New Approaches to Abdominal Aortic Aneurysm

Vascular surgeons review the possible symptoms of an abdominal aortic aneurysm, explain the role of diagnostic imaging, and discuss the considerations in choosing between endovascular stent-graft placement and open surgical repair.

By Thomas T. Terramani, MD, Sasan Najibi, MD, Alan B. Lumsden, MD, and Peter H. Lin, MD

Despite the more than 45,000 patients who undergo elective repair of abdominal aortic aneurysm (AAA) each year in the United States, approximately 15,000 patients die annually as a result of a ruptured aneurysm, making it the tenth leading cause of death. The incidence appears to be increasing, due in part to improvements in diagnostic imaging modalities and the growth of the elderly population. With early diagnosis and timely intervention, however, death from aneurysm rupture is largely preventable.

The conventional treatment of AAA involves replacing the aneurysmal segment of the aorta with a prosthetic graft. The operation is performed through a large abdominal incision. Techniques for this surgery have been refined, adapted, and extensively studied by vascular surgeons over the past four decades. Despite a well-documented low perioperative mortality rate of 2% to 3% in large academic institutions, the thought of undergoing an invasive open abdominal operation provokes anxiety in many patients because of the postoperative pain associated with the incision and the long recovery time needed before they can return to normal physical activity.

The most common location of aortic aneurysms is the infrarenal abdominal aorta. Endovascular stent-graft placement represents a revolutionary and minimally invasive treatment for infrarenal AAAs that only requires one to two days of hospitalization and allows the patient to return to normal physical activity within one week. This new treatment modality has captured the attention of patients with aortic aneurysms as well as physicians who practice vascular surgery. Physicians in general should be knowledgeable about the available treatment options for AAA in order to provide adequate evaluation and education to patients and their families. The purpose of this article is to outline these surgical treatment options and to address their advantages and potential complications.
 

PRESENTATION AND DIAGNOSIS

Approximately 75% of patients with an AAA remain asymptomatic. When symptoms do occur, they include abdominal pain or tenderness, back pain, limb ischemia from distal embolization of aneurysmal thrombus, and shock. Other symptoms that are possible, but occur much less frequently, include acute thrombosis of the aorta and symptoms resulting from aneurysmal compression of adjacent structures such as the proximal small bowel and the ureteral and pelvic venous systems.

A careful physical examination is the first step in the diagnosis of an AAA. The sensitivity of the examination depends on multiple factors such as the patient's body build and size of the aneurysm. Overall, the accuracy and positive predictive valve of the examination is moderate at best and should not be relied on to rule out an AAA. In nonemergent clinical situations, an imaging modality is necessary to exclude the presence of an AAA. Commonly used imaging techniques include computed tomography (CT), magnetic resonance imaging (MRI), and duplex ultrasonography. In an emergent situation, most patients need to go to surgery immediately for an exploratory laparotomy.

Because most patients with an AAA are asymptomatic, many are being diagnosed incidentally during the diagnostic work-up of unrelated problems, using the imaging modalities just mentioned and other techniques. Duplex ultrasonography is an excellent screening tool in the evaluation of patients for aortic aneurysmal disease and generally yields accurate results; it is also the least expensive and least invasive technique. (In an emergent situation, there may be time to perform ultrasonography to diagnose a symptomatic AAA.) A CT scan is also an excellent imaging modality for diagnosing AAAs and determining their precise anatomical configuration in various clinical settings. With new high-definition and spiral CT scanners, the three-dimensional configuration of an AAA can also be established. Computed tomography is the most common imaging modality used in the evaluation of patients for endovascular repair. It is excellent in providing the diameter measurements of the aortic neck and iliac arteries, and in many cases it may be the only imaging modality required.

Contrast arteriography is commonly used in the work-up of patients with AAAs, but it is usually not the first diagnostic study obtained. It is limited in terms of determining the size of an aneurysm in the presence of aortic intramural thrombus, and it tends to underestimate aortic neck diameter. Still, it is an important imaging tool, particularly for investigating concurrent renal, mesenteric, and iliac artery occlusive disease. It is also valuable in determining the appropriate required length of the aortic stent graft and the tortuosity of the iliac access vessels in patients being evaluated for endovascular repair.

The roles of MRI and magnetic resonance angiography (MRA) are currently evolving. These tests have an advantage over CT scanning in that they avoid exposing the patient to radiation and nephrotoxic contrast agents. However, their higher cost and the fact that they are not as readily available as CT scanning, as well as higher patient dissatisfaction, have limited their use in the evaluation of the patient with an AAA.

