Thoracic Aortic Aneurysms and Aortic Dissection


Figure 85-1. Normal anatomic aortic segments. The aortic root extends from the aortic annulus to the sinotubular junction. The aortic root is the largest diameter section of the aorta when measured at the level of the sinuses of Valsalva. The tubular or ascending segment of the aorta begins at the sinotubular junction and extends to the innominate artery. The aortic arch travels anterior to posterior and the left, giving rise to the head and upper extremity vessels. The descending thoracic aorta begins distal to the left subclavian at the level of the ligamentum arteriosum to the pulmonary artery (PA). The abdominal aorta begins at the level of the diaphragm.





Figure 85-2. A: Sagittal CT scan showing an ascending aortic aneurysm. Computed tomography (B) and angiogram (C) of a patient with a fusiform descending thoracic aortic aneurysm.



Table 85-1 Risk Factors for Thoracic Aortic Aneurysms



Prevalence and Classification


Thoracic aneurysms compose 25% of all aortic aneurysms. Approximately 50% of TAAs involve the ascending aorta, 10% the aortic arch, and 40% the descending aorta.15 True thoracoabdominal aneurysms constitute less than 5% of TAAs.16 Interestingly, during the first half of the 20th century, thoracic aneurysms were far more common than abdominal aortic aneurysms because of the predominance of infectious (syphilitic) aneurysm in the thoracic aorta. By 1964, however, less than half of aneurysms were in the thorax, primarily as a result of the decline in the incidence of syphilis.17


The true incidence and prevalence of TAAs is likely underreported. In a Swedish autopsy study, the prevalence of TAAs was reported as 489 per 100,000 men and 437 per 100,000 women.16 Stratified by age, 65-year-olds had approximately 400 asymptomatic thoracic aneurysms per 100,000 autopsies, whereas 80-year-olds had a prevalence of 670 per 100,000 autopsies.16 The mean age at diagnosis is between 60 and 70 years. Men are diagnosed 10 years earlier on average and have a 2:1 to 3:1 predominance compared to women.15 In a study of residents in Olmstead county, the prevalence of thoracic aneurysms was 5.9 cases per 100,000 people-years prior to 1980.18 This number increased to 10.4 cases per 100,000 people-years between 1980 and 1994.19


Etiology and Risk Factors


Although there has been a long-standing belief that aortic aneurysms represent a late degenerative stage of atherosclerosis, this concept has been seriously challenged, and aneurysms are now viewed in terms of genetic and molecular mechanisms.15 Atherosclerotic disease is now thought to be associated with, rather than causative of, aneurysmal disease. Aneurysms associated with atherosclerosis of the aorta most commonly involve the descending or thoracoabdominal segments, whereas atherosclerosis is rarely associated with ascending aortic aneurysms.


1 Abnormal proteolysis, the presence of elastolytic serum enzymes and deficiencies of collagen and elastin have been implicated as factors contributing to the development of these aneurysms.17 Atherosclerotic lesions of the thoracic aorta may ulcerate and penetrate the internal elastic lamina of the aortic wall, which can result in hemorrhage within the layers of the media leading to intramural hematoma, an entity on the continuum of aortic dissection. Once aneurysmal dilatation of the aorta has begun, it tends to progress. Whether this is due to a gradual but constant increase in size or episodic incremental increases is unknown. Associated chronic obstructive pulmonary disease (COPD), smoking, and hypertension are known to be risk factors and can increase the rate of aneurysm growth. Other disorders with genetic predisposition are also highly linked with thoracic aneurysms and include Marfan syndrome, Loeys–Dietz syndrome, Ehlers–Danlos syndrome, and Turner syndrome (Table 85-1).


Degenerative


The most common etiology for TAA formation is cystic medial degeneration. Although the underlying mechanisms for this process are unknown, pathologic features include elastic fiber fragmentation, smooth muscle cell apoptosis, and local production of matrix-degrading enzymes, specifically matrix metalloproteinases (MMPs). The degree of atherosclerosis present in the aorta is dependent on the anatomy of the aneurysm. Ascending aortic aneurysms are degenerative often without extensive atherosclerosis, although atherosclerosis is often a feature associated with degenerative descending aneurysms. The pathogenesis of aortic aneurysms appears to be linked to a reduction in functional extracellular matrix proteins, elastin and collagen, either through enzymatic degradation or deficiencies in these structural proteins. Moreover, in some diseases with genetic predisposition, there is a shift to increased expression of MMPs as well as a reduction in tissue inhibitors of MMPs (TIMPs) resulting in degradation of the extracellular matrix. For example, patients with bicuspid aortic valves are known to have altered expression of MMPs and TIMPs.20 As such, these patients have degenerative ascending aortic aneurysms often independent of any associated functional valve disease or atherosclerosis.




