Patient Care

When Should an Abdominal Aortic Aneurysm Be Treated?


 

Key Points

  1. AAA rupture is associated with significant morbidity and mortality.
  2. Risk of AAA rupture increases with size of the aneurysm, rate of growth, and female gender.
  3. Growth of AAAs is variable and affected by individual patient characteristics.
  4. Endovascular and open repair are the two surgical options for treatment of asymptomatic AAA and are comparable in long-term outcomes.
  5. Studies support ultrasound surveillance for AAA >3.0 cm; a six-month interval is recommended.
  6. Surgical repair is indicated for symptomatic AAAs or for those >5.5 cm.

Table 1. Risk Factors Associated With AAA

  1. Age > 65 years
  2. Male sex
  3. History of smoking
  4. First-degree family history of AAA requiring surgical repair
  5. Caucasian race
  6. Hypertension
  7. Elevated cholesterol
  8. Central obesity
  9. Aneurysms of femoral or popliteal arteries

Case

A generally healthy, 74-year-old man presents with sudden-onset abdominal pain due to acute pancreatitis. Computed tomography (CT) of his abdomen shows pancreatic inflammation and an incidental finding of a 4.5-cm abdominal aortic aneurysm. He had never had any imaging of his abdomen prior to this study and described no prior episodes of abdominal pain.

When should his abdominal aortic aneurysm be treated?

Overview

An abdominal aortic aneurysm (AAA) is an abnormal dilation of the abdominal aorta between the diaphragm and the aortic bifurcation of the iliac arteries. An AAA is usually defined as a dilatation with a diameter of >3 cm or 50% greater than the typical diameter. Most AAAs are located in the infrarenal aorta, proximal to the iliac bifurcation.

Population screening programs show a prevalence of AAA of 4% to 8% in men aged 65 to 80 years.1 AAA prevalence is approximately six times greater in men than women, though the prevalence in women might be increasing.1 AAA is most common in white men, with black men and those of Asian heritage having lower risk. A combination of genetic predisposition and environmental and physiologic factors lead to initiation and progression of AAAs; family history, male sex, advanced age, and history of smoking are major risk factors.

Mortality after AAA rupture is high. Approximately 62% of patients die prior to hospital arrival.2 Of those who undergo emergent AAA surgery, 50% will die.1

Aortic repair with a prosthetic vascular graft reduces morbidity and mortality from rupture, but the risks of repair are not trivial.2

Review of the Data

Risk of rupture. An AAA should be repaired when the risk of rupture outweighs the risks of surgical repair. Symptomatic aneurysms—such as those causing back or abdominal pain—have a higher risk of rupture than asymptomatic aneurysms. Most AAAs are asymptomatic and, in the absence of imaging, not identified until the time of rupture. Given the significant mortality associated with rupture, there is benefit to intervening on asymptomatic aneurysms before rupture.

The risk of AAA rupture has been studied in patients who either have been unfit for surgical repair or uninterested in intervention. Risk of rupture increases substantially with aneurysm size. Lederle et al estimated a two-year aneurysm rupture risk of 22.1% for AAA with a diameter of 5.0 to 5.9 cm, 18.9% for 6.0 to 6.9 cm, and 43.4% for a diameter ≥7.0 cm.3 In another study of 476 patients, the average risks of rupture in male and female patients with an AAA of 5.0 to 5.9 cm were 1.0% and 3.9% per year, respectively. For male and female patients with ≥6.0 cm AAAs, risks of rupture were 14.1% and 22.3% per year.4 Women with AAA have been found to have a higher risk of rupture in all studies in which female patients were included.

Because rupture risk increases with size, predicting the rate of growth is clinically important. Powell et al conducted a systematic review of growth rates of small AAAs.5 In 15 studies that examined 7,630 patients, the growth rate for a 3.5-cm aneurysm was estimated at 1.9 mm/year and for a 4.5-cm aneurysm was 3.5 mm/year. Given an exponentially increasing aneurysm diameter, this suggests an elapsed time of 6.2 years for a 3.5-cm aneurysm to grow to 5.5 cm, and 2.3 years for a 4.5-cm AAA to grow to 5.5 cm. This prediction does not account for individual variability in growth rate. Some AAAs grow quickly, others erratically, and others not at all. This growth variability is influenced by individual characteristics including cigarette smoking, sex, age, and other factors.

