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An aneurism is a weak spot in the wall of an artery or a vein that dilates or balloons out, forming a blood-filled sack or pouch. Aneurisms can occur almost anywhere in the body and are found in all age groups, although they occur primarily in the elderly. The foremost cause of aneurisms is atherosclerosis, or fatty deposits in the arteries. If an aneurism bursts, a massive amount of blood is released, which results in an almost instantaneous drop in blood pressure and can cause death. Surgery can be a successful treatment for unruptured aneurisms.

Three aneurisms can be seen in this section of cerebral artery removed from a human brain. © Martin Rotker/Phototake NYC Reproduced with permission.

Atherosclerotic aneurisms are the most common aneurisms. They are found primarily in the abdominal aorta (a larger elastic artery), typically occur after the age of 50, and are more common in men than women. Syphilitic (luetic) aneurisms occur primarily in the thoracic aorta and are due to syphilis, a sexually transmitted disease. Dissecting aneurisms occur when blood penetrates the arterial or venous wall, causing dissections between the wall's various layers. Aneurisms occurring from injury to the artery are known as false aneurisms.

Aneurisms are also classified by their size and shape. A berry aneurism is spherical, or circular, and usually 0.4-0.6 in (1-1.5 cm) in diameter. A saccular aneurism is a larger berry aneurism and may reach a size of 6-8 in (15-20 cm). Fusiform aneurisms are shaped like spindles, and cylindroid (or tubular) aneurisms are cylinder dilations that occur over a considerable length of the artery.

Although recent studies show that genetics can be a factor in the development of aneurisms, environmental factors, such as a bad diet resulting in high blood pressure, are a major cause of aneurisms. Cigarette smoking is considered to be the highest risk factor for developing an aneurism. As a result, treatment of hypertension through proper diet and prescription drugs and by refraining from smoking are approaches to preventing the likelihood of an aneurism forming.

A key to the successful treatment of aneurisms is to discover them before they burst. Diagnostic procedures for aneurisms include the use of ultrasound, computed tomography, and magnetic resonance angiography. Surgical "clipping" or "grafting" is the treatment of choice for most aneurisms. This process involves cutting out the aneurism and then reconnecting the artery or vein with a metallic clip or synthetic tube graft.

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