Intracranial aneurysms are classified by different types, depending on their morphology and the disease process that underlies it:

• saccular aneurysms (most common): caused by a congenital defect of one of the components of the arterial wall (tunica media)
• fusiform aneurysms: usually due to atherosclerosis
• infectious aneurysms: caused by an infection of the arterial wall supported by germs present in the circulation;
• post traumatic aneurysm: from artery injury (penetrating trauma or surgical manipulation)

Saccular aneurysms affecting the large arteries at the base of the brain (circle of Willis). In particular, they are formed in correspondence of the bifurcation points or the bending arteries, where the greater the pressure of the blood flow on the vessel wall.

Intracranial aneurysms is clinically manifested when it breaks with subarachnoid hemorrhage.  Sometimes an intact aneurysm can cause neurological symptoms due to compression of adjacent neural structures.