Abdominal aortic aneurysms (AAA) are focal dilatations of the abdominal aorta that are 50% greater than the proximal normal segment or >3 cm in maximum diameter.
The prevalence of AAAs increases with age. Males are much more commonly affected than females, with a ratio of 4:1. They are the tenth most common cause of death in the Western world. Approximately 10% of individuals older than 65 have an AAA.
Most AAAs are asymptomatic unless they leak or rupture, and are therefore diagnosed incidentally when the abdomen is imaged for other indications. Unruptured aneurysms may uncommonly cause abdominal or back pain, or a pulsatile mass if large.
The most common and feared complication is that of abdominal aortic rupture which presents with severe abdominal or back pain, hypotension and shock. The mortality rate from a ruptured AAA is high (59-83% of patients die before they make it to a hospital or undergo surgery). The operative mortality rate for those who make it to surgery tends to be around 40%. Elective surgery mortality is much lower (4-6%). Ruptured abdominal aortic aneurysms are discussed in more detail separately.
Other reported complications include:
- pseudoaneurysm from chronic contained leak/rupture
- aortic fistulas
- distal thromboembolism
- thrombotic occlusion of a branch vessel
- infection/mycotic aneurysm
- compression of adjacent structures if large (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 commonly extends into the common iliac arteries
- the vast majority of patients with CIA aneurysms have an AAA
- isolated CIA aneurysms are rare
popliteal artery aneurysm
- 10-14% of patients with an AAA have popliteal artery aneurysm
- 30-50% of patients with popliteal artery aneurysm have an AAA
intracranial cerebral aneurysm
- prevalence of ~10%, higher in females 13
Role of imaging
- detection of AAA
- monitoring of growth rate
- pre-operative planning
- post-operative follow-up
An aneurysm may be visible as an area of curvilinear calcification in the paravertebral region on either abdominal or lumbar spine radiographs performed for alternative indications. Radiographs are not optimal for detection or follow-up.
Ultrasound assessment is simple, safe and inexpensive. It has a reported sensitivity of 95% and specificity close to 100%. It is usually the preferred choice for the monitoring of small aneurysms. The measurement error for ultrasound evaluation of AAA is 4 mm 12; keep in mind that an aneurysm never decreases in size.
CT angiography (CTA) is considered the imaging gold standard 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 haematoma
- 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 10 mm over a 12-month interval or a diameter of 7 cm are also considered to be at high risk for rupture and warrant urgent repair.
Offers lack of ionising radiation, but is more costly, less widely available, and the examination is substantially lengthier.
Digital subtraction angiography (DSA) does not show the true aneurysm size if there is a mural thrombus but is superb at delineating branch vessels. Barring certain contraindications, endovascular repair of AAA is usually the first treatment choice. See Treatment and prognosis below.
When reporting a study with a AAA, a number of features and relevant negatives should be included in the radiology report. This is particularly true if this is a new or undocumented finding:
- the maximum diameter of the aneurysmal sac perpendicular to the axis of the vessel
- shape (excentric/fusiform)
- any major kink
- upper extent, relative to the renal arteries
- lower extent, including the extension into any branches
- any side or visceral branches arising form the aneurysm
- relevant anatomy
Also see: reporting tips for aortic aneurysms
Treatment and prognosis
The natural history of abdominal aortic aneurysms is that of slow expansion, with possible eventual rupture having devastating consequences.
The risk of rupture is proportional to the size of the aneurysm and the rate of growth. Differing rates of rupture for a given aneurysm size have been reported in the literature, but the general consensus is that aneurysms greater than 5.0 cm in diameter in women and 5.5 cm in men carry a significantly increased risk of rupture and should be treated. Furthermore, aneurysms that expand at a rate greater than 10 mm per annum are also at significant risk of rupture and are considered for treatment even when less than 5.0 cm.
In patients with a connective tissue disorder, especially those with a bicuspid aortic valve, surgical treatment may be considered even with a diameter smaller than 5.0 cm.
Follow-up intervals for imaging an enlarged infrarenal abdominal aorta from initial detection 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
Management options include:
- follow-up; see above
- endovascular aneurysm repair (EVAR)
- open surgical repair
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- acute aortic syndrome
- thoracic aortic aneurysm
- abdominal aortic aneurysm
- endovascular aneurysm repair (EVAR)
- reporting tips for aortic aneurysms
- aortic coarctation
- aortic pseudocoarctation
- cervical aortic arch
- interrupted aortic arch
- transposition of the great arteries
- variant anatomy of the aortic arch
- traumatic aortic injuries