Coloured angiogram of the supra-aortic trunks of a 62-year-old male patient with a thoraco-abdominal aortic aneurysm. A graduated catheter has been inserted up to the aneurysm. An aneurysm is a blood-filled dilation in a blood vessel wall. It is caused by a weakening of the vessel wall, which may be caused by hardening of the vessel (atherosclerosis), high blood pressure or injury. If left untreated an aneurysm can rupture, which is often fatal.