The etiology of thoracoabdominal aneurysms, in order of frequency, is medial degenerative disease (myxomatous or myxoid degeneration, senile aorta), dissection, Marfan syndrome (cystic medial necrosis), Ehlers-Danlos syndrome, infection (mycotic), aortitis (Takayasu's disease), and trauma ( Table 41-1 ). [1 ] – [3 ] Traditionally, many thoracic aortic aneurysms were termed “atherosclerotic aneurysms.” Although atherosclerosis and aortic aneurysms share common risk factors and frequently occur concomitantly, thoracic aortic aneurysms primarily are the result of age-related changes in elastin and collagen that lead to a loss of integrity and strength. Subsequent enlargement and aneurysm formation provide fertile ground for superimposed intimal atherosclerosis and further degeneration of the aortic wall.
Most etiologic conditions produce diffuse, fusiform aneurysmal dilatation. One exception to this is infection (mycotic aneurysm), which frequently produces a saccular aneurysm at localized areas of the aortic wall destroyed by the mycotic process. Characteristically, for unknown reasons, such mycotic aneurysms tend to occur along the lesser curvature of the transverse aortic arch or in the upper abdominal aorta immediately posterior to the origin of the visceral vessels. In such cases, only a portion of aortic circumference is affected and consequently localized weakening causes a diverticular or saccular outpouching. Saccular aneurysms of the thoracic aorta, taken as a whole, are more frequently secondary to atherosclerosis, although both mycotic and degenerative saccular aneurysms may be superimposed on or combined with fusiform, more generalized aneurysmal disease of the thoracoabdominal aorta.
In patients with Marfan syndrome, the aortic wall is weakened by fragmentation of elastic fibers and deposition of extensive amounts of mucopolysaccharides. [4 ] Many patients with Marfan syndrome have an abnormal mutation of the fibrillin gene located on the long arm of the 15th chromosome. [5 ] Abnormal fibrillin in the extracellular matrix decreases connective tissue strength in the aortic wall and produces abnormal elastic properties that predispose the aorta to dilatation from wall tension resulting from left ventricular ejection impulses (DP/DT). Laplace's law causes cycles of progressive dilatation as increasing luminal diameters produce greater wall tension. The usual histologic changes of the aging aorta include cystic medial necrosis, elastin fragmentation, fibrosis with increased collagen, and medial necrosis. [6 ]
The diameter of the aorta remains the single most important factor in the decision to repair a thoracic aortic aneurysm. The Ad Hoc Committee on Reporting Standards of the Society for Vascular Surgery and the North American Chapter of the International Society for Cardiovascular Surgery states that the definition of an aneurysm is “a permanent localized dilatation of an artery having at least 50 percent increase in diameter compared to the expected normal diameter of the artery in question.” [7 ] This definition can be applied to the thoracic aorta, but, it is first necessary to know the normal diameter. The average diameter of the mid-descending thoracic aorta is 28 mm for men and 26 mm for women; at the level of the celiac axis 23 mm for men and 20 mm for women; and the infrarenal aorta 19.5 mm for men and 15.5 mm for women. [8 ] Normal aortic diameters, however, vary according to age, gender, and body surface area. Aortic enlargement with advancing age is reported in a number of studies. [9 ] Even when corrected for age and body surface area, aortic size is statistically smaller in women than in men. On average, the aorta is 2 to 3 mm greater in diameter for men than for women. Body surface area is a better predictor of aortic size than height or weight and best correlates with aortic diameter in patients less than 50 years of age. [10 ]