Epidemiology of Aortic Aneurysmal Disease

Peter F. Lawrence, Gustavo S.C. Oderich, Kiran Bhirangi

Aneurysm of the aorta is a common disease, representing 83% of all noncerebral aneurysms diagnosed in the United States.1 Although necropsy studies indicated a predominance of thoracic aortic aneurysms in the beginning of the century, this figure has dramatically changed since then. This is probably due to the loss of syphilis as an etiologic factor and an increase in degenerative atherosclerotic aneurysms of the abdominal aorta. In 1994, 88% of the aortic aneurysms surgically repaired in the United States were located in the abdominal aorta, whereas thoracic and thoracoabdominal aneurysms accounted for 5-12% of the total.1 Abdominal aortic aneurysms (AAA) are responsible for 0.8% of all deaths in the United States, ranking 10th as cause of death in older males.

Due to the frequency of abdominal aneurysm deaths in the general population, and noninvasive access to the aorta by ultrasound, several large screening programs have been undertaken to diagnose and ultimately treat asymptomatic aneurysms. The detailed epidemiological information from the United Kingdom (UK) Small Aneurysm Trial published recently and the ongoing Aneurysm Detection and Management (ADAM) Study were designed to answer the question whether a policy of early diagnosis and elective repair of small abdominal aneurysms is preferable to ultrasonographic surveillance.2,3 The UK trial demonstrated no benefit for early elective surgery over ultrasonographic surveillance of small aneurysms (< 5.5 cm). However, this may be attributed to a higher than expected operative mortality rate (5.8%) in this study. Had this trial achieved an elective surgical mortality rate of 2-3%, it is likely that early surgery would have shown significant benefit. We reported an even higher mortality rate (8.4%) when analyzing the results of all abdominal aneurysms electively repaired in the United States, which may reflect discrepancies between high-volume vascular centers and less specialized hospitals.1 The ADAM study is continuing and will further elucidate the correct management of patients with asymptomatic small aneurysms.

Knowledge of the prevalence, incidence, risk factors, associated diseases and survival in patients with aneurysms is of paramount importance when planning future screening and treatment programs.

Prevalence

The prevalence of AAA detected through screening programs varies between 1.4-11.9%, depending on the definition of aneurysm and on the population studied.2

The Ad Hoc Committee on reporting standards of the Society for Vascular Surgery (SVS) and the International Society for Cardiovascular Surgery (ISCVS) (North American chapter) defined aneurysm as a permanent localized dilatation of an artery to a diameter greater than 50% (1.5 times) of its normal size. Normal diameter for an infra-renal aorta is estimated to be 21.4 ± 3.6 mm (maximum 25 mm) in males and 18.7 ± 3.3 mm (maximum 22 mm) in females. The ADAM study, which is the largest population-based screening study published to date, reported a prevalence of

AAA greater than 4.0 cm in 1.4% of the 73,451 veterans who were 50-79 years of age. If the size definition is decreased to 3.0 cm, the prevalence increases to 4.6%. Bengtsson et al also reported a prevalence of 3.3% using the 4.0 cm criteria and a significantly higher prevalence (8.5%) when the 3.0 cm criteria was adopted.

The prevalence of aortic aneurysms increases steadily with age (Fig. 2.1).2 In the ADAM trial, prevalence was on average 3 times higher among smokers, when compared to nonsmokers of the same age.2 This increased steadily, reaching a 9-fold peak prevalence within the 29-year interval. The prevalence was 0.3% among smokers with ages between 50 and 54, and 2.7% for those older than 75. This was also confirmed by Morris et al. In this study, prevalence was 2.3% in individuals 50-64 years of age, increasing to 8.8 and 11.9% in individuals 65-79 or more than 80 years of age, respectively. These authors used the 3.0 cm diameter criteria to define aneurysm.

Another method of obtaining an estimate of the prevalence of aortic aneurysms is through "epidemiological necropsy" studies. This method has been developed to counteract the influence of selection bias, such as an overrepresentation of patients with sudden and unexpected deaths, which more frequently are attributed to ruptured aneurysms. The "epidemiological necropsy" approach excludes patients with a suspected aneurysm. Prevalence estimates obtained by this technique are similar to those from screening surveys, ranging from 0.6-3.2% in the 3 largest series published to date.4 Bengtsson et al studied age- and sex-specific prevalence of AAA, and found that total aneurysm frequency was twice as high in men than in women.4 In men, the frequency increases rapidly after age of 55, reaching a peak prevalence of 5.9% at 85 years of age, whereas in women, there is a continuous increase after 70 years of age, reaching a 4.5% peak after the age of 90.6

Estimates for thoracic and thoracoabdominal aneurysms are not as precise as those for abdominal aneurysms. A Danish study of 6480 autopsies (not using an epidemiological method) reported a prevalence of thoracic aneurysms of 1.2 per million population per year, as compared to a 3.6 per million for abdominal aneurysms.

Incidence

Almost all studies have demonstrated an increasing incidence for both ruptured and nonruptured AAA since the 1950s. The SVS / ISCVS reported that the number of aneurysmorrhaphies more than doubled from 1979-1992 (19 vs. 46/100,000 population). In a report from Rochester, Minnesota, the average increase in incidence was 11% per year, higher than the 4% per year reported in the Western Australia study. This study showed an incidence of 117.2 and 33.9 per 100,000 person-years

Prevalence Aortic Dissection
Fig. 2.1. Age-specific prevalence rates in smokers and nonsmokers males: results from the ADAM study.

for males and females, respectively. Melton at al demonstrated an incidence of 36.5 per 100,000 person-years, without stratifying for sex. As incidence estimates can only be obtained by screening a defined population for a second time, accurate data for asymptomatic AAA are sparse and unreliable. In addition, comparison between studies is difficult due to variances in sex, age and risk factor distribution.

The incidence of ruptured abdominal aortic aneurysms was 3 per 100,000 person-years in the Western Australia study. This was slightly higher than the incidence of 1 per 100,000 person-years reported in the Goteburg study, which also demonstrated a seven-fold increase in incidence rates over a 36-year period. Bengtsson et al showed an increase in the incidence of ruptured AAA with age. Rupture was extremely rare before age 50 in men, increasing rapidly after age of 55. Women have a more delayed (15-20 years) increase in the incidence of ruptured AAA (Fig. 2.2).

Risk Factors

Several risk factors for abdominal aneurysms have been described (Table 2.1). Male sex and cigarette smoking are the strongest factors in multivariate analysis.2 Other factors associated with higher prevalence rates of AAA include age, white race, family history, hypertension, peripheral arterial occlusive disease (PAOD) and hypercholesterolemia.2

Male Sex

Male sex is associated with a relative risk for abdominal aneurysm of 6.5 (95% confidence interval: 5.9-7.2). The male:female ratio varies from 6:1 to 2:1 in population-based screening studies and necropsy surveys. We reported a 2.3:1 ratio for abdominal aneurysms diagnosed in the United States in 1994.1 The corresponding figure for thoracic aneurysms is 1:1. Female sex was negatively associated with AAA

Fig. 2.2. Sex-specific percentage by age in an epidemiological necropsy study. Adapted with permission from Bengtsson et al (5).
Table 2.I. Results adapted from the ADAM study: Risk factors associated with an increased prevalence of AAA in decreasing order of importance

Risk Factor

Odds Ratio

(95% Confidence Interval)

Positive association

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