Since 1951, when Dubost first performed repair of an AAA with a homograft, surgery has been the mainstay of treatment. Many refinements in technique have occurred during the interval, but none as significant as the stent-graft. In 1991, Parodi et al described a novel, less invasive technique for repairing AAAs by placing a graft from within the vessel. This technique was labor intensive and involved the customized construction of the graft for each patient by sewing the graft material to self-expanding metal skeleton. Today, designs are approved by the US Food and Drug Administration, and numerous devices are being used in clinical investigations.
Pathophysiology: Classically, AAAs have been attributed to a weakening of the arterial wall as a result of atherosclerotic vascular disease caused by the atheromatous lesions seen on pathologic examination. Recent evidence supports a multifactorial process in which atherosclerosis is involved. Other etiologic cofactors under investigation include changes in the matrix of the aortic wall with age, proteolysis, metalloproteinase changes, inflammation, infectious agents (eg, syphilis, mycotic infections), and a genetic predisposition (eg, Marfan syndrome, Ehlers-Danlos syndrome).
True aneurysms involve dilation of all 3 layers of the vessel wall, whereas false aneurysms are caused by the disruption of 1 or more layers of the vessel wall. Elastin and collagen are the primary structural elements of the aortic wall. Experimental findings have shown that the relative content of elastin and collagen are lowest in the infrarenal aorta and that, with the destruction of collagen and elastin, dilatation of the aorta ensues. Increased concentrations of several proteases capable of degrading collagen and/or elastin have been found in the walls of AAAs and in aortic occlusive disease; both are manifestations of atherosclerosis.
An immunologic component to atherosclerotic vascular disease has been recognized and is characterized by infiltration of the aortic wall by macrophages, T lymphocytes, and B lymphocytes; these are known to activate proteolytic activity. The nature of this response has led researchers to investigate autoimmunity in the pathogenesis of AAA. Recent reports describe Chlamydia pneumoniae antigens, in contrast to active infection, in the walls of AAA. After the infectious agent is cleared, an antigenic stimulus remains, stimulating proteolytic activity with weakening of the vessel wall and aneurysm formation.
Inflammatory aneurysms, once believed to be distinct entities, are currently considered one extreme in the spectrum of atherosclerotic aneurysms; these account for 3-10% of all AAAs. Clinical and imaging characteristics differentiate inflammatory from noninflammatory aneurysms.
The familial pattern of AAA has long been recognized with a 15-19% incidence among first-degree relatives. This observation suggests that one or more genes are related to AAA and atherosclerosis. The identification of these genes may enable the early detection and prevention of AAA in high-risk patients.
Frequency:
Mortality/Morbidity: In the United States 15,000 deaths per year are attributed to AAAs.
Race:
Sex: Depending on the published series, the male-to-female ratio is 1.6-4.5:1.
Age:
Anatomy: An aneurysm is defined as a localized dilation of an artery by at least 50% as compared with the expected normal diameter of the vessel. The term ectasia is used when the dilatation is less than 50%. If the arteries are diffusely enlarged by 50% or more, the condition is called arteriomegaly.
The Society for Vascular Surgery and the International Society for Cardiovascular Surgery have suggested the classification of aneurysms by their site, origin, histologic features, and clinicopathologic manifestations. The anatomic site and morphology of an aneurysm can be preoperatively determined by radiologic means.
The site of an aneurysm is related to its natural history, clinical presentation, and means of treatment. The site of abdominal aneurysms should be characterized as suprarenal, juxtarenal or pararenal, or infrarenal. Approximately 90-95% of AAAs involve the infrarenal abdominal aorta. Rarely do they extend above the renal arteries; however, extension into the common iliac arteries is fairly common.
Clinical Details: AAAs occur in 5-7% of the population older than 60 years. Although most patients with AAA are asymptomatic, they can present with symptoms of mass effect, compression of abdominal organs, or visceral or peripheral emboli originating from the wall of the aneurysm. Rarely, patients present with back pain, which can represent rupture of the aneurysm, a surgical emergency. Patients older than 60 years who smoke and who are known to have atherosclerosis, hypertension, and/or chronic obstructive pulmonary disease are at increased risk for AAA. Routine screening of these patients is warranted.
