The blood thinner given to Israeli Prime Minister Ariel Sharon after his mini-stroke may have backfired.

In Sharon's case, the medication might have contributed to the recent massive stroke he suffered, in what experts say is a classic illustration of this seeming paradox in stroke treatment.

After Sharon was stricken when a small clot traveled from his heart to his brain, doctors immediately put him on blood thinners, which may have been a factor in the far more devastating "bleeding" stroke he suffered.

The 77-year-old Israeli leader underwent five hours of surgery again to relieve a rise in pressure on his brain and more internal bleeding. He had been in a medically induced coma and on a respirator following several hours of brain surgery aimed at saving his life, according to hospital officials.

"The decision to put him on a blood thinner is a little questionable," said Dr. Stephan A. Mayer, an associate professor of neurology and neurosurgery at Columbia University Medical Center in New York City. "I don't know the details of this case, but the use of blood thinners to prevent ischemic strokes [those caused by clots] has been challenged by several studies. It is a curious decision."

When blood thinners are given, Mayer added, "they need to be regulated very closely. You want it thin, but not too thin."

But Dr. Vincent Marchello, vice president of medical affairs at Metropolitan Jewish Health System in New York City, said the standard treatment for the kind of stroke Sharon initially had is to thin the blood. The particular drug used, the anticoagulant enoxaparin, is given by injection and is considered safe, he added.

Using this or another blood thinner does decrease the chance of another ischemic stroke, but it doesn't address the issue of hemorrhagic stroke, which is much less common.

It was a hemorrhagic stroke -- or bleeding in his brain -- that Sharon suffered while being taken to the hospital from his ranch in southern Israel's Negev. He had then been scheduled for a heart operation at the hospital, Hadassah-Ein Kerem, to close a hole that had been discovered after his first stroke.

Sharon's situation was complicated by the small hole in his heart. "That basically sets up a chain of events that will allow a blood clot to form near the hole or in it and then be pushed into the brain by the prime minister's own heart beat," said Dr. Jeffrey Harris, director of the stroke program at Huntsville Hospital in Huntsville, Ala., and a spokesman for the American Heart Association.

Enoxaparin is a powerful blood thinner but it probably did not cause the second, massive stroke, Harris added.

"Most likely what happened is he had the second stroke and enoxaparin made it bigger," Harris said. "The usual cause of a bleeding stroke is not the presence of a blood thinner but problems with blood pressure." But any blood thinner would have had the same worsening effect, he noted.

To the experts, it appears Sharon was on the wrong end of a tricky risk-benefit calculation.

"We as stroke experts have to ask, what is the devil in front of us? What is the higher risk? Is it more likely for him to have another non-bleeding stroke or is he more likely to have a bleeding stroke," Harris said. "The odds were that he would have another non-bleeding stroke, especially with the hole in the heart."

Added Dr. Stanley Tuhrim, director of the Mount Sinai Stroke Center in New York City, "Anybody that has had a TIA [transient ischemic attack or mini stroke] or an ischemic stroke is at an increased risk for having another ischemic stroke relative to someone who hasn't had that kind of cerebral event. The overall risk is something like 10 to 15 percent in the first year after the initial event, but that risk can be reduced by the use of anti-thrombotic agents or anti-coagulants."

An ischemic stroke, which accounts for some 80 percent of all strokes, occurs when a blocked artery hinders blood flow and oxygen supply, creating a zone of damaged brain tissue.

That was the type of stroke TV personality Dick Clark suffered in December 2004. Quite often, a mini-stroke can signal the possibility of a full-blown stroke, which is why doctors often put patients on blood thinners to prevent a second stroke.

On the other hand, a hemorrhagic stroke, which is when a vessel ruptures, spilling blood into brain tissue, accounts for only about 15 percent of all strokes.

An intracerebral hemorrhage -- what Sharon suffered -- is the deadliest type of stroke, with 60 percent of patients dying within a year, experts say.

"These strokes tend to be much more serious because not only are you losing blood flow but there's also compression in the brain causing further damage," Marchello said.

"You're more likely to die and, if you live, you're left with a more severe deficit," Mayer added.

An estimated 700,000 strokes occur in the United States each year, making it the third leading cause of death and first leading cause of disability and suffering, according to the American Stroke Association.

Blood thinners can make matters worse. "When you're on a blood thinner, it's a dreadful problem," Mayer said. "You continue to bleed much more, and over a longer period of time."

Sharon was rushed into the operating room and underwent two sets of surgery to stanch the bleeding and reduce the pressure on his brain.

"If it was a massive stroke, this was a desperate attempt to save his life," Mayer said.

Although outside experts don't have enough information to truly evaluate Sharon's condition, Mayer said the prognosis is bleak.

"It's not looking good," he said.

"Dick Clark had a very mild ischemic stroke, on the complete opposite end of the spectrum," Mayer continued. "Sharon had the more serious type and, even worse, he was on a blood thinner and he was probably about as close to being dead when he went into the operating room as you can be."

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