A surprising 11 million Americans each year have strokes that are never detected because they cause no obvious symptoms, although over time they may lead to memory loss and other ills, a study concludes.
Doctors have long known that people can have insidious “silent strokes”—tiny spots of dead cells inside the brain that do not cause classic stroke symptoms. But the new study suggests they are extremely common, occurring in about 4 percent of the U.S. population each year.
Besides the 11 million Americans who have silent strokes annually, about 750,000 more have ones that cause classic stroke symptoms, such as slurred speech, dizziness and numbness on one side.
“Silent strokes are epidemic in this country,” said Dr. Megan C. Leary. While they occur in parts of the brain where they don’t cause symptoms right away, “the word ‘silent’ should be put in quotes, because their effects accumulate over the years.”
While a single silent stroke may have no impact, repeated ones lead to memory lapses, mood problems and difficulty walking. They are also a sign that people are especially prone to full-blown strokes.
Leary and colleagues from UCLA Medical Center released the results Friday at a meeting of the American Stroke Association in Fort Lauderdale.
“It’s an alarming estimate,” said Dr. Robert J. Adams of the Medical College of Georgia. “It’s surprisingly high but very possibly accurate.”
The researchers say silent strokes are rare before age 30. But after that, their prevalence doubles every 10 years. By the time people reach their 70s, one in three has a silent stroke every year.
The researchers also found that some people have more than one silent stroke in a year. When these repeat ones are added together, Americans have almost 22 million silent strokes annually. This means that only 3 percent of the total number of strokes in 1998, when the data were compiled, were actually diagnosed.
Leary based her estimate on two surveys involving brain scans on about 5,500 Americans.
“What’s sad is that silent strokes and symptomatic strokes are among the most preventable diseases in this country,” Leary said.
Doctors can see the scars left by silent strokes when they perform routine brain scans. But ordinarily they do not search for them. However, the new data raise the possibility that perhaps they should, since people who have one silent stroke are likely to have many more.
Strokes can be prevented by keeping blood pressure under control, lowering cholesterol, treating diabetes and stopping smoking. Leary said if people actually know they have had a silent stroke, they might be more willing to stick to blood pressure medicines and cholesterol-lowering drugs that cut their risk.
The new data suggest that diagnosed strokes “may be just the tip of the iceberg,” said Dr. Ralph Sacco of Columbia University. But while they raise the possibility that doctors should look for silent strokes and treat victims more aggressively, “we don’t have the data to show that yet.”
In a separate study, Dutch doctors did brain scans on a cross-section of 1,077 elderly people. They found that one-quarter of them had signs of stroke, and 80 percent of these were silent strokes.
“Up until now, we have not told people about silent strokes because we didn’t know what they mean,” said Dr. Sarah E. Vermeer of Erasmus Medical Center in Rotterdam. “Now we have evidence that silent strokes do count.”