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MANDATORY ANATOMICAL FEATURES

In September 1999, the Food and Drug Administration approved two stent-graft devices for AAA repair: the Ancure and AneuRx Endograft Systems (see illustrations). However, certain anatomical features must be considered when evaluating patients for endovascular repair. An infrarenal aortic neck length of at least 1.5 cm and a diameter of less than 26 mm are mandatory. If the aneurysmal disease extends into the common iliac arteries, stent-graft placement in the external iliac arteries may be necessary. Under such circumstances, coil embolization of the internal iliac artery may be required to prevent retrograde perfusion into the aortic aneurysm (also known as an "endoleak"). Significant tortuosity or circumferential calcification of the iliac artery may pose a technical challenge for stent-graft deployment. It is critical to assess the femoral and iliac access vessels in planning for endovascular graft repair. Forceful deployment of the stent-graft in iliac arteries in such condition may result in severe dissection or even rupture.

The Ancure Endograft System consists of a single-piece (unibody) design that is attached to the proximal infrarenal aorta and distal iliac arteries by a series of hooks. The device is deployed through bilateral femoral arteries exposed by two small groin incisions. The hooks are anchored to the wall of the aorta and iliac arteries by balloon angioplasty. The body of the endograft is made of a polyester material that is not supported by a stent.

The AneuRx Endograft System is a multiple-piece (modular) design that consists of a polyester graft secured to a nitinol exoskeleton stent. The device is deployed through bilateral femoral arteries exposed by two small groin incisions. The device has no hooks for attachment to the aorta but utilizes the unique self-expanding property of the nitinol stent. By oversizing the endograft device, it is secured in position by its constant outward radial force against the aorta and the iliac vessels.

 
Severe angulation of the proximal aortic neck may contribute to suboptimal attachment of the stent-graft, which can lead to an endoleak in the aortic aneurysm and even delayed aneurysm rupture. Since these devices are introduced through the groin, the common femoral and iliac arteries must have a diameter of at least 8 mm to allow safe passage.

Endovascular aneurysm repair is generally performed in the operating room with the patient under general, regional, or local anesthesia. Bilateral femoral arteries are exposed surgically, followed by placement of introducer sheaths. An aortogram is performed to provide an image for stent-graft deployment. Intravenous heparin is administered for anticoagulation. Endovascular repair is then begun by inserting the stent graft, under fluoroscopic guidance, into the aorta via the introducer sheaths. Clinical studies have uniformly supported the findings that endovascular repair is associated with less operative blood loss, shortened hospital stay, and faster postoperative recovery when compared to the conventional operation.
 

OPEN AAA REPAIR

General anesthesia is necessary for a conventional open AAA repair. While a retroperitoneal incision is a well-accepted surgical option, a midline abdominal incision remains the more common approach. Since the incision can lead to significant pain and discomfort, an epidural catheter may be inserted before the operation for a postoperative analgesic infusion.

Once the abdominal cavity is opened, the small intestine and transverse colon are retracted to expose the retroperitoneum overlying the abdominal aorta. The retroperitoneum is then opened and the proximal and distal segments of the AAA are isolated. Intravenous heparin is administered, followed by clamping of the proximal and distal segments of the aneurysm. The aneurysm sac is opened next and a prosthetic graft is used to reconstruct the aorta.

If the aneurysm involves only the abdominal aorta, a prosthetic tube graft can be used to replace the aorta (see illustration below). If the aneurysm extends distally to the iliac arteries, a prosthetic bifurcated graft is used for either an aorto-bi-iliac or aorto-bi-femoral bypass reconstruction. The overlying aneurysm sac and the retroperitoneum are then closed to cover the prosthetic bypass graft to minimize potential bowel contact with the graft. The small intestines and transverse colon are returned to the abdominal cavity and the abdominal fascia and skin closed.

In a conventional open aortic aneurysm repair, an abdominal incision is made to enter the peritoneal cavity. A prosthetic graft is used to replace the diseased segment of the aorta.

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ADVANTAGES AND RISKS OF ENDOVASCULAR REPAIR

The obvious advantage of an endovascular AAA repair is its minimally invasive nature. Typically, patients who undergo this procedure stay in the hospital for only 1 to 3 days, in contrast to the 5- to 10-day stay required after conventional open surgical repair. In our institution, patients who have had an endovascular repair are routinely transferred to a general vascular ward from the postanesthesia recovery unit, avoiding admission to a more costly intensive care unit (see sidebar).

Because an abdominal incision is not necessary in endovascular repair, the procedure is particularly beneficial in patients with severe pulmonary disease, such as chronic obstructive pulmonary disease or emphysema. Patients can sustain adequate breathing in the postoperative period, avoiding respiratory complications or prolonged mechanical ventilation. Because the abdominal cavity has not been entered, the risk of gastrointestinal complications, such as ileus, ventral hernia, or bowel obstruction due to intestinal adhesion, is also greatly reduced. Moreover, regional or epidural anesthesia can be used, avoiding the risks associated with general anesthesia in patients with severe cardiopulmonary dysfunction.

