Tran and Khoynezhad Etiology and pathogenesis The heart serves as a positive displacement pump to supply blood to organs in humans. For every cardiac cycle, the left ventricle pushes (displaces) a ﬁxed amount of blood into the aorta, which serves not only as a conduit but also as a modulator and damper to convert this pulsatile blood ﬂow into a even reservoir of blood supply to the issues. The aortic wall is made up of three layers: the inner layer of intima, the middle layer of media, and the outer layer of adventitia (Figure 1). The intima is made up of an endothelial lining, a highly active metabolically monolayer, which is supported by a sublayer of loose connective tissue. This intimal sublayer allows the intima to move relative to the media when the aorta expands or contracts during systole and diastole. The media is made up of a meshwork of concentric fenestrated lamellar layers of elastin fibers (about 50). Interposed between the elastin ﬁber layers are concentric smooth muscle cells and collagen ﬁbers, proteoglycans, and glycoproteins. Elastin ﬁbers are made of elastin, a highly stretchable material that is able to stretch up to 2–3x of its original length without rupture. Elastin is the material that gives the aorta its distensibility. Collagen ﬁbers, on the other hand, are very stiff and are up to 5,000 times stiffer than elastin. Collagen gives the aorta the structural integrity and resistance to hemodynamic stress as well as the shearing effects created by the force of blood ﬂow. In addition, the rings of smooth muscle cells in the media also contribute to aorta vascular stiffness. Under the inﬂuence of autonomic control, smooth muscle cells in the media contract and relax, giving the aorta its vascular tone. The outermost layer, the adventitia, is made of the tough collagen ﬁbers and connective tissue. Vasa vasorum (“vessels of the vessel”) are small blood vessels located in the adventitia and the outer part the media. These blood ves- sels provide nutrients to the media and adventitia since the wall of the aorta is too thick to deride nourishment from the blood in the aortic lumen via diffusion. Although it may appear overly simplistic, acute aortic dissection occurs when the aortic wall, already weakened by disease processes, is further subjected to elevated stress from systemic hypertension. Classic AAD begins with a tear in the intimal layer. Blood under pulsatile pressure subsequently forces the tear to open and dissects along the media layers in the diseased aortic wall, forming a false lumen(s) (Figure 2). The dissection can propagate distally, proximally, or both along the extent of the aorta. In the majority of cases ( 90%), as the dissection propagates, it also causes 1 secondary tear in the intimomedial layer, allowing blood to ﬂow back to the 54 true lumen of the aorta. While the primary entry site is usually in the thoracic aorta, the secondary site can be in the lower descending or abdominal aorta. AAD can be catastrophic because the dissecting false lumen can rupture, causing rapid exsanguination and death. The false lumen can also obstruct the arterial branches coming off the aorta, causing serious and potentially lethal complications. In type A AAD, these include stroke, acute myocardial infarction, pericardial tamponade, or aortic valve insufﬁciency. In type B AAD, potential complications include intestinal ischemia, renal failure, paraplegia, and other malperfusion syndromes. There are two similar pathological entities worth mentioning: intramural hematomas (IMH) and penetrating atherosclerotic ulcers (PAU). Patients with IMH or PAU have clinical presentations that are indistinguishable from AAD. IMH refers to a condition where there is a bleed within the aortic wall without the evidence of an intimomedial ﬂap. It is often considered a precursor of the classic aortic dissec- tion and thought to be the result of mechanical tearing of the vaso vasorum. The hematoma can expand inward causing a tear on the intimal side, causing blood to dissect along the extent of the aorta, resembling classic acute aortic dissection. PAU refers to a condition where there is an ulceration of atherosclerotic aortic plaques, usually in the descending thoracic and abdominal aorta. These lesions can lead to a penetration into the intimal borders and into the media, resembling a localized hematoma and an IMH. They can also start dissecting along the extent of the aorta, resembling a classic aortic dissection. The entry tear at the ulceration may be thicker and less mobile compared to the intimomedial ﬂap seen in the classic aortic dissection. PAU tends to affect the descending thoracic aorta and the abdominal aorta. Its management is similar to type B AAD. Risk factors Both acquired conditions and connective tissue disorders are known to cause certain individuals to be at risk for acute aortic dissection. Data from the International Registry of Aortic Dissection (IRAD)6,8 showed the following risk factors in AAD: male sex, age, a history of hypertension or atherosclerosis, prior cardiac surgery including aortic valve surgery, a history of bicuspid aortic valve, or a his- tory of Marfan syndrome. The average age for AAD was 63.1 years, with type B slightly older (66.3 vs 61.2 years). 65.3% of patients were male. 62.3% of the patients had type A dissection, 72.1% had a history of hypertension, and 4.9% had Marfan syndrome.6 Of particular interest to the physicians evaluating these patients in the emergency Vascular Health and Risk Management 2009:5