Aortic aneurysms. Coloured angiogram (X-ray) of a patient's chest showing two aneurysms (swellings, bottom centre and just above heart, pink, at upper right) in the aorta (main body artery, dark red). An aneurysm occurs when a weakened part of an artery wall balloons outwards. It may be caused by disease or a congenital weakness in the wall. This patient is suffering from aortic atheroma, in which the vessel walls degenerate due to the formation of fatty plaques and scar tissue. Aneurysms may be symptomless. However, they may rupture, causing a fatal haemorrhage, if the affected wall is not replaced with a graft. A contrast medium has been injected into the artery.