When Senator Joseph Biden underwent emergency brain surgery last week, he joined the ranks of 2 percent of adult Americans who suffer from aneurysms in the brain -- a weakening of an artery wall that can burst and often kill its victims.

Biden appears to be among the lucky ones. His aneurysm was detected in time for him to undergo surgery, and it was located in a region of the brain that made it possible for surgeons to operate. Others won't be so fortunate.

In this age of high-tech medical developments, aneurysms remain serious medical emergencies that kill or disable nearly 20,000 people each year. But important new treatments, such as laser surgery, better drugs and a special balloon procedure to treat inoperable aneurysms are under development, and health officials are optimistic that the outlook for aneurysm patients is steadily improving.

Named after the Greek word for widening, aneurysms "are a very common problem," says Dr. Harold P. Adams Jr., director of the Division of Cerebrovascular Diseases at the University of Iowa's College of Medicine. The condition accounts for approximately 8 percent of strokes in the U.S.

An aneurysm, which is an abnormal stretching or widening of a blood vessel wall, occurs when the middle layer of the blood vessel weakens. Like a worn, old tire, the weakened area bulges and eventually bursts if the pressure in the blood vessel builds high enough or the wall becomes weak enough. Aneurysms can be a single large structure -- as big as an inch in diameter -- or as small as a berry.

For some people, aneurysms may go undetected and never cause a problem. For others, aneurysms are a serious medical problem that can produce a deadly stroke.

But unlike other types of strokes, which generally afflict people 55 and older, aneurysms usually strike those ages 35 to 50. "One of the horrors of this condition is that it wipes out young people," says Dr. Murray Goldstein, director of the National Institute of Neurological and Communicative Disorders and Stroke (NINCDS).

Usually the aneurysm bursts without warning, quickly changing an apparently healthy adult into a seriously ill individual. In the United States, "about 28,000 people a year have an intracranial {or brain} aneurysm that bursts," says Dr. Neal Kassell, director of the Cooperative Aneurysm Study and a neurosurgeon at the University of Virginia at Charlottesville. "About 70 percent of those people will die or be disabled."

Aneurysms can occur anywhere in the body, but why they form is still not understood. Researchers suspect that there may be some kind of hormonal effect because women outnumber male patients, with about three women being afflicted for every two men. Smoking also increases the risk of developing an aneurysm, as does high blood pressure. As blood pressure rises, the strain on artery walls increases. If an artery has a weakened area, the wall begins to bulge outward -- and an aneurysm forms.

But many of those who develop aneurysms have normal blood pressure, a fact that leads researchers to believe that some people may be born with developmental aneurysms -- weakened regions that over years simply can't sustain the blood pressure and eventually produce an aneurysm.

However they form, only a small percentage of aneurysms "rupture and bleed," says the University of Iowa's Adams. But when these complications occur, the condition is often fatal. Only 8,000 of the 28,000 people whose aneurysms burst will survive without disability, according to a study by the University of Virginia's Kassell. Of the rest, 3,000 will die immediately at home or at work; 7,000 will be referred too late for treatment at a specialized medical center. An additional 10,000 people will die or be disabled from various complications.

"The hope for the future," says Kassell, "is to identify patients who have aneurysms through more accurate diagnosis and to get them prompt treatment at specialized centers."

When an aneurysm ruptures on the aorta, which supplies blood to the heart, the result is sudden death. If it bursts on an artery supplying the legs or the stomach, treatment must be swift or there is a danger of bleeding to death.

In the head, a ruptured aneurysm means that the blood supply is cut off to regions of the brain. Cells begin to die from lack of oxygen. In addition, blood itself is toxic to brain cells and causes more damage. Finally, as the bleeding continues, pressure builds in the skull, killing more brain cells. Without treatment, the pressure will literally push some of the brain into the spinal canal, says NINCDS's Goldstein.

The treatment for most aneurysms is surgery. But first, the aneurysm must be detected, which can sometimes be difficult. "Many aneurysms are misdiagnosed," says Dr. Louis Caplan, a neurologist and chairman of the American Heart Association's Council on Stroke.

Part of the difficulty in diagnosing an aneurysm is that it can mimic other conditions, including the flu. For example, an aneurysm that is leaking but hasn't burst yet may produce a very bad headache -- one described as the worst ever experienced -- accompanied by a feeling of restlessness and a little confusion. Other symptoms of aneurysms are vomiting, a stiff neck, fainting or a numb feeling in the arms and legs that passes quickly.

"For a minute or two you lose control of a hand or a leg," says Goldstein. "Or a side of your face slips and you drool, or you go blind in one eye and then feel all right."

The symptoms can last as long as 24 hours, but most occur for just a minute or two. "The important thing is that it is a medical emergency," says Goldstein. "It's not something that you pat yourself on the head and say, 'I ought to see a doctor one of these days.' Call your doctor right away."

To diagnose an aneurysm -- and to determine exactly where it is -- physicians often turn to angiograms, CT scans and sometimes to MRI -- magnetic resonance imaging. These techniques allow doctors to check for pooling blood in the brain. Spinal taps -- which detect blood in the cerebrospinal fluid, an indication of bleeding in the brain -- are another way to diagnose the condition.

Once doctors know an aneurysm exists, the next step is usually surgery. Surgeons used to wait two weeks before operating to stabilize the patient, but new results of the Cooperative Aneurysm Study show that early surgery, like the kind performed on Biden, can produce "superior results," says Kassell. In the operation, surgeons "clip" the aneurysm or cut it out. While this can eliminate the possibility of the vessel's leaking more blood into the brain, physicians still have not been able to prevent a mysterious spasm of the artery that occurs in about 40 percent of those who undergo an aneurysm operation. The spasm is fatal about half the time and seems "to have something to do with the blood," says Caplan. Researchers believe that an enzyme or derivative of red blood cells may somehow spark the spasm.

"Surgery appears to be very effective in preventing bleeding," says Iowa's Adams. "But it does nothing with vasospasm and stroke. Those {problems} continue to haunt us, and they can be devastating."

Studies presented last week at the American Heart Association's 13th International Joint Conference on Stroke and Cerebral Circulation suggest that a class of drugs known as calcium channel blockers may help. Two calcium channel blockers -- nimodipine and nicardipine -- selectively work on blood vessels in the brain and seem "promising" in treating spasms, says Goldstein.

For those who can't be operated on -- either because of the location of the aneurysm or because of other medical complications -- researchers are testing an experimental laser surgery, which may be able to reach otherwise difficult areas of the brain. Still in the developmental phase, the laser surgery can "cut" through hard-to-reach areas of the brain without causing much damage. and A new balloon technique is also emerging as a way to treat inoperable aneurysms.

Dr. Jafar Jafar of the University of Illinois at Chicago College of Medicine and his colleagues have treated 14 otherwise inoperable patients with the balloon technique. In a procedure performed under local anesthesia, physicians block the artery that contains the aneurysm (rather than surgically removing it) and let other blood vessels adjust by carrying extra blood to and from the brain.

To place the balloon at the site of the aneurysm, doctors thread a catheter through the femoral artery in the groin up to the brain, while monitoring their progress on a fluoroscope and measuring the blood flow in the brain. If the blood flow is adequate, the balloon is inflated, detached and left in place at the site of the aneurysm. Two special coils are also inserted to keep the balloon from moving should it deflate.

With all these developments, the future for treatment, says Kassell, "looks optimistic."