Aims To evaluate the acute and long-term prognosis of acute aortic syndrome (AAS) according to the disease entity [intramural haematoma (IMH) vs. aortic dissection (AD)] and the anatomical location (type A vs. B). Methods and results A total of 1012 patients [672 with AD and 340 with IMH (33.6%)] were enrolled between 1993 and 2015. Compared with AD patients, IMH patients were older and had higher frequency of female sex and distal aorta involvement. The overall crude in-hospital mortality of AAS was 8.6%; type A AD [15.0%; adjusted hazard ratio (aHR) 30.4; 95% confidence interval (CI) 8.62–107.3; P < 0.001], type A IMH (8.0%; aHR 4.85; 95% CI 1.29–18.2; P = 0.019), type B AD (5.0%; aHR 3.51; 95% CI 1.00–12.4; P = 0.051), and type B IMH [1.5%; aHR 1.00 (reference)]. During a median follow-up duration of 8.5 years (interquartile range: 4.0–13.5 years), AD (aHR 2.78; 95% CI 1.87–4.14; P < 0.001) and type A (aHR 2.28; 95% CI 1.45–3.58; P < 0.001) was associated with a higher risk of aortic death. After 90 days, a risk of aortic death was no longer associated with anatomical location (aHR 0.74; 95% CI 0.40–1.36; P = 0.33), but remained associated with disease entity (aHR 1.83; 95% CI 1.10–3.04; P = 0.02). Conclusion The clinical features, response to treatment strategy, and outcomes of IMH patients were distinct from those of AD patients. Both early and late survival was better for IMH than for AD. In addition to the anatomical location of AAS, the disease entity is an independent factor associated with both acute and long-term mortality in patients with AAS. Further investigation is necessary to confirm the prognostic implication of disease entity in different patient populations.

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