June 30, 2008 at 2:15 PM CDT - Updated July 2 at 4:15 AM
By Ed Edelson, HealthDay Reporter
If you're an older American with no major health problems, chances are about one in 10 that you've had a stroke and didn't know it.
It was probably not severe enough to cause recognizable symptoms, such as vision problems, facial weakness or trouble walking, but it was still a blockage of a brain artery, and it reduced your thinking powers just a bit.
That estimate comes from a new study of 2,040 people, average age 62, in the long-running Framingham Offspring Study. MRI scans showed that 10.7 percent of them had experienced what study author Dr. Sudha Seshadri, an associate professor of neurology at Boston University, called "a silent brain infarct."
It's the cerebral equivalent of what physicians call a myocardial infarct -- blockage of a blood vessel that causes damage to tissue. In the case of a silent stroke, the blockage and the damage occurs in the brain, without symptoms.
A silent stroke is different from a transient ischemic attack (TIA), a momentary loss of brain function, Seshadri said. A TIA causes some symptoms, while a silent stroke, by definition, doesn't. But both are warning signs to pay attention to the well-known risk factors for stroke, such as cholesterol levels, blood pressure, obesity and smoking, she said.
The incidence found in the Framingham Offspring study "was within the ballpark of what prior studies have suggested," Seshadri said. "But this was a group of people who were younger than in most of the prior studies. The fact that one in 10 persons had silent attacks that had subtle side effects on the brain is something we should be concerned about and should address."
The report was published in the online issue of Stroke.
The effects of a silent brain infarct show up on an MRI scan as "small lesions in various parts of the brain," Seshadri said. "We can't tell from that whether they had a symptomatic attack." And the MRI scans give no clues as to when the silent stroke occurred.
Testing showed that "on average, compared to age-matched controls, those with lesions do have subtle signs, such as loss of flexibility of talk," she said.
The incidence seen in the study did not startle Dr. Claudette Brooks, director of the neurovascular laboratory at West Virginia University Health Sciences Center.
"When I look for the cause of headaches and similar problems, it doesn't surprise me when I see these lesions, and other colleagues tell me they see them," Brooks said.
An even higher rate of silent strokes might be expected in a study of black Americans, she noted. "They have a higher incidence of hypertension [high blood pressure], atherosclerosis, and hyperlipidemia [excess blood fat]," Brooks said.
Nothing special needs to be done to reduce the risk of silent stroke, Seshadri and Brooks both said.
"I wouldn't recommend that people rush out to have an MRI," Seshadri said. "It's up to the medical and public health community to emphasize the importance of controlling risk factors."
"The whole thing boils down to modifying risk factors," Brooks said. "If you don't have risk factors such as high cholesterol, obesity and diabetes, try to keep yourself out of the group that does. If you do, modify them by keeping blood pressure and cholesterol down, things like that."