Sen. Harry Reid, D-Nev., said the condition of Sen. Tim Johnson, D-S.D., "looked very good" after surgery late last night for bleeding in his brain caused by a congenital condition.
The brain-bleeding episode, which came about with little warning, has left many wondering whether they too could be at risk.
The senator was admitted to George Washington University Hospital last night, where he underwent late-night emergency surgery.
The underlying cause, known as arteriovenous malformation, or AVM, "is an abnormal tangle of blood vessels where arteries connect directly to veins without the normal smaller vessels in between that regulate flow," said Dr. Gene Barnett, of the department of neurological surgery at the Cleveland Clinic.
The direct connection results in abnormally high blood pressure in the area, which makes bleeding more likely.
AVM is rare. Only about one out of 700 Americans has it. And for most people, perhaps 80 percent or more, AVMs pose no problems.
However, it is not a normal practice to screen for AVMs in the brain, so many people do not know they have an AVM until they experience a complication.
Experts say not enough information has been released to say for certain whether the senator will recover fully.
"[The prognosis] depends entirely on things we do not know," said Dr. J.P. Mohr, director of Columbia University Medical Center's Stroke Center and lead researcher of a trial that examines patients with AVMs that have not ruptured. He said the size of the senator's AVM, as well as how deeply it is located in his brain tissue, will likely determine the extent of his recovery.
And he said Johnson's difficulties in speaking before his hospitalization suggest the AVM could have been affecting the speech center of his brain.
"It is not clear if this was the only thing that was affected, or just the main thing," Mohr said. "We can hope that he was still able to walk, since this gives us hope that the bleeding was minor."
"When it bleeds, there can be bleeding into the brain tissue, which can lead to the kind of symptoms we're talking about," said Dr. Howard Kirshner, vice chairman of neurology and director of the stroke center at Vanderbilt University Medical Center. Kirshner discussed the condition with ABC News medical editor Dr. Tim Johnson on the ABC News Now "Healthy Life" program this morning.
On average, about one in 10 people who suffer their first episode of bleeding in the brain due to an AVM die. However, the South Dakota senator could experience subsequent bleeding. Each bleeding episode puts him at greater risk of death and permanent brain damage.
"The likelihood and timetable for recovery depend largely on the extent of bleeding, where in the brain the bleeding occurred, and whether the AVM will bleed further," Barnett said. "If all went well, he could be back to work in several weeks to a few months; however, he will likely need further treatment of the AVM."
This additional treatment could involve additional surgery or radiation therapy to treat the AVM before it ruptures again.
Although symptoms of AVMs may occur at any age, it is less common for problems to occur later in life.
Some experts said the news of a successful surgery is a good sign.