Aneurysms: What are they and why do they occur?

Most of us have had the experience of seeing a tire or inner tube that has a bulge in its side or a balloon that has a weak spot and doesn't fill normally. Some of us have also had the experience of what can happen when the tire, inner tube or balloon that is weakened by one of these "blebs" loses air and needs to be replaced. In the body an area of blood vessel weakness can have the same effect. In this case, instead of air being released, the contents of the blood vessel (blood) are released from this rupture. This can lead to death (if the blood vessel is a large one like the aorta), stroke, or other catastrophe.

In the body there are two major types of aneurysm. One is called saccular and looks like a sac protruding from the artery, the second is called fusiform and it looks like the whole artery is bulging at just one point.

Why do aneurysms occur? There are several reasons for people to develop weak spots in the blood vessels. The most common is a combination of long-term elevated blood pressure (hypertension) and the aging and sclerosis of the blood vessel that are accompanied by arteriosclerosis. Blood vessel walls are comprised of multiple layers of muscle and connective tissue. When plaque deposits form on the inside of the artery, oxygen and other nutrients cannot penetrate to get to the middle layer of tissue. The lack of nourishment weakens this tissue. The consistent shearing force of blood coursing into the blood vessel stretches the damaged vessel wall. Increased blood pressure is a great strain on the weakened artery wall over time. The larger an aneurysm is, the more there is tension on its wall, and the more likely it is to grow. In the aorta, growth of an aneurysm is usually slow when the aneurysm is less than 4 centimeters. After the aneurysm gets past 5 cm and beyond there is a greater risk that it will burst, or be the area in which a blood clot may form.

There are also other factors that lead to aneurysm formation. These include genetic factors that lead to abnormalities in the matrix of the artery, and certain connective tissue disorders. Aneurysms present at birth, or at a young age are most often not due to arteriosclerosis and are most often related to an inherited predisposition (such as Marfan's syndrome). Some families have a very high incidence of "berry-like" aneurysms in the brain and individuals may need to be screened for this disorder. Less frequently repeated injury or trauma or even bacterial infection might be a source of arterial wall compromise. The most common infecting bacteria found are Salmonella.

Aneurysms may be seen as "beating bulges" (they expand and contract with the heart beat) if they are close to the skin. Most aneurysms however, are found by doctors in the course of a complete physical examination, an x-ray or an ultrasound test. Some are found when an MRI (magnetic resonance image) or CAT scan (computerized axial tomogram) of the chest or abdomen is done for another reason. They are much more common in patients with high blood pressure and a smoking history. They are easier to find in thinner people and harder to find in people with a lot of fat in their belly (this is because the aorta, which is the main vessel that can have a life threatening aneurysm, is located close to the spine and it requires a lot of pushing for the doctor's hand to feel anything back there due to the excess fat).