AAA Repair: When Will EVAR Become the Gold Standard?

VASCULAR DISEASE MANAGEMENT 2012:9(6):E90-E96

Abstract

Endovascular aneurysm repair (EVAR) is considered relatively safe and effective for treating infrarenal abdominal aortic aneurysms (AAA). In patients with favorable anatomy, it is widely considered the therapy of choice among clinicians. The indications for EVAR are expanding, particularly in high-risk patients, due to advances in device technology and with the increased experience of physicians skilled in performing EVAR procedures. In contemporary trials and clinical series, EVAR has demonstrated the advantages of lower perioperative rates of mortality and major complications, shorter hospital stays, and quicker return to functional status. However, lifelong prevention of aneurysm-related mortality remains a challenge to be better understood and managed. Issues such as the management of endoleaks, surveillance schedule, and prevention of post-EVAR sac enlargement and actual aneurysm rupture must be fully assessed. In addition, the restrictions imposed by current instructions for use of approved devices make clear that not all patients with an aortic aneurysm can be repaired using the endovascular approach with the expectation of long-term safety. The most common reason for this is the inability of currently available devices to handle aneurysms with unfavorable neck or iliac anatomy.

EVAR has been adopted as the gold standard for patients with AAA anatomy that fit within the parameters recommended for available devices. Development over the coming years will expand to more complex anatomies but must include advances in issues such as sac management, fixation at the neck, and improvement of delivery caliber to be recognized as the gold standard technique for all patients with AAA.

Abdominal aortic aneurysm (AAA) rupture is usually fatal and rupture risk increases with aneurysm diameter. For more than 50 years prophylactic open repair (OR) has been practiced as the first choice treatment; it is a major surgical procedure performed under general anesthesia with hand sewn placement of a prosthetic graft after aortic cross clamping. Many patients in the open-only era who had significant comorbidities were denied treatment, or suffered perioperative death as the result of treatment. Endovascular aneurysm repair (EVAR) was introduced in the early 1990s as a less invasive method for managing patients at high risk for traditional open surgical repair. EVAR evolved into a common treatment option, as prospective randomized trials comparing EVAR and open AAA surgery show a marked benefit with respect to shorter hospital stays, lower 30-day mortality, and more rapid return to previous functional status.^1,2 Open repair has a 3% to 5% perioperative mortality for standard risk patients and 5% to 10% for higher risk patients. There is also the likelihood of fewer reinterventions and excellent graft durability generally for 20-30 years that usually lasts the rest of the patient’s life. The long-term durability of EVAR is less than with OR. This has been demonstrated by prospective trials (EVAR-1 and DREAM) and registries (European Collaboration on Stent/Graft Techniques for Aortic Aneurysm Repair [EUROSTAR],³ UK Registry for Endovascular Treatment of Aneurysm [RETA]).⁴ These studies were developed in the last 10 years and compared the results of open repair with the results of first and second generation aortic stent grafts. EVAR progressed since these trials and registries with improvements in technique, education, pre-case planning, aneurysm neck fixation systems, grafts to handle larger necks, lower profile delivery, and other features that may lead to fewer complications and better long-term results.

AAA Diameter and Rupture Risk

AAA rupture has been most closely correlated with diameter and this has been our primary indicator for assessing the risk of the natural history of the untreated aneurysm. There is likely some predisposition to rupture based upon other aspects of the aneurysm, including configuration, the ratio of aneurysm diameter to the size of the person, whether there are any blebs in the aneurysm wall, and whether there are some areas where wall thickness and collagen degeneration are substantial. Until we understand these issues in more detail, we will continue to respond primarily to AAA diameter and aneurysm-related symptoms. Various trials have analyzed the risk/benefit of the treatment of small AAA (4.5 cm-5 cm) versus surveillance (PIVOTAL, CAESAR, UK-EVAR1, DREAM).^5,6  There is no evidence to indicate that an elective repair is appropriate in uncomplicated AAAs smaller than 5 cm, but they should be closely followed. An analysis of survival and time without rupture in patients with AAA under ultrasound surveillance reported an annual risk of rupture for AAA smaller than 5 cm in diameter to be quite low, probably less than the risk of repair. The annual rupture risk increases somewhat from 5 to 6 cm and increases dangerously for aneurysms of more than 6 cm.^7,8

