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|Aortic Aneurysm - Abdominal
(diameter > 3
U.S. Preventive Services Task Force, Guidelines from Guide to Clinical Preventive Services, (2nd Edition) 1996
Screening for Abdominal Aortic Aneurysm:
An aneurysm is usually defined as a focal dilation of the aorta at least 150% of the normal aortic diameter. Given a normal aortic diameter in older men of 2 cm (range 1.4-3.0 cm), an aortic diameter above 3 cm usually indicates an aneurysm.
Few aneurysms less than 4 cm in diameter will rupture. Overall, 3-6% of aneurysms greater than 4 cm in diameter will rupture annually, but the rate of rupture is directly related to the size of the aneurysm. The natural history of most aneurysms is one of gradual enlargement; growth rates have been estimated to average 0.2 cm/year for aneurysms under 4 cm, and 0.5 cm/year for those over 6 cm.
Surgical resection and repair with an artificial graft is a very effective
treatment for AAA.
If the patient survives the immediate postoperative period, long-term survival is comparable to similar persons without aneurysms, but late postoperative complications (graft infection, graft occlusion, and aortoenteric fistula) may result in additional deaths and morbidity. The high prevalence of cardiovascular disease in patients with AAA and competing causes of morbidity and mortality in older patients may diminish the benefit of detecting asymptomatic aneurysms in older populations.
Risk of elective surgery must be balanced against the risk of rupture of an untreated aneurysm, which is directly related to aneurysm size. Most vascular surgeons currently recommend surgery for asymptomatic aneurysms 5 cm or larger, since the risk of rupture (25-41% over 5 years) is substantially higher than risks from surgery. While more aggressive management of smaller aneurysms (4-5 cm) has been recommended by some, others have suggested that asymptomatic, slow-growing aneurysms under 6 cm can be successfully followed by serial ultrasound. A large community-based screening program, which employed this conservative strategy over 8 years, observed two cases of rupture among 29 subjects with aneurysms 5-5.9 cm, for a rate of 1.5%/year. A model fitted to data from 13 studies of untreated aneurysms supports a relatively low risk of rupture in aneurysms less than 6 cm; estimated annual rates of rupture for aneurysms 4-4.9 cm, 5-5.9 cm, 6-6.9 cm, and over 7 cm were 1%, 3%, 9%, and 25%, respectively. These data, which are based largely on incidentally detected cases, may not reflect accurately the prognosis of asymptomatic aneurysms discovered by routine ultrasound screening. Furthermore, decisions to forgo surgery in patients with larger aneurysms were likely to have been influenced by factors (e.g., age, comorbidity, lack of symptoms) that may have independently influenced the risk of rupture. Trials are currently ongoing to determine the optimal management of patients with AAA that are 4-5.4 cm in size.
The draft of this chapter was prepared for the U.S. Preventive Services Task Force by Paul S. Frame, MD, and David Atkins, MD, MPH.