Thoracic Aortic Aneurysms

Anatomically the thoracic aorta is divided into a several distinct segments (Figure 1.1). The ascending aorta extends from the left ventricle (at the aortic annulus) and rises in the anterior mediastinum to the innominate artery. The base of the ascending aorta is referred to as the aortic root. The root is the widest aortic segment and is comprised of three coronary sinuses, which bulge outward, and serves as the support structure for the aortic valve cusps. The portion of the ascending aorta above the root is narrower and tubular in shape. Distal to the ascending aorta is the aortic arch, which moves posteriorly and to the left in the superior mediastinum, extending from the innominate artery to the ostium of the left subclavian artery. Thereafter, the descending aorta courses posteriorly, adjacent to the vertebral column, and continues to the level of the diaphragm, after which it becomes the abdominal aorta.

The true incidence of thoracic aortic aneurysms is difficult to determine, as many go undiagnosed. However, in a Mayo Clinic sampling from 19801994, the incidence in Olmstead County, Minnesota, was 10.4 per 100,000 person years1. This was significantly higher than the incidence in the same

Bicep Muscle Diagram
Figure 1.1. Thoracic aortic anatomy. © Massachusetts General Hospital Thoracic Aortic Center. Used with permission.

population prior to 1980 but may have reflected advances in diagnostic imaging techniques. Thoracic aortic aneurysms are classified according to the segment of aorta involved—either ascending, arch, or descending thoracic aortic aneurysms. Aneurysms of the descending thoracic aorta that extend below the diaphragm are known as thoracoabdominal aortic aneurysms. The anatomical distinctions are important because the etiology, natural history, and treatment of thoracic aneurysms differ for each of these segments. Based on the most current data, approximately 60% of thoracic aneurysms involve the ascending aorta, 10% involve the arch, 40% involve the descending aorta, and 10% involve the thoracoabdominal aorta2.

Thoracic aortic aneurysms most often result from cystic medial degeneration, which appears histologically as smooth muscle cell drop-out and elastic fiber degeneration, resulting in the presence in the media of cystic spaces filled with mucoid materia. The histologic changes occur most frequently in the ascending aorta but in some cases may involve the entire aorta. The medial degeneration results in a weakening of the aortic wall, which in turn leads to progressive aortic dilatation and eventually an aneurysm.

CYSTIC MEDIAL DEGENERATION Hypertension

Cystic medial degeneration is known to occur to some extent with aging, but this process is accelerated by hypertension3. Hypertension leads to intimal thickening, degradation of the extracellular matrix, loss of elastic fibers, and smooth muscle cell necrosis. As a consequence, the aortic wall becomes stiff and progressively dilates. Thus, advanced age and hypertension are, collectively, important risk factors for the development of thoracic aortic aneurysms.

Marfan Syndrome

On the other hand, when cystic medial degeneration occurs at younger ages, it is classically associated with recognized connective tissue disorders, such as Marfan syndrome (see Chapter 2) or, less commonly, Ehlers-Danlos syndrome (see Chapter 2) or Turner syndrome. Among those with Marfan syndrome, thoracic aortic aneurysms predominantly involve the aortic root in a pattern known as annuloaortic ectasia. Penetrance is variable, and in some with Mar-fan syndrome the aortic root is significantly aneurysmal by the teenage years, whereas others have much slower progression of disease, and still others have minimal or no aortic dilatation.

Bicuspid Aortic Valve

Other congenital conditions can predispose to thoracic aortic aneurysms. It is now well recognized that those with a congenital bicuspid aortic valve have a significantly increased risk of aortic dilatation, aneurysm, and dissection. Echocardiography studies of young people with normally functioning (neither stenotic nor regurgitant) bicuspid aortic valves have shown that about 50% have dilatation of the ascending aorta4. In the majority of cases the dilatation involves the tubular portion of the ascending aorta, whereas in a minority it involves primarily the aortic root (annuloaortic ectasia). A number of studies have identified cystic medial degeneration as the culprit. In one series, of those with bicuspid aortic valve undergoing aortic valve replacement surgery, 75% had biopsy proven cystic medial degeneration, compared with a rate of 14% among those with tricuspid aortic valves undergoing similar surgery5. One possible mechanism for the association of cystic medial degeneration and bicuspid aortic valve is that inadequate production of fibrillin-1 during embryogenesis results in both the bicuspid aortic valve and a weakened aortic wall6. No single gene responsible for bicuspid aortic valve has yet been identified, and it may well be genetically heterogeneous.

Familial Thoracic Aortic Aneurysm Syndrome

Cystic medial degeneration has also been found as the cause of thoracic aortic aneurysms among many of those with neither an overt connective tissue disorder nor a bicuspid aortic valve. Moreover, while such cases of thoracic aortic aneurysms may be sporadic, they are often familial in nature and have now been termed the familial thoracic aortic aneurysm syndrome.

In an analysis using a large database of patients with thoracic aortic aneurysms, the Yale group found that at least 19% of patients without Marfan syndrome had a family history of a thoracic aortic aneurysm7. Moreover, they found that those with familial syndromes presented at a mean age of 57 years, which was significantly younger than the sporadic cases, who presented at a mean age of 64 years. Most pedigrees have suggested an autosomal dominant mode of inheritance, but some have suggested a recessive mode and possibly X-linked inheritance as well7. In a study of 158 patients referred for surgical repair or thoracic aortic aneurysms or dissections, Biddinger et al. found that first-degree relatives of probands had a higher risk (RR 1.8 for fathers and sisters, RR 10.9 for brothers) of thoracic aortic aneurysms or sudden death compared with controls8.

