Some of the patients originally assigned to the "no surgery" group did undergo the operation during the study, the researchers noted.
A total of 1,979 operations were performed. Among these the risk of stroke within 30 days was 3 percent, including 26 minor and 34 disabling or fatal strokes, Halliday's team reported.
Over an average of five years of follow-up, 4.1 percent of those who underwent the procedure suffered a stroke, compared with 10 percent of those who did not have the operation.
At 10 years, 10.8 percent of those who had the operation had suffered a stroke, compared with 16.9 percent of those who were not operated on, the researchers found.
The study authors noted that both groups included patients receiving blood pressure-lowering and anti-clotting drugs.
In addition, over the years of the study, patients in both groups were also taking cholesterol-lowering drugs. Even these medications did not affect the benefit of the surgery, the researchers said.
The benefit of the operation was seen for people who had the surgery before they were 75, but not among patients older than that, Halliday's group found.
"This trial took more than 15 years to complete, because we wanted to know about the long-term effects of surgery," Halliday said in a statement.
"The finding that successful carotid artery surgery can substantially reduce the stroke risk for many years is remarkable, because it means that most of the risk of stroke over the next five years in patients with a narrowed carotid artery is caused by that single carotid lesion. The definite benefits that we have found will be of practical value to doctors and patients deciding in the future whether to take the immediate risk of having such surgery," she said.
Overall, the study found benefit of endarterectomy in selected patients, Goldstein said.
The rates of stroke were reduced from about 2.2 percent a year
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