EDITOR'S NOTE - Strokes caused by blood clots or aneurysms in the brain hit more than 400,000 strokes Americans a year. They kill 40 percent of their victims outright and often condemn the others to years of struggle, as Science Writer Warren E. Leary reports in the final installment of a four-part series on sudden killers.

WASHINGTON (AP) _ Without warning, they lose their lives. Or part of their lives.

They are the victims of stroke, more than 400,000 of them a year. Nearly 160,000 victims die every year, and those who survive can be condemned to paralysis, depression and a seemingly endless struggle to regain a part of themselves that disappeared when a million brain cells died.

A recent victim is William Schroeder, the Indiana man who successfully received an artificial heart in Louisville, Ky., last Nov. 25. He was recovering so well from the surgery that doctors had hoped to send him home for Christmas. But Dec. 13, he suffered a series of unexpected strokes from which he has not yet recovered.

The strokes left Schroeder with amnesia about events since Dec. 13, and he cannot retain much new information.

As with Schroeder, strokes usually seem to come out of the blue. But only recently, experts realized that what used to be considered symptoms of aging may warn of an impending stroke days or weeks ahead of time, giving a chance for preventive treatment.

Stroke is the third leading cause of death in the United States, right after heart disease and cancer. The condition hits primarily older people, but almost one-third of all stroke victims are younger than 65.

The damage is done by cutting blood supply to the brain, which is more dependent than other organs on a continuous flow. Brain cells die if deprived for even a few minutes of vital oxygen and nutrients from the blood.

Strokes occur in four forms:

-A clot forms in an artery carrying blood to a section of the brain and blocks the flow. These clots of blood or debris often occur in vessels damaged by buildup of fatty material inside their walls.

-A wandering clot travels until it wedges into an artery or smaller vessel and chokes off flow.

-A defective artery within the brain, or one damaged by a head injury, breaks and spills blood into surrounding tissues. The blood loss deprives cells of oxygen, while the buildup of blood outside the vessel can put pressure on surrounding brain tissue, interfering with its function.

-A weak spot in an artery wall balloons out into an aneurysm, a condition associated with high blood pressure. If one of these balloons breaks inside the brain, the hemorrhaging results in a stroke.

Rather than killing suddenly, a stroke is likely to incapacitate its victim for a time, making him vulnerable to death from complications or a later stroke.

''It is uncommon that a stroke will cause sudden death, which is when the person is dead within 30 minutes,'' said Dr. Oscar Reinmuth, professor of neurology at the University of Pittsburgh.''Less than one in 100 stroke events kill in that time span.''

When sudden death does accompany a stroke, it most likely results from cardiac arrest or other heart problems that result from its reaction to bleeding in the brain, said Reinmuth, past chairman of the American Heart Association's Stroke Council.

Dr. Murray Goldstein of the National Institutes of Health said the rates of stroke and deaths from stroke have been dropping for more than 20 years and took a sudden shift downward in the 1970s. Stroke deaths declined 40 percent in the last two decades.

Reasons for the decline are unclear, he said, but experts suspect better diagnosis and treatment, as well as control of such risk factors as high blood pressure.

In many instances of what appears to have been a sudden stroke, there were clues to the impending event that either went unnoticed or unrecognized. Had these hints been detected, it might have been possible to prevent or delay the result.

Goldstein, director of the National Institute of Neurological and Communicative Disorders and Stroke, said common warning signals included sudden and temporary weakness in the face or limbs on one side of the body, temporary loss of intelligible speech and unexplained dizziness and falls.

Many strokes are preceded by days or weeks by ''little strokes'' that cause some of the warning signs. Recent recognition of the importance of these events, called transient ischemic attacks or TIAs, is an important development in understanding and, possibly, warding off strokes, he said.

''We now recognize that TIAs are a medical emergency and, in a reasonable number of cases, are treatable,'' Goldstein said. ''Everyone use to think these kind of things - lightheadedness, numb hands and such - were normal in aging and accepted them. We know better now.''

Using new diagnostic tools, such as brain scans, doctors can look for evidence of damage from TIAs and hope to spot problems that could lead to a full-blown stroke, such as arteries that are clogged or ballooning out at a weak point.

More doctors are using new surgical techniques to remove obstructions from arteries or repair weakened parts of vessel walls. While data still is coming in on the effectiveness of these interventions in preventing strokes, some initial results look promising.

Other preventive approaches include lowering high blood pressure through diet or drugs and putting patients on medicines to reduce blood clotting, including daily regimens of aspirin.

Goldstein said there was little evidence that people who had not had ''little strokes'' or shown other signs of problems could do much to prevent stroke, other than controlling their blood pressure.

While lifestyle changes such as reducing weight and dietary fats, quitting smoking and getting exercise seem to help reduce risks of cardiovascular disease, their usefulness in preventing stroke is questionable, he said.

''In many ways, the factors leading to strokes are not like those of heart disease,'' Goldstein said.

''Blood vessels in the brain are different from vessels anywhere else in the body,'' he continued. ''They arise from different embryotic tissue and differ in metabolism and other things. We have a lot more to learn.''

Reinmuth, however, was more optimistic about the chances of reducing stroke risk through lifestyle changes, even without hard evidence.

''Do we have to wait for a controlled study to say what is reasonable?'' he asked.

Reinmuth thinks stress may be a big factor in strokes that has not received enough attention. Stress in general has been linked to strokes, he said, but all types of stress are not alike.

''It's hard to study stress because it's hard to measure,'' he said. ''But I can't help but think there may be a particular kind of stress that plays a role in stroke. And perhaps amelioration of that stress could be a big factor in cutting stroke.''