The risk of rupture for AAA has been reported by diameter: for AAA diameter 4.0-4.9 cm, it is about 1% per year, and for 5.0-5.9 cm, it is 1%-11% per year, with a level of evidence 2a. Other factors may also predispose a given patient to rupture, such as female gender, smoking, hypertension, AAA expansion rate, and peak AAA wall stress, with a level of evidence 2b to 3b. The rapid increase of intraluminal thrombus, increased AAA wall stiffness, increased AAA wall tension, the low forced expiratory volume in 1 second (FEV1), and transplant patients have been associated in individual studies to higher risk of AAA rupture.⁹ In the future, we will likely have a better way of determining risk of rupture than diameter with new abilities to perform more complex morphologic evaluations of the AAA using 3 dimensional CT reconfiguration and analysis techniques to calculate wall stress areas of increased metabolic activity.

Specific evidence to differentiate between the risk of rupture of a 5.0 cm AAA and a 5.5 cm AAA is lacking. However, when smaller aneurysms were studied, the subgroup with a 4.9 cm to 5.4 cm diameter AAA were most likely to rupture and substantially more likely to rupture than those with smaller aneurysms.⁷ We recommend that AAAs larger than 5.0 cm should be repaired in patients who can tolerate repair. In these patients, the option of OR versus EVAR must be analyzed in each case with respect to adequacy of anatomy for stent grafts, patient age, life expectancy, and also patient choice after adequate information from the appropriate specialist.

In contemporary series, the risk of perioperative death with OR is in the range of 3%-5% (Table 1).^10-14 There can be significant morbidity and substantial hospital lengths of stay. After recovery from the operation, it is a very durable and long-lasting repair with a low rate of long-term complications. The advantages of EVAR are lower morbidity and mortality rates, shorter hospital stays, less pain, and sooner return to function. All of this comes at a price that is both monetary and clinical; EVAR is associated with more costs due to the greater need for surveillance, more reintervention, and a small annual risk of post-EVAR rupture (Table 2).¹⁵

EVAR Limitations

In 2006 nearly 22,000 EVAR procedures were performed in the United States, exceeding the number of OR of AAA for the first time.¹⁶ EVAR cannot be offered to all patients with an AAA, even if they are not acceptable for OR due to high perioperative risk. Baseline aortoiliac arterial anatomic characteristics are required for appropriate patient selection in endovascular repair and these seem to help determine long-term results. The instructions for use (IFU) of devices approved for EVAR in the U.S. include recommendations to guide patient selection since it defines adequate anatomic characteristics for each particular device (including aortic neck diameter, aortic neck length, and iliac artery length and morphology) (Figure 1).

In a recent 10-year period, the necessary anatomic characteristics to be deemed suitable for EVAR were analyzed and patients with both pre-case planning and follow-up CT scans after EVAR were assessed.¹⁷ Variables included aortic neck diameter, aortic neck angle, and common iliac artery diameter, which were found to be independently associated with aortic aneurysm sac enlargement when patients were treated with anatomies that were outside the IFU. Only 42% of patients had anatomy consistent with the most conservative IFU. When outside the IFU, the incidence of sac enlargement after EVAR was 41% at 5 years. This study was retrospective and evaluated only patients who had follow-up CT scans sent for 3-D analysis so there may be a bias toward inclusion of patients that were found to have problems such as sac growth on follow-up. Nevertheless, the overall concept is that when patients with poorly suited anatomies are treated with EVAR, there is a correlation with sac enlargement during follow-up.

In a recent review of EVAR versus OR in the U.S. Medicare population, including 22,830 patients during the 2001 to 2004 period with follow-up until 2005, EVAR was performed for older and higher risk patients. The perioperative mortality with EVAR was lower than in the OR group (1.4% vs 4.8%, P<0.001) and the reduction in mortality risk increased with advancing age. After 3 years the survival curves converged. Four years after AAA repair, reinterventions were more common after EVAR (9.0% vs 1.7%, P<0.001) although most reinterventions were minor. In contrast, surgery for laparotomy-related complications and hospitalizations for surgery were more likely among the patients that underwent OR. Overall mortality rates for elective and ruptured AAA decreased over time and the survival advantage observed in older patients also increased over time.¹⁸

EVAR Trials

There are 2 trials, EVAR-1 trial in the UK and the DREAM trial in the Netherlands, that randomized patients with AAA 5.5 cm or larger in diameter to undergo either EVAR or OR in those who were a suitable risk for either. Patients with prohibitively high risk for OR were randomized to EVAR versus observation (best medical treatment) in the EVAR-2 trial.^19,20

