The aorta is the largest artery in the body, channeling all the blood pumped by the heart through the chest up toward the neck. Branch arteries lead to the head & each arm, while the aorta curves down the chest and abdomen to around the belly-button, where it divides into two narrower arteries to each leg (see Diagram — Circulatory System).
An aneurysm is a ballooning out of an artery if the wall is weak. The danger is that it can eventually rupture, which then carries a high mortality rate (almost 100% without surgery). In the chest, this usually happens right after the aorta leaves the heart, though it can also occur further along. Aneurysms develop due to atherosclerosis, caused by factors like smoking, hypertension, high cholesterol. They likely also have a genetic component, even if there are no other family members affected. Some cases occur in younger patients with auto-immune or other conditions (e.g. Marfan’s Syndrome).
There are no symptoms until the artery ruptures, usually after 60 years-old. Then there’s severe chest pain, often described as “tearing” (“tear” rhymes with “hair,” not “here”), and may be felt in the back. If there’s a complete rupture, pain begins immediately; if bleeding occurs into the wall of the artery (“dissection”), it may evolve a little more gradually.
Diagnosis of rupture or dissection can sometimes be made by chest x-ray, but more easily & certainly by a chest CT scan. Virtually everyone dies without surgery. Occasionally, intact thoracic aneurysms are found incidentally by x-rays obtained for unrelated reasons, then confirmed by CT scan. Surgery is recommended if the diameter is >5.0 cm., or >4.5 cm. if the person also has a bicuspid aortic heart valve (can only found by echocardiogram). Results are most often successful, as opposed to the 50% mortality with emergency operations after rupture.