Please note: In 2003, the CTF updated its Grades of Recommendations to include an "I Recommendation" for situations where insufficient evidence exists to allow a recommendation to be made.  (Formerly, these situations were captured under a "C Recommendation".)  This change is not retroactive, and all "C Recommendations" made prior to 2003 have not been reevaluated in light of the new "I" recommendation grade.  For a discussion of these recommendation grades, please link to the 2003 article in the Canadian Medical Association Journal here.

Screening for Abdominal Aortic Aneurysm

Overview
The prevalence of abdominal aortic aneurysm (AAA) rises sharply with age. Risk factors include male sex, a positive family history of aneurysm, smoking, hypertension and other vascular risk factors. Small aneurysms enlarge slowly, but probably as an exponential function. Symptoms occur late, and rupture may be the first indication of disease. Elective surgery has a mortality of less than 5%, rupture carries a mortality of 50 to 70% even when surgery is performed. Abdominal palpation is sensitive for large aneurysms. Abdominal ultrasound is sensitive and specific for aneurysms of all sizes. While there is insufficient evidence to recommend for or against screening with physical examination or ultrasound, the prudent physician may choose to include a targeted physical examination for aneurysm in males over age 60 in the periodic health examination.

Burden of Suffering
Abdominal aortic aneurysm, a localized abnormal dilatation of the aorta, is usually due to atherosclerotic changes affecting the arterial intima. It is defined as a dilatation of the aorta greater than 3 cm or 150% of the aortic diameter at the diaphragm (usually 2 cm in men aged 65-74 years). Most commonly AAA arise below the renal arteries, and remain asymptomatic for many years. Symptoms may occur from pressure effects on adjacent structures, (e.g. causing back pain or abdominal throbbing), from embolization of intramural thrombus, or in association with other vascular complaints such as intermittent claudication. As the aneurysm enlarges the incidence of rupture increases, particularly in saccular aneurysms. The characteristic presentation of rupture includes excruciating back pain, hypovolemic shock and pulsatile abdominal mass. The classic triad is present in approximately 70% of cases reaching hospital. Aortic rupture is a surgical emergency of the most urgent type, and even with surgery, mortality rates of 50 to 70% are common.

The prevalence of aneurysms is related to age and vascular risk factors. It is more common in men and in those with a positive family history. In community surveys, the prevalence of AAA is reported to be between less than 1%<1> and 5.4%.<2> In hospital outpatient clinics dealing with hypertension, vascular or cardiac disorders, the prevalence is between 5.3 and 21%.<3,4>

The incidence of aneurysm has been estimated at 52 per 100,000 per annum in men aged 55-64 years<5> and 499 per 100,000 per annum in men over 80 years.<6> There are approximately 520 hospital separations and 270 deaths annually from AAA in Canada.<7>

Maneuver
For screening or case finding there are two practical detection measures, physical examination and ultrasound.

The accuracy of physical examination depends on many factors such as the skill of the examiner, the size of the aneurysm, and habitus of the individual. Where the prevalence of aneurysm is between 5 and 10%, sensitivity is from 22-50%, with specificity of 71-94%. Where the prevalence of large AAA is high, in settings such as preoperative vascular clinics, the sensitivity is above 80%, with high specificity.<8-10> There is evidence to suggest that examination specifically seeking aneurysm is significantly more sensitive than routine palpation.

Abdominal ultrasound is an ideal technique for detecting AAA and estimating its size. Sensitivity approaches 100% in detecting AAA in the presence of a pulsatile abdominal mass.<10> Ultrasound may overestimate the size of an aneurysm compared to intraoperative measurement. Technical problems arise with ultrasound when the patient is obese or if there is excessive intestinal gas at the time of examination. In one study there was inadequate visualization of the aorta in 18%.<11> In most case series failure to visualize the aorta occurred in only a few percent. Community studies have demonstrated poor compliance rates in screening programs: less than 60%<2> and less than 50%.<11>

The only community-based screening program to describe outcome was recently published from Norway. Two thousand six hundred and fifty-four males over 60 years were invited to attend for ultrasound screening for a modest fee (150 Norwegian krona). One thousand two hundred and fifty-six (47%) complied and 92 aneurysms were detected – 7.3% of attendees. Of these 92, 69 were smaller than 4.0 cm and 23 were 4.0 cm or larger. Seventeen of these underwent elective surgery within 18 months of screening, with elective surgery within 18 months of screening, with no mortality and no serious complications.<11>

