What is a Cerebral Aneurysm?

An aneurysm, in general, is a bulging or ballooning of a thin or weak spot in a blood vessel. The bulge fills with blood.

A cerebral aneurysm, in particular, can take place anywhere in the brain, but most are located along a loop of arteries that run between the underside of the brain and the base of the skull.

The bulging aneurysm can put pressure on a nerve or surrounding brain tissue. It may also leak or rupture, spilling blood into the surrounding tissue (called a hemorrhage). Some cerebral aneurysms, particularly those that are very small, do not bleed or cause other problems.

Larger aneurysms, however, are very serious and can cause instant death if they were to rupture.

Classification of Aneurysms

Cerebral aneurysms are classified by type and size:

• The most common form is called a saccular or "berry" aneurysm is a rounded or pouch-like sac of blood that is attached by a neck or stem to an artery or a branch of a blood vessel. It is typically found on arteries at the base of the brain and occur most often in adults.
• A lateral aneurysm appears as a bulge on one wall of the blood vessel.
• A fusiform aneurysm is formed by the widening along all walls of the vessel.
• Small aneurysms are less than 11 millimeters in diameter (about the size of a standard pencil eraser).
• Larger aneurysms are 11-25 millimeters (about the width of a dime).
• Giant aneurysms are greater than 25 millimeters in diameter (more than the width of a quarter).