Acute Aortic Syndromes: Focus on Pathogenesis

Katarzyna J. Macura, Frank M. Corl, Elliot K. Fishman, and David A. Bluemke
The Russell H. Morgan Department of Radiology and Radiological Science
Johns Hopkins Medical Institutions, Baltimore, MD

Acute Aortic Syndromes refer to the spectrum of aortic emergencies that include: aortic dissection, intramural hematoma, penetrating aortic ulcer, aortic aneurysm leak and rupture, and traumatic aortic transection. This exhibit focuses on physiopathological mechanisms that lead to the development of aortic emergencies.

A classic aortic dissection begins with a laceration of the aortic intima and inner layer of the aortic media, forming an entrance tear, which allows entering blood to split the aortic media [1]. The splitting of the media is responsible for formation of a double channel aorta, with aortic dissection flap dividing the aortic lumen into true and false lumina. The intima and the inner part of the aortic media form the intimomedial flap. The flap tissue is composed mainly of aortic media delaminated from the aortic wall [2]. The outer part of the aortic media forms with the adventitia the false channel. There are usually re-entrance tears in the intima creating additional communication between the true and false lumina in the distal aorta.

Cystic medial necrosis associated with connective tissue disorders was once believed to contribute to aortic medial degeneration leading to aortic dissection. However, it was demonstrated that minority of patients with AD exhibited medial degeneration [3]. It was shown that in most patients the primary event was the intimal tear that allowed the blood to spread through the aortic media. When present, the degenerative changes within the media with loss of the elastic tissue reduce resistance of the aortic wall to hemodynamic stress, leading to subsequent dissection. Hypertension-related spontaneous rupture of the aortic vasa vasorum might lead to intramural hematoma and subsequently to intimal tear. Intramural hematoma precedes intimal rupture because hemorrhage of the vasa vasorum weakens the media, allowing the blood to leak into the media from the aortic lumen owing to arterial pressure [1]. Atherosclerosis was once thought to cause AD. However, it was shown that only in a small number of patients with AD there was a relationship between atheroma and location of dissection [1]. Dissection in the region of gross atherosclerosis is usually limited by neighboring fibrosis and calcification.

Mechanical forces contributing to AD include: flexional forces of the vessel at fixed sites, the radial impact of the pressure pulse, and the shear stress of the blood.

During the cardiac cycle, the heart and aorta produce rhythmic movements, allowing all but fixed segments to move. These fixed points of the aorta are exposed to the most significant flexional forces. Classic type A and B aortic dissections produce an intimal tear at the areas of greatest hydraulic stress: the right lateral wall of the ascending aorta, or the descending aorta in proximity to the ligamentum arteriosum. Hypertension with increased aortic blood pressure adds to a mechanical strain on the aortic wall and the shearing forces exerting a longitudinal stress along the aortic wall. Decreased vasa vasorum flow, occurring in arterial hypertension [4], may increase the stiffness of the outer ischemic media of the aorta to produce interlaminar shear stresses contributing to the development of aortic dissection.

Aortic intramural hematoma may occur as a primary event in hypertensive patients in whom there is spontaneous bleeding from vasa vasorum into the media or secondary to a penetrating atherosclerotic ulcer. IMH may also develop following blunt chest trauma with aortic wall injury. IMH is thought to begin with rupture of the vasa vasorum of the aortic wall with propagation of a hematoma that disrupts the medial layer of the aorta [2]. Consequently, IMH weakens the aorta and may progress to either outward rupture of the aortic wall or inward disruption of the intima, which leads to communicating aortic dissection [5]. IMH can be distinguished from mural thrombus by identification of the intima [6]; mural thrombus lies on top of the intima, which is frequently calcified, whereas IMH is subintimal. It has been documented [7] that type A IMH, has a high frequency of complications and, if possible, should be treated surgically. Type B IMH, uncommonly progresses to complications and frequently resolves completely (Table1.).

In penetrating aortic ulcer, an atheromatous plaque ulcerates and disrupts the internal elastic lamina, burrowing deeply through the intima into the aortic media [2,8]. When an atherosclerotic plaque penetrates into the media, the media is exposed to pulsatile arterial flow, causing hemorrhage into the wall leading to intramedial hematoma [9]. The plaque may precipitate a localized intramedial dissection associated with a variable amount of hematoma within the aortic wall, may break through into the adventitia forming a pseudoaneurysm, or may rupture. The rate of rupture in PAU 42%, is higher than in IMH 35% and in AD 3.6-7% [1]. Ulceration of an aortic atheroma occurs in patients with advanced atherosclerosis. In the same time, the presence of atherosclerotic changes limits the extent of disease.

The wall stress related to blood pressure in the non-aneurismal aorta is relatively low and uniformly distributed, whereas within the aortic aneurysm, regions of high and low stress distribution are present [10]. Increased tension stress results in progressive vessel dilatation and weakening of the aortic media. According to the LaPlace's Law, the wall tension is proportional to the radius for a given blood pressure. When an artery wall develops a weak spot and expands as a result, it might seem that the expansion would provide some relief, but in fact the opposite is true. The expansion subjects the weakened wall to even more tension. The weakened vessel continues to expand. A localized weak spot in an artery might gain some temporary tension relief by expanding toward a spherical shape, since a spherical membrane has half the wall tension for a given radius. Unfortunately, in an expanding aneurysm, forming a near-spherical shape is not possible to give sufficient tension relief. Aortic aneurysm rupture is believed to occur when the mechanical stress on the wall exceeds the strength of the wall tissue. It was documented that the abdominal aortic aneurysm expansion averaged 2-4 mm per year for aneurysms smaller than 4 cm, 2-5 mm for aneurysms 4-5 cm, and 3-7 mm for those larger than 5 cm. The rupture risk at four years was 2, 10, and 22% respectively [11].

One of the accepted mechanisms for traumatic aortic rupture from rapid deceleration involves a combination of traction, torsion, and hydrostatic forces created by differential deceleration of thoracic structures. Unequal horizontal shear forces that are applied during high-speed deceleration cause mobile ascending and descending aorta lag behind the transverse aortic arch, which is relatively fixed by the brachiocephalic vessels [12]. Deceleration forces place the maximal stress on those segments of the aorta and great vessels at points of attachments, the aortic isthmus and the aortic root. Another hypothesis involves the osseous-pinch theory [13], when the aorta is pinched between the spine and the anterior bony thorax during chest compression caused by abrupt deceleration. Compressive forces cause the manubrium, clavicle, and first ribs to rotate posteriorly and inferiorly with impaction of the anterior osseous structures on the vertebral column. This allows the osseous elements to shear interposed vascular structures. Most common injury occurs just distal to the left subclavian artery. The ligamentum arteriosus and the intercostal vessels fix the distal arch and descending thoracic aorta in apposition to the vertebral bodies. The superior portion of the arch is held in place by the great vessels extending from the thoracic inlet into the neck. Therefore, the relatively fixed proximal descending aorta cannot move away from the bony structures as they pinch and transect it. The spectrum of TAT includes: incomplete rupture (intramural hematoma without tear, intimal tear, intimomedial tear with pseudocoarctation or pseudoaneurysm), and complete rupture. The most common sites of arterial injuries are: aortic rupture alone (81%), aortic arch branches alone (16%), aorta and aortic branches (3%). Among the aortic injuries, 96% occur at the aortic isthmus distal to left subclavian artery, 1% at isthmus and proximal ascending aorta, 1% at proximal ascending aorta only, 1% at distal ascending aorta only, and < 1% at descending aorta [14].

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