The genetics of the familial thoracic aortic aneurysm syndrome are being actively investigated. Milewicz et al. have identified a mutation on 3p24.2-25 that can cause both isolated and familial thoracic aortic aneurysms9. Aortic histopathology of these families reveals cystic medial degeneration. There appears to be dominant inheritance, yet there is marked variability in the expression and penetrance of the disorder, such that some inherit and pass on the gene but show no manifestation. More recently, two studies of familial thoracic aortic aneurysm syndromes have mapped mutations to at least two different chromosomal loci, whereas other families mapped to neither of these—suggesting at least a third locus10,11. The genetics may be rather complex; indeed, the fact that there is such variable expression and penetrance suggests that this may be a polygenic condition.

Atherosclerosis

Atherosclerosis is infrequently the cause of ascending thoracic aortic aneurysms and, when it is, tends to be associated with diffuse aortic atherosclerosis. Aneurysms of the aortic arch are most often contiguous with aneurysms of the ascending or descending thoracic aorta. Arch aneurysms may be due to atherosclerotic disease but are often due to cystic medial degeneration and syphilis or other infections. Conversely, atherosclerosis is the predominant etiology of aneurysms of the descending thoracic aorta. These aneurysms tend to originate just distal to the origin of the left subclavian artery and may be either fusiform or saccular. The pathogenesis of such atherosclerotic aneurysms in the descending thoracic aorta may resemble that of abdominal aneurysms but has not been extensively examined.

Syphilis

Whereas syphilis was once the most common cause of ascending thoracic aortic aneurysms, accounting for up to 80% of cases, in the current era of aggressive antibiotic treatment of the disease it is now rarely the cause. The latent period from initial spirochetal infection to aortic complications is most commonly 10-25 years (range 5 to 40 years). During the secondary phase of the disease, spirochetes directly infect the aortic media, with the ascending aorta most often affected. The infection and attendant inflammatory response destroys the muscular and elastic medial elements, leading to weakening of the aortic wall and progressive aneurysmal dilatation.

Vasculitis

Takayasu's arteritis typically causes obliterative lesions of the aorta, producing signs and symptoms of vascular insufficiency, but less often can produce aortic aneurysms. Takayasu's arteritis primarily affects young the young, typically those 10-30 years old, with females affected in 90% of the cases. It occurs most often in Asian populations. On the other hand, giant cell arteritis tends to affect an older population, especially those over the age of 55, but again with females affected far more often than males. In most cases, giant cell arteritis presents with signs and symptoms of temporal arteritis. When the aorta is affected, it may result in thoracic aortic aneurysms, most often involving the arch or descending aorta.

Other Causes

Infectious aneurysms may result from a primary infection of the aortic wall causing aortic dilatation with the formation of fusiform or saccular aneurysms. Thoracic aortic aneurysms can also result from aortic trauma (see Chapter 2) or aortic dissection (see below).

Natural History

The natural history of thoracic aortic aneurysms is affected by the size and location of the aneurysm. The best data presently available on the natural history of thoracic aortic aneurysms come from a longitudinal report by Davies et al. from the Yale group in which 304 patients with thoracic aortic aneurysms at least 3.5 cm in size were followed for a mean of more than 31 months12. The mean rate of growth for all thoracic aortic aneurysms was 0.1 cm per year. However, the rate of growth was significantly greater for aneurysms of the descending aorta (0.19 cm per year) than for those of the ascending aorta (0.07 cm per year). In addition, dissected thoracic aneurysms grew significantly more rapidly (0.14 cm per year) than did nondissected ones (0.09 cm per year). Not surprisingly, those with Marfan syndrome also had more rapid aneurysm growth.

In this same study population, the mean rate of aortic rupture or dissection was 2% per year for thoracic aortic aneurysms less than 5 cm in diameter, 3% per year for aneurysms 5.0-5.9 cm, and 7% per year for aneurysms 6.0 cm or larger. In a multivariate logistic regression analysis of the predictors of dissection or rupture, the relative risk of an aneurysm diameter of 5.0-5.9 cm was 2.5, an aneurysm diameter of 6.0 cm or larger was 5.2, Marfan syndrome was 3.7, and female gender was 2.9. Other natural history studies that focused on thoracic and thoracoabdominal aneurysms have found that the odds of rupture are increased by chronic obstructive pulmonary disease (RR 3.6), advanced age (RR 2.6 per decade), and aneurysm-related pain (RR 2.3)13.

Thoracic aortic aneurysm size is an important predictor of rate of growth. Dapunt et al. monitored 67 patients with thoracic aortic aneurysms and found that aneurysms that were 5.0 cm or smaller grew more slowly than did those larger than 5.0 cm, and the only independent predictor of rapid expansion (>0.5 cm per year) was an initial aortic diameter greater than 5.0 cm14. Nevertheless, even when controlling for initial aneurysm size, substantial variation was still seen in individual aneurysm growth rates, thus making such mean growth rates of little value in predicting aneurysm growth for a given patient.

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