In the EVAR-1 trial, 1,082 patients (from 1999 through 2004) were randomly assigned at 37 hospitals in the United Kingdom to OR versus EVAR. The mean AAA diameter was 6.5 cm. The 30-day operative mortality was 1.8% for EVAR and 4.3% for OR. Secondary procedures were higher in the endovascular group (9.8%) than in the open group (5.8%). However, despite the lower upfront morbidity of EVAR, no differences were observed in aneurysm related mortality or total mortality in the long-term (8 years) follow-up. EVAR requires diligent imaging surveillance at least yearly and was associated with more costs. The rate of secondary procedures required due to graft-related complications following EVAR was 2 to 3 times that for open repair.²¹

In the Dutch Randomized Endovascular Aneurysm Management (DREAM) trial, 345 patients with AAA minimum diameter of 5.0 cm were randomized to receive OR vs EVAR.²² The 30-day mortality was 4.5% for OR versus 1.2% for EVAR, with a higher incidence of major complication or death in the OR (9.8%) than in the EVAR group (4.7%). The DREAM trial also concluded that the early survival benefit provided by EVAR disappeared over about 2 years of follow-up.

Four hundred and four patients with large AAA (≥5.5 cm in diameter), who were considered to be physically ineligible for OR, were randomly assigned to undergo either EVAR or no repair from 1999 through 2004 at 33 hospitals in the United Kingdom.²³ The mean aneurysm diameter was 6.7 cm. The 30-day operative mortality was 7.3% in the EVAR group. The overall rate of aneurysm rupture in the no-intervention group was 12.4 per 100 people/year. EVAR was associated with a significantly lower rate of aneurysm-related mortality than no repair, but it was not associated with a reduction in the rate of death from any cause. The rates of graft-related complications and reinterventions were higher with endovascular repair, and it was more costly (Table 3). However, the EVAR-2 trial conclusions are highly debatable because of all the bias linked to this study. Therefore some recent studies have evaluated the results of endovascular exclusion of AAAs in this high-risk patient group in high volume centers and their conclusions are in opposition with the EVAR-2 trial. ²⁴

Both implant technique and the implant devices have become safer, simpler, and faster than when all of these trials were completed. These improvements in EVAR help make it smoother and potentially more durable for good risk patients and may be a huge benefit to those at high risk. The newer, less invasive procedures, such as the percutaneous access in EVAR, and techniques to minimize contrast use and radiation times may be very good for those patients who are poor candidates for surgery. Endovascular repair has become better now than 10 years ago, when most of the trials were designed.

EVAR Durability Issues

In the EVAR-1 trial, the rates of graft-related complications and reinterventions were higher among patients in the EVAR group than among those in the OR group. In the DREAM trial, it was observed that in the first 9 months after randomization the rate of reintervention after EVAR was near 3 times the rate after OR, and concluded that the perioperative survival advantage with EVAR compared to OR is limited to the first postoperative year (Table 4). However, there was a difference between follow-up protocols after OR vs EVAR that could result in bias. After EVAR, follow-up CT scans were performed at 1 month, 3 months, 6 months, and annually but were not routinely obtained after OR, where reinterventions are usually major procedures and mostly due to symptomatic presentations. This strict follow-up after EVAR allows detection and treatment of potential problems in asymptomatic patients (like type II endoleaks), and most of the reinterventions following EVAR could be classified as relatively minor procedures, solved with endovascular techniques in most of the cases.

In patients undergoing EVAR, there are issues about durability and long-term complications, as many factors have now been associated with a risk of rupture post-EVAR. These factors include the presence of endoleaks, especially type 1 endoleaks, migration of the stent graft, or the development of sac enlargement during surveillance. Ruptures have been reported in both open and endovascular procedures, but are more common after EVAR. Data from the EVAR I trial reported a series of 27 ruptures that occurred after EVAR, with a high mortality and a frequent association with previous serious complications identified during surveillance (63%). Few ruptures after EVAR seem to be spontaneous.²⁵ Sac growth was identified in 15 patients with some type of endoleak observed in 12 out of 15. Beyond the perioperative period, ruptures seemed to occur at a very low but constant rate over the years.¹⁹

