ABDOMINAL AORTIC ANEURYSMS

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Arterial aneurysms are one of the most common vascular disease causes of disability and death.Although it occurs in most arteries throughout the body, it is most common in the infrarenal abdominal aortic portion and is particularly common in the elderly.The definition of an aneurysm is a permanent focal dilation of an artery of at least 50% increase in diameter compared to the expected normal diameter of the artery in question.
Aneurysms are further classified into true versus false aneurysms.The former involves all three layers of the arterial wall while the latter involves a portion of the arterial wall with presence of blood flow outside the normal layers of arterial wall.False aneurysms (or pseudoaneurysms) are associated with complications from needle punctures, infections, or arterial anastomotic disruption, and lacks three true layers of the aneurysm wall.

Historical Background
The history of the treatment of aortic aneurysms can be traced as far back as the 2nd century AD, where Antyll, performed proximal and distal ligation of aneurysms, thus providing the first record of the cause and treatment of aneurysms.Vesalius first diagnosed an abdominal aortic aneurysm (AAA) in 1555 and the great British surgeons John and William Hunter performed multiple ligations of peripheral aneurysms.In 1817 Astley Cooper was the first to ligate the abdominal aorta for a ruptured iliac aneurysm.In 1888, Rudolph Matas performed the first definitive repair, known as endoaneurysmorrhaphy, by ligating the branches of a brachial artery aneurysm from inside the aneurysm sac.
In 1951 Dubost managed to preserve blood flow during aneurysm repair when he replaced an abdominal aortic aneurysm with a thoracic aortic homograft.Voorhees published their series of aortic aneurysms repaired with Vinyon "N" cloth.Their work launched the modern age of using synthetic grafts in aneurysm repair.Open repair has been refined since the 1960s, and continues to prove itself to be a durable operation with an excellent track record, with perioperative mortality for open repair of abdominal aortic aneurysms (AAAs) ranging from as low as 1.2% to 3.8%.
With the dawn of a new era in endovascular management of vascular pathology, Parodi reported the first repair of an aortic aneurysm with an aortic stent-graft in 1991.Since then, a rapid transition from open to endovascular repair of AAA may be noticed.In the United States today, data indicates that the majority of AAA repairs are performed via an endovascular approach.Thoracic endografts are also now widely used to treat aneurysms of the thoracic aorta as well as in traumatic disruptions.As endograft technology and endovascular techniques progressed and refined over the 2000s, new innovations including branch and fenestrated endografts allow treatment of complex thoracoabdominal lesions without compromising flow to visceral branches.

Abdominal Aortic Aneurysms
Abdominal aortic aneurysms account for the majority of aneurysm disease in the body.It usually results from degeneration in the media of the arterial wall, leading to a slow and continuous dilatation of the lumen of the vessel.Other less common causes include infection, cystic medial necrosis, arteritis, trauma, inherited connective-tissue disorders, and anastomotic disruption.
It generally affects the elderly, especially white men.Several risk factors are associated with its development, with smoking being the strongest.Other factors include increased height, weight, body mass index, and body surface area.A genetic component is also present, with familiar clustering been noted in 15-25% of patients undergoing surgical repair of AAA.Negatively associated factors include female sex, African American race, and diabetes mellitus.

Anatomy
The abdominal aorta has three distinct tissue layers: intima, media, and adventitia.The intima is composed of endothelium in the luminal surface and subendothelial extracellular matrix.The media comprises smooth muscle cells surrounded by elastin, collagen, proteoglycans, and is bounded by the internal and external elastic laminae.This layer accounts for the structural and elastic properties of the artery.The adventitia consists primarily of collagen, loose connective tissue, fibroblasts, capillaries, immunomodulatory cells, and adrenergic nerves.
The diameter of the aorta decreases in size from its thoracic portion to the abdominal and infrarenal portions.There is a change in vessel wall composition, with a reduction in medial elastin layers from the thoracic area to the abdominal portion.There is also a reduction in elastin and collagen content as it progresses from thoracic to abdominal aorta.AAAs may be described as fusiform, or saccular.Most cases of AAA involve the segment below the renal arteries and just above the iliac arteries.Occasionally it can be located close to the renal arteries (juxtarenal AAA) or involve the renal arteries altogether (pararenal AAA).There can also be associated iliac and hypogastric artery aneurysms as well.These have important anatomic surgical and endovascular considerations, as preservation of these arterial branches is desirable.

