Surgery can halve the odds of a stroke, one of the leading killers worldwide, in high-risk patients, scientists said on Friday.

Severe narrowing of the carotid artery that carries blood to the brain raises a person’s chance of having a disabling or fatal stroke, which accounts for one in 10 of the 55 million deaths that occur each year worldwide.

But researchers at St George’s Hospital in London said a two-hour operation, under general or local anaesthetic, can alleviate the narrowing.

“It’s clear from our trial that immediate surgery is the best option for some patients with severe narrowing of the carotid artery,” said Alison Halliday, a vascular surgeon who reported the finding in The Lancet medical journal.

In a trial of 3,000 patients in 126 hospitals in 30 countries, she and her colleagues found that surgery halved the risk of stroke from 12 to six percent in high-risk patients after a five-year follow-up.

“Most of the patients we found who benefited were under the age of 75,” Halliday said in an interview.

Stroke is the second leading cause of death, after ischemic heart disease, according to the World Health Organization. It is also among the five most important causes of disability and occurs when a blockage stops the flow of blood to the brain.

But it is also largely preventable with efforts to reduce blood pressure and raised cholesterol levels.

Although surgery itself can cause stroke, the risk is much less than it would be without an operation.

Anyone with a 70-90 percent blockage in the artery would be a candidate for surgery, according to Halliday. Blockages occur in the carotid artery just as they do in arteries linked with heart attack.

People who have high blood pressure, who smoke, who have diabetes or have High cholesterol have an increased risk of stroke.

Ultrasound screening on the neck will detect narrowing of the carotid artery.

“We hope this research provides hope for people,” said Halliday, adding she plans to follow up patients for an additional five years.