A true aneurysm is a bulge in all three layers of a blood vessel wall. If there is a bulge in some portion of the blood vessel wall but not in all three layers (commonly seen in aneurysms formed as a result of injury), it is a pseudoaneurysm.
The aorta could be described as a smooth tube that is fairly constant in diameter throughout its course from the heart to the abdomen until it divides. An aneurysm occurs when a portion of the aorta bulges more than 3 centimeters in diameter. After it initially forms, it is likely to increase in size as you age.
As an aneurysm expands, the tension on the blood vessel wall increases. This in turn causes the aneurysm to expand further, which puts even more tension on the wall. This cycle continues, and the larger the aneurysm gets, the greater the chances that it will grow larger and eventually burst.
Understanding that aneurysms expand is a key to managing them: the larger they become, the higher the rate of rupture, and the more important it is to treat them.
The location of the aneurysm is also important. Aneurysms can be located in any blood vessel in the body. Aortic aneurysms can be found in the thoracic portion, the abdominal portion, or both. The treatment, surgical approach, and outcome of aneurysms involving the thorax and the abdomen can be quite different.
For example, if an abdominal aortic aneurysm is found below the kidneys, the aorta is cross-clamped below the arteries that supply blood to the kidneys (renal arteries), allowing a normal blood flow and a less risky operation. On the other hand, if the aneurysm is located above the kidneys, the clamp may have to be placed above the renal arteries and thus blood flow to the kidneys is limited. This increases the risk for kidney failure.
Another distinction should be drawn between aneurysms based on their cause. While most arise from chronic changes in the arterial wall and take many years to develop, a small percentage are caused by infection or inflammation. These are called inflammatory aneurysms. While the surgical principles are the same for all aneurysms, inflammatory aneurysms may be more severe and occasionally involve other blood vessels and adjacent organs.
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