Introduction
Aneurysms of the thoracic aorta account for fewer than 10% of aortic aneurysms. Medial degeneration, chronic dissection, vasculitis, and collagen-vascular disease (Marfan’s syndrome or Ehlers-Danlos syndrome) are common causes; syphilis is now a rare cause of thoracic aneurysm. Traumatic aneurysms occur at the ligamentum arteriosus just beyond the left subclavian artery and result from shearing injury during rapid-deceleration automobile accidents.

Thoracoabdominal aneurysms are categorized by the Crawford classification: type I extends from the left subclavian artery to the renal arteries, type II from the left subclavian artery to the iliac bifurcation, type III from the midthoracic to the infrarenal region, and type IV from the distal thoracic aorta to the infrarenal region. The prevalence of each type of thoracoabdominal aneurysm is roughly equal, but type IV aneurysms have the lowest operative mortality (2-5%) and the lowest risk of postoperative neurologic deficits (2-10%).

Clinical Findings

A. Symptoms and Signs
Clinical manifestations depend largely on the size and position of the aneurysm and its rate of growth. Most are asymptomatic and are discovered during a diagnostic procedure undertaken for other reasons. Some patients present with substernal, back, or abdominal pain. Others develop dyspnea, stridor, or a brassy cough from pressure on the trachea, dysphagia from pressure on the esophagus, hoarseness from pressure on the left recurrent laryngeal nerve, or neck and arm edema from external compression of the superior vena cava. Aortic regurgitation due to distortion of the aortic valve annulus may occur with aneurysms of the ascending aorta.

B. Imaging
An aneurysm suspected on chest radiography must be differentiated from other anterior mediastinal masses, including lung neoplasm, thymoma, cyst, and substernal goiter. CT scan and MRI are the most sensitive and accurate means of imaging thoracic aneurysms. Aortography may be necessary to assess involvement of the arch vessels. The coronary vessels and the aortic valve should also be studied if aortic root replacement is anticipated.

Treatment
Control of hypertension and use of ß-blockers may slow aneurysmal growth. Indications for surgical treatment include the presence of symptoms, rapid expansion, or size greater than 5 cm. Operative risk from comorbid medical conditions must be considered when recommending repair of asymptomatic aneurysms. Morbidity and mortality are higher than with abdominal aortic aneurysms; the 30-day operative mortality is 8-20% with repair of type I and type II thoracoabdominal aneurysms. The thoracotomy incision is associated with a higher risk of pulmonary complications and more challenging postoperative pain management. Proximity to the recurrent laryngeal nerve, the phrenic nerve, and the carotid and subclavian arteries makes injury to these structures possible. The great radicular artery (artery of Adamkiewicz) arises from an intercostal artery between T8 and L1 and is the dominant artery to the spinal cord in 80% of patients, imposing a 5-30% risk of paraplegia during thoracic aneurysm repair. Use of left heart bypass to femoral bypass to preserve retrograde perfusion below the level of the cross-clamp or antegrade perfusion by direct cannulation of selected arteries reduces end-organ ischemia. Lumbar drains have been proven to reduce the rate of paraplegia.

Endovascular repair of thoracic aortic aneurysms reduces cardiopulmonary risk, but the location of the aneurysm may preclude endovascular repair by current methods. Recent investigations involve development of branched stent grafts for repair of arch and thoracoabdominal aneurysms.

Prognosis
Five-year survival for patients with unrepaired thoracic aneurysms greater than 6 cm is 20-25%. Most deaths are due to rupture or to the complications of generalized atherosclerosis.

Chuter TAM et al: Multi-branched stent-graft for type III thoracoabdominal aortic aneurysm. J Vasc Interv Radiol 2001; 12:391.

Elefteriades JA: Natural history of thoracic aortic aneurysms: indications for surgery, and surgical versus nonsurgical risks. Ann Thorac Surg 2002;74:S1877.

Gowda RM et al: Endovascular stent grafting of descending thoracic aortic aneurysms. Chest 2003;124:714.