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Mr Punjabi Prakash
Mr Nawaz M Asghar
Dr. Kok Meng John Chan
Mr Macys Antanas
Ascending aortic aneurysm may be fatal due to its liability to dissect or rupture (1). Approximately 50% of patients with acute untreated ascending aortic dissection die within 48 hours, and those undergoing emergency surgery have 15-26% mortality (3, 4). Elective surgery lowers mortality to only 3-5% (2). Understanding the pathophysiology of ascending aortic aneurysms can help reducing the morbidity and mortality from aortic dissections or ruptures by indicating timely elective surgery.
The normal ascending aorta functions as an elastic reservoir to enhance arterial flow. It stores energy during systole and dissipates it during diastole. The medial layer of the aorta consisting of elastin, collagen, and smooth muscle cells, ensures the elasticity and the tensile strength of the aortic wall. The ascending part of the aorta is more elastic than the descending part due to its greater concentration of elastic fibres.
a) Degenerative changes in elastic media
Ascending aortic aneurysm is an enlargement (in width and/or in length) of a weakened area in the ascending aorta Whereas other aneurysms of the aorta are associated with atherosclerosis, aneurysms of the ascending aortaare mostly associated with degenerative changes in elastic media. Degenerative processes in ascending aorta - cystic medial degeneration - affect the medial layer by varying degrees of elastic fibre fragmentation and smooth muscle loss. Although fragmentation of elastic fibre is a normal process of aging, it may occasionally accelerate and cause partial or complete loss of elastic fibres and smooth muscle cells in the medial layer of the aorta.The aortic wall then loses its strength and elasticity, becoming aneurysmal, it may then dissect or rupture. b) Annulo-aortic ectasia and genetic connective tissue disorders Annulo-aortic ectasia is a combination of 1) ascending aortic aneurysm, 2) dilatation of the sinuses of Valsalva, and 3) dilatation of the aortic annulus. Annulo-aortic ectasia can be an isolated condition or can occur as an as part of a generalised connective tissue disorder, e.g. Marfan syndrome, a genetic disorder affecting fibrillin synthesis with Fibrillin and elastin being basic components of elastic fibre. The aorta in Marfan syndrome exhibits the typical features of cystic medial degeneration with disruption of elastic fibres and fibrosis of media. Up to 85% of patients with Marfan syndrome have aortic root dilatation with or without associated aortic regurgitation (5). Ehlers-Danlos syndrome is another genetic disorder of connective tissue affecting synthesis of collagen. Type IV causes spontaneous arterial rupture, particularly in the mesenteric artery. This syndrome also may be associated with fatal outcomes of ascending aortic rupture without dissection however, manifestation of ascending aortic aneurysms, ,in Ehlers-Danlos syndrome is less common (6). c) Atherosclerosis and acute aortic dissection Aneurysmal degeneration of ascending aorta may also be caused by atherosclerosis, although atherosclerosis less commonly affects the ascending aorta. Atherosclerotic processes in aortic media cause extensive disruption of elastic fibres and smooth muscle cells which may then be replaced by extracellular matrix and lipids. Various invasive atheromas develop and this progressive destruction weakens the aortic wall. Aneurysmal dilatation of ascending aorta can develop secondarily in patients who survive acute aortic dissection. Such aneurysms mostly evolve from the false lumen of the dissected aorta and have a greater tendency to expand and a higher rate of rupture compared to other types of ascending aortic aneurysms. d) Valve malformations and infections Ascending aortic aneurysms are associated with aortic valve malformations such as bicuspid and unicuspid aortic valves. Predisposition to aneurysmal formation may be related to the same developmental anomaly manifesting as anomalous valve similar to cystic medial degeneration occurring in ascending aorta of patients with bicuspid or unicuspid aortic valves (7). In this aneurysmal dilatation primary structural abnormality of aortic wall is also contributed to, by secondary process of post-stenotic aortic dilatation in patients having stenotic bicuspid or unicuspid aortic valves (8). Aneurysms of ascending aorta caused by infection are referred to as mycotic aneurysms. These aneurysms are rare and most commonly seen after an episode of valvular endocarditis. The infection of the aortic “jet” lesion or infection of the intraluminal clot in an atherosclerotic aneurysm may give rise to a mycotic aneurysm. Syphilis was the most common cause of mycotic aneurysm of ascending aorta in the past, but is rarely seen now. Syphilitic infection initiates aortitis and obliterative endarteritis followed by destruction of the vasa vasorum and degeneration of the medial elastic fibers. Successful anti-syphilitic therapy usually does not prevent the formation of aortic aneurysm as there is no reversal of post syphilitic vascular lesions. e) Arteritis, trauma and pseudoaneurysm Systemic arteritis such as Takayasu arteritis, Kawasaki disease, Behcet disease or giant cell arteritis may involve ascending aorta and initiate formation of an aneurysm. Aseptic inflammatory process in the medial layer of aorta is presumed to induce medial destruction which leads to weakening of aortic wall and potential aneurysm formation or dissection. Chronic traumatic aneurysms in ascending aorta are very rare. Blunt chest traumas most commonly involve the descending rather than the ascending aorta and patients with acute ascending aortic injuries have a very low survival rate due to the development of cardiac tamponade and/or associated cardiac injuries. Pseudoaneurysms do not contain all layers of the aortic wall. In ascending aorta they may develop after trauma, surgical interventions or infections. Postoperative pseudoaneurysms may develop at sites of cannulation or suture lines of ascending aorta.
