aorta is responsible for much of its tensile strength and elasticity. Multiple structural proteins comprise the normal medial layer of the human aorta. Of these, collagen and elastin are probably the most important. The elastin content of the ascending aorta is high and diminishes progressively in the descending thoracic and abdominal aorta. The infrarenal aorta has a relative paucity of elastin fibers in relation to collagen and compared with the thoracic aorta, possibly accounting for the increased frequency of aneurysms in this area. In addition, the activity and amount of specific enzymes is increased, which leads to the degradation of these structural proteins. Elastic fiber fragmentation and loss with degeneration of the media result in weakening of the aortic wall, loss of elasticity, and consequent dilation.
Hemodynamic factors probably play a role in the formation of aortic aneurysms. The human aorta is a relatively low-resistance circuit for circulating blood. The lower extremities have higher arterial resistance, and the repeated trauma of a reflected arterial wave on the distal aorta may injure a weakened aortic wall and contribute to aneurysmal degeneration. Systemic hypertension compounds the injury, accelerates the expansion of known aneurysms, and may contribute to their formation.
Hemodynamically, the coupling of aneurysmal dilation and increased wall stress is defined by the law of Laplace. Specifically, the law of Laplace states that the (arterial) wall tension is proportional to the pressure times the radius of the arterial conduit (T = P x R). As diameter increases, wall tension increases, which contributes to increasing diameter. As tension increases, risk of rupture increases. Increased pressure (systemic hypertension) and increased aneurysm size aggravate wall tension and therefore increase the risk of rupture.
Aneurysm formation is probably the result of multiple factors affecting that arterial segment and its local environment.
Presentation
Most patients with aortic aneurysms are asymptomatic at the time of discovery. Thoracic aneurysms are usually found incidentally after chest radiographs or other imaging studies. Abdominal aortic aneurysms may be discovered incidentally during imaging studies or a routine physical examination as a pulsatile abdominal mass.
The most common complication of abdominal aortic aneurysms is rupture with life-threatening hemorrhage manifesting as pain and hypotension. The triad of abdominal pain, hypotension, and a pulsatile abdominal mass is diagnostic of a ruptured abdominal aortic aneurysm, and emergent operation is warranted without delay for imaging studies.
Patients with a variant of abdominal aortic aneurysm may present with fever and a painful aneurysm with or without an obstructive uropathy. These patients may have an inflammatory aneurysm that can be treated with surgical repair.
Other presentations of abdominal aortic aneurysm include lower extremity ischemia, duodenal obstruction, ureteral obstruction, erosion into adjacent vertebral bodies, aortoenteric fistula (ie, GI bleed), or aortocaval fistula (caused by spontaneous rupture of aneurysm into the adjacent inferior vena cava [IVC]). Patients with aortocaval fistula present with abdominal pain, venous hypertension (ie, leg edema), hematuria, and high output cardiac failure.
Patients with thoracic aneurysms are often asymptomatic. Most patients are hypertensive but remain relatively asymptomatic until the aneurysm expands. Their most common presenting symptom is pain. Pain may be acute, implying impending rupture or dissection, or chronic, from compression or distension. The location of pain may indicate the area of aortic involvement, but this is not always the case. Ascending aortic aneurysms tend to cause anterior chest pain, while arch aneurysms more likely cause pain radiating to the neck. Descending thoracic aneurysms more likely cause back pain localized between the scapulae. When located at the level of the diaphragmatic hiatus, the pain occurs in the mid back and epigastric region.
Large ascending aortic aneurysms may cause superior vena cava obstruction manifesting as distended neck veins. Ascending aortic aneurysms also may develop aortic insufficiency, with widened pulse pressure or a diastolic murmur, and heart failure. Arch aneurysms may cause hoarseness, which results from stretching of the recurrent laryngeal nerves. Descending thoracic aneurysms and thoracoabdominal aneurysms may compress the trachea or bronchus and cause dyspnea, stridor, wheezing, or cough. Compression of the esophagus results in dysphagia. Erosion into surrounding structures may result in hemoptysis, hematemesis, or gastrointestinal bleeding. Erosion into the spine may cause back pain or instability. Spinal cord compression or thrombosis of spinal arteries may result in neurologic symptoms of paraparesis or paraplegia. Descending thoracic aneurysms may thrombose or embolize clot and atheromatous debris distally to visceral, renal, or lower extremities.
Patients who present with ecchymoses and petechiae may be particularly challenging because these signs probably indicate disseminated intravascular coagulation (DIC). The risk of significant perioperative bleeding is extremely high, and large amounts of blood and blood products must be available for resuscitative transfusion.
The most common complications of thoracic aortic aneurysms are acute rupture or dissection. Some patients present with tender or painful nonruptured aneurysms. Although debate continues, these patients are thought to be at increased risk for rupture and should undergo surgical repair on an emergent basis.
Indications
Indications for surgery of thoracic aortic aneurysms are based on size or growth rate and symptoms. Because the risk of rupture is proportional to the diameter of the aneurysm, aneurysmal size is the criterion for elective surgical repair. Elefteriades published the natural history of thoracic aortic aneurysms and recommends elective repair of ascending aneurysms at 5.5 cm and descending aneurysms at 6.5 cm for patients without any familial disorders such as Marfan syndrome.[13, 14] These recommendations are based on the finding that the incidence of complications (rupture and dissection) exponentially increased when the size of the ascending aorta reached 6.0 cm (31% risk of complications) or when the size of the descending aorta reached 7.0 cm (43% risk).[15, 14] Patients with Marfan syndrome or familial aneurysms should undergo earlier repair, when the ascending aorta grows to 5.0 cm or the descending aorta grows to 6.0 cm.
