One of the first successful repairs of a thoracoabdominal aortic aneurysm (TAAA) in the United States was reported in 1955 by Etheredge (1). Utilizing a 5-mm aortic shunt, an in situ aortic homograft repair of an extent IV aneurysm was performed via a thoracoabdominal incision (Video 1). This included anastomoses of the celiac and superior mesenteric arteries. That same year, Charles Rob, an English surgeon, also reported on his experience of 33 abdominal aortic aneurysms, six of which required lower thoracic aortic clamping approached via a thoracoabdominal incision. Rob’s report predated Etheredge’s manuscript, and he is credited with the first description of a TAAA repair (2). Cooley and DeBakey’s initial report of a descending thoracic aortic aneurysm (likely an extent I) approached via a thoracoabdominal incision in 1953 predated Etheredge’s manuscript as well (3), but this repair did not involve direct manipulation of the abdominal visceral arteries and thus Etheredge and Robb remain the first surgeons to describe such a technique. Shortly thereafter, DeBakey reported on four cases similar to Etheredge’s patient utilizing aortic homograft as the conduit, concluding that aortic shunting was imperative to success (4). His subsequent report in 1965 included 42 patients in whom knitted Dacron grafts were utilized as the initial shunt and then converted to the formal conduit by stepwise side branch anastomoses of the visceral arteries (5). Most of these initial reports involved aneurysmectomy which prolonged operative times and led to significant morbidity, and Crawford is credited with pioneering the technique of an intra-aortic anastomosis after longitudinal division of the sac. In addition, he also described the technique of a single pedicled visceral segment anastomosis including the celiac, superior mesenteric, and right renal arteries, followed by a left renal anastomosis with sequential reperfusion after the completion of each (6). His results were superb with only one death in 23 consecutive cases, a mortality rate that is rarely achieved in modern practice. Along with the utilization of cardiopulmonary bypass, hypothermic circulatory arrest, and cerebrospinal fluid drainage, Crawford’s approach most resembles contemporary techniques performed at major centers today.
Definition, etiology, and risk factors
Thoracoabdominal aortic aneurysms result from continuous dilation of the descending thoracic aorta extending into the abdominal aorta. Multiple configurations occur anywhere along the continuum from the origin of the left subclavian artery to the aortoiliac bifurcation. The medial layer of the aortic wall, comprised mainly of structural proteins such as collagen and elastin, contributes to aortic capacitance and elasticity. Degradation of these structural proteins or a defect in their composition leads to medial degeneration and weakening of the aortic wall. Subsequent dilatation results from hemodynamic forces on the arterial wall as well as intrinsic changes in the composition of the arterial wall itself. As defined by the law of Laplace, wall tension is proportional to the pressure multiplied by the radius of the arterial conduit; thus, as the diameter of the aorta increases, wall tension increases, creating a vicious cycle. Hypertension clearly exacerbates this process.
Several genetic disorders are known etiologies of aortic aneurysms. Patients with Marfan syndrome, an autosomal dominant condition resulting in abnormal fibrillin, commonly develop aortic aneurysms. Ehlers-Danlos syndrome, a collagen disorder, also causes similar clinical findings in some patients. Other disorders associated with aortic aneurysms include Turner’s syndrome, polycystic kidney disease, Loeys-Dietz syndrome, syphilis, arteritis, and traumatic injury (7).
It has been postulated that atherosclerosis plays a role in aneurysmal formation as well, and it is clear that if this relationship is not causal, the two conditions occur simultaneously in the majority of patients. The vast majority of TAAAs are the result of atherosclerotic disease, and these are deemed ‘degenerative’. Consequently, risk factors for TAAAs are similar to those for atherosclerosis itself and include smoking, hypertension, obesity, hyperlipidemia, chronic obstructive pulmonary disease (COPD), and family history. It is important to note, however, that in contrast to abdominal aortic aneurysms (AAA) in which the presence of concomitant coronary artery disease (CAD) is greater than 70%, patients with TAAAs have a much lower incidence of CAD, often cited as less than 30%. This seems to indicate that the respective etiologies of aortic dilatation differ somewhat between the thoracic and the abdominal aorta (8,9).
Aortic dissection is another risk factor for development of TAAAs with up to 40% of patients with chronic dissection eventually requiring repair (10). Hypertension, specifically diastolic blood pressure greater than 100 mmHg, seems to be the most consistent risk factor associated with dissection progressing to aneurysm formation (11,12).
Natural history and incidence
The natural history of aortic aneurysms is dissection or rupture. Population studies have revealed the incidence of thoracic aortic aneurysms to be in the range of 10 new aneurysms per 100,000 person-years (13). Up to 80% of these will eventually rupture, owing to a 10-20% five-year survival of patients who remain untreated. Females generally develop TAAAs later in life than men but are at a higher risk of rupture, and in both sexes advanced age is also associated with a higher risk. With respect to comorbidity, COPD had been shown to significantly increase the odds of rupture by 3.6 times (14). The transverse diameter of the aneurysmal aorta is directly related to the risk of rupture. It has been shown that for every 1 cm of growth over 5 cm in the descending thoracic aorta, the risk of rupture nearly doubles (14), resulting in an annual rupture risk for aneurysms greater than 6 cm of 7% (15). Among patients with TAAAs greater than 7 cm, 43% will eventually progress to dissection or rupture (16).
In 1986, Crawford described the first TAAA classification scheme based on the anatomic extent of the aneurysm (17). Type I involves most of the descending thoracic aorta from the origin of the left subclavian to the suprarenal abdominal aorta. Type II is the most extensive, extending from the subclavian to the aortoiliac bifurcation. Type III involves the distal thoracic aorta to the aortoiliac bifurcation. Type IV TAAAs are limited to the abdominal aorta below the diaphragm. Safi’s group modified this scheme by adding Type V, which extends from the distal thoracic aorta including the celiac and superior mesenteric origins but not the renal arteries (18). See Figure 1.
Indications for repair
While it may seem obvious, it is important to note that all symptomatic aortic aneurysms regardless of size or anatomic extent should be addressed surgically. These symptoms most commonly present as pain or pressure. In the case of the descending thoracic aorta, aneurysmal pain is often described as intrascapular or chest pain radiating to the back, with a ‘tearing’ or ‘stabbing’ quality. Unfortunately, very few patients present with symptoms prior to an acute aortic event, with up to 95% of the events occurring in the absence of any heralding symptoms (19).
The size criteria for repair of asymptomatic TAAAs have been extensively debated in the literature, with groups advocating elective intervention at anywhere from 5 to 10 cm. Elefteriades et al. have published extensively on the natural history and rupture risk of the thoracic aorta stratified by diameter and provide the following guidelines for repair of the descending thoracic aorta (20):
(II) Acute dissection resulting in malperfusion or other life altering complications
(III) Symptomatic states
(i) Pain consistent with rupture and unexplained by other causes
(ii) Compression of adjacent organs
(IV) Documented enlargement ≥1 cm/year or substantial growth approaching absolute size criteria
(V) Absolute size >6.5 cm or >6.0 cm in patients with connective tissue disorders
Absolute size criteria must be adjusted in patients of extreme size, and nomograms are available to assist the surgeon in decision making for these patients. It is also important to realize that the size criteria above are based on the premise that the appropriate operative time is realized when the annual risk of rupture exceeds the mortality of the proposed procedure. Surgeons at individual institutions must weigh this fact in conjunction with their own mortality rates to best guide the appropriate therapy.