Have you had a stroke without realizing it?

Stroke is the No. 3 cause of death in the United States, behind heart disease and cancer, but a new study shows that there is a significant group of people who could suffer a stroke and not even know it. Perhaps you are one of these.

Ordinarily, the warning signs of a stroke include changes in balance, slurred speech or a droopy face, weakness in one side, partial vision loss and severe headache, according to the American Stroke Association. But sometimes there are no symptoms at all. In the study, routine brain scans were conducted on a group of middle-aged people, and these showed that 10 percent of them had suffered a stroke without knowing it. This raises their risk for further strokes and memory loss, researchers say.

People with atrial fibrillation, the most common type of irregular heart beat in people over 65, had more than twice the rate of these silent strokes. Silent cerebral infarctions (often called SCIs) are brain injuries caused by a blood clot that interrupts blood flow to the brain.

"The findings reinforce the need for early detection and treatment of cardiovascular risk factors in midlife," Dr. Sudha Seshadri of Boston University School of Medicine and colleagues wrote in the journal Stroke. This is especially true since SCIs have been associated with an increased risk of cognitive impairment.

The researchers based their findings on routine MRI scans from about 2,000 people with an average age of 62. Brain scans of these people found that 10.7 percent had a stroke even though they showed no stroke symptoms. Of these, 84 percent had only a single lesion in the brain. The researchers then looked back to their medical records to see if these people had a risk profile that might predict a stroke.

What they found was that atrial fibrillation more than doubled the risk of silent stroke. In atrial fibrillation, the heart's two upper chambers do not pump effectively enough to empty properly, leaving blood to pool and raising the risk of blood clots. 3 to 5 percent of people over age 65 have this problem.

Silent stroke can also be triggered by high levels of blood homocysteine, a sulfur-containing amino acid found in the blood. Other factors include carotid artery disease or high blood pressure (especially high systolic blood pressure -- the top number in the blood pressure reading).

About two-thirds of people over the age of 65 have high blood pressure, according to the National Heart, Lung, and Blood Institute. High blood pressure has long been implicated as a risk factor for silent strokes.

Dave