Despite its many advantages, endovascular repair does have potential complications. Since the stent-graft device is attached endoluminally within the abdominal aorta, an endoleak due to incomplete stent-graft exclusion of the aneurysm can occur. With this type of leak, blood flow persists outside the lumen of the endoluminal graft but within an aneurysm sac. A recent meta-analysis of 1,118 patients who underwent successful endovascular repair found an endoleak incidence of 24%. While a small endoleak usually poses little clinical significance because it will typically become thrombosed spontaneously, a large or persistent endoleak may lead to continuous aneurysm perfusion and ultimately to aneurysm rupture. The rupture rate following an endovascular AAA repair has been reported to be less than 0.8%.

Stent-graft iliac limb dysfunction resulting in thrombosis has been reported following endovascular repair. One possible cause is aneurysm remodeling, resulting in a shortening in the aortic length, which can cause the stent-graft to kink. Alternatively, progression of an underlying iliac atherosclerotic lesion may cause compression of the iliac limb and ultimately result in graft-limb occlusion. Treatment options include thrombolysis or graft thrombectomy to determine the underlying cause and possibly additional stent-graft placement. Renal artery occlusion may occur due to improper stent-graft positioning or migration. Graft-limb separation or dislocation has also been reported.

In patients with AAA and concurrent iliac artery aneurysms who undergo preoperative coil embolization of the internal iliac artery, 20% to 45% experience symptoms of pelvic ischemia. These symptoms may include buttock claudication, impotence, gluteal skin sloughing, and colonic ischemia. Other complications pertaining to endovascular repair relate to the access site and include groin hematoma and wound infection. Occasionally, the stent-graft device can malfunction by either failing to deploy or dislodging during the deployment procedure. If the device cannot be salvaged or rescued endoluminally, open surgical repair of the aneurysm may be necessary.
 

ADVANTAGES AND RISKS OF OPEN REPAIR

The main advantage of a conventional open repair is that the AAA is permanently eliminated because it is entirely replaced by a prosthetic aortic graft. The risk of aneurysm recurrence or delayed rupture no longer exists. As a result, long-term imaging surveillance is not needed with these patients. In contrast, the long-term efficacy of endovascular repair remains unclear. Consequently, long-term imaging surveillance is critical to ensure that the aortic aneurysm remains properly sealed by the stent-graft.

Other potential advantages of open repair include direct assessment of the circulatory integrity of the colon. If signs of colonic ischemia become evident after aortic bypass grafting, a concomitant mesenteric artery bypass can be performed to revascularize the colonic circulation. In addition, open repair permits the surgeons to explore for other abdominal pathologies, such as gastrointestinal tumors, liver mass, or cholelithiasis.

As for the risks associated with open repair, cardiac complications, in the form of either myocardial infarction or arrhythmias, remain the most common morbidity, with an incidence between 2% and 6%. Another significant complication is renal failure or transient renal insufficiency as a result of perioperative hypotension, atheromatous embolization, inadvertent injury to the ureter, preoperative contrast-induced nephropathy, or suprarenal aortic clamping. While the incidence of renal failure is less than 2% in elective aneurysm repair, it can occur in more than 20% of patients after repair of a ruptured AAA.

Ischemic colitis is a devastating potential complication after open repair. The likelihood of such a complication is highest in those who had a prior colon resection and undergo repair of a ruptured AAA, due to the loss of collateral blood supply to the rectosigmoid colon. It is estimated that 5% of patients who undergo elective aneurysm repair will develop partial-thickness ischemic colitis but without significant clinical sequelae. However, if the partial-thickness ischemia progresses to full-thickness gangrene and peritonitis, mortality can be as high as 90%.

The incidence of prosthetic graft infection ranges between 1% and 4% after open repair. It is more common in those who undergo repair of a ruptured AAA. If the prosthetic graft is not fully covered by the aneurysm sac or retroperitoneum, intestinal adhesion with subsequent bowel erosion may occur, resulting in an aorto-enteric fistula. The predominant sign of such a complication is massive hematemesis, and it typically occurs years after the operation. Despite these potential complications, however, the majority of patients who undergo successful elective open repair have an uneventful recovery.
 

REVOLUTIONARY DEVELOPMENT

The management of AAA has been revolutionized by developments in endovascular technology. This minimally invasive treatment represents an exciting alternative to the conventional open repair. Short-term clinical reports have shown that endovascular stent-grafting is technically feasible and efficacious. Moreover, the procedure is associated with less operative blood loss, shorter hospital stay, decreased morbidity, and reduced convalescence period compared to the conventional operation.

While our understanding of endovascular technology continues to evolve, this new treatment clearly provides the greatest benefit to high-risk patients who otherwise would not tolerate an open repair. Minimizing the risk of endoleaks will be critical to the long-term success of this procedure. Judicious application of this new technology and long-term patient surveillance are essential to ensuring positive outcomes with endovascular repair of AAA.

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