# Thoracic Aortic Aneurysm and Thoracic Endovascular Aortic Repair (TEVAR)

## Overview

A thoracic aortic aneurysm (TAA) is characterized by a dilation of the descending thoracic aorta to at least 1.5 times its normal diameter, which typically measures around 2.5 to 3.0 cm. Although less common than abdominal aortic aneurysms (AAA), TAAs carry a significant risk of rupture and death. The incidence of TAA is approximately 5 to 10 cases per 100,000 person-years. The advent of thoracic endovascular aortic repair (TEVAR) has revolutionized the treatment of descending thoracic aortic pathology, offering a less invasive alternative to open surgery. It is important to note that aneurysms involving the ascending aorta and aortic arch are managed by cardiac surgeons and are not covered in this discussion.

## Etiology and Pathophysiology

The most common cause of TAA in the descending thoracic aorta is degenerative or atherosclerotic disease. Connective tissue disorders such as Marfan syndrome, Ehlers-Danlos syndrome type IV, and Loeys-Dietz syndrome also predispose patients to aneurysm formation. Chronic aortic dissection can lead to aneurysmal degeneration of the false lumen. Inflammatory conditions like giant cell arteritis and Takayasu arteritis may contribute to aneurysm development. Infectious or mycotic aneurysms caused by organisms such as Salmonella and Staphylococcus species represent another etiology. Post-traumatic aneurysms may arise as chronic pseudoaneurysms following blunt aortic injury. Additionally, iatrogenic causes include aneurysms developing after surgical procedures, such as prior coarctation repair.

## Natural History

Thoracic aortic aneurysms typically grow at an annual rate of 1 to 4 millimeters, with larger aneurysms tending to expand more rapidly. The risk of rupture increases exponentially with aneurysm diameter. For aneurysms less than 5 cm, the annual rupture risk is approximately 2%. This risk rises to 3-5% for aneurysms measuring 5 to 6 cm and escalates dramatically to 7-14% for those exceeding 6 cm. The 6.0 cm diameter represents a critical threshold or "hinge point," beyond which complications increase sharply; this is analogous to the 5.5 cm threshold used for abdominal aortic aneurysms. Without treatment, the five-year survival rate for patients with large TAAs ranges from 20 to 50%.

## Landing Zone Classification (Ishimaru/Mitchell)

The landing zone classification is essential for planning TEVAR, as it defines the proximal seal zone for the endograft. Zone 0 corresponds to the ascending aorta at the origin of the innominate artery and typically requires total arch debranching or a hybrid approach. Zone 1 lies between the innominate and left common carotid arteries and usually necessitates innominate artery debranching. Zone 2 is located between the left common carotid and left subclavian arteries and may require revascularization of the left subclavian artery (LSA). Zone 3 is distal to the left subclavian artery within 2 cm and represents the standard landing zone for TEVAR. Zone 4 corresponds to the mid-descending thoracic aorta and is considered the ideal landing zone. Zones 5 through 10 refer to the distal descending aorta and the visceral segment.

| Landing Zone | Location | Required Adjunct |
|-------------|----------|-----------------|
| 0 | Ascending aorta (innominate origin) | Total arch debranching or hybrid repair |
| 1 | Between innominate and left CCA | Innominate artery debranching |
| 2 | Between left CCA and left subclavian | LSA revascularization (recommended) |
| 3 | Distal to LSA within 2 cm | Standard TEVAR landing zone |
| 4 | Mid-descending thoracic aorta | Ideal landing zone (no adjuncts needed) |
| 5–10 | Distal descending / visceral aorta | Visceral vessel management |

<image>Diagram of thoracic aortic landing zones (Zones 0-4) showing the relationship to the great vessels (innominate artery, left common carotid artery, left subclavian artery) with annotations indicating which zones require arch debranching or subclavian revascularization before TEVAR</image>

## Indications for Repair

Repair of thoracic aortic aneurysms is generally indicated based on size thresholds, symptomatology, and complication status. For degenerative aneurysms, repair is recommended when the diameter reaches 5.5 to 6.0 cm. In patients with connective tissue diseases such as Marfan or Loeys-Dietz syndrome, earlier intervention is advised at diameters of 4.5 to 5.0 cm due to their increased risk of rupture. Rapid aneurysm growth, defined as more than 5 mm in six months or over 10 mm per year, also warrants repair. Symptomatic aneurysms causing pain or compressing adjacent structures like the esophagus, trachea, or recurrent laryngeal nerve require intervention. Complicated aneurysms presenting with rupture or malperfusion demand urgent repair. Additionally, aneurysmal degeneration of the false lumen following aortic dissection, when reaching 5.5 to 6.0 cm, is an indication for treatment.

