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An abdominal aortic aneurysm (AAA) is a focal dilation of the aortic wall measuring greater than 1.5 times the normal diameter of the aorta. An aortic diameter of 3 cm at the level of the renal arteries meets the definition of an AAA. Most aneurysms produce little to no symptoms unless they rupture.
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CT can accurately detect an AAA, mural thrombosis, aneurysmal leakage, and rupture. CT findings of a ruptured aneurysm include anterior displacement of the kidney, indistinct aortic wall, free intraperitoneal fluid, or a retroperitoneal hematoma in the presence of an aneurysm. Leakage and rupture of an AAA can be visualized without the use of IV contrast; however, contrast is necessary to delineate a patent aortic lumen from a mural thrombus and can provide anatomic information about the shape and location of the aneurysm. A normal aortic diameter on CT excludes the diagnosis of an AAA.
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ED bedside ultrasonography is very accurate in establishing the presence of an AAA; however, it cannot reliably detect leakage or rupture. In a hemodynamically unstable patient, a ruptured AAA is presumed when it is visualized by ultrasonography.
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Clinical Implications
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An asymptomatic aneurysm may be detected during the evaluation of an unrelated medical problem, and can be followed closely as an outpatient. A leaking or ruptured AAA is a true emergency. Clinically stable patients should undergo CT scanning with IV contrast. If the patient is unstable, a bedside ultrasound can be obtained to determine if an aneurysm is present.
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When performing bedside ultrasonography be careful to measure the aortic diameter from outer wall to outer wall. If the aneurysm contains a thrombus, take care not to mistake the edge of the thrombus for the aortic wall. Measuring the inner rim of the thrombus will result in a smaller measurement and may lead to the incorrect conclusion that an aneurysm is not present.
Aneurysms measuring greater than 5 cm, or demonstrating an increase in size of greater than 1 cm over a period of 6 months are at increased risk of rupture.
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