Abdominal Aortic Aneurysm Workup
- Author: Saum A Rahimi, MD, FACS; Chief Editor: Vincent Lopez Rowe, MD more...
More than 80% of patients with ruptured abdominal aortic aneurysm (AAA) present without a previous diagnosis of AAA, which contributes to an initial misdiagnosis rate of 24-42%. A rational approach to the diagnostic evaluation is predicated on a high degree of suspicion.
No specific laboratory studies exist that can be used to make the diagnosis of AAA. Laboratory testing may be used to aid in diagnosis of other pathology or associated medical disorders. Options for radiologic evaluation of AAA include ultrasonography, plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), and angiography.
A complete blood count with differential is used to assess transfusion requirements and the possibility of infection. A metabolic panel (including kidney and liver function tests) is indicated for ascertaining the integrity of renal and hepatic function and thus help assess operative risk and guide postoperative management. Blood must be typed and crossmatched to prepare for the possibility of transfusion, including clotting factors and platelets.
Because synthetic material is used in the intervention, any potential foci of infection should be assessed and eliminated preoperatively with the aid of urinalysis.
The preoperative workup should also include assessment of pulmonary function to help evaluate operative risk and determine postoperative care. Patients who can climb a flight of stairs without excessive shortness of breath generally do well. If the patient’s pulmonary status is in question, blood gas measurement and pulmonary function tests are helpful.
Ultrasonography is the standard imaging tool for AAA (see the image below). When performed by trained personnel, it has a sensitivity of nearly 100% and a specificity approaching 96% for the detection of infrarenal AAA. Ultrasonography can also detect free peritoneal blood.
Ultrasonography is noninvasive and may be performed at the bedside. Bedside emergency ultrasonography should be performed immediately if AAA is suspected. Elderly patients with abdominal pain are prime candidates for bedside ultrasonography screening. (See Bedside Ultrasonography Evaluation of Abdominal Aortic Aneurysm.)
Screening for AAA reduces the mortality from rupture and is cost-effective. The US Preventive Services Task Force recommends ultrasound screening in men aged 65-75 years who have smoked.[4, 5] Abdominal ultrasonography can provide a preliminary determination of aneurysm presence, size, and extent. In addition, it is a cost-effective modality for monitoring patients whose aneurysms are too small for surgical intervention. It is also useful for follow-up after endovascular surgery to assess the durability of the repair.
Limitations of ultrasonography in this setting are few but include inability to detect leakage, rupture, branch artery involvement, and suprarenal involvement. In addition, the ability to image the aorta is reduced in the presence of bowel gas or obesity.
Significant portions of the abdominal aorta (at least one third of its length) are not visualized on bedside emergency ultrasonography in 8% of nonfasting patients. This rate is higher than reported for fasting patients receiving elective ultrasonography for evaluation of their aortas.
Plain radiography is often performed on patients with abdominal complaints before the diagnosis of AAA has been entertained. Using this method to evaluate patients with AAA is difficult because the only marginally specific finding, aortic wall calcification, is seen less than half of the time. Aortic-wall calcification (see the images below) may appear without aneurysm rim calcification, resulting in a high false-negative rate.
Plain radiography should not, however, be ordered for the sole purpose of evaluating suspected AAA; because of its low diagnostic yield, its use can waste time, delay care, and place the patient at risk for aortic rupture and death.
Chest radiography may be employed to gain a preliminary assessment of the status of the heart and lungs. Concurrent pulmonary or cardiac disease may have to be addressed before the AAA is treated.
CT has a sensitivity of nearly 100% for detecting AAA, and it has certain advantages over ultrasonography for defining aortic size, rostral-caudal extent, involvement of visceral arteries, and extension into the suprarenal aorta (see the image below). CT permits visualization of the retroperitoneum, is not limited by obesity or bowel gas, detects leakage, and allows concomitant evaluation of the kidneys. Spiral (helical) CT allows three-dimensional (3D) imaging of abdominal contents, facilitating detection of branch vessel and adjacent organ involvement.
Preoperative CT is helpful for more clearly defining the anatomy of the aneurysm and other intra-abdominal pathologic conditions. Nonenhanced CT is used to size aneurysms. As important as sizing the aneurysm is determining the anatomic relations that are relevant to surgical repair. These include the location of the renal arteries, the length of the aortic neck, the condition of the iliac arteries, and the presence of anatomic variants such as a retroaortic left renal vein or a horseshoe kidney.
Enhanced spiral CT of the abdomen and pelvis with multiplanar reconstruction and CT angiography is the modality of choice for preoperative evaluation for open and endovascular repair (see the image below).
In 10-20% of AAA cases, CT scans show focal outpouchings or blebs that are thought to contribute to the potential for rupture. The wall of the aneurysm becomes laminated with thrombus as the blebs enlarge. This process can yield the appearance of a relatively normal intraluminal diameter in spite of a large extraluminal size.
CT is the best modality for determining whether a patient is a candidate for endovascular aneurysm repair (EVAR). It can assess the aneurysm neck diameter, length, and angulation, as well as thrombus within the neck. The CT scan is also useful for assessing iliac vessel diameter, calcification, and tortuosity, which are important for determining whether the endovascular device can be advanced from the femoral artery.