Aneurysm diameter is the main indication for elective surgical intervention, as it correlates strongly with the risk of the ascending aortic aneurysm dissecting or rupturing (1,9). Diameter of anormal aorta is influenced by age and body mass index (10).Indications for replacement of ascending aorta are influenced by aetiology, diameter and rate of growth of the aneurysm, and are as follows (11,12):
1. Davies RR, Goldstein LJ, Coady MA, et al. Yearly rupture or dissection rates for thoracic aortic aneurysms: simple prediction based on size. Ann Thorac Surg. 2002;73(1):17-27. 2. Isselbacher EM. Thoracic and abdominal aortic aneurysms. Circulation 2005;111:816-828. 3. Hagan PG, Nienaber CA, Isselbacher EM, et al. The International Registry of Acute Aortic Dissection (IRAD): New insights into an old disease. JAMA. 2000;283(7):897 - 903. 4. Ehrlich MP, Ergin A, McCullough JN, et al. Results of immediate surgical treatment of all acute type A dissections. Circulation 2000;102(Suppl III):248-252. 5. Marsalese DL, Moodie DS, Vacante M, et al. Marfan's syndrome: natural history and long-term follow-up of cardiovascular involvement. J Am Coll Card 1989;14:422. 6. Lindsay ME, Dietz HC. Lessons on the pathogenesis of aneurysm from heritable conditions. Nature. 2011;473(7347):308-316. 7. Nistri S, Sorbo M, Basso C, et al. Bicuspid aortic valve abnormal aortic elastic properties. J Heart Valve Dis 2002;11:369–374. 8. Della Corte A, Bancone C, Quarto C, et al. Predictors of ascending aortic dilatation with bicuspid aortic valve: a wide spectrum of disease expression. Eur J Cardiothorac Surg 2007;31(3):397-404. 9. Coady MA, Rizzo JA, Hammond GL, et al: Surgical intervention criteria for thoracic aortic aneurysms: A study of growth rates and complications. Ann Thorac Surg 1999;67:1922. 10. Hannuksela M, Lundqvist S, Carlberg B. Thoracic aorta: dilated or not? Scand Cardiovasc J 2006;40:175-178. 11. HiratzkaLF, Bakris GL, Beckman JA, et al. 2010 ACCF/AHA/AATS/ACR/ASA/SCA/SCAI/ SIR/STS/ SVM guidelines for the diagnosis and management of patients with thoracic aortic disease. J Am Coll Cardiol, 2010;55:27-129. 12. Cozijnsen L, Braam RL, Waalewijn RA, et al. What is new in dilatation of the ascending aorta? Review of current literature and practical advice for the cardiologist. Circulation. 2011;123:924-928.
Macys Antanas, Chan K.M. John, Ravi Patni, Nawaz M. Asghar, Punjabi Prakash P Department of Cardiothoracic Surgery, Imperial College Healthcare NHS Trust, Hammersmith Hospital, Du Cane Road, London W12 0HS, United Kingdom.
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