Thoracic Endovascular Aortic Repair (TEVAR)

In 1994, Dake et al first reported the use of thoracic "stent-grafts" for the treatment of descending thoracic aortic aneurysms (TAAs) in patients who were believed to be at excessive risk with conventional open surgery. ^[1]  They showed that placement of thoracic stent grafts, otherwise known as thoracic endovascular aortic repair (TEVAR), could be performed from a technical standpoint with relatively low morbidity; however, they noted that long-term follow-up would be required. The initial stent grafts were actually constructed by the implanting physicians themselves; later, the devices were restricted to those under investigational study.

In 2005, the US Food and Drug Administration (FDA) approved the first commercially available thoracic stent graft, the W. L. Gore TAG endograft system (Flagstaff, AZ); in 2008, the FDA approved the Cook Zenith TX2 (Bloomington, IN) ^[2]  and the Medtronic Talent (Santa Rosa, CA) thoracic endograft systems. All three of these devices were approved for use in the treatment of TAAs.

The randomized clinical trial that led to the approval of the TAG device demonstrated that in patients with appropriate anatomy, TEVAR could be performed with lower operative mortality than open surgical repair (2.1% vs 11.7%) and with less spinal cord ischemia (3% vs 14%), respiratory insufficiency (4% vs 20%), and renal failure (1% vs 13%). ^[3]  However, TEVAR yielded more vascular access–related complications. Importantly, a small percentage of patients who were undergoing TEVAR did not have their aneurysms entirely excluded from the aortic circulation at 1- and 2-year follow-up. As in the 1994 study by Dake et al, the authors maintained that all patients undergoing TEVAR required close long-term follow-up.

Dake et al and investigators in Europe later showed that TEVAR was technically feasible in patients with descending thoracic aortic dissections (patients with tears in the wall of their aortas). ^[4]  Nonetheless, ongoing studies are still trying to identify which patients with thoracic aortic dissection can benefit from TEVAR. Patients with complicated dissections, including those involving malperfusion (where a blood supply is impeded by flaps of aortic tissue caused by the dissection), appear to benefit from TEVAR to seal the site of dissection and reappose aortic wall layers. ^[4]

It should be noted that historically, open surgical approaches in patients with dissection-related malperfusion have had a high mortality. ^[5] Accordingly, a US clinical trial examined TEVAR for patients with complicated dissection (STABLE trial, Cook Inc, Bloomington, IN). This multicenter study included patients with complicated type B aortic dissection (cTBAD) who were treated with an endovascular system consisting of proximal TX2 thoracic stent grafts and distal bare-metal dissection stents (Zenith Dissection Endovascular System; Cook Medical, Bloomington, IN). Indications for enrollment included the following:

• Branch-vessel malperfusion
• Impending rupture
• Aortic diameter ≥40 mm
• Rapid aortic expansion
• Persistent pain or hypertension despite maximum medical therapy

In this trial, the 30-day mortality was 5% (2/40); two more deaths occurred after 30 days, leading to a 1-year survival rate of 90%. The investigators noted aortic remodeling with a decrease in false lumen diameter and an increase in true lumen diameter, with complete thrombosis of the false lumen in 31% at 1 year.

It would seem logical that TEVAR could also be used in patients with uncomplicated dissections. To address this question, a prospective randomized clinical trial was completed in Europe, the INSTEAD XL trial (INvestigation of STEnt grafts in patients with type B Aortic Dissection). ^[6] Initially, at 1 year, this trial did not show a mortality benefit ^[6] ; however, 5-year follow-up revealed an aorta-specific survival benefit in patients treated with TEVAR vs medical therapy (6.9% vs 19.3% risk of aorta-specific mortality), as well as less disease progression risk (27.0% vs 46.1%). ^[7]

Since the 2005 FDA approval of the TAG device for thoracic aneurysms, the use of endovascular stent grafts for thoracic aortic disease has increased dramatically. Surgeons have subsequently devised novel techniques to facilitate the use of TEVAR in higher-risk patients with more complex anatomy. Branch vessels that would have been occluded by the stent grafts can often be bypassed, and the landing zone can be moved more proximally to allow an adequate seal. ^{[5, 8, 9]}  

Additionally, trials are currently under way that are aimed at addressing thoracic aortic pathology encroaching on or involving the arch branch vessels, such as the Gore Thoracic Branch Endoprosthesis (Gore TBE) trial (see the first image below), the Medtronic MonaLSA trial (see the second image below), and the Bolton Dual Arch branch graft trial.

A landmark paper from 2013 by Patterson et al, using prospectively collected data on over 1000 TEVARs from the Medtronic Thoracic Endovascular Registry (MOTHER), performed a subgroup analysis of patients undergoing TEVAR for TAA, chronic type B aortic dissection (CTBAD), and acute type B aortic dissection (ATBAD). ^[10]  TEVAR was protective from aortic-related mortality, with aortic-related deaths less than 1 per 100 patient-years (equivalent to 1%/y). Freedom from all-cause mortality by life-table analysis over the whole follow-up period was 56% in the TAA group, 64% in the CTBAD group, and 42% in the ATBAD group. Freedom from aortic death was 93%, 96%, and 85%, respectively.

At present, there are four thoracic aortic stent grafts available for commercial use in the United States, each with unique characteristics that should be considered in operative planning and device choice:

• Valiant Navion thoracic stent graft (Medtronic)
• TAG conformable thoracic endoprosthesis (W. L. Gore & Associates)
• Zenith Alpha thoracic device (Cook Medical)
• RelayPlus system (Terumo Aortic, previously Bolton Medical)

All four devices have FDA indications for use in treating aneurysmal disease; the TAG conformable thoracic endoprosthesis and the Valiant thoracic stent graft have indications for the treatment of all descending thoracic aortic lesions.

