Baltimore County Executive Roger B. Hayden has lost much of his right-side vision after suffering a ruptured blood vessel in his brain Sunday, a Johns Hopkins Hospital neurologist said yesterday.

Dr. Kyra Becker, an intensive care neurologist who was treating Mr. Hayden, said "a large portion of his vision on the right side is gone," but she added that the condition should improve with time.

Asked if the county executive will regain normal vision, Dr. Becker said, "It would be hard to say."

She said such cases are difficult to predict. Mr. Hayden might have no further problems, or he may have more ruptures, she said.

However, she said he "looks great" and is in "very good condition."

Mr. Hayden, 49, is to remain at Hopkins for several more days of testing, Dr. Becker said.

In a a telephone interview yesterday from the intensive care unit, Mr. Hayden said the problem was the result of a condition he has had about 20 years.

He said a tangle of tiny blood vessels over his right eye periodically has caused him painful headaches, but this is the first time one has burst.

Asked how he felt, he said, "I'm hanging in there."

Robert Hughes, spokesman for the executive, said Mr. Hayden awoke at his north county home Sunday morning with a severe headache and called Kirk G. McCleary, his police driver-bodyguard.

Officer McCleary took Mr. Hayden, who lives alone, to St. Joseph Hospital in Towson. After examination there, he was transferred to Hopkins. The executive remained conscious and alert the entire time, Mr. Hughes said.

Dr. Becker said Mr. Hayden's arteries, which bring blood to the occipital lobe at the back of his head, have no capillaries connecting them to veins, which return blood to the heart.

As a result, she said, the flow of blood is faster, creating more pressure. In addition, the artery-vein connections sometimes grow larger over the years, leading to ruptures.

Dr. Becker described the loss of right-side vision as being from both eyes. For example, she said, if Mr. Hayden were sitting in a chair, surrounded by a circle of people, he would see only those directly in front of him and to his left.

The condition is called a "vascular malformation," she said, and can range in seriousness from having no effect to the kind of rupture that Mr. Hayden experienced.

She said an angiogram planned for the county executive would use dye injected into his blood to show where fluid is going inside his head. He also will undergo a magnetic resonance imaging test.

Mr. Hughes said the executive intends to work from his hospital room.