An eye treatment in which a laser is used to seal leaking blood vessels halves the possibility of blindness or damage to vision from a common complication of diabetes, according to a new study.

Researchers found the laser effective in treating macular edema. An estimated 400,000 diabetics in the United States have that condition, in which fluid leakage at the center of the retina can severely impair ability to recognize faces, read or drive.

The laser therapy represents a "major clinical advance in treating diabetic eye disease," said Dr. Morton F. Goldberg, editor of the Archives of Ophthalmology, which published the study in this month's issue.

In the study, sponsored by the National Eye Institute, patients with macular edema were randomly assigned to two groups, one to be treated with the laser and the other to be watched without treatment. After three years, vision had deteriorated in 24 percent of the untreated but in only 12 percent of the treated.

Lasers have been widely used since the 1970s to treat a more serious eye complication of diabetes in which abnormal blood vessels grow across the retina and often bleed unpredictably, causing sudden loss of vision. Ophthalmologists prevent such bleeding by creating many small laser burns along the periphery of the retina to seal abnormal vessels.

The new application expands recommendations for laser therapy to include many diabetics with macular edema, a less severe complication which, if left untreated, would produce blindness within five years in an estimated 66,000 people who now suffer from it.

To treat macular edema, an ophthalmologist trains a laser beam on areas near the macula, a spot near the center of the retina and critically important to sharp vision. Short bursts of energy burn and seal leaking points on tiny blood vessels. The patient is awake and "sees stars" when hit by the laser but usually feels no pain, according to Dr. Lloyd Aiello of Boston's Joslin Diabetes Center.

The burns scar the retina but slow or stop visual damage from fluid leakage. In as many as 30 percent of treated patients, vision improves, according to Dr. Frederick L. Ferris of the National Eye Institute, a co-chairman of the study.

The findings underscore the need for diabetics to see an ophthalmologist yearly, Ferris said, since retinal disease can develop without symptoms.

An estimated 10 million Americans have diabetes. Virtually all patients with type 1 diabetes, the kind that usually starts in childhood, suffer retinal complications by the time they have been diabetic for 25 years, according to Dr. Lawrence Rand of the Joslin Diabetes Foundation. As many as 70 percent of those with type 2 diabetes, the more common type starting in adulthood, develop eye complications, he said.