Over the course of time, smaller-profile TEVAR devices have been investigated through trials. Currently, the Cook Alpha and the Medtronic Navion are the smallest-profile devices that are commercially available and may be considered in patients with small iliac arteries. Terumo’s Relay Pro low-profile device is currently in the trial phase. Improved device designs result in better conformability with a significant reduction in endoleaks (to as low as 1-2%). ^{[11, 12, 13]}

TEVAR has been around for two decades and has been established as the first line of treatment for most descending thoracic aortic pathology (including aneurysms, dissections, and penetrating aortic ulcers), yielding significant reductions in morbidity and mortality as compared with open surgical repair. Improved materials, reduced delivery sheath sizes, enhanced conformability, tapered grafts, and a wider range of sizes have improved TEVAR applicability and outcomes over this initial decade of widespread use. 

Descending thoracic aortic aneurysm

The official FDA-approved "on-label" indication for the stent grafts currently available commercially in the United States (W. L. Gore C-TAG, Cook Alpha, Medtronic Valiant and low-profile, Navion, and Bolton Relay) is for treatment of descending TAAs with a diameter at least two times greater than that of the adjacent aorta. Furthermore, there must be sufficient aorta (typically 2 cm) of normal dimensions on either side of the aneurysm (the so-called proximal and distal landing zones) to allow the stent graft to adhere (seal) to the aortic walls and achieve exclusion of the aneurysm.

Complicated descending thoracic aortic dissection

TEVAR is increasingly being used as the optimal treatment for patients complicated descending thoracic aortic dissections in preference to open surgery. ^{[4, 5]} Clinical experience over time, the ongoing evolution of endovascular devices, and several multicenter trials have led to FDA approval of Medtronic and Gore thoracic stent graft devices for use in patients with acute and chronic dissection. The Zenith Dissection Endovascular Stent is now FDA-approved for patients with chronic dissections and intended to be used as a distal component to support delaminated segments of nonaneurysmal aorta with dissection distal to a Zenith TX2 Dissection Endovascular Graft with Pro-Form.

Focal penetrating ulcer

Patients with focal penetrating ulcers in the thoracic aorta constitute another group in which TEVAR may prove beneficial because these patients have defined, limited areas of the thoracic aorta where loss of endothelial integrity can lead to potentially life-threatening rupture. Coverage of the ulcer with a stent graft can be performed with minimal morbidity. However, patients with penetrating ulcers often have extensive peripheral vascular disease, which may limit their suitability for TEVAR. ^{[6, 14]}

Aortic trauma

TEVAR can be life-saving and has now become the standard of care for patients with thoracic aortic trauma resulting in intimal tears, pseudoaneurysms, or frank ruptures. ^[15]  

Patients undergoing TEVAR must have anatomy that is suitable for deployment of the endografts. The following would be contraindications for placement of TEVAR devices:

• Proximal or distal landing-zone aortic diameter beyond the range of 18-42 mm (fusiform and saccular aneurysms/penetrating ulcers), 18-44 mm (blunt traumatic aortic injuries), or 20-44 mm (dissections)
• Proximal and distal aortic neck lengths < 20 mm (fusiform and saccular aneurysms/penetrating ulcers, blunt traumatic aortic injuries); the proximal extent of the landing zone is dissected
• Extensive circumferential thrombus or calcification of the aortic wall at the desired landing zones; this is a relative contraindication for TEVAR and leads to type I endoleaks
• Iliac/femoral access vessel morphology that is not compatible with vascular access techniques, devices, or accessories; however, an iliofemoral or aortofemoral conduit may be created to facilitate the use of TEVAR for the thoracic aortic pathology
• Involvement of branch vessels (including the celiac and subclavian or carotid arteries); however, these branch vessels can often be bypassed to create landing zones in so-called hybrid techniques, ^[16] or the use of branch graft trial devices may be considered

Patients with connective tissue disorders in whom a high likelihood exists of further tissue degeneration (eg, those with Marfan disease) were specifically excluded from the trials that led to FDA approval of the devices available in the United States (Cook Alpha, C-TAG, Medtronic Valiant, Bolton Relay).

Hong et al retrospectively analyzed the long-term outcomes of proximal TEVAR in 48 patients with chronic Stanford type B aortic dissection (mean age, 58.3 ± 10.6 y; 40 men, 8 women). ^[17] Patients were followed at 1, 3, 6, and 12 months after treatment and annually thereafter (mean follow-up, 48.7 ± 40 mo; range, 1-120). Changes from preoperative status to status at final follow-up were as follows:

• Descending aortic level - True lumen, 19.2 ± 7.01 mm vs 36.9 ± 9.53 mm; false lumen, 30.47 ± 15.89 vs 19.16 ± 15.33 mm; maximum diameter, 49.67 ± 13.96 vs 56.66 ± 14.95 mm
• Diaphragm level - True lumen, 16.24 ± 5.41 vs 24.41 ± 8.04 mm; false lumen, 12.37 ± 11.49 vs 14.92 ± 12.25 mm; maximum diameter, 34 ± 7.81 vs 38.04 ± 7.7 mm

At 5 years, 81% of patients were free from reintervention, and at 10 years, 50.6% were. ^[17] The overall 10-year survival rate was 83%, and the aortic-related survival rate was 92.3%.