Ask The Doctors: July 2014
Q. I was diagnosed with an abdominal aortic aneurysm, but my doctor wants to watch it and not repair it. I have it checked regularly, but I’m nervous. How long can you take a watch-and-wait approach with these things?
A. It’s wonderful that you are taking an active role in the care of your aorta, the largest artery in the body. An aortic aneurysm is a segment of the aorta which has ballooned out beyond its normal size. Women tend to have smaller diameter aortas than men, and taller people have larger diameter aortas than shorter people. But for most individuals, a “normal” size is considerably less than 3 cm in diameter. An aortic aneurysm, where a portion of the vessel is greater than 3 cm in diameter, can occur in the chest where it is called a thoracic aneurysm, or in the abdomen where it is called an abdominal aortic aneurysm (AAA, pronounced “triple-a”).
The most dreaded consequence of AAA is rupture, which has a sobering mortality rate of approximately 75 percent. At the same time, open surgical repair of AAA is a relatively high-risk procedure, with a mortality rate of approximately 5 percent, and should not be performed on patients with minimal risk of rupture. When the aorta is under 4 cm in diameter, the risk of rupture is essentially zero, while at 5 cm, the annual risk rises to approximately 5 percent, and at 6 cm, that risk doubles to 10 percent. The rate of aneurysm enlargement is also important, as a small AAA with an expansion rate of greater than 0.5 cm per year is considered at high risk for rupture. Our job is to determine the right time to intervene and fix the aneurysm, when the benefits of the procedure outweigh its risks and those of the disease. Whether or not to proceed with AAA repair depends mainly on aortic size. Current guidelines suggest that AAA repair should be performed for asymptomatic aneurysms greater than 5.5 cm in diameter, and for all aneurysms causing symptoms.
Q. My doctor has mentioned the value of a “cardiac MRI” in evaluating heart valve problems. Is this the best screening test for valve disease? And I was told that because I have kidney disease, I can’t have an MRI that uses a contrast agent. Is an MRI without contrast helpful?
A. Most people’s experience with magnetic resonance imaging (MRI) occurs when they are undergoing an orthopedic (bones and joints) or neurological (brain, spinal cord, nerves) workup. MRI can be very helpful in determining the size, structure, composition, and spatial interrelations between bodily organs. More specifically, cardiac MRI has developed tremendously over the past 20 years or so. It is used to assess for large structural abnormalities of the heart and aorta, such as those found in cases of inborn (congenital) problems, aortic aneurysms, heart failure, scars, tumors, and inflammation of the heart muscle (myocarditis) or the sac around the heart.
Cardiac MRI can also be used to evaluate heart valves: their shape and flexibility, and whether they are narrowed or leak. However, it is not the best first-line test to evaluate for valvular disease. An echocardiogram, or “echo” for short, is the test of choice. With regards to your other question, MRI pictures are generally improved, and the results are thus more helpful, when a contrast agent is administered through an IV. Instead of the iodine-based contrast agents used for CT scans, MRI uses gadolinium-based contrast. As you mention, people with kidney disease often cannot receive gadolinium contrast. This is due to their elevated risk of a rare complication known as nephrogenic systemic fibrosis. If your doctor is concerned about the possibility of valvular heart disease, an echocardiogram should be the first test. Cardiac MRI without contrast would be used only if these two types of echocardiograms did not provide adequate information about the heart’s valves.