Effectiveness of Screening and Treatment
The natural history of a small aneurysm is gradual enlargement, with an increasing risk of rupture as the size increases. For aneurysms less than 5 cm, the mean growth rate lies between 0.17 and 0.48 cm per year. In a community based retrospective study, no aneurysm less than 3.5 cm had ruptured by 8 years; between 3.5 and 4.9 cm, 5% had ruptured by 9 years, and for those greater than 5 cm 25% had ruptured by 8 years.<12> In several series of untreated AAA, survival at one year was between 59 and 84%, at 2 years between 44 and 58%, and at 5 years less than 20%. Modelling studies suggest that mean expansion rate is an exponential function.<13>

While correction of risk factors, particularly hypertension, may play a role in slowing the enlargement of aneurysm, the only definitive treatment for AAA is replacement graft. Most centres now perform elective AAA replacement graft with a surgical mortality of less than 5%.<8> Mortality is increased by large aneurysm size, impaired renal function, blood loss greater than 4 units, and the presence of coronary artery disease.<14> If operation is delayed until symptoms are present but before rupture has occurred, mortality of the procedure is 5-33%. In the presence of aortic rupture, surgical mortality remains extremely high, in many cases exceeding 50%. Following successful surgery, subsequent mortality is strongly influenced by coexistent disease. When there is no coexistent disease the survival curve does not differ significantly from that of age-matched controls. Subsequent mortality is higher in hypertensives and those with vascular disease.<15>

When an aneurysm smaller than 5 cm is detected, the recommended approach includes serial ultrasound (at 3-6 monthly intervals) and treatment of hypertension and other risk factors.<16> A clinical trial is underway to determine whether beta-adrenergic blocking drugs slow the rate of enlargement of small AAA. If an aneurysm greater than 5 cm is detected, referral to a vascular surgeon is indicated, providing the patient is suitable for surgery.

Cost Effectiveness
Several cost effectiveness analyses of screening for AAA have been published.<17> The most recent simulation was based upon screening a cohort of 10,000 men between the ages of 60 and 80 years.<18> It was concluded that using the "most probable" values for the simulation parameters, a single screen by abdominal palpation followed by abdominal ultrasound for positive screens is estimated to gain 20 life-years at a cost of US$ 28,741 per life-year. A single ultrasound screen gains 57 life-years at a cost of US$ 41,550 per life-year. A repeat ultrasound screen after five years gains one additional life-year at a cost of US$ 906,769.

Recommendations of Others
Oboler and Laforce recommend abdominal examination for aneurysm in men over age 60 as a prudent maneuver.<19>

Conclusions and Recommendations
Population screening for AAA produces a very low yield of aneurysms of sufficient size to warrant consideration of surgical treatment. While physical examination is insensitive for small aneurysms, sensitivity to detect aneurysms large enough to be considered for surgery is as high as 80-90%. As the prevalence and incidence of AAA is age and sex dependent, targeted physical examination of the abdomen for men over the age of 60 may be considered a prudent maneuver, although there is insufficient evidence to recommend for or against inclusion in the periodic health examination (C Recommendation). While ultrasound screening is more sensitive and specific than physical examination, it is more expensive, and community studies have demonstrated poor compliance with the maneuver. There is therefore poor evidence for or against a recommendation to screen for AAA using ultrasound (C Recommendation). Ultrasound should be reserved for those where any suspicious pulsation is detected by examination, or if the abdominal aorta is impalpable due to obesity. In the older male smoker with hypertension, claudication, evidence of other vascular disease, or a positive family history of AAA, a more liberal policy of case finding with ultrasound could be considered. Costs of screening with ultrasound are substantially higher than physical examination.

Unanswered Questions (Research Agenda)
The following have been identified as research priorities:

1. To determine in the primary care setting, the characteristics of physical examination for the detection of AAA of different sizes using ultrasound as the gold standard.
2. To determine the natural history of small aortic aneurysms discovered by screening, using serial ultrasound.
3. To define and quantify risk factors for the development rapid growth and rupture of AAA.
4. To determine the value of ultrasound screening for AAA in high risk individuals.
5. To determine the best method of disseminating the risks and benefit of surgery in elective and symptomatic situations.

This review was initiated in October 1993 and updates a report published in 1991.<8> Recommendations were finalized by the Task Force in January 1994.

Full Citation
Patterson C. Screening for abdominal aortic aneurysm. In: Canadian Task Force on the Periodic Health Examination. Canadian Guide to Clinical Preventive Health Care. Ottawa: Health Canada, 1994; 672-78.