Aneurysm Structure

Like people, no two aneurysms are the same. They vary is size, shape and location. Thus, the appropriate course of treatment is dependent on the particular structure and location of the aneurysm. It is important to familiarize yourself with the indices used to measure aneurysms, such as the neck-to-dome ratio.

Size

The size of an aneurysm is measured by its diameter in millimeters.

3-D CT scan reconstruction of small aneurysm at bifurcation of arteriole (arrow designates aneurysm) - Click image for a larger view.
Courtesy of Dr. Kieran Murphy, Johns Hopkins University, Department of Radiology

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3-D CT reconstruction using Vitrea software of giant aneurysm in the posterior Communicating artery - Click image for a larger view.
Courtesy of Dr. Kieran Murphy, Johns Hopkins University, Department of Radiology

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Shape

There are two main types of aneurysms: saccular and fusiform. The more common of the two, saccular or "berry" aneurysms, bulge out of one side of the artery wall. Saccular aneurysms can connect to the artery either with a distinct, well-defined neck or with a wider, broad-based opening to the artery. Fusiform aneurysms are a swelling of both sides of an artery wall and have no defined neck or stem. The shape of an aneurysm is a major factor in the choice of treatment.

Aneurysms usually occur along the major arteries deep within the brain structures. They occur, as mentioned above, in areas of relatively higher blood pressure such as bifurcations, where one artery splits into two.

They can occur either in the front part of the brain (anterior circulation) or back part of the brain (posterior circulation).

Anatomy of the Brain (posterior vs. anterior circulation, circle of Willis, basilar tip, etc.)

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Location

Much of the risk of rupture and risks of treatments depends on the location of the aneurysm. The brain has several main arteries as well as the network of smaller vessels (small arteries are called arterioles) branching out from those, any of which can develop an aneurysm. These arteries are responsible for delivering blood from the heart to the tissues of the brain.

3-D CT reconstruction of intracranial arterial and venous anatomy. - Click image for a larger view.
Courtesy of Dr. Kieran Murphy, Johns Hopkins University, Department of Radiology

The blood is then collected by veins and delivered back to the heart. However, because the arteries are the vessels receiving blood pumped straight from the heart (via the carotid artery), they experience much higher pressure than the veins. They are therefore more susceptible to aneurysms.

3-D reconstruction CT scan with aneurysm located in the paraopthalmic area. - Click image for a larger view.
Courtesy of Dr. Kieran Murphy, Johns Hopkins University, Department of Radiology

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75% of SAH cases are the result of a ruptured saccular aneurysm in the "circle of Willis" or a branch artery (Sengupta and McAllister, 1986 cited in Wardlaw and White, 2000).

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Rupture Rates

Patients with a history of subarachnoid hemorrhage (bleeding in the brain resulting from among other things a ruptured aneurysm) have higher probabilities for another rupture. However, for people with a previously ruptured aneurysm, size is not a factor in re-rupture. The only clear predictor of future rupture for them is location in the basilar tip, with has a higher chance for rebleed. With aneurysms 10 mm or smaller, patients with a history of SAH have a 0.5% rupture rate, 11 times higher than that of patients with no history. However, patients with a history have a lower chance for rupture for aneurysms 10mm or larger than do patients with no history.

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