Abdominal Aortic Aneurysm Workup

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 abdominal aortic aneurysm (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, CT scan, MRI, and angiography. For more information, see Emergent Management of Abdominal Aortic Aneurysm Rupture and Bedside Ultrasonography Evaluation of Abdominal Aortic Aneurysm.

Screening for AAA reduces the mortality from rupture and is cost-effective.^[8] The US Preventive Services Task Force recommends ultrasonography screening in men aged 65-75 years who have ever smoked.^[4] 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.

Ultrasonography is the standard imaging tool for AAA. When performed by trained personnel, it has a sensitivity and specificity approaching 100% and 96%, respectively, for the detection of infrarenal AAA. An ultrasonogram of AAA is shown below. 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. The primary role of ultrasonography is to screen patients at risk for AAA, to determine the size of the aneurysm, and to observe the aneurysm over time.

Limitations of the study 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 abdominal aorta (at least one third of its length) are not visualized on bedside emergency ultrasonography in 8% of nonfasting patients.^[9] This rate is higher than reported for fasting patients receiving elective ultrasonography for evaluation of their aortas.

Plain radiographs often are obtained 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 may appear without aneurysm rim calcification, which leads to a high false-negative rate. Aortic wall calcification is shown in the radiographs below.

Do not order plain radiography for the sole purpose of evaluating suspected AAA because it wastes time, delays care, and places the patient at risk for aortic rupture and death because of its low diagnostic yield.

CT scan sensitivity for detecting AAA is nearly 100%, and the study offers certain advantages over ultrasonography in defining aortic size, rostral-caudal extent, involvement of visceral arteries, and extension into the suprarenal aorta. CT scanning permits visualization of the retroperitoneum, is not limited by obesity or bowel gas, detects leakage, and permits concomitant evaluation of the kidneys. A CT scan is shown below.

Spiral CT scan allows 3-dimensional imaging of abdominal contents, enhancing the ability to detect branch vessel and adjacent organ involvement.

Major disadvantages of CT scanning include technician availability, cost, longer study time, exposure to radiation and contrast, and the need to send patients with possible rupture out of the emergency department for an extended time.

MRI permits imaging of the aorta comparable to that with CT scanning and ultrasonography without subjecting the patient to dye load or ionizing radiation. MRI may offer superior imaging of branch vessels compared with CT scan or ultrasonography, 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. An MRI is shown below.

Limitations of MRI in the assessment of AAA are the lack of widespread availability, need for a stable patient, incompatibility with monitoring equipment, and high cost.

Angiography is helpful in determining aortic anatomy and has been advocated for preoperative use if suspicion of suprarenal or thoracic aortic aneurysm, femoral or popliteal aneurysm, renal artery stenosis, unexplained impairment of renal function, occlusive iliofemoral disease, or visceral ischemia exists. Examples of arteriograms are shown below.

Angiography is limited by its invasiveness, cost, lack of operator availability, time involved, and risk of complications (eg, bleeding, perforation, embolization). Routine use of angiography in evaluation of AAA is not recommended.

Digital subtraction angiography (DSA) requires less time, uses less contrast material, and is less invasive than conventional angiography. The technique is not widely available and offers no real advantage over conventional CT scanning.

AAAs contain a chronic inflammatory infiltrate and neovascularity of varying degrees.

Inflammatory AAAs may contain germinal centers.