Unruptured brain aneurysms are found more often. Because brain MRIs are done for many reasons. Should unruptured brain aneurysm be treated? That depends on aneurysm location, size, and rupture risk. Why do brain aneurysms rupture? Dr. Anton Titov, MD. What are the factors that determine the risk of rupture of any given brain aneurysm? Dr. Mika Niemela, MD. The problem is that aneurysms miss an essential part of normal blood vessels in the brain. Brain aneurysms start to grow. Aneurysm's wall becomes degenerated. Because there is the inflammatory processes inside brain aneurysm. Inflammation causes myo-intimal hyperplasia and degeneration. The wall of aneurysm becomes weak. Then at some point the wall of brain aneurysm cannot hold the stress. Stress on the wall of aneurysm comes from the blood pressure. Then aneurysm bursts. Dr. Anton Titov, MD. But, obviously, not all brain aneurysms rupture. So this is a very difficult to determine the reasons in any particular person of all the factors that cause brain aneurysm to rupture. Dr. Mika Niemela, MD. Yes, between 2% to 6 % of all people harbor unruptured intracranial aneurysms. Not all of brain aneurysms rupture. Rupture of aneurysm depends on the risk factors. It is smoking, hypertension, female gender. Sometimes there's a male who is 40 years old. He is otherwise active, doesn't smoke, no hypertension. His brain aneurysm rupture risk is different than that of another person. Other person is a 55-year-old woman with hypertension and smoking. She has the risk of brain aneurysm rupture to be much higher regardless of the size of the brain aneurysm. Dr. Anton Titov, MD. Of course, brain aneurysm size matters a lot. Dr. Mika Niemela, MD. But also many risk factors affect the risk of brain aneurysm rupture. But even smaller brain aneurysms can rupture, at least with time.