What is the most common cause of abdominal aortic aneurysm?

Most Common Cause of Abdominal Aortic Aneurysm

Atherosclerosis is the most common cause of abdominal aortic aneurysms, accounting for over 90% of cases, with severe atherosclerotic changes involving more than 70% of the aortic surface in the vast majority of patients. 1, 2

Primary Pathophysiologic Mechanism

Atherosclerotic degeneration leads to structural breakdown of the aortic wall through multiple mechanisms:

• Massive intimal thickening with fibrosis and calcification compromises the integrity of the vessel wall, with degradation of the extracellular matrix by histiocytic cells 1
• The increased distance between the endothelial layer and media impairs oxygen and nutrient supply to the vessel wall 1
• Medial thinning occurs secondary to smooth muscle cell necrosis and fibrotic changes in elastic structures, increasing vessel stiffness and vulnerability to shear stress 1, 3
• These degenerative changes particularly affect the infrarenal aorta, where 85% of atherosclerotic aneurysms are located 1

Critical Risk Factors

Hypertension is the single most important modifiable risk factor:

• Present in 85% of patients with ruptured aneurysms and 52% of those with non-ruptured aneurysms 1, 2
• Directly increases wall stress according to the law of Laplace, where stress is proportional to pressure and radius 4

Smoking demonstrates a strong dose-response relationship:

• Adjusted odds ratio of 9.55 for ≥50 pack-years compared to never smokers 5
• Smoking, male sex, and positive family history are the three most important risk factors 3

Male predominance is striking:

• Men outnumber women by 3:1 to 4:1 in most aneurysm series 1, 2
• Most common in men >65 years of age 3

Important Clinical Distinction

It is critical to distinguish atherosclerotic AAA from mycotic (infected) aneurysms, which represent a completely different entity:

• Mycotic aneurysms are caused by bacterial infection, most commonly Staphylococcus aureus (50-60% of cases) and gram-negative bacilli including Salmonella (30-40%) 1
• Mycotic aneurysms typically occur in the suprarenal aorta, whereas atherosclerotic aneurysms are predominantly infrarenal 1
• Infected aneurysms present with fever (≥70% of cases) and back pain (65-90%), which are uncommon in atherosclerotic AAA 1, 6

Key Clinical Pitfall

Do not confuse risk factors for AAA with those for occlusive atherosclerotic disease:

• Hypercholesterolemia shows weak association, with 60% of AAA patients having cholesterol <240 mg/dL 1, 2
• Neither clinical hypercholesterolemia nor serum lipid levels show strong association with AAA 5
• Diabetes mellitus is inversely associated with AAA (OR 0.32), unlike its strong association with occlusive atherosclerosis 5
• This suggests AAA pathophysiology differs fundamentally from typical atherosclerotic occlusive disease 5, 7