Types of and location of cerebral aneurysms

Types of and Location of Cerebral Aneurysms

Cerebral aneurysms are classified both by size and shape.

Small aneurysms have a diameter of less than 15 mm.
Larger aneurysms include those classified as large (15 to 25 mm.), giant (25 to 50 mm.), and super giant (over 50 mm.)
Saccular aneurysm refers to any aneurysm with a saccular outpouching including berry aneurysms. Saccular aneurysms are the most common form of cerebral aneurysm.
Berry aneurysm, is a type of saccular aneurysm with a neck or stem resembling a berry.
A fusiform aneurysm describes an aneurysm without a stem.

Cerebral aneurysms usually occur at the bifurcations and branches of the large arteries located at the Circle of Willis. Approximately 85% of aneurysms develop in the anterior part of the circle of Willis, and involve the internal carotid arteries and their major branches that supply the anterior and middle sections of the brain.

The most common sites include the:

Anterior Communicating artery (30 - 35%)
Bifurcation of the Internal Carotid and Posterior Communicating artery (30 - 35%)
Bifurcation of Middle cerebral (20%)
Basilar artery bifurcation (5%)
Remaining posterior circulation arteries (5%)

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The majority of cerebral aneurysms occur in the anterior portion of the Circle of Willis.