Melissa P. Taylor and Abelardo DeAnda Jr.
This chapter is a revision and update of that included in previous editions of the TSRA Review written by Ibrahim Sultan (2nd edition), Mani A. Daneshmand (1st edition), and Jason A. Williams (1st edition).
Definition
Aneurysms are present when there is an enlargement of the aortic diameter of more than 1.5 times normal. Normal diameters vary on sex and location. The average diameter for mid-descending thoracic aorta for men is 28 mm and 26 mm for women. The aorta progressively gets smaller as it descends toward the iliac bifurcation. Thoracoabdominal aneurysms (TAAAs) are classified by their anatomic location extending from distal to the left subclavian artery into the abdominal aorta. The celiac artery is a landmark that divides thoracoabdominal aneurysms into descending thoracic aneurysms and abdominal aortic aneurysms.
Classification
The Crawford classification system, based on the anatomy of the aneurysms, classifies TAAAs:
- Extent I: Proximal descending thoracic aorta to the suprarenal aorta
- Extent II: Proximal descending thoracic aorta to aortoiliac bifurcation (most extensive)
- Extent III: Distal thoracic aorta (T6) to aortoiliac bifurcation
- Extent IV: Abdominal aorta only
The Crawford classification system was modified in 1999 by Safi and Miller, adding:
- Extent V: distal thoracic aorta (T6) to suprarenal aorta
Epidemiology and screening
In the early 1900s tertiary syphilis was the most common cause of thoracic aneurysms. Due to advances in detection of the infection and antibiotics, this is no longer true today. The most common types of descending and thoracoabdominal aneurysms now are the result of age-related medial degeneration. Identified causative factors including aortic dissection, connective tissue disorders, aortitis, aortic coarctation, infection, and trauma have led to improved screening resulting in increasing incidence and prevalence over time.
Pathogenesis
The aorta is a constantly remodeling tissue. It is supported by a dynamic system of synthesizing, degrading, and repairing the extracellular matrix. Breakdown of this system at any point will result in aneurysm formation. Risk factors include genetic disorders which cause defects in the aortic extracellular matrix, chronic aortic dissection, atherosclerosis, and risk factors for atherosclerosis itself such as smoking, hyperlipidemia, obesity, hypertension, and COPD.
Age-related medial degeneration
Elastin fragmentation, fibrosis with increased collagen over time reducing the aortic integrity and strength. Subsequent enlargement increases risk of intimal atherosclerosis and medial degeneration usually resulting in diffuse, fusiform aortic dilatation.
A misnomer, cystic medial degeneration is seen on microscopy by patches or “cystic” scarring and loss of smooth muscle cells and elastic fibers.
Chronic aortic dissection
Dissections occur in the medial layer, creating a weakness in the outer aortic wall. This makes the aorta susceptible to progressive aneurysmal dilation especially with the presence of a pressurized false lumen.
Genetic disorders
Marfan syndrome is the most common genetic cause of thoracic aneurysms (36%). The defective fibrillin-1 gene mutation is autosomal dominant and causes fragmentation of elastin fibers and deposition of mucopolysaccharides. Most Marfan aneurysms are the result of chronic dissection.
Loeys-Dietz syndrome (autosomal dominant). Mutations in TGF- ß (1 and 2) receptors cause abnormalities in elastin and collagen I and II. Aneurysms in these patients are more aggressive and tend to rupture or dissection at smaller sizes. This disorder is associated with widely spaced eyes, cleft palate, or bifid uvula, and tortuous aortas and bicuspid aortic valve.
Ehlers-Danlos syndrome. These patients have defects in type III collagen synthesis.
Autoimmune disorders
Takayasu’s arteritis, giant cell arteritis, and rheumatoid aortitis can cause destruction of the aortic media resulting in aneurysmal dilation.
Coarctation of the aorta
Aneurysms may occur in repaired or unrepaired coarctation. Post repair aneurysms are commonly associated with a patch graft aortoplasty.
Infection
Saccular “mycotic” aneurysms tend to occur along the lesser curvature of the transverse aortic arch or upper abdominal aorta adjacent to visceral vessels.
Natural history and clinical presentation
Thoracic aortic aneurysms grow at an average rate of 0.10 cm/year. The rate of growth is 0.19 cm/year for descending and 0.07 cm/year for ascending aortic aneurysms; however, this varies with increasing size. Laplace’s Law explains the findings that as luminal diameter increases, increasing wall tension contributes to progressive dilatation. At a critical size of 7.0 cm the likelihood of rupture is 43% with a yearly rate of rupture (3.6%), dissection (3.7%), death (10.8%). Aneurysms grow faster in the setting of a chronic dissection or collagen vascular disease.
Most patients with TAAAs are asymptomatic. They are typically diagnosed incidentally via CT scan. Up to 40% of patients with TAAAs will become symptomatic at some point, usually presenting with abdominal or back pain. Symptoms result from stretching of the aortic wall or extrinsic compression of neighboring structures. Dysphagia or hoarseness can occur if the aneurysm is compressing the esophagus or the recurrent laryngeal nerve (Ortner’s syndrome), respectively. Severe acute pain may represent rupture or dissection.
