An aneurysm is a bulging out of part of the wall of a blood vessel. It forms where the wall has weakened, often due to the build-up of plaque. It may also be an inherited condition or a complication of high blood pressure (hypertension). Left untreated, aneurysms may tear or burst (a ruptured aneurysm). Ruptures are very painful events that cause massive internal bleeding. The patient must be treated within minutes in order to have a chance of survival. If an aneurysm bursts in the brain, it could cause a hemorrhagic stroke. If an aneurysm bursts in the chest, there is only a 20 percent chance of survival. Therefore, early diagnosis and treatment are critical. Because aneurysms often produce either no symptoms or mild symptoms (e.g., back pain), routine physical examinations are strongly encouraged so that a physician can regularly test for warning signs.

There are a number of different types of aneurysms, which include the following:

• Aortic aneurysm. A general condition characterized by the distention, or ballooning out, of part of the wall of the aorta. The aorta is the main artery carrying oxygen-rich blood from the heart to the rest of the body. Typically, the widened part of the aorta is considered to be an aneurysm when it is more than 1.5 times its normal size. If accompanied by an aortic dissection, it is classified as a dissecting aortic aneurysm.
• Cerebral aneurysm. Also known as a berry aneurysm, this is a bulge in the wall of a blood vessel in the brain (one of the cerebral arteries). A cerebral aneurysm is typically found where the arteries branch at the base of the brain. The cause is unknown, but smoking increases a person's risk of developing this condition.
• Ventricular aneurysm. A bulging or ballooning out of part of the wall of one of the heart's lower chambers (ventricles), usually the left. If the aneurysm is present in the wall between the ventricles (the ventricular septum), it is also known as a ventricular septal aneurysm, an interventricular aneurysm or simply a septal aneurysm. It may be present since birth (congenital) or may be a complication of a heart attack or other trauma.
• Atrial aneurysm. A bulging or ballooning out of part of the wall of one of the heart's upper chambers (atria). If the aneurysm is present in the wall between the atria (the atrial septum), it is also known as an atrial septal aneurysm (ASA), an aneurysm of septum primum or an aneurysm of the septum secundum. An ASA has been associated with an increased risk of stroke and is often accompanied by the presence of a patent foramen ovale (PFO), an opening between the upper chambers (atria) of the heart. Normally, this opening is present in the developing fetus, and closes shortly after birth. It is often present since birth (congenital).
• Cirsoid aneurysm. The enlargement of a network of blood vessels that commonly occurs on the scalp and may result in the formation of a tumor.

Strategies for preventing an aneurysm include managing high blood pressure and preventing the build-up of plaque.