A brain aneurysm is a weak bulging spot on the wall of a brain artery very much like a thin balloon or weak spot on an inner tube. As blood flows through the brain, the artery pushes against the thinned portion and forces it to swell outwards. A build up of pressure can cause the aneurysm to rupture and allow blood to leak into the space surrounding the brain.
Brain aneurysms that have not yet ruptured are relatively common, with some experts believing that as many as one in 20 could have the condition. Statistically, an unruptured aneurysm—depending on its size—might have a one to two percent chance of bleeding each year. Ruptured brain aneurysms are very rare. In the United States, they number at 30,000 to 50,000 cases a year; while in the UK the cases are around 12,500 in a year.