Indications and timing of intervention
The risk of rupture is directly proportional to the diameter of the aneurysm. Thus, size is the best predictor of when to intervene for TAAA. Risk factors for rupture include age, COPD, and the presence of chest or back pain. Nearly 80% of ruptures occur in women. Intervention is recommended in patients with an asymptomatic aortic diameter greater than 6 cm, greater than 1 cm/year increase in dilation, or 5.0 cm in connective tissue disease. Symptomatic disease should be resected regardless of size.
Procedural morbidity and mortality
The mortality for TAAA repair ranges from 3% to 21% depending on the series. Paraplegia is one of the most significant and morbid complications of TAAA repair. The rates of permanent paraplegia range from 1.5 to 5%. Hence, spinal cord protection is critical in these patients. This is accomplished by intraoperative monitoring of somatosensory or motor evoked potentials, selective intercostal artery reimplantation, distal aortic perfusion, hypothermia, and CSF drainage. The most common complications postoperatively include respiratory failure, MI, stroke, acute kidney injury, and bleeding. Five-year survival rates are about 65%. Mid and long-term mortality is typically not related to the surgery itself but likely a result of the risk factors such as atherosclerosis, hypertension, and COPD.
Elective repair should take into consideration pulmonary function, cardiac status, and renal function. Cardiac disease is the causative factor for 49% of early postoperative deaths after TAAA repair and coronary revascularization should be performed where indicated prior to aneurysmal repair in asymptomatic patients. Poor renal function secondary to proximal occlusive disease are revascularized at operation with the expectation renal function will stabilize or improve. Optimization of pulmonary function prior to elective repair may reduce postoperative pulmonary complications, the most common cause of postoperative morbidity. This includes smoking cessation, weight loss, and exercise.
Surgical repair
The contemporary gold standard of TAAA intervention is open surgical repair. Preoperative planning is critical as these patients may have significant comorbidities. Patients benefit from insertion of a lumbar drain to manage CSF pressure and drainage postoperatively to decrease the risk of paraplegia. Somatosensory or motor evoked potentials may also be used to monitor the spinal cord but require avoidance of muscle paralytics. Swan-Ganz catheters are routinely used for hemodynamic monitoring as well as a right radial arterial line. Lung isolation with double lumen endobronchial tube is also important for exposure of extent I-III repairs. Prolonged aortic cross clamp may result in acidosis and sodium bicarbonate and mannitol can be given prior to cross clamp to enhance renal perfusion and avoid acidosis.
Other strategies for spinal cord and visceral protection during repair include moderate heparinization (1 mg/kg), permissive mild hypothermia (32–34°C), sequential aortic clamping when possible, selective perfusion of visceral and renal arteries, reattachment of segmental arteries between T8 and L1, and left heart bypass during proximal anastomosis. Use of cell-saver and meticulous surgical hemostasis can significantly reduce the use of blood and blood products.
The operation is performed with the patient in a right lateral decubitus position with the hips supine to allow for femoral cannulation. The left chest is entered through the intercostal space that correlates with the proximal extent of the aneurysm. The diaphragm is divided, and the retroperitoneal aorta is exposed.
Most surgeons employ partial bypass for very proximal aneurysms. This can be accomplished using the left atrium as the outflow and the femoral artery or descending aorta as the inflow. An alternative is to use femoral-femoral partial bypass with an oxygenator in the circuit. In patients where a proximal clamp site cannot be safely dissected out, deep hypothermic circulatory arrest can be employed. Several patients may benefit from an LV vent in this situation, particularly patients with aortic insufficiency. This can be placed through the left pulmonary veins.
Endovascular repair
Most TAAAs occur in the elderly whose pulmonary reserve can be suboptimal, particularly in the setting of COPD. Endovascular repair of the descending thoracic aorta is associated with less mortality and morbidity in the early postoperative period than open repair. As a result, endovascular procedures continue to become more and more popular. Other factors necessary for consideration of endovascular repair include specific anatomy details including landing zones, vessel angulation, excessive intramural thrombus or vessel calcifications, and vascular access as well as the presence of connective tissue disorders. The left subclavian artery can be covered to increase the proximal landing zone however a carotid-subclavian bypass may be necessary to prevent subclavian steal. “Hybrid” procedures are more prevalent than total endovascular repairs. A hybrid procedure involves debranching or extra-anatomic bypass of the visceral vessels followed by endovascular deployment of the aortic stent graft. Total endovascular repairs employ premade fenestrated grafts and require complex preoperative planning to ensure success and restoration of visceral blood flow.
Suggested Readings
- Frederick J and Y Woo. Thoracoabdominal aortic aneurysm. Ann Cardiothorac Surg. 2012;1(3):277-285.
- Elefteriades J. Natural history of thoracic aortic aneurysms: indications for surgery, and surgical versus nonsurgical risks. Ann Thorac Surg. 2002;74(5):S1877-S1898.
- Estrera A, Miller C, Azizzadeh A, Safi H. Adjuncts during surgery of the thoracoabdominal aorta and their impact on neurologic outcome: distal aortic perfusion and cerebrospinal fluid drainage. Multimed Man Cardiothorac Surg. 2006;2006(1009):mmcts.2006.001933.
- Preventza O, and J Coselli. Descending Thoracic and Thoracoabdominal Aneurysm. STS Cardiothoracic Surgery. e-Book.
