Brain Aneurysm Resource Center

Brisman, Song, Newell. N Engl J Med 2006; 355:928-939August 31, 2006DOI: 10.1056/NEJMra052760

Basic Brain Aneurysm Anatomy

Michael Chen, MD 

A useful understanding of your brain aneurysm, management options as well as what the future holds takes a little bit of homework. This may be the first time you have ever read about brain diseases.  As with many other disciplines, there are a few basic terms and concepts that should be understood prior to any type of meaningful conversation with your physician.

Brain arteries:  These are the pipes that transport blood, pumped by your heart to your brain.  As you can imagine, they are lined with a muscular layer to help maintain the pressure of the blood such that the furthest regions of your brain can get a steady stream of blood.  The cells that make up the wall of the artery are not static.  In fact, these cells are constantly growing and remodeling in response to the pressure from the blood within it to attempt to maintain, as much as possible, a smooth cylindrical shape.

Brain veins:  Veins are thin walled vessels which lack a muscular wall that drain blood after it has passed through tissues or organs.  After they have passed through tissues or organs, they lose oxygen and are under very little pressure as it returns it to the heart to get pumped to the lungs.  Veins outside the brain are usually more visible as blue vessels near the skin surface.

Brain aneurysms:  When the cells that line the walls of arteries are not able to effectively maintain a smooth, cylindrical shape, the blood pressure within the arteries causes a bulge.  Over varying amounts of time, the bulge may grow into a sac, or an aneurysm.  Therefore, you are generally not born with brain aneurysms.  As you might expect, they tend to occur on the outer curve of vessels or at branching points.  Furthermore, because of the anatomic configuration of the major arteries at the base of the brain, aneurysms occur in fairly predictable locations.  The reason why we worry about aneurysms is because the wall lacks the normal muscular layer which may allow spontaneous rupture when the force of the pressurized blood in the artery exceeds the tensile strength of the thinned artery wall. 

Subarachnoid Hemorrhage:  The major brain arteries and brain are normally suspended within the skull, bathed in spinal fluid.  This is anatomically referred to as the subarachnoid space.  The spinal fluid is clear and plays an important role in maintaining normal function in the brain as well as serving a protective roll.  The most common location for brain aneurysms to bleed is within this subarachnoid space.  Part of what makes bleeding from aneurysms so devastating is because brain arteries have no additional protective tissues surrounding it, and are simply ‘floating’ in this subarachnoid fluid space.  Furthermore, in adults, the skull represents a fixed volume, such that any further increase in volume in the skull, in this case blood, causes a rapid, dangerous rise in pressure within the skull.