Once an aneurysm is identified, it should be repaired or followed up with imaging, depending on the clinical scenario and the size of the aneurysm at the time of diagnosis. Most aneurysms (80%) demonstrate progressive enlargement. The diameter of an aneurysm is directly related to its risk of rupture. For aneurysms smaller than 4 cm in diameter, the risk of rupture is less than 10%. Once an aneurysm is 4-5 cm in diameter, the risk of rupture increases to almost 25%, with an associated mortality rate as high as 75%. The accepted surgical mortality rate remains less than 5% with the elective repair of these 4- to 5-cm aneurysms.
The morphologic features, including the maximum diameter in both the anteroposterior and lateral dimensions and the length of the aneurysm, should be reported. The shape of the aneurysm (fusiform or saccular) and its relationship to branch vessels should be described. Arterial wall complications such as the expansion over time, compression or erosion into adjacent structures, rupture, dissection, and thrombotic occlusion should be documented as well.
With the advent of the endoluminal repair of aneurysms, several additional morphologic characteristics should be recorded. These determine if endovascular repair is possible, and if so, what type of device can be used. These features include the following: (1) greatest mural diameter, (2) extent of aneurysm (eg, length of proximal and distal neck, extension into iliac arteries), (3) tortuosity of the aorta, (4) anatomy of the iliac arteries (eg, iliac artery occlusive disease, tortuosity, caliber, patency of internal iliac arteries and relation of aneurysm to them, presence of concomitant iliac artery aneurysms), (5) presence and degree of intraluminal thrombus, (6) presence and degree of calcification in the neck and iliac arteries, and (7) anatomy of the femoral arteries (eg, caliber, degree of calcification or occlusive disease).
Preferred Examination: Because of portability, lack of ionizing radiation, cost, and availability, ultrasonography (US) should be the initial imaging modality when an asymptomatic, pulsatile abdominal mass is palpated.
If the aneurysm is approaching 5 cm or more or if rapid enlargement is seen on serial US images, a CT or CT angiogram (CTA) should be ordered to better delineate the extent of disease prior to conventional surgery or treatment with the insertion of an endovascular graft. In patients whose renal function does not permit the administration of iodinated contrast material, MRI and magnetic resonance angiography (MRA) provide good alternatives.
Angiographic examination may be requested because of a clinical concern that concomitant renal artery stenosis or peripheral vascular disease may require surgical intervention during AAA repair. At some institutions, CTA and MRA have replaced routine diagnostic angiography in the preoperative evaluation of AAA.
In urgent situations in which the clinical diagnosis is fairly certain or rupture is imminent or suspected and in which the patient's condition is stable, CT and/or CTA may be the initial and only examination required.
Imaging of the aorta does not end with the repair of the aneurysm. After repair with, either a traditional open surgical procedure or an endovascular procedure, follow-up imaging is necessary. In the case of conventional surgical repair, follow-up imaging is performed yearly, usually with US. For endovascular grafts, the follow-up is more stringent, with immediate postprocedural CT as well as 6-month and then yearly CT follow-up.
When the evaluation for AAA is performed with CT or MRI, note the extent of the aneurysm, any involvement of major branch vessels, and the existence of a retroaortic or circumaortic left renal vein. Note if the aneurysm has significant wall thickening, a typical characteristic of an inflammatory aneurysm, because the surgical approach for this condition differs from that needed for the more common, predominately atherosclerotic aneurysm.
Limitations of Techniques: With conventional radiography in the anteroposterior or lateral projection, calcification of both opposing abdominal aortic walls must be present to outline an AAA. However, this finding is present in less than 50% of cases. A tortuous, calcified aorta may mimic an AAA unless both walls can be seen clearly. The lack of overlying bony structures in the lateral projection may allow clearer definition of the aneurysm.
US is considered the screening examination of choice; however, it may not adequately depict the entire abdominal aorta if a large amount of bowel gas is present or if the patient is obese.
With or without contrast enhancement, CT is an excellent screening examination for AAA. CT depicts the absolute size of the aneurysm. However, the extent of mural thrombus and the presence of dissection cannot be evaluated without the administration of contrast material.
