Abdominal aortic aneurysm

Last revised by Mohammad Taghi Niknejad on 15 Mar 2024

Abdominal aortic aneurysms (AAA) are focal dilatations of the abdominal aorta measuring 50% greater than the proximal normal segment or >3 cm in maximum diameter.

They can be confined to the abdominal aorta or continue from the thoracic aorta as part of a thoraco-abdominal aneurysm (TAAA).

The feared complication is rupture, which is a surgical emergency due to its high mortality. Imaging has a crucial role in active surveillance.

Abdominal aortic aneurysms represent the tenth most common cause of death in the Western world:

  • prevalence increases with age

    • ~10% of patients older than 65 years have an AAA

  • males are much more commonly affected than females (4:1 male/female ratio)

  • common iliac artery aneurysm

    • AAA extends into the common iliac arteries in 25% of cases 14

    • the vast majority of patients with CIA aneurysms have an AAA

    • isolated CIA aneurysms are rare 15

  • popliteal artery aneurysm

    • 4% of patients with an AAA have a peripheral femoral or popliteal artery aneurysm 16

    • 30-50% of patients with a popliteal artery aneurysm have an AAA

  • intracranial cerebral aneurysm

    • prevalence of ~10%, higher in females 13

Since most abdominal aortic aneurysms are asymptomatic unless they leak or rupture, they are commonly diagnosed incidentally during imaging for other indications.

Uncommonly, unruptured aneurysms may present with abdominal or back pain. Large aneurysms may present as a pulsatile abdominal mass.

The most significant complication is abdominal aortic rupture, which presents with severe abdominal or back pain, hypotension, and shock.

  • the mortality rate from a ruptured AAA is high

    • ~70% (range 59-83%) of patients die before hospitalization or surgery

    • for those who undergo operative repair, the mortality rate is ~40%

    • for comparison, mortality from elective surgical repair is 4-6%

  • ruptured abdominal aortic aneurysms are discussed in more detail separately

Other complications include:

Role of imaging includes

  • detection of abdominal aortic aneurysm

  • monitoring of growth rate

  • preoperative planning

  • postoperative follow-up

An abdominal aortic aneurysm may be visible as an area of curvilinear calcification in the paravertebral region on either abdominal or lumbar spine radiographs. Although not adequate for AAA detection or follow-up, an x-ray may be sufficient for initial detection and diagnosis.

Ultrasound is optimal for general AAA screening and surveillance because it is fast, spares the use of ionizing radiation and intravenous contrast, and is relatively inexpensive. The sensitivity and specificity approach is 100% 19. However, it should be noted that visualization is poor in 1% to 3% of patients due to patient habitus or overlying bowel gas 19.

Although excellent for following lesions, ultrasound does not provide sufficient detail for procedural planning or more complex lesions. Given a reported range in the measurement error of 4 mm 12, ultrasound cannot be reliably used to evaluate endovascular treatments and assess regional branch vessels.

CT angiography (CTA) is considered the gold standard for evaluation but exposes patients to high radiation doses. It is excellent for preoperative planning as it accurately delineates the size and shape of the AAA and its relationship to branch arteries and the aortic bifurcation. Oblique reformations enable accurate measurements in non-orthogonal planes. CTA is superior to ultrasound in detecting and measuring common iliac artery aneurysms.

Signs of frank rupture include:

Signs of impending rupture or contained leakage:

An increasing diameter of the aneurysmal sac of 5 mm over a 6-month interval or a diameter of 7 cm is also considered to be at high risk for rupture and warrants urgent repair.

Dual-energy CT imaging of aortic aneurysms can discern the difference between iodinated contrast, calcified atheroma, and previous grafts or surgical materials. Post-processing techniques can create virtual non-calcium or non-enhanced images.

Dual-energy CT has several advantages over single-energy CT including 22:

  • delivering lower radiation doses

  • uses lower volumes of contrast media

  • removing calcified plaques from the image to allow assessment of the degree of stenosis

  • better assessment of endoleak

Catheter-based angiography alone is inadequate for the preprocedural evaluation of AAA. While digital subtraction angiography (DSA) is superb for delineating regional branch vessels, it can be misleading and mask true aneurysm size in the setting of mural thrombus.

MR angiography (MRA) offers a lack of ionizing radiation but is more costly, less widely available, and the examination is substantially lengthier.

Certain features and relevant negatives regarding AAA should be included in the radiology report - especially if this is a new or undocumented finding:

Also see: reporting tips for aortic aneurysms

The natural history of abdominal aortic aneurysms is variable. Some small aneurysms do not appear to change, while others slowly expand and become at risk for eventual rupture 19,21. A number of clinical factors (e.g. smoking, sex, blood pressure) are known to contribute. Ultimately, the primary clinical question is whether and when to intervene to avoid aortic rupture.

In terms of imaging, there remains debate about the best criteria for predicting AAA rupture and, therefore, indications for operative intervention. Prognostic imaging criteria include:

  • maximum transverse diameter

    • most widely used and validated method 19,20

    • Society of Vascular Surgery 2018 recommendations generally recommend intervention for AAA ≥5.4 cm and surveillance for smaller diameter lesions 19

    • young, healthy (especially female) patients may benefit from the intervention for lesions between 5.0-5.4 cm 19

    • most study data is based on fusiform aneurysms; it is debated whether the more uncommon saccular aneurysm is at higher risk for rupture at smaller transverse diameter 19

  • rate of aneurysm growth

    • enlargement in transverse diameter ≥5 mm in 6 months may be an indication for intervention 17

  • symptomatic lesions 19

In patients with a connective tissue disorder (e.g. Marfan syndrome), especially those with a bicuspid aortic valve, surgical treatment may be considered even with a diameter smaller than 5.0 cm.

Recommended imaging follow-up intervals for enlarged infrarenal abdominal aorta vary by organization. The American College of Radiology (2013) recommends 11:

  • <2.5 cm: follow-up not needed

  • 2.5-2.9 cm: 5-year interval

  • 3.0-3.4 cm: 3-year interval

  • 3.5-3.9 cm: 2-year interval

  • 4.0-4.4 cm: 1-year interval

  • 4.5-4.9 cm: 6-month interval

  • 5.0-5.5 cm: 3-6 month interval

  • >5.5 cm: treatment

The Society for Vascular Surgery (2018) recommends 19:

  • >2.5-2.9 cm: rescreen after 10 years

  • 3.0-3.9 cm: 3-year interval

  • 4.0-4.9 cm: 1-year interval

  • 5.0-5.4 cm: 6-month interval

Management options include:

  • surveillance (see above)

  • endovascular aneurysm repair (EVAR) 23

    • if the anatomy permits, EVAR is preferred because of its less invasive nature vs open surgical repair

    • aneurysm-related mortality is much lower with EVAR vs open surgical repair

  • resection (open surgical repair)

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