Pathophysiology
AAA is primarily a degenerative condition resulting from a failure of the major structural proteins of the aorta.Biochemical studies have shown decreased quantities of elastin and collagen but an increased ratio of collagen to elastin in aneurysm walls.AAA develops following degeneration of the media, ultimately leading to widening of the vessel lumen and loss of structural integrity.
There has been significant debate regarding the cause and pathophysiology of AAA development.More than 90% of the AAA's are associated with atherosclerosis and this has traditionally been considered as the primary cause.However, although atherosclerosis is uniformly present in aneurysm walls and it shares common risk factors with occlusive atherosclerosis, the concept of atherosclerosis as the sole contributor to AAA development has been challenged..There are indications that more factors other than atherosclerosis may be involved in AAA development.Most patients with aneurysmal disease do not have concomitant aortoiliofemoral atherosclerotic occlusive disease.This has seriously challenged the long held theory of atherosclerosis causing vessel wall weakening leading to aneurysmal degeneration.Many authors suggest that atherosclerosis might be a coincidental or a facilitating process rather than a primary cause for aneurysmal disease.
Studies have identified multiple processes involved in the development of AAA, including proteolytic degradation of aortic wall connective tissue, inflammation and immune responses, biomechanical wall stress, and genetic modulation.Surgical specimens of AAA have also revealed inflammation, infiltration of lymphocytes and macrophages, thinning of the media; and marked loss of elastin.
Elastin is the main load-bearing element in the aorta.The number of medial elastin layer decreases markedly as one progresses from proximal thoracic aorta to the infrarenal aorta.In aneurysm walls, elastin degeneration and fragmentation have been observed.
Proteolysis of the aortic media in AAA has been observed.There is a relative increase in proteolytic enzymes to their inhibitors in the abdominal aorta with age.The role of metalloproteinases (MMP) in tissue remodeling has also been implicated.Reports have found increased expression and activity of this enzyme in individuals with AAA.MMP and other proteases are secreted into the extracellular matrix by macrophages and smooth muscle cells.Although MMP is responsible for vessel wall remodeling in normal aortic tissue, in AAA, there is increased MMP activity relative to its inhibitor activity, leading to net degradation of elastin and collagen.AAA is also an inflammatory state, whereby a chronic adventitial and medial inflammation is present on histological examination.Lymphocyte and macrophage infiltration of the vessel wall in AAA release various cytokines, particularly IL1, 6, 8 and TNF-alfa, all of which can trigger protease activation.

Etiology
Patients at greatest risk for AAA are men older than 65 years with peripheral atherosclerotic vascular disease and history of smoking.The US Preventive Services Task Force recommends ultrasonography screening in men aged 65-75 years who have ever smoked.
Cooley and DeBakey followed several months later in the United States.In 1954, Blakemore and TREATMENT OF CARDIOVASCULAR DISEASES -Abdominal Aortic Aneurysms -Chuo Ren Leong, Rajeev Gupta, Mark Kissin, Robert W. Chang and John B. Chang ©Encyclopedia of Life Support Systems (EOLSS) TREATMENT OF CARDIOVASCULAR DISEASES -Abdominal Aortic Aneurysms -Chuo Ren Leong, Rajeev Gupta, Mark Kissin, Robert W. Chang and John B. Chang ©Encyclopedia of Life Support Systems (EOLSS) TREATMENT OF CARDIOVASCULAR DISEASES -Abdominal Aortic Aneurysms -Chuo Ren Leong, Rajeev Gupta, Mark Kissin, Robert W. Chang and John B. Chang ©Encyclopedia of Life Support Systems (EOLSS) Figure 1.An example of a CT Scan
Endovascular Repair of an Iliac Artery Aneurysm 1.30.Endovascular Procedures in Azotemic Condition Using CO 2 .
Preoperative evaluation of the patient with AAA should include medical optimization of cardiac, pulmonary, renal disease, trans-thoracic echo, and further cardiac work up depending on the risk factors.THE PREVENTION AND TREATMENT OF CARDIOVASCULAR DISEASES -Abdominal Aortic Aneurysms -Chuo Ren Leong, Rajeev Gupta, Mark Kissin, Robert W. Chang and John B. Chang ©Encyclopedia of Life Support Systems (EOLSS) Criado FJ, Clark NS, McKendrick C, Longway J, Domer GS.. (2003) Update on the Talent LPS AAA stent graft: results with "Enhanced Talent."Semin Vasc Surg; 16:158-165.[Follow up report on the results of Talent aortic stent-graft in the endovascular treatment of infrarenal abdominal aortic aneurysms] De Bruin JL, Baas AF, Buth J. (2010) Long-term outcome of open or endovascular repair of abdominal aortic aneurysm; N Engl J Med;362:1881-9.[Six year results of a multicenter, randomized, controlled trial comparing open repair with endovascular repair of abdominal aortic aneurysm] EVAR trial participants.(2005) Endovascular aneurysm repair and outcome in patients unfit for open repair of abdominal aortic aneurysm (EVAR trail 2): randomized controlled trial.Lancet; 365: 2187.[A landmark article comparing endovascular repair of abdominal aortic aneurysm to no intervention in patients unfit for open repair of abdominal aortic aneurysm] stent-graft: a preliminary report from the United States phase I and II trials.J Vasc Surg;33:S146-S149.[Initial report on the safety and efficacy of the Talent aortic stent-graft (Medtronic World Medical, Sunrise, Fla) in the treatment of infrarenal abdominal aortic aneurysms]