MRI with contrast enhancement provides an alternative to CT in patients with renal insufficiency. MRI has several absolute contraindications, including cardiac pacemakers and intracranial aneurysm clips. Claustrophobia and a patient's inability to remain motionless are likely to yield a nondiagnostic study. MRI is not as available as CT and US.
Angiography is also a safe procedure. However, because it is an invasive procedure, a small but definite risk to the patient exists. The true size of the aneurysm may not be discernible because of a mural thrombus; therefore, underestimation of the true extent of the aneurysm is possible. The role of angiography is in planning surgical or endovascular repair.
RADIOGRAPH
Findings: Calcification of the abdominal aortic wall is frequently evident on plain radiographs of the abdomen. Calcification is best seen on lateral views when the spine does not obscure the opposing walls of the vessel. When calcification can be clearly identified in the opposing aortic walls, AAA can be diagnosed with the plain radiographic findings.
Degree of Confidence: If the classic eggshell appearance is present (see Image 2), the degree of confidence is approximately 100%; however, this finding is present only in 50% of patients. Occasionally, only the anteroposterior or lateral abdominal image demonstrates the findings clearly. If AAA is suspected, perform abdominal US or CT for confirmation. As such, negative plain radiographic findings do not exclude the diagnosis in any way.
False Positives/Negatives: A tortuous, calcified aorta can mimic AAA unless both walls can be seen clearly. If the opposing walls are not calcified, the diagnosis cannot be made with certainty. In these cases US, CT or MRI must be performed if AAA is clinically suspected.
CT SCAN
Findings: CT accurately demonstrates dilation of the aorta , and involvement of major branch vessels proximally and distally. This information helps in determining the appropriate intervention, which may be either surgical or endovascular repair. CT also shows the other organs in the abdomen and demonstrates involvement or displacement of organs that can confuse the clinical picture. The location and number of the renal arteries, caliber of the aneurysm, degree of calcification, lengths of the neck and iliac artery, and presence of mural thrombus are readily assessed. CTA allows multiplanar assessment of the aneurysm and associated relevant vessels (visceral arteries, iliac and femoral arteries).
Degree of Confidence: CT has emerged as the diagnostic imaging standard for the evaluation of AAA with an accuracy that approaches 100%. A well-performed CT examination can reveal the extent of the aneurysm, as well as the involvement of other organs. Intravenously administered contrast agent is needed to obtain the full benefit of CT; however, a nonenhanced study accurately depicts AAAs. Three-dimensional reconstructions of state-of-the-art, multidetector-row, helical CT scans can help in preoperative planning and may replace the need for preoperative diagnostic angiography.
False Positives/Negatives: The administration of contrast material is essential for detecting dissection or ulceration of a vessel that might be missed without it. In the acute setting (eg, in a patient with back pain or an aneurysm), a false-positive diagnosis of rupture is possible if fluid resulting from another cause is seen in the abdomen. Conversely, an aneurysm or rupture can be missed in a patient who has recently undergone barium study because artifact can obscure the aorta.
MRI
Findings: MRI and MRA can be used to define the extent of AAA, similarly CT . The absence of iodinated contrast material and radiation are advantages of this modality. However, MRI is more sensitive to motion than CT because a patient must remain motionless for longer than with current multidetector-row helical CT technology. In addition, the remaining organs in the abdomen are not seen as well on MRIs because of motion.
Degree of Confidence: In technically well performed MRI and MRA, degree of confidence approaches 100%. These examinations clearly reveal the extent of the aneurysm; however, motion can cause artifacts that can render the results nondiagnostic. Patients must be able to remain motionless for longer periods than with CT to enable a diagnostic examination.
False Positives/Negatives: If prior abdominal surgery has been performed and if metal clips or devices were used, MRI may not be possible. If the metal is close to the aneurysm or if branch vessels or heavy calcification is seen, artifacts may obscure the vessel and result in a nondiagnostic study.ULTRASOUND
Findings: US is the screening examination of choice as a result of its relative availability, speed, and low cost ). US is operator dependent, unlike other modalities; therefore, operator experience is important. The abdominal aorta normally tapers as it extends distally. Any increase in its diameter exists is considered abnormal.
