This study aims to outline the ‘true’ natural history of ascending thoracic aortic aneurysm (ATAA) based on a cohort of patients not undergoing surgical intervention.

The outcomes, risk factors, and growth rates of 964 unoperated ATAA patients were investigated, over a median follow-up of 7.9 (maximum of 34) years. The primary endpoint was adverse aortic events (AAE), including dissection, rupture, and aortic death. At aortic sizes of 3.5–3.9, 4.0–4.4, 4.5–4.9, 5.0–5.4, 5.5–5.9, and ≥6.0 cm, the average yearly risk of AAE was 0.2%, 0.2%, 0.3%, 1.4%, 2.0%, and 3.5%, respectively (P < 0.001), and the 10-year survival free from AAE was 97.8%, 98.2%, 97.3%, 84.6%, 80.4%, and 70.9%, respectively (P < 0.001). The risk of AAE was relatively flat until 5 cm of aortic size, at which it began to increase rapidly (P for non-linearity <0.001). The mean annual growth rate was estimated to be 0.10 ± 0.01 cm/year. Ascending thoracic aortic aneurysms grew in a very slow manner, and aortic growth over 0.2 cm/year was rarely seen. Multivariable Cox regression identified aortic size [hazard ratio (HR): 1.78, 95% confidence interval (CI): 1.50–2.11, P < 0.001] and age (HR: 1.02, 95% CI: 1.00–1.05, P = 0.015) as significant independent risk factors for AAE. Interestingly, hyperlipidemia (HR: 0.46, 95% CI: 0.23–0.91, P = 0.025) was found to be a significant protective factor for AAE in univariable Cox regression.

An aortic size of 5 cm, rather than 5.5 cm, may be a more appropriate intervention criterion for prophylactic ATAA repair. Aortic growth may not be an applicable indicator for intervention.