Recently, it has become possible to diagnose unruptured cerebral aneurysm, owing to the development of diagnostic imaging techniques such as magnetic resonance angiography. However, the natural history and optimal treatment for unruptured cerebral aneurysm remain unknown. In Japan, the Unruptured Cerebral Aneurysm Study of Japan (UCAS-Japan) is now on-going. Preliminary results indicate that the overall annual risk of rupture is about 1%, and factors that are associated with increased risk of rupture include location and size of the aneurysm. The guidelines for Brain Dock 2008 recommend surgical indications for unruptured aneurysm. These are as follows: 1) patient's life expectancy is 10～15 years, 2) aneurysm size is over 5～7 mm, 3) aneurysm is located in the posterior circulation, the anterior communicating artery or the internal carotid-posterior communicating artery in cases of aneurysm size less than 5～7 mm, 4) symptomatic unruptured aneurysm, 5) irregularly shaped aneurysm, 6) aneurysm where the ratio of the dome to the neck is large. For patients with conservative treatment, periodic observation with imaging devices, control of hypertension and no smoking are required. We have have adopted these guidelines. Currently, we consider surgery is the treatment of choice for unruptured aneurysm, and the second choice is intravascular treatment. For aneurysm located in the paraclinoid IC or posterior circulation, we recommend intravascular treatment.
