"Thick" Blood May Increase Stroke Risk

Thick or viscous blood tends to coagulate and form unwanted blood clots that can block blood flow to the heart or brain, causing a heart attack or stroke.

Coagulation potential was assessed by measuring three 'factors,' which are components in the blood that are involved in blood clotting. The factors are von Willebrand factor, factor VIIIc and fibrinogen. Researchers also examined whether blood components -- such as white blood cell count -- that are an indication of inflammation can predict stroke risk. Inflammatory by-products in the blood, due to infections or other causes, have been associated with heart attack.

Individuals with the highest levels of these factors had the highest risk for stroke.

"Inflammation and elevated levels of these substances in blood have previously been associated with increased risk of coronary heart disease, but prior to the new study, there had been scant clinical evidence of their role in ischemic stroke, caused by a clot in a blood vessel in the brain," says lead researcher Aaron R. Folsom, M.D., of the division of epidemiology in the School of Public Health at the University of Minnesota, Minneapolis.

"This study modestly supports the hypothesis that von Willebrand factor, factor VIIIc and fibrinogen can identify groups of middle-aged adults at increased risk of stroke," concludes Folsom.

Researchers also say that the blood coagulation factors may not just be a 'marker' of risk, but could play a role in causing stroke by increasing the risk of a blood clot in the arteries that serve the brain.

Von Willebrand factor and factor VIIIc are produced by the cells that line the blood vessels. They increase blood clotting by causing platelets, the disk-shaped blood components, to stick together. Fibrinogen, a protein involved in blood coagulation, and white blood cell count, an indicator of inflammation, were also found to increase stroke risk, but to a lesser degree.

"Some of these factors can be detected in routine blood tests, but the value of screening for and modifying these new risk markers as an additional means of preventing stroke clearly remains to be established," says Folsom.

"High levels of some of these factors could be altered by getting rid of the other underlying risk factors," Folsom says. "Fibrinogen, in particular, is known to be elevated in people who smoke or have diabetes. So if you want to reduce your risk of stroke, it's important to eliminate as many of the proven major risk factors as possible."

Researchers emphasize that there is more potential benefit in focusing the concern of physicians and patients on well-established cardiovascular risk factors such as smoking, diabetes, obesity, high blood pressure and high blood cholesterol, since these may directly affect the levels of stroke-related coagulation protein in the blood.

During the Atherosclerosis Risk in Communities (ARIC) Study, researchers measured these substances in 14,700 participants ages 45 to 64, who were free of cardiovascular disease when the study began and followed them for six to nine years.

Researchers identified 191 study participants as having ischemic strokes. Ninety-four were men and 97 were women.

After adjusting for factors such as age, race, high blood pressure, diabetes, smoking habits and cholesterol, researchers found that those with the highest levels of von Willebrand factor and factor VIIIc had the highest risk of stroke.

The participants with the highest levels -- the top quarter -- of von Willebrand factor were shown to have a 1.7 times greater risk of developing ischemic stroke than those in the lowest quarter. Only one small previous prospective study has been reported on the relationship between von Willebrand factor and stroke, and ARIC is believed to be the first prospective study ever to examine the relationship between factor VIIIc levels and stroke risk. A prospective study follows healthy individuals over time to look for disease risk factors. Compared to whites, black participants had 15 to 20 percent higher levels of von Willebrand factor and factor VIIIc and 3 percent higher levels of fibrinogen in their blood. Whether such variables could help explain a statistically higher incidence of stroke among African Americans remains unclear, Folsom says. Compared with whites, young African Americans have a two to three times greater risk of ischemic stroke.

Co-authors were Wayne D. Rosamond, Ph.D.; Eyal Shahar, M.D.; Lawton S. Cooper, M.D.; Nena Aleksic, Pharm.D., Ph.D.; F. Javier Nieto, M.D., Ph.D.; Mandy L. Rasmussen and Kenneth K. Wu, M.D. for the Atherosclerosis Risk in Communities (ARIC) Study Investigators.