A brain aneurysm is a weak, bulging area in an artery in the brain, analogous to a thin balloon or a weak spot on a tire’s inner tube. Because its walls may be weak and thin, an aneurysm is at risk of rupturing. If an aneurysm ruptures, blood spills into the space between the skull and the brain, a life-threatening type of stroke known as a subarachnoid hemorrhage. 

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Saccular aneurysms, also called “berry” aneurysms because they look like berries, are the most common type of brain aneurysm. Saccular aneurysms have a “neck” that connects the aneurysm to its main “parent” artery and a larger, rounded area called the dome. These aneurysms bulge on only one side of the artery wall. A less common type is a fusiform aneurysm, in which the artery is widened on both sides. Fusiform aneurysms do not have a defined neck.