Some patients with severe sickle cell disease may need a lifetime of blood transfusions to reduce the chances of suffering a stroke, data from a new study show.

Scientists had hoped that patients with the blood disease could be treated with a limited number of transfusions. But the study, published in this week’s New England Journal of Medicine, found that the stroke risk reappeared after blood exchanges were stopped.

“We hoped that maybe we were dealing with something that was relatively short-lived over a few years in a child’s life,” said Robert Adams, chief author of the study, referring to the need for a continued fresh supply of donated red blood cells.

The study’s result, he said, was “a disappointment.”

Sickle-cell disease is a genetic disorder that causes normally flexible red blood cells to contort into a crescent-moon shape that makes them clump, blocking blood vessels and causing most patients bouts of intense pain.

About 1 in 650 African-Americans and up to 1 in 1,000 Latinos in the United States has some form of sickle cell disease.

Roughly 10 percent of those patients, who have narrowed blood vessels in the brain, face a higher-than-average chance of stroke. Those patients with the higher risk have a 1 in 10 chance of actually having a stroke in a given year.

Regular transfusions of red blood cells typically cut the stroke risk by 90 percent.

Because the blood-flow pattern in the brain seems return to normal in many patients who receive transfusions, doctors had hoped that the need for extra blood was only temporary.

To test that assumption, the team led by Adams, a stroke specialist at the Medical College of Georgia, decided to halt the transfusions in 50 children, but continue them in another 50.

Preliminary results, after only 79 patients were included in the study, showed that discontinuing the transfusions would be dangerous.

Two children who did not receive fresh blood cells regularly suffered a stroke and 14 others showed a dangerous increase of blood flow in the brain.

There were some exceptions, Adams said.

“There are a few kids who tolerate being taken off transfusion. It’s just that we can’t really figure out in advance who they are,” the researcher told Reuters.

Even when doctors perform regular ultrasound testing to track changes in blood flow through the brain, “you do have the chance of a breakthrough stroke, as has happened in two of the cases,” Adams said.