Figure 85-3. Reconstruction of a CT scan of an aortic root aneurysm in a patient with Marfan syndrome. Commonly the sinuses of Valsalva are involved with sparing of the aortic valve.


Genetically Triggered


Although the risk of TAA disease increases with age, thoracic aneurysms in young patients are most likely due to genetic predisposition or familial syndromes. Recently, several genetic etiologies linked to thoracic aneurysms have been elucidated. In particular, insights have been gained from a registry for the genetically triggered TAAs and cardiovascular conditions (GenTAC) which has genetic information on more than 1,000 patients with thoracic aneurysms secondary to hereditary or genetic predisposition.16 The most common and notorious disease is Marfan syndrome, which is an autosomal dominant defect of the fibrillin gene located on chromosome 15. Mutation of fibrillin leads to instability of the elastic fibers in the aortic media. The prevalence of aortic dilatation in Marfan syndrome is 70% to 80% and most commonly involves the aortic root and sinuses of Valsalva, rendering the aorta prone to rupture or dissection (Fig. 85-3). Marfan syndrome presents at an early age and tends to be more common in men than women. The cardinal features of the disorder include tall stature, ectopia lentis, mitral valve prolapse, aortic root dilatation, and aortic dissection. About three quarters of patients have an affected parent, while new mutations account for the remainder of cases.


Recently, mutations of transforming growth factor receptors I and II (TGFBR I, TGFBR II) termed Loeys–Dietz syndrome have been linked with thoracic aneurysms. These patients have a classic bifid uvula. Ehlers–Danlos syndrome encompasses a group of more than 10 disorders characterized by defects of collagen synthesis. The disease can range from hypermobility of the joints to extremely fragile skin and extreme muscle weakness. The common vascular Ehlers–Danlos (type 4) is caused by an autosomal dominant disorder of type III collagen. These patients are characterized by fragility of their blood vessels. Familial thoracic aortic aneurysms and dissections (FTAAD) is caused by mutations of the α actin II or myosin heavy chain (MYH11).21 Turner syndrome is defined by the loss of an X chromosome (XO). Patients with Turner are prone to aneurysmal degeneration of their ascending aorta (Table 85-2).


Infectious and Inflammatory


In current practice, primary infections of the aorta are rare but do lead to aneurysms. Mycotic aneurysms can be caused by bacterial, fungal, spirochetal, or viral agents. Prior to the antibiotic era, syphilis accounted for approximately 75% of all aneurysms. Mycotic aneurysms occur due to localized infections in the aortic or arterial walls, usually as the result of bacteremia. Infection of atherosclerotic plaques themselves can also result in mycotic aneurysms. As a result of trauma or due to infected lymph nodes, infection may also spread to blood vessel walls contiguously. Pre-existing aortic aneurysms may also become infected, usually from bloodstream seeding. Although any pathogen may infect aneurysms, Salmonella species show a special proclivity for vascular tissues. Other organisms associated with mycotic aneurysms are Staphylococcus species, Streptococcus species, Pasteurella multocida, Legionella anisa, and Escherichia coli. Treatment consists of long-term antibiotics prior to and after surgical aneurysm resection.



Table 85-2 Known Genetic Mutations Associated with Aortic Aneurysms



A host of rare inflammatory diseases of the aorta and great vessels including Takayasu arteritis, Behçet disease, Kawasaki disease, and giant cell arteritis can result in TAAs and can require surgical treatment. Giant cell arteritis is a systemic arteritis that occurs in elderly patients, that commonly affects the temporal artery which can be biopsied for diagnosis.


Traumatic Pseudoaneurysm


Pseudoaneurysms are associated with previous operations or trauma to the aorta. Iatrogenic pseudoaneurysms occur following aortic surgery and can form at anastomotic or cannulation sites or areas where an aortic cross-clamp was placed. Infection should be strongly considered when encountering postsurgical pseudoaneurysms. Classically, rapid decelerating blunt trauma leads to aortic transection. In more than 50% of cases, the location of the aortic injury is just distal to the fixed ligamentum arteriosum, the remnant of the ductus arteriosus.22,23 For the 20% of patients who survive this acute event, the rupture is partially contained by the aortic adventitia and pleura. Aneurysmal formation around the site of injury can occur and often dilates faster because it is an unsuspected injury, and utmost care with regard to their antihypertensive therapy has to be taken to avoid rupture. Given multisystem injuries often present in these high-speed injuries, many consider endovascular aortic repair the preferred approach for most of these patients.24


Diagnosis


Most TAAs are asymptomatic and are found incidentally. Symptomatic aneurysms are often diagnosed based on their anatomic sequelae. Patients with aneurysms of the ascending aorta may also present with fullness in the chest or signs and symptoms of aortic valve insufficiency. Acute chest pain may be caused by either expansion of the aneurysm or dissection of the aortic wall. Arch and descending thoracic aneurysms can present with hoarseness secondary to stretching of the left recurrent laryngeal nerve, stridor from tracheal compression, or dysphagia from esophageal compression.


