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Abdominal aortic aneurysm

An abdominal aortic aneurysm (AAA) refers to a focal dilatation of the abdominal aorta that is 50% greater than the proximal normal segment or that is greater than 3cm in maximum diameter.

Epidemiology

Its prevalence increases with age. Males much more commonly affected than females (with a male:female ratio = 4:1).

Clinical presentation

Most AAAs are asymptomatic unless they leak or rupture. Unruptured aneurysms may uncommonly cause abdominal or back pain, or a pulsatile mass, if large. 

Ruptured aneurysms present with severe abdominal or back pain and hypotension / shock. The mortality rate from a ruptured AAA is high : 59-83% of patients succumb to death before they make it to hospital or undergo surgery. The operative mortality rate for those who make it to surgery tends to be ~ 40%. 

Pathology

The natural history of abdominal aortic aneurysms (AAA) is that of slow expansion and rupture with 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 women and 5.5 to 6.0 cm in men carry a significantly increased risk of rupture and should be treated.  Furthermore, aneurysms that expand greater than 10 mm per year are also at significant risk of rupture and are considered for treatment even when less than 5.0 cm.

Causes
Associations
Common iliac artery (CIA) aneurysm
  • AAA commonly extends into common iliac arteries
  • vast majority of patients with CIA aneurysms have a AAA
  • isolated CIA aneurysms are rare
Popliteal artery aneurysm

Radiographic features

Role of imaging
  • detection of AAA
  • monitoring of rate of growth
  • pre-operative planning
  • post-operative follow-up
Plain film

The aneurysm may be visible as an area of curvilinear calcification in the para-vertebral region on either abdominal or lumbar spine films performed for alternative indications.

Ultrasound

Simple, safe and inexpensive. Has reported rates of ~ 95% sensitivity and ~ 100% specificity. It is usually the preferred choice for monitoring of small aneurysms.

CT angiography

This is considered the imaging gold standard but has a high radiation dose. Excellent for pre-operative planning as it accurately delineates the size and shape of the AAA and its relationship to branch arteries and aortic bifurcation. Also superior to ultrasound in detecting and sizing common iliac artery aneurysms.

Signs of rupture or impending rupture include:

MR angiography

Same as for CTA but can be more costly and less widely available.

DSA - angiography

Does not show true aneurysm size if there is mural thrombus but good at delineating branch vessels. Usually performed intra-operatively during treatment.  

Complications

Reported complications include

  • rupture
  • pseudoaneurysm from chronic contained leak / rupture
  • distal thromboembolism
  • thrombotic occlusion of branch vessel
  • infection
  • compression of adjacent structures if large (rare)
  • vertebral erosion

Treatment and prognosis

Management options include

  1. endovascular aneurysm repair (EVAR)
  2. open surgical repair

Treatment is recommended in surgically fit patients if the aneurysm is greater than 5.0 cm diameter in women and 5.5 cm diameter in men. Treatment is also considered if aneurysm growth rate exceeds 10 mm per year in smaller aneurysms.

Differential diagnosis

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