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An aneurysm (or aneurism) is a localized dilation or ballooning of a blood vessel. Aneurysms most commonly occur in the arteries at the base of the brain (the circle of Willis) and in the aorta (the main artery coming out of the heart). (See aortic aneurysm.)

Aneurysms can be broken down into two groups: true aneurysms and false aneurysms. A true aneurysm involves an outpouching of all three layers of a blood vessel: the tunica intima (commonly known as the intima), the tunica media (commonly known as the media), and the tunica adventitia (commonly known as the adventitia). True aneurysms can be due to congenital malformations, infections, or hypertension.

A false aneurysm, also known as a pseudoaneurysm, involves an outpouching of only the intima. Pseudoaneurysms can be due to trauma involving the intima of the blood vessel. Pseudoaneurysms are a known complication of percutaneous arterial procedures.

More inclusively, aneurysm is known as ectasia, ectasis, and ectasy. For example,

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