Ascending aortic aneurysms: pathophysiology and indications for 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 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

An 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 aorta are 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  part of a generalised connective tissue disorder, e.g. Marfan's syndrome, a genetic disorder affecting fibrillin synthesis with fibrillin and elastin being basic components of elastic fibre. The aorta in Marfan's 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's 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 the 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.
An aneurysmal dilatation of the 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 with 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 an anomalous valve similar to cystic medial degeneration occurring in ascending aortas of patients with bicuspid or unicuspid aortic valves. (7) In this aneurysmal dilatation, the primary structural abnormality of the aortic wall is also caused by the secondary process of post-stenotic aortic dilatation in patients having stenotic bicuspid or unicuspid aortic valves. (8)
Aneurysms of the 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 the 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 antisyphilitic therapy usually does not prevent the formation of an aortic aneurysm as there is no reversal of postsyphilitic vascular lesions.

e) Arteritis, trauma and pseudoaneurysm

Systemic arteritis such as Takayasu's arteritis, Kawasaki disease, Behçet's disease or giant cell arteritis may involve ascending aorta and initiate formation of an aneurysm. An aseptic inflammatory process in the medial layer of the aorta is presumed to induce medial destruction, which leads to weakening of the aortic wall and potential aneurysm formation or dissection. Chronic traumatic aneurysms in the 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 the ascending aorta, they may develop after trauma, surgical interventions or infections. Postoperative pseudoaneurysms may develop at sites of cannulation or along suture lines of the ascending aorta. 

Indications for surgery

Aneurysm diameter is the main indication for elective surgical intervention because it correlates strongly with the risk of the ascending aortic aneurysm dissecting or rupturing. (1,9) The diameter of an anormal aorta is influenced by age and body mass index. (10) Indications for replacement of the ascending aorta are influenced by aetiology, diameter, and rate of growth of the aneurysm. The indications are as follows: (11,12)

1. Asymptomatic ascending aortic aneurysm >5.0 cm in diameter.
2. Symptomatic aneurysm irrespective of size.
3. Asymptomatic ascending aortic aneurysm >4.5 cm in patients with Marfan's syndrome.
4. Acute dissection or rupture of the ascending aortic aneurysm.
5. Pseudoaneurysm or traumatic aneurysm in the ascending aorta.
6. Ascending aortic aneurysm >4.5 cm in patients undergoing aortic valve surgery.
7. Growth rate of >0.5 cm/y when the ascending aorta is <5.0 cm in diameter.