An aortic dissection is a tear in the inner lining of the aorta (the main artery that carries oxygen-rich blood from the heart to the rest of the body), creating a space between the inner and outer layers. If blood leaks into that space between the layers, it could cause potentially fatal conditions or events such as:

When accompanied by a widening or ballooning part of the wall of the aorta (aortic aneurysm), the condition is referred to as a dissecting aortic aneurysm.

Dissections can be either ascending or descending depending on whether the vessel damage occurred in the part of the aorta that rises up from the heart (a type A dissection) or the part that descends down through the chest (a type B dissection). While both are serious, the ascending aortic dissection is considered the more dangerous of the two and usually requires emergency surgery. Descending aortic dissections are more common and can usually be treated with medical therapy.

The incidence of an aortic dissection is estimated to occur in 2 to 4 per 100,000 person-years, and roughly 2,000 new cases of aortic dissections are reported each year in the United States. However, the condition may be underreported because it is difficult to determine whether death was caused by an aortic dissection, a heart attack or sudden cardiac death without an autopsy.

When untreated, an acute aortic dissection is fatal to more than a third of patients within 24 hours, half of patients within 48 hours and three-quarters of patients within two weeks of onset.