Full Text Review
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
Prepared by Christopher Patterson, MD, FRCPC,
Professor and Head, Division of Geriatric Medicine, McMaster University,
Hamilton, Ontario
These recommendations were finalized by the Task Force in January 1994
Contents
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:
-
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.
-
To determine the natural history of small aortic aneurysms
discovered by screening, using serial ultrasound.
-
To define and quantify risk factors for the development
rapid growth and rupture of AAA.
-
To determine the value of ultrasound screening for AAA
in high risk individuals.
-
To determine the best method of disseminating the risks
and benefit of surgery in elective and symptomatic situations.
Evidence
The literature was identified with a MEDLINE search
of the years 1980
to October 1991
by
exploding the terms aortic aneurysm and aorta, abdominal, costs and cost
analysis, and decision making, and a MEDLINE search from 1991
to
1993
with the major headings aortic aneurysm and mass screening.
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
Link to Structured Abstract of
this review
Link to Summary Table of this
review
Link to Selected References list of this review
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