What is an aneurysm as noted in the novel, One Last Thing Before I Go?

An aneurysm is a balloon like structure that occurs in blood vessels when the wall of the vessels has been weakened. This balloon like structure allows blood to pool until the pressure of the blood volume causes the weakened vessel wall to burst. This can be a fatal condition. Silver has one of these aneurysms in his heart, a condition that is allowing blood clots to flow freely through his blood stream, causing his small strokes. It could burst, which would cause Silver to bleed to death.