abdominal aortic aneurysm
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.
- represent the tenth most common cause of death in the Western world
- prevalence increases with age
- ~10% patients older than 65 years have an AAA
- males are much more commonly affected than females (4:1 male/female ratio)
Since most AAAs 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:
- pseudoaneurysm from chronic contained leak/rupture
- aortic fistulas
- distal thromboembolism
- thrombotic occlusion of a branch vessel
- infection/mycotic aneurysm (actually a pseudoaneurysm)
- compression of adjacent structures from large aneurysms (rare)
- anterior vertebral scalloping
- atherosclerosis (most common)
- inflammatory abdominal aortic aneurysm
- chronic aortic dissection
- vasculitis, e.g. Takayasu arteritis
- connective tissue disorders
- mycotic aneurysm
- traumatic pseudoaneurysm
- anastomotic pseudoaneurysm
- common iliac artery (CIA) aneurysm
- AAA extends into the common iliac arteries in 25% of cases
- the vast majority of patients with CIA aneurysms have an AAA
- isolated CIA aneurysms are rare
- popliteal artery aneurysm
- 4% of patients with an AAA have a peripheral femoral or popliteal artery aneurysm
- 30-50% of patients with a popliteal artery aneurysm have an AAA
- intracranial cerebral aneurysm
- prevalence of ~10%, higher in females
Role of imaging includes
- detection of AAA
- monitoring of growth rate
- preoperative planning
- postoperative follow-up
An 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 100% ; however, it should be noted that visualization is poor in 1% to 3% of patients due to patient habitus or overlying bowel gas .
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 , ultrasound cannot be reliably used in evaluation for endovascular treatments and assessment of regional branch vessels.
CT angiography (CTA) is considered the gold standard for evaluation but exposes the patients to high radiation doses. It is excellent for pre-operative 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:
- retroperitoneal hematoma
- para-aortic fat stranding
- contrast extravasation from the aorta into the retroperitoneum
Signs of impending rupture or contained leakage:
- draped aorta sign (contained rupture)
- high-attenuation crescent sign
- thrombus fissuration
- focal discontinuity of intimal calcification
- tangential calcium sign
An increasing diameter of the aneurysmal sac of 5 mm over a 6-month interval or a diameter of 7 cm are also considered to be at high risk for rupture and warrant urgent repair.
Imaging of aortic aneurysms with dual-energy CT can be used to 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 delivering lower radiation doses, lower volumes of contrast, removing calcified plaques from the image to allow assessment of the degree of stenosis, and allows better assessment of endoleak .
Catheter-based angiography alone is inadequate for the pre-procedural 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 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:
- maximum transverse diameter of the aneurysmal sac
- must be measured perpendicular to the longitudinal aortic axis
- keep in mind that an aneurysm never decreases in size!
- longitudinal length
- shape (saccular/fusiform/eccentric)
- any major kink
- upper extent, relative to the renal arteries
- lower extent, including extension into any branches
- any side or visceral branches arising from the aneurysm
- maximum transverse diameter of the aneurysmal sac
- relevant anatomy
Also see: reporting tips for aortic aneurysms
Treatment and prognosis
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 . A number of clinical factors (e.g. smoking, gender, 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
- 2018 Society of Vascular Surgery recommendations generally recommend intervention for AAA ≥5.4 cm, and surveillance for smaller diameter lesions
- young, healthy (especially female) patients may benefit from intervention for lesions between 5.0 - 5.4 cm
- 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
- rate of aneurysm growth
- enlargement in transverse diameter ≥5 mm in 6 months may be an indication for intervention
- symptomatic lesions
Follow-up intervals for imaging an enlarged infrarenal abdominal aorta from initial detection :
- <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
Management options include:
- surveillance (see above)
- endovascular aneurysm repair (EVAR)
- if the anatomy permits, EVAR is preferred vs open surgical repair
- aneurysm-related mortality has been shown to be much lower with EVAR vs open surgical repair
- resection (open surgical repair)