Where in the brain do aneurysms develop?

The internal carotid arteries are the two large arteries -- located on either side of the head and neck -- that supply blood to the brain. The most common types of aneurysms are found in the arteries that connect the two internal carotid arteries:

30% of aneurysms form in the anterior communicating artery (AComm) and 25% form in the posterior communicating artery (PComm).

The frequency of aneurysms in other locations:

• 4% in the pericallosal artery, which is a segment of another important pair of arteries called the anterior carotid arteries
• 7.5% in the internal carotid arteries themselves, called ICA bifurcation
• 20% in the middle cerebral artery (MCA), which supplies blood to the front of the brain
• 7% in the tip of the basilar artery, which runs along the middle of the brain, where it divides into two smaller arteries
• 3% in the posterior inferior cerebellar artery (PICA), which supplies blood to the bottom portion of the brain
• A small percentage form in other areas of the brain as well.

The vast majority of brain aneurysms that develop involve the internal carotid arteries and their major branches that supply the anterior and middle sections of the brain.

The internal carotid arteries are two major arteries, one on each side of the head and neck, which supply blood to the brain.

Most aneurysms develop in the frontal part of the "Circle of Willis", where several arteries join together at the bottom side of the brain. The internal carotid arteries branch into smaller arteries that supply oxygenated blood to over 80% of the cerebrum.

Aneurysms are outpouchings of blood vessels in areas where the wall of the blood vessel becomes weak. Conditions such as high blood pressure or cigarette smoking lead to chronic vessel injury and may lead to aneurysms. Wherever there is a blood vessel, there can be an aneurysm. In the brain, common locations are the main vessels of the “Circle of Willis” and branches of these vessels, such as the posterior communicating artery, ophthalmic artery, and middle cerebral artery. The main risk that aneurysms pose is bleeding from bursting of the weak wall of the aneurysm, which can lead to subarachnoid hemorrhage. 

An aneurysm is a weakening of a blood vessel wall which then forms an outpouching that fills with blood. Aneurysms can form anywhere along an artery's path. In the brain, the most common location for an aneurysm is along the vascular tree at the base of the brain, called the Circle of Willis. Appropriately 85% of cerebral aneurysms are found in the anterior portion of the Circle, most commonly where the arteries bifurcate or divide. 

Aneurysms can form spontaneously, but there are certain conditions that increase your risk of developing them including but not limited to connective tissue disease, significant head trauma, and high blood pressure. While many aneurysms are small, asymptomatic and benign, aneurysms of a certain size should be monitored or even removed, as they may bleed or rupture, which can be a life threatening event. If you or someone you know develops a new, severe, sudden onset headache with nausea, double vision, numbness, or weakness, a physician should be made aware immediately to assess whether or not the condition sounds like a potential aneurysmal rupture.