The incidence of small abdominal aortic aneurysms and the change in normal infrarenal aortic diameter: implications for screening.

Aim: to study the incidence of small abdominal aortic aneurysms (AAA), and to investigate what proportion of normal infrarenal aortic diameters (IAD) expand with age.

Methods: longitudinal follow-up in a population-based aneurysm screening programme. The infrarenal aortic diameter (IAD) was measured by ultrasound. A second scan was performed in subjects with a normal aorta after an average of 5.5 years.

Results: data were analysed from 4072 subjects, 464 with a small AAA and 3608 with a normal aorta. The infrarenal aorta expanded in 15% of subjects, but significant growth (>5 mm) occurred in only 7%. Age and initial diameter were independent predictors for aortic dilatation. The effect of diameter at first screen was non-linear. The relative risk for expansion increased dramatically for IADs over 2.5 cm (test for departure of trend: chi2=52, p<0.0001). The effect of age was also non-linear, the risk of expansion was highest in the 60-69 year old age group; test for departure of trend (chi2=13, p=0.002). The incidence of new aneurysms was 3.5 per 1000 person-years (py) (95% CI: 2.8-4.4). The highest incidence of new aneurysms was found in the 60 to 69 year old age group.

Conclusion: only a small proportion of the population is prone to aortic dilatation. Patients over 70 with an IAD <2.5 cm can be discharged from follow-up.