One theory suggests that congenital defects in the arterial wall are related to the formation of cerebral aneurysms. These congenital defects mainly occur in some of the most common sites of cerebral aneurysms, where congenital lack of smooth muscle layer in the arterial wall can lead to cystic bulging of the intracranial arterial wall and the formation of intracranial aneurysms. Another theory is that of acquired degeneration of the arterial wall. In this theory, it is believed that atherosclerosis and hypertension in the intracranial arteries can cause destruction of the elastic plates in the arteries, resulting in progressive bulging of the intracranial arteries and formation of cystic aneurysms.