Compliance with anatomic guidelines for EVAR has been correlated with the presence or absence of post-EVAR sac enlargement in a recent observational study. It was considered an anatomic risk factor for EVAR to have an aortic neck diameter >28 mm, an aortic neck angle >60˚ or a common iliac artery length of <20 mm. In patients without these risk factors the freedom from sac enlargement was 87% at 3 years, but in patients with 2 risk factors, it was 68% at 3 years. In those with 3 risk factors, sac enlargement was present in 66% of the patients at 3 years post-EVAR.¹⁷

New Techniques to Manage Patients with Complex AAA

Chimney grafts/fenestrated branched stent grafts

Although EVAR has reached a certain form of maturity for the treatment of infrarenal aneurysms with sufficient neck lengths, the use of endovascular techniques to treat complex AAA remains debatable. Recent improvements of fenestrated or branched stent grafts have demonstrated promising results but the use of such devices mandates highly precise planning, a manufacturing delay of 6 to 12 weeks, and an important cost because the devices are customized for each patient’s anatomy. In many countries where fenestrated or branched stent grafts are not commercially available yet, the use is limited to a few investigational centers.

To solve this situation “chimney” grafts have been advocated as a possible endovascular option for aortic aneurysms involving critical side branches such as renal and superior mesenteric arteries. This technique involves concurrent deployment of a standard aortic endograft associated with covered stents in the target arteries, and thus it has the advantage of being immediately available. The published literature represents an encouraging but limited preliminary experience but there is no consensus or recommendation currently about the suitable type of chimney grafts. Recent reports from European centers are considering chimney grafts a promising approach to treat juxtarenal AAA including this technique in their therapeutic algorithm of aortic pathologies.^26,27 When the number of visceral vessels involved is 1 or 2, the chimney graft may be an option. Fenestrated or side-branched grafts will be requited when more branches are involved and these grafts are likely to be widely used for patients with aortic neck anatomy outside current IFU once they become more readily available.²⁶

AAA Repair Algorithm

Based on available data, we follow an algorithm in our practice for treatment of AAA (Figure 2). If the patient has anatomy suitable for EVAR, it should be offered as the first option of treatment. Exceptions to this general approach include inability to comply with surveillance, patient preference, or specific patient-related contraindication to EVAR. If the anatomy of the aneurysm is challenging but the patient is at high risk for open surgery, EVAR should be considered and the patient advised of the risks of either approach. Options offered for use in this case could include internal or external conduits for complex iliac access, the use of chimney grafts or branched/fenestrated grafts in cases of short and/or angulated aortic necks, or placement of a balloon expandable stent to help manage an angulated neck. OR still remains the first option for patients with unsuitable anatomy for EVAR who are also good surgical candidates, especially if they are young and have a juxtarenal aneurysm.

Before Declaring Gold Standard

Several aspects of stent graft technology are evolving rapidly. Fixation concepts have moved from passive fixation to active stent graft fixation (hooks & barbs, suprarenal fixation, and now staples). In the coming years, we hope that advances in endovascular devices will permit the treatment of challenging anatomies with expected improvements of long-term results. Experience in some centers suggests that fenestrated or branched stent grafts are a reasonable alternative for complex anatomies and these will play a role in the near future. Improvements in sac management and post-EVAR procedures during the follow-up are also needed to improve the effectiveness and durability of EVAR against AAA rupture.

Current devices have limitations in handling complex anatomies, principally with short, angulated, and cone-shaped necks, and with small and short iliac arteries. Use of EVAR outside the device specific IFU has been associated with sac enlargement. The most accurate and effective surveillance schedule and the optimal management of endoleaks remain to be determined. It is highly likely that technology will continue to improve and the IFU will expand to more safely treat a broader range of anatomies.

Conclusion

Based upon available data and clinical experience, we consider EVAR to be the “gold standard” for patients with AAA anatomy that fits within the IFU for each device, and we can offer EVAR as the first-line treatment of choice for those with acceptable anatomy. Over the coming years the advances and improvements in stent grafts will expand this option of treatment to include most anatomies. Needed advances include improvement in graft fixation at the neck and sac management.

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Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The author reports no conflicts of interest regarding the content herein.

Manuscript submitted November 22, 2011, provisional acceptance given December 9, 2011, final version accepted January 19, 2012.

Address for correspondence: Fernando F. Gallardo, Sr., MD, Vascular Surgery Fellowship CHU, Divison of Vascular Surgery,  A Coruña, Galicia 15172, Spain. Email: fatiax@hotmail.com