Vice President Dick Cheney is scheduled for surgery to bypass a bulging blood vessel behind his knee, news sources report.

Cheney is reported to have a popliteal aneurysm (pronounced POP-li-teel AN-yur-ism). That means that there's a bulge in the artery running behind the knee.

Abdominal aortic aneurysms are more common -- and, because they tend to rupture, far more deadly. But popliteal aneurysms usually don't rupture. Instead, they tend to throw off blood clots. When that happens, the leg may have to be amputated.

When a popliteal aneurysm is found, surgery is the best option, says vascular surgeon Peter Henke, medical director of the Noninvasive Diagnostic Vascular Laboratory at the University of Michigan. Without treatment, patients who already have symptoms run a one in five risk of losing a leg and a one in 20 chance of dying.

"Patients generally should have it fixed, because the risk of complication does not correlate with the size of a popliteal aneurysm," Henke tells WebMD. "A small one is just as dangerous as a big one."

How the Surgery Is Done

Fixing a popliteal aneurysm is relatively simple. It means taking a blood vessel from the lower leg and using it to bypass the bulge in the artery. The graft usually runs from above the knee to below the knee.

"Overall this works quite well," Henke says. "Overall, 80 percent to 90 percent of the grafts are still fine after five years."

However, it's not a good sign to have a popliteal aneurysm. Some 70 percent of people who have these aneurysms also have abdominal aortic aneurysms, Henke says. Fixing an abdominal aortic aneurysm is a much more involved surgery, although less invasive techniques can be used for some patients. That technique -- endovascular repair, which avoids open surgery -- is also showing surprisingly good results for popliteal aneurysms in early trials.

Fortunately, popliteal aneurysms are relatively rare. Only 0.1 percent of the U.S. population gets them -- and for reasons not yet understood, nearly all of them occur in men.

The main risk for popliteal aneurysms is smoking.

"The typical patient is a male who smokes, with high blood pressure and high cholesterol," Henke says. "Diabetes does not increase the risk."

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Stein, R. The Washington Post, Sept. 17, 2005. News release, University of Michigan. Peter K. Henke, MD, medical director, Noninvasive Diagnostic Vascular Laboratory, University of Michigan, Ann Arbor.