There are multiple imaging modalities to diagnose TAAs and dissections. Plain chest radiograph can be suggestive whereas computed tomography (CT), magnetic resonance imaging (MRI), and transesophageal echocardiography (TEE) are definitive studies.25 CT is widely available and provides detailed information about the size, location, and extent of aortic disease as well as anatomic detail of surrounding mediastinal and thoracic structures. In particular, CT angiography with arterial phase contrast of the chest, abdomen, and pelvis is essential for operative planning. Critical elements to be gained from the CT include the following:


1. Aneurysm size, location, and extent


2. Presence of acute or chronic aortic dissection or intramural hematoma


3. Involvement and perfusion of branch vessels For endovascular considerations, the following elements are also essential to determine:


4. Angulation of Aortic Arch and Descending Aorta


5. Size of femoral and iliac vessels


6. Proximal and distal landing zones (a minimum of 2 cm of “normal” aorta is required)


7. Tortuosity of aorta and iliac vessels


8. Involvement of great vessels and/or visceral vessels


MRI/MRA offers detailed anatomy as CT does, but does not require radiation. Moreover, MRI may be particularly useful in pregnant patients. However, MRI requires lengthy image acquisition and processing times, is not as readily available as CT, and cannot be performed in patients with significant metallic implants, such as pacemakers and defibrillators. In addition, gadolinium contrast used in MRA is also nephrotoxic.


TEE avoids the use of these agents and offers good visualization of the ascending and descending thoracic aorta as well as cardiac structures. Due to tracheal shadowing, TEE does not provide reliable visualization of the aortic arch. After CT scanning, TEE is the second most common study used to diagnose aortic dissection with 90% to 99% sensitivity. In unstable patients with a high degree of clinical suspicion for aortic dissection, a confirmatory TEE in the operating room may be safer and more expedient than CT scanning. Although widely available, TEE does require specialized training and operators.


Management and Natural History


Currently, the only assessment tool to determine the risk of catastrophic aortic dissection or rupture is related to aneurysm size and rate of growth. It is likely that other biologic factors may help stratify risk, but as yet, these have yet to be proven. Data from the Yale Center for Thoracic Aortic Disease, which have followed more than 3,000 patients with TAAs, have suggested that the risk of aortic complications in the ascending aorta increases significantly as the size approaches 6.0 cm, and in the descending thoracic aorta as the size approaches 7.0 cm.26 Similarly, the Olmstead County registry (Minnesota) reported that a rupture rate of 16% at 3 years for 6-cm aneurysms compared to 31% for 7-cm aneurysms.19 The Yale group has reported the annual risk of death or aortic-related complication exceeds 15% in patients with aneurysms of 6.0 cm or greater compared to 5% in patients with aneurysms smaller than 5.0 cm. Other studies have supported this pattern, reporting a mean rate of rupture or dissection of 2% per year for small aneurysms, 3% for aneurysms 5.0 to 5.9 cm, and 6.9% for aneurysms 6.0 cm or greater in diameter.27,28 These data have formed the basis for size criteria to intervene prior to the attainment of the previously mentioned dimensions in an asymptomatic patient. Similar to other aneurysms, the growth of TAAs is indolent. The typical rate of growth for ascending aortic aneurysms is 0.1 cm/yr and for descending aneurysms is 0.3 cm/yr.26


Medical Management


Medical management of aortic aneurysms targets mechanisms to reduce aortic wall stress.29 Lifestyle changes to avoid high-intensity exercise, such as weight lifting or sprinting, should be encouraged. Anti-impulse therapy is the primary pharmacologic therapy for aortic aneurysms, with a goal of reducing overall blood pressure and the rate of change in aortic pressure over time (dP/dT). While β-blockers and angiotensin-converting enzyme inhibitors/angiotensin receptor blockers are common treatments, their efficacy is unproven and studies are currently underway in patients with aortic aneurysms. Smoking cessation may be helpful in patients who have not developed COPD. The diagnosis of COPD is a significant risk factor for aortic complications. It is unclear if risk factor modification of COPD once diagnosed can influence the likelihood of aortic rupture. Frequent imaging of these patients to assess changes in size should be performed to determine if surgery should be recommended. Present surveillance recommendations from the American Heart Association are repeat imaging at 1, 3, 6, and 12 months and, if stable, annually thereafter.29


