Abdominal aortic aneurysm. Computer artwork showing a large aneurysm (round, centre) in the abdominal section of the aorta. The organs and nerves in the retroperitoneal space are also shown; as are the lumbar vertebrae (L1-L5). An aneurysm is a localised dilation of a blood vessel and is usually caused by a weakening of the vessel's wall. Abdominal aortic aneurysms tend to cause no symptoms but can be life-threatening as they may rupture, spilling large amounts of blood into the abdominal cavity. This can lead to death within minutes. See image: C007/0900 for the labelled diagram.