Major disadvantages of CT include potential difficulties with technician availability, higher cost, longer study time, exposure to radiation and contrast material, and the need to send patients with possible rupture out of the emergency department for an extended period.
Magnetic Resonance Imaging
MRI permits imaging of the aorta comparable to that achievable with CT and ultrasonography, but without subjecting the patient to a dye load or ionizing radiation (see the image below). It may offer better imaging of branch vessels than either CT or ultrasonography does, but it is less valuable in assessing suprarenal extension and is not suitable in patients who are unstable. MRI may have a role in very stable patients with a severe dye allergy.
Limitations of MRI in the assessment of AAA are the lack of widespread availability, the need for a stable patient, potential incompatibility with monitoring equipment, and high cost.
Because of advances in CT imaging with 3D reconstruction capability, angiography (see the images below) currently is less often used in preoperative evaluation of AAA than it once was. Arteriography may miss an AAA if there is a lack of calcification because of the laminated thrombus within the AAA making a more normal-appearing aortic lumen. It is primarily used intraoperatively to facilitate endovascular repair.
Limitations on the use of angiography include the invasiveness of the procedure, the cost, the potential lack of operator availability, the considerable time involved, and the risk of complications (eg, bleeding, perforation, and embolization). Routine use of angiography in the evaluation of AAA is not recommended.
Digital subtraction angiography (DSA) requires less time, uses less contrast material, and is less invasive than conventional angiography. However, DSA is not widely available and offers no real advantage over conventional CT.
Intra-aortic CT angiography (IA-CTA) has good sensitivity for locating the Adamkiewicz artery (AKA) in patients with thoracoabdominal aortic aneurysms. In one study, the AKA was visualized by IA-CTA in 27 of 30 cases (90%) before surgery for aneurysm or dissection of the thoracoabdominal aorta. Continuity with the aorta was satisfactorily seen in 26 of 31 (84%) cases. Spinal angiography by selective catheterization confirmed the results of IA-CTA in 75% of cases in which the AKA was visualized.
In a number of centers, magnetic resonance angiography (MRA) is replacing traditional angiographic assessment of aneurysms. MRA provides excellent anatomic definition and 3D assessment of the problem. Gadolinium-enhanced MRA can provide excellent images, even though regional variations in quality are reported.
Because of the fluid shift involved during the operative repair of AAA, cardiac function should be assessed by means of echocardiography. Ascertaining the ejection fraction of the patient facilitates planning of the operative intervention and institution of cardiac protective measures as needed. This study is particularly indicated in patients with a history of congestive heart failure or known cardiac enlargement.
Assessment of pulmonary function is of paramount importance in AAA patients. Because surgical intervention requires an abdominal incision, preoperative assessment of the patient’s pulmonary status allows postoperative care to be appropriately tailored to the patient’s condition.
Assessment of cardiac status is mandatory in all patients with vascular disease. If one vascular bed is involved with an atherosclerotic process, others may be involved as well. Electrocardiographic findings provide a baseline assessment of cardiac rhythm and old disease processes. A stress test can be performed to uncover unsuspected cardiac ischemia. Significant coronary disease may have to be addressed before the AAA can be repaired.
On histologic examination, AAAs contain a chronic inflammatory infiltrate and neovascularity of varying degrees. Inflammatory AAAs may contain germinal centers.
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|Lowest Risk||Moderate Risk||High Risk|
|Age < 70 y||Age 70-80 y||Age 80 y|
|Physically active||Active||Inactive, poor stamina|
|No clinically overt cardiac disease||Stable coronary disease; remote MI; LVEF >35%||Significant coronary disease; recent MI; frequent angina; CHF; LVEF < 25%|
|No significant comorbidities||Mild COPD||Limiting COPD; dyspnea at rest; O2 dependency; FEV1 < 1 L/sec|
|...||Creatinine 2.0-3.0 mg/dL||...|
|Normal anatomy||Adverse anatomy or AAA characteristics||Creatinine >3 mg/dL|
|No adverse AAA characteristics||...||Liver disease (↑ PT; albumin < 2 g/dL)|
|Anticipated operative mortality, 1%-3%||Anticipated operative mortality, 3%-7%||Anticipated operative mortality, at least 5%-10%; each comorbid condition adds ~3%-5% mortality risk|
|AAA—abdominal aortic aneurysm; CHF—chronic heart failure; COPD—chronic obstructive pulmonary disease; FEV1 —forced expiratory volume in 1 second; LVEF—left ventricular ejection fraction; MI—myocardial infarction; PT—prothrombin time.|
|AAA Diameter (cm)||Rupture Risk (%/y)|
|AAA—abdominal aortic aneurysm.|
|Low Risk||Average Risk||High Risk|
|Diameter||< 5 cm||5-6 cm||>6 cm|
|Expansion||< 0.3 cm/y||0.3-0.6 cm/y||>0.6 cm/y|
|Family history||No relatives||One relative||Numerous relatives|
|Hypertension||Normal blood pressure||Controlled||Poorly controlled|
|Wall stress||Low (35 N/cm2||Medium (40 N/cm2||High (45 N/cm2)|
|COPD—chronic obstructive pulmonary disease.|