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Descending Thoracic and Thoracoabdominal Aortic Aneurysms: General Principles and Open Surgical Repair
The first reported repairs of descending thoracic aortic aneurysms were published in the early 1950s. The first to successfully perform a thoracoabdominal aortic aneurysm repair were Samuel Etheredge and colleagues in 1955 in Oakland, California. One year later, Michael DeBakey and colleagues performed a true thoracoabdominal repair using a homograft. Subsequently the same group devised an ingenious method of using a Dacron tube graft from the descending thoracic aorta to the infrarenal aorta, sequentially bypassing the celiac axis, superior mesenteric artery, and both renal arteries. This technique decreased the load on the heart and the ischemic time to the intestines and the kidneys. Stanley Crawford's contributions to thoracoabdominal aortic aneurysm repair incorporated three principles of vascular surgery: the Matas inclusion technique, whereby the aneurysm sac is opened but not resected; the reattachment of the intercostal arteries to the graft, as described in an animal model by Spencer in 1950 ; and the reattachment of the small arteries into a large artery, attributed to Carrel and Guthrie in 1906. Crawford accrued 28 cases by 1973 and published his results of a large group of aneurysm cases treated for the first time with the same clamp-and-sew technique. This technique depended on expeditious repair with short aortic clamp times because the spine and viscera suffered ischemia during repair. Several adjunctive methods for spinal cord and visceral protection later emerged, including distal aortic perfusion, cerebrospinal fluid (CSF) drainage, and systemic or regional profound hypothermia.
By the end of the 1980s the preferred method of treatment was still the clamp-and-sew technique, but it continued to be associated with high complication rates, predominantly because of spinal neurologic deficits and renal failure. Since 1992, supported by animal models and clinical experience, the clamp-and-sew technique has been largely abandoned in favor of distal aortic perfusion with the addition of perioperative CSF drainage and moderate passive hypothermia. This chapter provides a review of the authors' experience in the repair of descending thoracic and thoracoabdominal aortic aneurysms, including its natural history, pathology, diagnosis, open surgical treatment, postoperative complications, and management.
Natural History
The adjusted incidence of thoracic aortic aneurysms in population-based studies is 10 per 100,000 person-years. The incidence appears equal in men and woman but increases with age. The goal of aneurysm repair is to prevent rupture, complications from dissection, and death. Open repair of descending thoracic and thoracoabdominal aortic aneurysms is one of the most technically demanding operations in vascular surgery. Adjunctive surgical techniques have led to significant reductions in morbidity, especially from spinal cord ischemia. These improvements underscore the importance of a multidisciplinary approach to this complex problem.
Nearly 10,000 people die annually from thoracic aortic aneurysm, making it the nineteenth leading cause of death in the United States. Overall, aortic aneurysm and dissection accounted for 3.1 deaths per 100,000 people in the United States in 2013. This rate dramatically rises to 7.8 and 20.8 per 100,000 in the age groups of 65 to 74 and 75 to 84 years, respectively. Death from thoracic aortic aneurysm is primarily due to rupture. The rate of rupture increases with aneurysm expansion. The natural history of thoracic aortic aneurysm is expansion at a median 0.1 to 0.3 cm/year. After 5 cm, growth and rupture can occur unpredictably.
In the Olmsted County study, no ruptures occurred in thoracic aneurysms less than 4 cm in size. The 5-year rupture risk increased to 16% for aneurysms less than 6 cm and was greater than 30% for aneurysms larger than 6 cm. The risk of rupture or dissection occurs at nearly 7% per year. After rupture, the mortality rate is greater than 90% in the first 24 hours. Most patients die before reaching the emergency room. However, elective surgically repaired thoracic aortic aneurysms fare much better than those medically managed or after emergency surgery.
The most common thoracic aneurysm occurs in the ascending aorta, but descending thoracic aortic aneurysms account for 45%, whereas thoracoabdominal aortic aneurysms account for approximately 5% of all thoracic aortic aneurysms. Based on the natural history and risk of rupture, descending thoracic and thoracoabdominal aortic aneurysms are considered for repair when maximum diameter reaches 5 to 6 cm. Saccular aneurysms and those with growth greater than 5 mm in 6 months are also considered for repair, regardless of size. Aneurysms in patients with Turner syndrome, Marfan syndrome, and other connective tissue diseases are repaired at smaller diameters.
Pathology and Etiology
An aneurysm is a localized or diffuse dilatation greater than 50% of the reference diameter. Most aneurysms are due to medial degeneration and appear to occur sporadically, but genetic factors are increasingly recognized. Up to 20% of individuals with thoracic aortic aneurysms and aortic dissections (TAADs) have a first-degree relative with thoracic aortic disease. Marfan syndrome is an inherited connective tissue disorder caused by any mutation—missense, nonsense, exon deletion or insertion—of fibrillin-1 protein that is encoded in the FBN1 gene ( Fig. 36.1 ). Other syndromes that predispose individuals to abdominal aortic aneurysms include Loeys-Dietz syndrome, variants of Ehlers-Danlos syndrome, and Turner syndrome. Mutations in α-actin, such as ACTA2 and FOXE3, highlight the importance of vascular smooth muscle function in aortic aneurysm and dissection. Familial TAAD is diagnosed based on three criteria: (1) the presence of dilatation and/or dissection of the thoracic aorta; (2) the absence of clinical features of Marfan syndrome, Loeys-Dietz syndrome, or vascular Ehlers-Danlos syndrome; and (3) the presence of a family history of TAAD. TGFBR2 , TGFBR1 , MYH11 , ACTA2 , MYLK , SMAD3 , and two loci on other chromosomes, AAT1 (FAA1), and AAT2 (TAAD1) are associated with familial TAAD. The recognition of genetic variants in the development of aortic aneurysm will surely increase because many patients report multiple family members with thoracic aortic disease despite a negative genetic workup.
