4.0 Pathophysiology of Non-A Non-B dissection

The wall of the aorta comprises three layers, the tunica intima, tunica media which largely is constituted of structural proteins including elastin and collagen and adventitia (Levy et al. , 2020; Frederick and Woo, 2012). These layers form a thick aortic wall able to withstand high pulsatile pressure and shear stress.
Aortic Dissection is a condition characterised by the separation of these aortic layers. Classically it involves the breaching of the tunica intima, resulting in blood being diverted into a newly created channel within the medial layer of the aorta, known as the false lumen. A tear in the intimal layer tends to arise in locations where the rise in blood pressure is the greatest, commonly 2-2.5cm above the aortic root (Levyet al. , 2020). The separation of these layers paves the way for the formation of a false lumen. Increase in size of the false lumen can lead to an aortic rupture which has a high mortality rate or a second intimal tear which allows blood to re-enter the intima to form a double-barrelled aorta (Gawinecka, Schönrath and von Eckardstein, 2017). This tear can occur in any part of the aorta including the ascending, arch and descending aorta.
Other origins of an aortic dissection include an intramural haematoma and aortic ulceration. The former occurs due to the formation of a haematoma in the media, as a result of bleeding into the aortic wall from the vasa vasorum (Alomari et al. , 2014; Nienaber et al. , 2016). The latter, also referred to as a penetrating aortic ulcer and linked to atherosclerotic disease, is a penetration of the elastic lamina and can result in a haematoma forming in the tunica media (Hayashi et al. , 2000). Such haematomas can contribute to an aneurysm forming prior to aortic dissection.
The formation of an aortic aneurysm is thought to be more likely as a consequence of weakening of the tunica media, through the degeneration of collagen and elastin, which in turn increases higher wall stress. This is explained by Laplace’s Law which states that ‘wall stress is directly proportional to pressure (i.e. hypertension) and radius, and inversely proportional to vessel wall thickness (Patel and Arora, 2008). Compromising the integrity of the aortic wall raises the risk of an aortic dissection. This is key to the risk factors associated with the condition.
Should an aortic dissection progress, through the passage of blood further down the false lumen of the tunica media, it has potential to stretch past the aorta and into the major blood vessels, leading to ischaemia. Dissections can progress in an anterograde or retrograde fashion. This progression of the dissection can result in pressure differences which may lead to the compression or obstruction of the true lumen by the false lumen. Following this, fenestration may re-communicate the false with the true lumen or there is a risk the dissection may rupture into the surrounding cavities (Patel and Arora, 2008).
Carino et al. in their systematic review demonstrated in their analysis that 88% of non-A non-B aortic dissection patients had a complicated disease course and that 29% of these patients had signs of malperfusion; defined as a loss of blood supply to vital organs resulting in end-organ ischaemia (Carino et al. , 2019; Deeb, Patel and Williams, 2010). This percentage formed a considerably larger proportion than observed in type B aortic dissections (Pape et al. , 2015; Estrera et al. , 2007; Ziganshin, Dumfarth and Elefteriades, 2014).