What is the natural history of abdominal aortic aneurysm (AAA), and when is intervention indicated?
Aneurysms grow over time. Increased expression and activity of matrix metalloproteinases (MMPs) and decreased activity of their inhibitors result in the loss of aortic wall structural integrity, which leads to AAA formation and expansion. The rate of growth is generally 1 to 4 mm per year for aneurysms less than 4 cm in diameter, 4 to 5 mm per year for aneurysms 4 to 6 cm, and 7 to 8 mm per year for aneurysms greater than 6 cm. Intervention is typically indicated when the aneurysm reaches 5.5 cm, although many clinicians will intervene at the 5-cm threshold. The risk of rupture increases as the aneurysm grows: diameter greater than 5 cm = 20% eventual, 4% annual; diameter greater than 6 cm = 40% eventual, 7% annual; diameter greater than 7 cm = 50% eventual, 20% annual. Given that aneurysms grow, studies are ongoing to determine the benefit of earlier intervention when aneurysm diameters are smaller and patients are relatively healthier.