Feb. 14, 2000 NEW ORLEANS (Feb. 10, 2000) -- After steady decline in the 1960s and 1970s, the incidence of stroke may be slowly rising, according to a study presented here today.
Results were announced at the American Stroke Association's 25th International Stroke Conference. The American Stroke Association is a division of the American Heart Association.
According to researchers at the Mayo Clinic in Rochester, Minn., there were 751,000 strokes in the United States in 1999 and another 500,000 transient ischemic attacks (TIA), or "mini-strokes," affecting a total of more than 1.2 million people.
The American Heart Association estimates about 600,000 new and recurrent strokes occur in Americans each year. That figure does not count TIAs.
"When we applied our findings to 1999 estimated Census data, we found about 1.2 million strokes or TIAs occur in Americans each year – and that's a conservative estimate," says the study's lead author, Robert D. Brown, Jr., M.D., associate professor of neurology at Mayo Medical School and a staff neurologist in the department of neurology at the Mayo Clinic. "The new data are important in our understanding the overall burden of stroke in the population."
Using population data from Rochester -- an important source of health information for several decades – investigators found stroke incidence hadn't changed over the past 15 years, suggesting previously published numbers of strokes per year may be too low.
Incidence is the number of new cases of a disease that develop in a population during a specific time period, usually a year.
Mayo Clinic researchers have collected detailed health data regarding stroke in residents of Rochester that have occurred since 1935. Brown says that data show a decline in stroke incidence from the 1960s until the mid-1970s when the decline began to level off. The latest research shows that not only is the incidence of stroke in Rochester no longer declining, but it is actually slightly higher than 10 to 15 years ago.
"We like to think everything is going in the right direction, but these data suggest we need to work harder to reduce stroke risk factors such as high blood pressure, cigarette smoking, diabetes, and high cholesterol," Brown says. Examining data from 1990 through 1994, Brown's group found the total incidence of all stroke types -- including first, second, and all subsequent strokes occurring in Rochester residents during the 5-year period -- was 256 per 100,000 each year.
"Our prior estimates showed there are over 500,000 Americans that have their first stroke each year," says Brown. "By considering first-ever strokes only, one underestimates the occurrence of stroke by 37 percent."
When Brown and his team added together the number of strokes and the number of TIAs, they calculated it to be an incidence of 481 per 100,000 people per year. Counting TIAs is difficult, say the investigators, because the symptoms -- such as weakness or numbness -- are variable and are sometimes ignored by patients.
"When we cite incidence rates, we are referring to the first lifetime event of that particular type, such as a first stroke or a first heart attack," says Brown. "To obtain a better estimate of the overall burden of a disease in the population, it is important to consider second, third, and other recurrent events. Those recurrences result in significant health care expenditures, and greatly increases the burden of the disease."
Brown's group used data from Rochester and added to research by Dr. Joseph Broderick of the University of Cincinnati.
Broderick's research -- published in the February 1998 issue of Stroke: Journal of the American Heart Association -- counted first-ever and recurrent strokes and included a more racially-diverse population than other studies from Rochester and Framingham, Mass. Broderick's team estimated that 731,000 Americans have strokes each year.
"Combining the data from Dr. Broderick's study, which estimates the number of blacks having strokes, with our new figures on whites gives us the most detailed data available," says Brown. "Even though we determine all strokes, including those presenting to a hospital, outpatient clinic, occurring at a nursing home, and even those not reaching medical care before dying, there are occasionally cases of minor stroke that are never seen by a health care provider."
The study was funded by the National Institutes of Health.
Co-authors are Jack P. Whisnant, M.D.; JoRean D. Sicks, M.S.; Theresa Christianson, B.S.; W. Michael O'Fallon, Ph.D; and David O. Wiebers, M.D.
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