Surgical Indications


Since thoracic aneurysms increase in size until they rupture, the decision to intervene is critical. All symptomatic TAAs meet indications for surgical intervention. Preemptive surgical therapy should be applied at a size of 5.5 cm for ascending and arch aneurysms and 6.0 to 6.5 cm for descending aortic aneurysms in suitable surgical candidates to avoid aortic complications or death (Table 85-3).28,30 These surgical threshold criteria should be modified for patients with Marfan syndrome or connective tissue disease in which preemptive surgery is appropriate at 4.5 cm for root and ascending aneurysms and 5.5 cm for descending aneurysms.31 Some have further suggested that patients with Turner syndrome be treated surgically if the aortic diameter exceeds 4.0 cm due to the aggressive nature of aortic disease in these patients. Similarly, for patients with bicuspid aortic valves a size threshold of 5.0 cm is recommended for ascending aneurysm intervention.32 Furthermore, any patient with symptoms or rapid growth as seen on sequential CT scans should have surgical intervention in a timely manner. Growth rates exceeding 5 to 7 mm/yr for aneurysms or greater than 5 mm per 6 months for chronic aortic dissections are considered harbingers of aortic complications, and earlier intervention should be considered.



Table 85-3 Typical Size Indications for Aortic Surgery



Open Surgical Management of TAA


2 Careful planning by experienced medical personnel is paramount to providing optimal treatment in patients with TAAs as the morbidity and mortality for surgical intervention are significant. Over the years there have been substantial improvements in the intraoperative and postoperative management in patients with TAA. The approach and techniques of repair vary widely based on the location of the thoracic aneurysm.


Once the need for intervention has been determined, a full cardiac evaluation should be completed including angiographic evaluation of coronary arteries and echocardiography. The basic premise of open surgical repair for aneurysms, regardless of the situation, is replacement with an interposition graft. Cardiopulmonary bypass, cerebral and visceral organ perfusion, and protection strategies are determined by the location and extent of aneurysm resection. Intraoperative management is essential with central venous pressure and arterial monitoring as well as large-volume intravenous infusion lines. Intraoperative TEE is useful to monitor fluid status and cardiac function during the operation.


Aortic Root and Ascending Aortic Aneurysms


3 Open surgical repair remains the mainstay for aortic root, ascending aortic, and aortic arch aneurysms with very acceptable outcomes and low morbidity when performed in centers of excellence. Operations to replace aortic root and ascending aorta are approached via a median sternotomy. All pathologic segments of the aorta are evaluated and replaced if abnormal, including the aortic valve, sinuses of Valsalva, the tubular portion of the ascending aorta, and the aortic arch. Cardiopulmonary bypass is established through right atrial (venous) cannulation and either aortic, femoral, or axillary arterial cannulation. The left ventricle is vented via the right superior pulmonary vein. The use of the axillary artery for cannulation allows for selective antegrade cerebral perfusion if circulatory arrest is needed during the replacement of the aorta (Fig. 85-4).33,34 The axillary artery is exposed via a 4-cm transverse incision 2 cm below and parallel to the clavicle. Once through the pectoral muscles, the axillary vein is isolated and retracted, exposing the axillary artery. After systemic heparinization, an 8- or 10-mm Dacron graft is anastomosed in an end-to-side fashion onto the artery. The arterial cannula is then secured to the Dacron graft. Myocardial protection is achieved by infusion of antegrade blood cardioplegia into the coronary ostia and retrograde cardioplegia into the coronary sinus accompanied by a topical hypothermia.


If the aneurysm is contained to the root or ascending aorta and does not involve the aortic arch or head vessels, the heart can be arrested with the cross-clamp proximal to the innominate artery, and mild hypothermia can be used for systemic protection while the patient is maintained on cardiopulmonary bypass. A tube graft is then anastomosed distally and proximally using running monofilament suture. The aortic valve is also assessed with the aorta open and the valve repaired or replaced as necessary (Fig. 85-5). If the dilation of the aorta extends into the arch, deep HCA allows for complete resection and reconstruction of the diseased aorta and an open distal anastomosis in a bloodless field. Cerebral protection can be augmented by maintaining antegrade cerebral perfusion via the right axillary cannula and clamping the proximal innominate artery. The distal anastomosis is then performed in a timely manner. The cross-clamp is then reapplied to the graft, and full cardiopulmonary bypass can then resume. The proximal anastomosis is then completed encompassing all diseased portions of the ascending aorta and the aortic root.




Figure 85-4. Cannulation of the right axillary artery with the use of a side graft. Once established, axillary artery cannulation can allow for antegrade cerebral perfusion throughout the aortic replacement procedure.

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May 5, 2017 | Posted by in GENERAL SURGERY | Comments Off on Thoracic Aortic Aneurysms and Aortic Dissection
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