Profile X-ray of the skull of a 49-year-old patient with an aneurysm of the posterior communicating artery treated using endovascular coils (centre). Endovascular coils are used to block the flow of blood from the artery. Aneurysms are localised, blood-filled balloon-like bulges in the wall of a blood vessel. They form when the wall is weakened by disease or another cause. If they burst there is the danger of death or other complications due to severe hemorrhaging (bleeding).