Degree of Confidence: If the abdominal aorta can be seen in its entirely, US provides a reliable, low-cost screening examination. Any increase in the size as the aorta travels distally is abnormal. However, in a patient who is obese or in whom the bowel is distended with gas, a complete examination of the aorta and proximal iliac arteries may not be technically possible. In such instances, another cross sectional imaging study (eg, CT, MRI) should be obtained.
False Positives/Negatives: A technically unsatisfactory examination may result from a large patient body habitus or a large amount of bowel gas, which results in incomplete visualization of the aorta. Thus, a false-negative result is possible if these limitations are not recognized.
INTERVENTION
Intervention: The Society for Vascular Surgery and the International Society for Cardiovascular Surgery guidelines for the repair of AAA include the following: (1) Any patient with a documented rupture or suspected rupture; (2) a symptomatic or rapidly expanding aneurysm, regardless of its size; (3) aneurysms larger than 4 cm in diameter; (4) complicated aneurysms with embolism, thrombosis, or symptomatic occlusive disease; and (5) atypical aneurysms (eg, dissecting, mycotic, saccular). These guidelines must be weighed against the existing clinical risk factors in each patient. With the advent of endoluminal repair, patients who are poor surgical candidates have a possible alternative to open repair. Careful screening of these patients is critical for good outcomes.
Endovascular repair, such as stent-graft placement, is evolving as an alternative to conventional, open surgical repair (see Image 6). The US Food and Drug Administration has approved several devices for use in the endovascular repair of aneurysms. Each device has benefits and limitations.
The primary factors that determine suitability for endovascular repair are the diameter and length of the proximal neck of the aneurysm, the tortuosity of the aorta, and the anatomy of the iliac arteries.
Endovascular devices rely on radial force (and, for some devices, hooks) to engage the more normal segments of the aorta and iliac arteries and to exclude blood flow from the aneurysmal sac. If the proximal neck is too wide or too short or densely calcified, a good seal cannot be achieved at the attachment site. The sac remains pressurized, and the aneurysm is still at risk of rupture, with endotension or an endoleak.
The flexibility of an endovascular graft is an important consideration in selecting a patient for endovascular repair. If the angle between the neck of the aneurysm and the aorta is too great, the graft may be displaced from its intended position with a subsequent leak at the attachment site. The leak can occur acutely (type I) or later, as aneurysm shrinkage and remodeling occurs.
The tortuosity of one or both of the iliac arteries can also preclude endovascular repair. If the common iliac arteries are too large, the limb of the stent is not well opposed to the wall of the artery, and a leak at the attachment site results. Embolization of a hypogastric artery can be performed to allow extension of a graft limb to a nonaneurysmal external iliac artery if needed. If the iliac arteries are too small or too tortuous, advancing the stent-graft deployment system into position may be impossible.
The presence of circumferential calcification at the neck is increasingly recognized as a negative prognostic indicator for primary seal formation, and it may indicate an increased risk of rupture during the procedure. Extensive intraluminal thrombus may similarly affect the ability to obtain a secure, long-term seal at the proximal part of the neck.
Concomitant embolization of one or, rarely, both internal iliac arteries may be required prior to graft placement if the iliac arteries are aneurysmal to or beyond the distal common iliac artery. Embolization can lead to buttock claudication and, in rare cases, colonic ischemia or infarction.
Medical/Legal Pitfalls:
- Clinicians order radiologic studies during the course of patient examinations. The clinical history should be considered when reviewing the radiologic studies, though it may be misleading in some cases. When AAA is suspected, it is rarely missed.
- When an examination, especially a plain radiograph is ordered for a reason other than the evaluation of AAA, curvilinear calcifications should be carefully assessed because most AAAs are asymptomatic. When they are discovered, the referring clinicians should be notified of the abnormal and unexpected findings. In some cases, referring clinician might be reminded of the need for appropriate follow-up and the time interval.
- At minimum, radiologists should follow several guidelines to ensure good patient care:
- The ordering physician should be notified and US or CT should be recommended when findings suggestive of AAA are seen on plain radiographs.
- Recommend appropriate surveillance for aneurysms both before and after their repair.
- Notify the ordering physician when evidence of arterial wall complications is present. Such evidence includes expansion over time, compression or erosion into adjacent structures, rupture, dissection, and thrombosis.
