A potential complication of abdominal aortic aneurysm [AAA] is rupture, which can cause exsanguinating hemorrhage, and if it involves its branches, can lead to severe visceral ischemia and damage. AAA arises secondary to degeneration of the media, with atherosclerotic damage as the most common cause. Systemic diseases such as Marfan’s, Ehler Danlos and collagen vascular diseases may also be complicated by aneurysms. The inflammatory damage causes loss of structural integrity of the aorta leading to luminal dilatation. Co-morbid states of advanced age, obesity, smoking and uncontrolled hypertension contribute to the damage. AAA is described as suprarenal or infrarenal as the management depends on the location.
Transabdominal ultrasound with color Doppler serves as an effective screening modality for the detection of AAA. Ultrasound is usually the initial modality used as most of these patients with AAA are asymptomatic or may present with vague pain in the abdomen. A fusiform or saccular dilatation of the aorta is demonstrated in patients with AAA. Aortic diameter that is 1.5 times larger than the normal aorta is considered aneurismal [about 3 cm for infrarenal aorta]. With ultrasound, periodic monitoring of the uncomplicated cases to assess its caliber, identification of patients for surgery and follow up of operated cases of AAA with endovascular stents placed can be easily carried out.
Ultrasound most often demonstrates the aortic diameter, the site and extent of involvement, calcifications, presence of thrombus and may reveal involvement of the visceral vessels including the renals and iliacs, depending on the degree of visualization in the patient. Non-visualization of a part of the aortic segment due to technical factors is a major drawback of this exam.
A rapid increase in size of the aneurysm is considered evidence of an increased risk of rupture. Thrombosis of the aneurysm is a known complication and recent studies suggest that the rapid growth of thrombus over a period of time may be a better indicator of the risk of rupture of AAA than the aortic diameter. Most surgeons consider an aneurysm measurement of 5.5 cm or more as an indication for surgery, depending on the clinical status of the patient. Smaller aneurysms ranging from 3-5 cm are most often followed up by ultrasound to monitor for any increase in width or for the development of complications.
Rupture of the aneurysm may not be easily diagnosed by ultrasound, and requires other imaging modalities, usually CT. In an extremely unstable patient presenting with shock due to the rupture of AAA, bedside ultrasound with color Doppler may be the only modality available to demonstrate these aneurysms. A hematoma may be seen as a hypoechoic or anechoic area surrounding the aorta. Free complex fluid suggestive of intra or retroperitoneal bleed may also be seen. A recent study by Blaivas, et al demonstrated that in at least 8% of non-fasting patients, the aorta was not visualized in its entirety on bedside ultrasound. So, if a rupture is suspected and the ultrasound fails to reveal the pathology, further diagnostic studies are definitely indicated.
Endovascular repairs are now being increasingly offered as a form of treatment for these aneurysms. Intravascular ultrasound using an intracardiac probe is emerging as the technique that depicts the anatomy of the aorta as well as details of the area of stent placement. The other modalities that accurately demonstrate abdominal aortic aneurysms are helical/ multidetector CT angiography, MR angiography and DSA.