Probabilities of progression of aortic aneurysms: estimates and implications for screening policy

Background: Screening for abdominal aortic aneurysm, and intervention with elective repair, can reduce the incidence of aneurysmal rupture by a half. If a screening programme is implemented, it is essential to determine appropriate follow up intervals for rescreening. This paper estimates probabilities of progression growth of aortic diameter to provide evidence for this. Methods: Data were taken from 2342 men aged 65-80 screened in the Chichester randomised control trial, who have been followed up for an average of 11 years. Aortic diameter was modelled as a Markov process with four categories: <30 mm (normal), 30-44 mm, 45-54 mm, and greater than or equal to55 mm. Estimates of the probabilities of progressing to each higher category were obtained. Results: The probabilities of progression increased with greater initial aortic diameter. The estimated rates/year were 0.018 (95% confidence interval 0.014 to 0.023), 0.16 (0.12 to 0.20), and 0.49 (0.35 to 0.70) respectively for moving up one category. The probabilities of moving from <30 mm to greater than or equal to55 mm were estimated as 1% in 5 years and 12% in 15 years, while the corresponding figures for moving from 45-54 mm to greater than or equal to55 mm were 91% and 99%. There were differences in rates of progression according to age, with men over 70-years having rates about three times those of men under 70. Conclusions: It seems unnecessary to follow up men with normal aortic diameter as they experience a low probability of reaching criteria for surgery even within 15 years. However, follow up intervals should be progressively shorter for those with greater aortic diameter, especially in those aged over 70. Active follow up, for example every 3 months, is appropriate for men with an aortic diameter of 45-54 mm.