Medical prevention of progression and rupture. Studies have assessed whether modification of risk factors can delay progression of growth of AAAs. In a small aneurysm trial in the United Kingdom, self-reported smoking status was associated with an incrementally increased growth rate of 0.4 mm per year.5 Each year of smoking increases the relative risk of AAA by 4%, and continued smoking leads to more rapid AAA expansion.6 There is no clear relationship between cholesterol levels and AAA expansion rate. Observational studies suggest that aneurysm expansion decreases with statin use, but there is not sufficient evidence to recommend statin therapy for AAA alone.6

Many patients with AAA, however, are candidates for statins because of concomitant coronary artery or peripheral vascular disease. Small, randomized controlled trials have shown that macrolides and tetracycline antibiotics might inhibit AAA growth, but prescribing them for this purpose is not currently the standard of care.7 Elevated mean blood pressure has been associated with rupture, but there is not good evidence showing delay of progression with treatment of hypertension.6 Early observational studies suggested that beta-blocker use would decrease AAA progression, but further evidence has not supported their benefit in slowing progression of size.8 Likewise, use of angiotensin-converting enzyme inhibitors has also shown no growth inhibition.7 An ongoing Cochrane review is evaluating evidence for these medical treatments of AAA.9

Surgical prevention of rupture. There are two surgical methods of AAA repair: open repair and endovascular aneurysm repair (EVAR). Both involve use of a prosthetic graft to prevent the aneurysm from enlarging. The EVAR procedure typically involves entry at the femoral artery, with use of catheters and guide wires to advance a graft to the desired location and anchor it in place. Because this utilizes an endovascular approach, regional rather than general anesthesia can be used.

Multiple investigators have evaluated for differences in outcomes between the two methods of surgical AAA repair. Studies have shown increased 30-day postoperative mortality with open repair, as well as significantly higher rates of postoperative cardiac, pulmonary, and renal complications. One randomized controlled study found 30-day operative mortality of 1.8% in the EVAR group and 4.3% in the open repair group.10 However, after a median six-year follow-up of patients after EVAR or open repair, there is no difference in total mortality or aneurysm-related mortality.10 Compared with open repair, the need for long-term surveillance and re-intervention post-EVAR is higher, with endoleak and graft migration the most common complications. This accounts for the loss of early survival advantage in post-EVAR patients. By two years post-operation, complication after repair with either technique is not statistically different. De Bruin et al found that six years after randomization for repair type, cumulative survival rates were 69.9% for patients after open repair and 68.9% with EVAR.11

Studies also have focused on subgroups of patients with a higher operative risk and shorter life expectancy, such as the elderly.12 A pooled analysis of 13,419 patients aged ≥80 years from six observational studies showed 8.6% immediate mortality after open repair and 2.3% after EVAR (risk difference 6.2%, 95% CI 5.4-7.0%).13 Pooled analysis of three longer-term studies showed similar overall survival at three years after EVAR and open repair.13 When EVAR is not available, open repair has acceptable short- and long-term survival in patients aged ≥80 years with an AAA at high risk of rupture.14

Screening. A Cochrane review evaluated the effect of ultrasound screening of asymptomatic AAA on mortality. In 127,891 men and 9,342 women aged 65 to 79, researchers found a significant decrease in mortality in men aged 65 to 79 who were screened (odds ratio 0.6, 95% CI 0.47-0.78) but no benefit to screening of women.15 The current U.S. Preventive Services Task Force (USPSTF) guidelines recommend one-time ultrasound-guided (USG) screening for AAA in men aged 65 to 75 who have any history of tobacco use. For men in this age group who have never smoked, the balance between benefits and harms of screening is too close for the USPSTF to make recommendations. Because of the lower prevalence in women, the USPSTF recommends against screening women for AAA.18

The United States Preventive Services Task Force (USPSTF) recommends one-time ultrasound-guided screening for AAA in men ages 65 to 75 who have ever smoked. This is a Grade B recommendation.

Timing of repair. Early repair of small AAAs (4 cm to 5.5 cm) has no long-term survival benefit compared to ultrasound surveillance without repair.16,17 Therefore, AAAs <5.5 cm should be followed with regular ultrasound surveillance every six months, with referral to surgery if the diameter reaches 5.5 cm, or grows >1 cm a year. The size at which surgery should be performed might be lower in women, given that their risk of rupture is higher than men. A thoughtful discussion of individual risks should take place in every case, but in many patients, even the elderly, repair of a large asymptomatic AAA is indicated. 5

Back to the Case

Our patient should have repeat imaging of his AAA in six months and regular surveillance afterward to monitor for growth every six months. When the AAA is >5.5 cm or if it grows >1 cm a year, he should be evaluated for EVAR or open repair.