Like atherosclerosis, aortic aneurysm pathophysiology includes a robust inflammatory component. However, unlike atherosclerosis, which primarily affects the intima and causes occlusive disease, the inflammation in aneurysms occurs in the media and adventitia. The histology of degenerative aortic aneurysms is characterized by thinning of the media with distraction of the smooth muscle and elastic tissue (elastin). There is infiltration of inflammatory cells, including mast cells, with neovascularization ( Fig. 36.2 ). There is also activation of proteolytic enzymes and their inhibitors within the aortic wall.
A small percentage of aneurysms are related to infection, so-called mycotic infection, or to trauma, such as pseudoaneurysm. Bacteria or septic emboli may seed the atherosclerotic or ulcerated aortic wall. Infection may also spread from an adjacent empyema or lymph nodes. Organisms associated with aortic infection include Salmonella species, Haemophilus influenzae, Staphylococcus species, Mycobacterium tuberculosis, and Treponema pallidum . These aneurysms are usually saccular and carry a high risk of rupture.
Traumatic injuries of the thoracic aorta are also prone to rupture and should be repaired unless the injury is confined to the intima. Consistent with consensus guidelines, endovascular repair is commonly used for injuries of the distal aortic arch and descending thoracic aorta. Open repair is indicated when anatomy is not suitable for endovascular repair.
Clinical Manifestation
Many thoracic and thoracoabdominal aneurysms are discovered incidentally and most often develop without symptoms. Indications for surgical intervention depend on the size of the aneurysm and the presence or absence of symptoms. Symptoms are usually due to sudden expansion of the aneurysm, which can cause a vague pain in the back or sometimes a sharp pain that may denote the presence of rupture or impending rupture. Other symptoms are related to pressure on adjacent structures, such as pressure on the bronchus, that can cause respiratory distress or pressure on the recurrent laryngeal nerve, causing vocal hoarseness ( Fig. 36.3 ). Pressure on the esophagus can cause difficulty swallowing. On rare occasions, paraplegia and paraparesis can be caused by occlusion of critical intercostal arteries. There is also a risk of distal embolization causing “blue toe” syndrome ( Fig. 36.4 ).
Certain manifestations of rupture in 10% to 20% of thoracoabdominal aortic aneurysms include the sudden onset of severe or sharp pain radiating to the back and causing a drop in hemoglobin and blood pressure. In a review of more than 100 ruptured descending thoracic and thoracoabdominal aortic aneurysms, the most common site of rupture was the pleural cavity. Other sites of rupture occurred into the lung, esophagus, mediastinum, retroperitoneum, and peritoneal cavity. Thus, in addition to back, chest, or abdominal pain, patients may also have hemoptysis or hematemesis.
Diagnosis
Computed tomography (CT) and magnetic resonance imaging (MRI) represent the preferred diagnostic modalities. CT allows for visualization of the aortic wall, the aneurysm size, the presence or absence of clot formation, intercostal arteries, and effects on the surrounding tissues. The disadvantage of CT is the effect of ionizing radiation and the contrast media, which can exacerbate poor renal function. Three-dimensional reconstruction of axial imaging provides an additional view of thoracoabdominal aortic aneurysms ( Fig. 36.5 ). MRI is now widely available and has the advantage of not using ionizing radiation or requiring an intravenous contrast agent. However, MRI has long acquisition times, provokes claustrophobia, and is contraindicated for patients with cerebral aneurysm clips, certain mechanical valves, or other internal metallic hardware. It is used less frequently because of these limitations.
Transesophageal echocardiography (TEE) provides excellent views of the ascending and descending aorta as well as heart function. TEE is an important tool in the diagnosis of acute aortic dissection. The limitation of TEE is that it is invasive, requires an expert echocardiographer, and does not provide adequate imaging of the aortic arch or areas below the level of the diaphragm.
Intravascular ultrasound (IVUS) is emerging as a useful adjunct, especially in circumstances of aortic dissection or when there is diagnostic uncertainty, as in traumatic aortic injury ( Fig. 36.6 ). This modality allows for high-resolution imaging of the aortic wall, dynamic visualization of intimal lesions, and delineation of branch-vessel involvement. The drawback is that it is operator-dependent, requires invasive intraarterial access, and adds additional cost.
Thoracic Aneurysm Classification
Descending thoracic aortic aneurysms are divided into three extents ( Fig. 36.7A ). Extent A designates an aortic aneurysm from the distal origin of the left subclavian artery to the level of the sixth thoracic intercostal space (T6). Extent B is from T6 to the 12th thoracic intercostal space (T12). Extent C denotes the entire descending thoracic aorta from the left subclavian artery to T12.
Thoracoabdominal aortic aneurysms are classified in five categories (see Fig. 36.7B ). Extent I is from the left subclavian to above the most proximal renal artery. Extent II is from the left subclavian artery to below the renal arteries. Extent III is from the sixth intercostal space to below the renal arteries. Extent IV is from T12 to below the renal arteries. Extent V, which was introduced in the last 2 decades, is from T6 to just above the renal arteries. The importance of the classification scheme is that it correlated with the incidence of neurologic deficits and mortality. Currently we consider extents II and III to be higher risk for spinal ischemia and early mortality compared with the others.
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