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.
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.
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.
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.
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.
Figure 7.1 ▪ Abdominal Aortic Aneurysm.
Atherosclerotic calcifications delineate the extensive abdominal aortic aneurysm on this plain radiographic image.
Figure 7.2 ▪ Abdominal Aortic Aneurysm.
A, B: Contrast-enhanced CTA demonstrates the extent of mural thrombus visible as low density (arrows) adjacent to the enhanced lumen of the vessel. The aneurysm extends throughout the suprarenal and infrarenal portions of the abdominal aorta. C, D: Transaxial and sagittal ultrasound images in the same ...