An aneurysm is a weak or thin spot on an artery wall. Over time, these weak spots stretch or balloon out. The thin walls of ballooning aneurysms can rupture, causing blood to gush into the space next to the brain and raise the intracranial pressure to dangerously high levels. Small cerebral aneurysms, less than three millimeters in diameter, are common. They usually do not cause symptoms unless there is a family history of bleeding aneurysms. The risk of bleeding is increased if there is a family history of bleeding aneurysms, or if the aneurysms are large (greater than seven millimeters in diameter). In those instances, neurosurgical or intra-arterial aneurysm repair is considered.