In addition, relative aortic aneurysm size in relation to body surface area may be more important than absolute aortic size in predicting complications.[16] Using the aortic size index (ASI) of aortic diameter (in cm) divided by body surface area (m2), patients are stratified into 3 groups: ASI < 2.75 cm/m2 are at low risk for rupture (4%/y), ASI 2.75-4.25 cm/m2 are at moderate risk (8%/y), and ASI >4.25cm/m2 are at high risk (20-25%/y).[16, 17]
Rapid expansion is also a surgical indication. Growth rates average 0.07 cm/y in the ascending aorta and 0.19 cm/y in the descending aorta.[14] A growth rate of 1 cm/y or faster is an indication for elective surgical repair.
Symptomatic patients should undergo aneurysm resection regardless of size. Acutely symptomatic patients require emergent operation. Emergent operation is indicated in the setting of acute rupture. Rupture of the ascending aorta may occur into the pericardium, resulting in acute tamponade. Rupture of the descending thoracic aorta may cause a left hemothorax.
Patients with acute aortic dissection of the ascending aorta require emergent operation. They may present with rupture, tamponade, acute aortic insufficiency, myocardial infarction, or end-organ ischemia. Acute dissection of the descending aorta does not require surgical intervention, unless complicated by rupture, malperfusion (eg, visceral, renal, neurologic, leg ischemia), progressive dissection, persistent recurrent pain, or failure of medical management.
Patients who undergo surgery for symptomatic aortic insufficiency or stenosis with an associated enlarged aneurysmal aorta should have concomitant aortic replacement if the aorta reaches 5 cm in diameter. Concomitant aortic replacement should be consider for patients with bicuspid aortic valves with an aorta >4.5 cm in diameter.
As one may imagine, the quality of data and level of evidence supporting these recommendations widely vary. Given this discrepancy and important subject matter, in 2010, a joint task force spearheaded by the American College of Cardiology Foundation and the American Heart Association, and composed of members of many professional societies specializing in treatment of diseases of the thoracic aorta, produced an Executive Summary detailing guidelines for diagnosis and management of this disease, which will be annually updated.[18]
Summary of indications
- Aortic size
- Ascending aortic diameter ≥5.5 cm or twice the diameter of the normal contiguous aorta
- Descending aortic diameter ≥6.5 cm
- Subtract 0.5 cm from the cutoff measurement in the presence of Marfan syndrome, family history of aneurysm or connective tissue disorder, bicuspid aortic valve, aortic stenosis, dissection, patient undergoing another cardiac operation
- Growth rate ≥1 cm/y
- Symptomatic aneurysm
- Traumatic aortic rupture
- Acute type B aortic dissection with associated rupture, leak, distal ischemia
- Pseudoaneurysm
- Large saccular aneurysm
- Mycotic aneurysm
- Aortic coarctation
- Bronchial compression by aneurysm
- Aortobronchial or aortoesophageal fistula
Relevant Anatomy
Ascending aortic aneurysms occur as proximally as the aortic annulus and as distally as the innominate artery. They may compress or erode into the sternum and ribs, causing pain or fistula. They also may compress the superior vena cava or airway. When symptomatic by rupture or dissection, they may involve the pericardium, aortic valve, or coronary arteries. They may rupture into the pericardium, causing tamponade. They may dissect into the aortic valve, causing aortic insufficiency, or into the coronary arteries, causing myocardial infarction.
Aortic arch aneurysms involve the aorta where the innominate artery, left carotid, and left subclavian originate. They may compress the innominate vein or airway. They may stretch the left recurrent laryngeal nerve, causing hoarseness.
Descending thoracic aneurysms originate beyond the left subclavian artery and may extend into the abdomen. Thoracoabdominal aneurysms are stratified based on the Crawford classification. Type I involves the descending thoracic aorta from the left subclavian artery down to the abdominal aorta above the renal arteries. Type II extends from the left subclavian artery to the renal arteries and may continue distally to the aortic bifurcation. Type III begins at the mid-to-distal descending thoracic aorta and involves most of the abdominal aorta as far distal as the aortic bifurcation. Type IV extends from the upper abdominal aorta and all or none of the infrarenal aorta. Descending thoracic aneurysms and thoracoabdominal aneurysms may compress or erode into surrounding structures, including the trachea, bronchus, esophagus, vertebral body, and spinal column.
Contraindications
Aneurysm surgery has no strict contraindications. The relative contraindications are individualized, based on the patient's ability to undergo extensive surgery (ie, the risk-to-benefit ratio). Patients at higher risk for morbidity and mortality include elderly persons and individuals with end-stage renal disease, respiratory insufficiency, cirrhosis, or other comorbid conditions. For descending thoracic aneurysms, endovascular stent grafting is less invasive and is an ideal alternative (with appropriate anatomic considerations) to open repair for patients at high risk for complications of open repair. Stent grafts are also a reasonable alternative (with the appropriate anatomy) to open repair in patients who are not at high risk for complications. Patients must understand that life-long follow-up is required and that long-term durability is unknown.
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