A brain aneurysm (also called an cerebral aneurysm or intracranial aneurysm) is a bulging, weakened area in the wall of an artery in the brain, resulting in an abnormal widening, ballooning, or bleb. Because there is a weakened spot in the aneurysm wall, there is a risk for rupture (bursting) of the aneurysm.
A brain aneurysm more frequently occurs in an artery located in the front part of the brain that supplies oxygen-rich blood to the brain tissue. Arteries anywhere in the brain can develop aneurysms. A normal artery wall is made up of three layers. The aneurysm wall is thin and weak because of an abnormal loss or absence of the muscular layer of the artery wall, leaving only two layers.
The most common type of brain aneurysm is called a saccular, or berry, aneurysm, occurring in 90 percent of brain aneurysms. This type of aneurysm looks like a "berry" with a narrow stem. More than one aneurysm may be present.
Two other types of brain aneurysms are fusiform and dissecting aneurysms. A fusiform aneurysm bulges out on all sides (circumferentially), forming a dilated artery. Fusiform aneurysms are often associated with atherosclerosis.
A dissecting aneurysm results from a tear along the length of the artery in the inner layer of the artery wall, causing blood to leak in between the layers of the wall. This may cause a ballooning out on one side of the artery wall, or it may block off or obstruct blood flow through the artery. Dissecting aneurysms usually occur from traumatic injury, but they can also happen spontaneously. The shape and location of the aneurysm may determine which treatment is recommended.
Most brain aneurysms (90 percent) present without any symptoms and are small in size (less than 10 millimeters, or less than four-tenths of an inch, in diameter). Smaller aneurysms may have a lower risk of rupture.
Although a brain aneurysm may be present without symptoms, the most common initial symptom of a cerebral saccular aneurysm is a sudden headache from a subarachnoid hemorrhage (SAH). SAH is bleeding into the subarachnoid space (the space between the brain and the membranes that cover the brain) and not into the brain tissue. Minor subarachnoid hemorrhage most frequently occurs following head trauma. Major subarachnoid hemorrhage is most commonly from a ruptured cerebral saccular aneurysm (80 percent). A sudden headache associated with SAH is a medical emergency.
Increased risk for aneurysm rupture is associated with aneurysms that are over 10 millimeters (less than four-tenths of an inch) in diameter, a location (circulation in the back portion of the brain), and/or previous rupture of another aneurysm. A significant risk of death is associated with the rupture of a brain aneurysm.