Pathogenesis of Aortic Dissection: Elastic Fiber Abnormalities and Aortic Medial Weakness

Abstract

A lthough aortic dissection was previously a disease with an extremely poor prognosis, recent advances in diagnostic imaging and therapeutic modalities may improve its prognosis.1, 2) However, much of its etiology and pathogenesis remains unknown. In addition, new pathological variants of aortic dissection, such as intramural hematoma (IMH) and penetrating atherosclerotic ulcer, and related diseases have been identified, raising a new problem. Against this background, this paper revisits the basic pathology of aortic dissection, and discusses the structural abnormalities of the aortic media and mechanism of aortic dissection, as extrapolated from them. Medial Weakness: A Pathological Problem with Aortic DissectionAortic dissection used to be clearly defined as a pathological condition characterized by the presence of an aortic intimal tear and medial dissection, that is, a condition in which blood flows from the entry site into the false lumen.3) However, advances in diagnostic imaging have resulted in the identification of aortic dissection with no entry tear or false lumen flow, and these variants were included in the category of aortic dissection.4) For the differentiation between the two types of aortic dissection, the former is referred to as classic or communicating aortic dissection, and the latter as IMH or non-communicating aortic dissection. In IMH, no entry tear or false lumen flow is observed, strongly suggesting that no intimal tear exists anatomically. However, the limitations of diagnostic imaging make it difficult to establish its presence or absence. Therefore, it is doubtful whether IMH should be treated as the same entity as classic aortic dissection. However, in some cases, IMH has a poor prognosis in that it progresses to classic aortic dissection, or ruptures. Thus, clinically, it is believed that it is not contradictory to treat IMH as a variant of aortic dissection. 5,6) In contrast, pathologically, since the diagnosis can be confirmed by autopsy, IMH is clearly defined as a dissection without an intimal tear. 7,8) However, at the same time, the important question of whether dissection with a tear (classic aortic dissection) and that without a tear (IMH) is the same disease arises. This is related to the long-standing debate over which occurs first, intimal tear or medial dissection, in the development of aortic dissection. The former is the hypothesis that a crack (intimal tear) first forms in the intima overlying a site of aortic medial weakness, followed by blood inflow from the lumen, resulting in medial dissection.9) This is a very natural way of thinking in terms of the direction of blood flow. However, from this viewpoint, it follows that no dissection occurs without an intimal tear, making it contradictory to include IMH in the category of aortic dissection. On the other hand, the latter is based on the fact that no intimal tears were identified in about 10% of autopsied patients with aortic dissection, leading to a hypothesis that bleeding from the medial vasa vasorum at a site o...

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