## TEVAR: Technique and Devices

### Devices

Several commercially available endografts are used for TEVAR, including the Gore TAG and its conformable variant cTAG, Medtronic’s Valiant Captivia and Navion, and Cook’s Zenith TX2 and Alpha Thoracic devices. These stent grafts typically range from 20 to 46 mm in diameter and 10 to 20 cm in length. They are delivered through large-bore systems, usually between 18 and 24 French.

### Technique

The procedure begins with femoral artery access, which can be obtained via surgical cutdown or percutaneous puncture. A stiff guidewire is advanced into the ascending aorta to provide support for device delivery. Precise positioning of the endograft is achieved using fluoroscopy and radiopaque markers on the device. Deployment is performed under controlled hypotension, maintaining a mean arterial pressure (MAP) of 60 to 70 mmHg to prevent the graft from being displaced by blood flow, a phenomenon known as windsocking. After deployment, balloon molding is performed proximally and distally to ensure proper apposition of the graft to the aortic wall. Completion angiography confirms successful exclusion of the aneurysm and verifies patency of the great vessels.

### Left Subclavian Artery (LSA) Management

In some cases, coverage of the left subclavian artery is necessary to achieve an adequate proximal seal, particularly when landing in Zone 2. Coverage of the LSA can lead to complications such as left arm ischemia, which is usually tolerated due to collateral circulation. However, it may also cause posterior circulation strokes by compromising the vertebral artery, spinal cord ischemia since the LSA contributes to the anterior spinal artery, and compromise of the left internal mammary artery, which is critical in patients with prior coronary artery bypass grafting using the left internal mammary artery (LIMA). To mitigate these risks, revascularization of the LSA is recommended. Options include open surgical techniques such as carotid-subclavian bypass or carotid-subclavian transposition, the latter being preferred when feasible. Endovascular approaches like chimney or periscope stenting are alternatives. The Society for Vascular Surgery (SVS) guidelines advocate routine LSA revascularization when coverage is planned.

## Spinal Cord Protection

### Spinal Cord Blood Supply

The spinal cord receives its blood supply primarily from the artery of Adamkiewicz, a major radiculomedullary artery that typically originates between the T8 and L2 vertebral levels. This artery supplies the anterior spinal artery in the thoracolumbar region. Because its origin is variable and cannot be reliably preserved during TEVAR, spinal cord perfusion depends on a collateral network that includes intercostal and lumbar arteries, the internal iliac (hypogastric) arteries, the subclavian artery via the vertebral artery, and the inferior mesenteric artery. Spinal cord ischemia (SCI) following TEVAR occurs in approximately 2 to 8% of cases.

### Risk Factors for SCI

Several factors increase the risk of spinal cord ischemia after TEVAR. These include extensive aortic coverage exceeding 20 cm, coverage of the LSA without revascularization, prior or concurrent infrarenal aortic repair which sacrifices lumbar arteries, occlusion of hypogastric arteries, perioperative hypotension, renal failure, and emergency procedures.

### Protective Strategies

Cerebrospinal fluid (CSF) drainage is a key protective strategy. A lumbar CSF drain is placed preoperatively to maintain CSF pressure below 10 to 12 mmHg, thereby optimizing spinal cord perfusion pressure, which is calculated as MAP minus CSF pressure. The use of CSF drainage is somewhat controversial; some advocate for routine use in high-risk cases such as those involving long segment coverage or prior aortic repair, while others recommend selective use only if SCI symptoms develop. Risks associated with CSF drainage include epidural or subdural hematoma, meningitis, and headache. Most centers employ CSF drainage for high-risk patients and adopt a selective approach for those at lower risk.

