Abdominal aortic aneurysm

An abdominal aortic aneurysm (AAA) is defined as an enlargement of the aorta of at least 1.5 times its normal diameter or greater than 3 cm diameter in total (1)

• natural history is ongoing expansion which vary considerably, with some remaining stable for years and others growing rapidly
• usually they are asymptomatic until rupture
  • rupture is often lethal with a mortality of 85 to 90%
  • size of the aneurysm is the best known predictor of rupture (2)

Majorty of AAAs are usually located between diaphragm and the aortic bifurcation. It can be classified

• suprarenal - origin of one or more visceral arteries are involved
• pararenal - involves the origins of the renal arteries
• infrarenal - begins beyond the renal arteries
  • 85% of AAAs are infrarenal
  • normal infrarenal aortic diameters in patients older than 50 years are 1.5 cm in women and 1.7 cm in men (2)

The disease is caused by a degenerative process involving all layers of the vessel wall. Four events can be identified in the pathophysiology of the AAA:

• lymphocyte and macrophage infiltration of the vessel wall
• destruction of elastin and collagen in the media and adventitia by proteases, including matrix metalloproteinases
• loss of smooth-muscle cells with thinning of the media
• neovascularisation (2)

Most AAAs are asymptomatic, and they are often diagnosed opportunistically during clinical examination or investigation for another condition. Because of this it is difficult to establish their prevalence. There is a national screening programme which enrols men at age 65 and suggests a prevalence of about 1.3% in this population. The prevalence is falling (3).

The prevalence of AAAs is approximately 6 times lower in women, but the rate of aneurysm rupture is significantly higher (3).

Reference:

• (1) NICE (February 2009). Endovascular stent-grafts for the treatment of abdominal aortic aneurysms
• (2) Kent KC. Clinical practice. Abdominal aortic aneurysms. N Engl J Med. 2014;371(22):2101-8
• (3) NICE (March 2020).Abdominal aortic aneurysm: diagnosis and management