Bottom Line

The current USPSTF guidelines recommend one-time ultrasound-guided (USG) screening for AAA in men aged 65 to 75 with a history of smoking. If an AAA >3 cm is found, the patient should undergo regular USG screening every six months. The AAA should be repaired if >5.5 cm or symptomatic, via either an endoscopic or open approach.


Dr. Best is a hospitalist at University of Washington Medicine at Harborview and associate program director of the internal-medicine residency program at the University of Washington, Seattle. Dr. Carpenter is a fellow in the division of geriatrics at the University of California at San Franscisco.

References

  1. Nordon IM, Hinchliffe RJ, Loftus IM, Thompson MM. Pathophysiology and epidemiology of abdominal aortic aneurysms. Nat Rev Cardiol. 2011;8, 92-102.
  2. Ernst CB. Abdominal aortic aneurysm. N Engl J Med. 1993;328(16):1167-1172.
  3. Lederle FA, Johnson GR, Wilson SE, et al. Rupture rate of large abdominal aortic aneurysms in patients refusing or unfit for elective repair. JAMA. 2002;287:2968-2972
  4. Brown PM, Zelt DT, Sobolev B. The risk of rupture in untreated aneurysms: the impact of size, gender, and expansion rate. J Vasc Surg. 2003;37:280-284.
  5. Powell JT, Sweeting MJ, Brown LC, Gotensparre SM, Fowkes FG, Thompson SG. Systematic review and meta-analysis of growth rates of small abdominal aortic aneurysms. British Journal of Surgery. 2011;98:609-618.
  6. Brown LC, Powell JT. Risk factors for aneurysm rupture in patients kept under ultrasound surveillance. UK Small Aneurysm Trial Participants. Ann Surg. 1999;230(3):289-297.
  7. Baxter BT, Terrin MC, Dalman RL. Medical management of small abdominal aortic aneurysms. Circulation. 2008;117:1883-1889.
  8. The Propranolol Aneurysm Trial Investigators. Propranolol for small abdominal aortic aneurysms: results of a randomized trial. J Vasc Surg. 2002:35:72-79.
  9. Ruhani G, Robertson L, Clarke M. Medical treatment for small abdominal aortic aneurysms. Cochrane Database for Systematic Reviews 2012. Sep 12;9:CD009536. doi: 10.1002/14651858.
  10. The United Kingdom EVAR Trial Investigators. Endovascular versus open repair of abdominal aortic aneurysm. N Engl J Med. 2010; 362:1863-1871.
  11. De Bruin JL, Baas AF, Buth J, et al. Long-term outcome of open or endovascular repair of abdominal aortic aneurysm. N Engl J Med. 2010;362:1881-1889.
  12. Jackson RS, Chang DC, Freischlag JA. Comparison of long-term survival after open vs. endovascular repair of intact abdominal aortic aneurysm among Medicare beneficiaries. JAMA. 2012;307(15):1621-1628.
  13. Biancari F, Catania A, D’Andrea V. Elective endovascular vs. open repair of abdominal aortic aneurysm in patients aged 80 years and older: systematic review and meta-analysis. Eur J Vasc Endovasc Surg. 2011;42:571-576.
  14. Schermerhorn ML, O’Malley AJ, Jhaveri A, Cotterill P, Pomposelli F, Landon BE. Endovascular vs. open repair of abdominal aortic aneurysms in the Medicare population. N Engl J Med. 2008;358:464-474.
  15. Cosford PA, Leng GC, Tomas J. Screening for abdominal aortic aneurysm. Cochrane Database for Systematic Reviews 2007, Issue 2. Art. No.: CD002945. DOI: 10.1002/14651858.CD002945.pub2.
  16. Filardo G, Powell JT, Martinez MA, Ballard DJ. Surgery for small asymptomatic abdominal aortic aneurysms. Cochrane Database for Systematic Reviews 2012, Issue 3. Art. No.:CD001835. DOI: 10.1002/14651858.CD001835.pub3.
  17. The UK Small Aneurysm Trial Participants. Final 12-year follow-up of surgery versus surveillance in the UK small aneurysm trial. Brit J Surg. 2007;94:702-708.
  18. U.S. Preventive Services Task Force. Screening for Abdominal Aortic Aneurysm: Recommendation Statement. AHRQ Publication No. 05-0569-A, February 2005.

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