Additional protective measures include maintaining a MAP above 80 mmHg postoperatively, often with vasopressor support, performing staged repairs for extensive aneurysms to allow collateral development, avoiding intraoperative and postoperative hypotension, preserving hypogastric arteries to maintain pelvic collateral pathways, and revascularizing the LSA when covered. Some centers also utilize neuromonitoring techniques such as somatosensory and motor evoked potentials to detect early spinal cord ischemia.

<image>Illustration of spinal cord blood supply relevant to TEVAR, showing the anterior spinal artery, the artery of Adamkiewicz arising from an intercostal artery, and the collateral network including subclavian, intercostal, lumbar, and hypogastric artery contributions, with zones at risk during thoracic aortic coverage highlighted</image>

## Outcomes of TEVAR

TEVAR is associated with a perioperative mortality rate of 2 to 5% in elective cases, which is significantly lower than the 5 to 15% mortality seen with open thoracic repair. Stroke occurs in 2 to 4% of patients, while spinal cord ischemia affects 2 to 8%. Access site complications occur in 5 to 10% of cases. Endoleaks, classified similarly to those seen in endovascular abdominal aneurysm repair (EVAR), can occur and require monitoring. A rare but catastrophic complication is retrograde type A dissection, occurring in 1 to 2% of cases. Long-term surveillance is necessary to monitor for device migration, endoleaks, and aortic remodeling.

## Open Thoracic Aortic Repair

Open repair involves a left thoracotomy through the posterolateral approach, typically at the fourth to sixth intercostal space. The aorta is managed using either a clamp-and-sew technique or with left heart bypass employing devices such as the Gott shunt or centrifugal pump. An interposition Dacron graft is used to replace the diseased segment. Reimplantation of intercostal arteries between T8 and T12 is performed to protect the spinal cord. Open repair carries higher morbidity, including paraplegia rates of 5 to 10%, respiratory complications, and renal failure. This approach is generally reserved for young patients, those with connective tissue disease, or cases where TEVAR is unsuitable due to anatomical considerations.

## Surveillance After TEVAR

Post-TEVAR surveillance involves computed tomography angiography (CTA) at 1 month, 6 months, 12 months, and then annually. This imaging assesses for endoleaks, aneurysm sac behavior, device migration, and graft integrity. The principles of surveillance are similar to those applied after EVAR.

## Clinical Pearls

TEVAR has largely replaced open repair for most descending thoracic aortic pathologies, but proficiency in open repair remains essential for complex cases and emergencies. When the left subclavian artery is covered during TEVAR, revascularization is recommended, especially if the patient has a dominant left vertebral artery, a functioning LIMA graft, or left upper extremity dialysis access. Spinal cord ischemia may present up to 72 hours post-procedure, so maintaining a high MAP and having CSF drainage available during the early postoperative period is critical. Retrograde type A dissection is a devastating complication of TEVAR, with risk increased by aggressive oversizing and landing in a diseased proximal aorta. Oversizing of the endograft is typically 10 to 20% relative to the proximal landing zone diameter; excessive oversizing can cause infolding, bird-beaking, and retrograde dissection. Preservation of at least one hypogastric artery is vital to maintain the spinal cord collateral network.

<image>Post-TEVAR CT angiography 3D reconstruction showing a thoracic endograft extending from Zone 2 (with left subclavian artery coverage) to the mid-descending aorta, with a patent carotid-subclavian bypass visible, and the excluded aneurysm sac beginning to thrombose</image>

## References
- Riambau V, et al. ESVS 2017 Clinical Practice Guidelines on Descending Thoracic Aorta. Eur J Vasc Endovasc Surg. 2017;53(1):4-52.
- Matsumura JS, et al. The Society for Vascular Surgery practice guidelines: management of the left subclavian artery with TEVAR. J Vasc Surg. 2009;50(5):1155-1158.
- Buth J, et al. Outcome of endovascular repair of thoracic aortic aneurysms (EUROSTAR/UK registries). J Vasc Surg. 2007;46(1):1-10.
- Acher C, et al. Cerebrospinal fluid drainage for thoracoabdominal aortic aneurysm repair. Semin Vasc Surg. 2012;25(2):100-103.
- Griepp RB, et al. The anatomy of the spinal cord collateral circulation. Ann Cardiothorac Surg. 2012;1(3):350-357.