Only days after stroke-stricken Dick Clark resumed his role as king of New Year's Eve, Israeli Prime Minister Ariel Sharon suffered a devastating stroke just weeks after his first.

Each suffered very different strokes, with very different causes.

How can stroke mean so many different things? Why do different strokes have such different outcomes? For answers to these and other stroke questions, WebMD turned to Keith A. Siller, MD. Siller is medical director of the NYU Comprehensive Stroke Care Center and assistant professor of psychiatry and neurology at the NYU School of Medicine.

What Is a Stroke? Get the Facts

Ischemic Strokes

Strokes happen when part of the brain loses its blood supply. This can happen in two major ways.

More than 80 percent of strokes occur when a blood vessel (an artery) in the brain or neck gets blocked by a blood clot. This is called an ischemic stroke.

"Typical strokes are ischemic and are due to clogged arteries," Siller tells WebMD.

It's not clear which kind of stroke Dick Clark had. But Clark has made public the fact that he has diabetes. People with diabetes tend to have atherosclerosis — clogged arteries — so Siller guesses that it is likely Clark suffered an ischemic stroke.

Doctors further classify ischemic strokes by where the blood clot originated. When a clot forms in an artery, stays in place, and blocks the artery, it's called a thrombus — and a person has a thrombotic stroke. More than half of ischemic strokes are thrombotic strokes.

"Another mechanism of ischemic stroke is a clot arising from the heart: an embolism," Siller says. "That is a clot from one part of the body going to another."

When these traveling clots wedge into a blood vessel leading to the brain, a person has an embolic stroke.

Hemorrhagic Strokes

Strokes also occur when an artery supplying blood to the brain bursts. These are hemorrhagic strokes — the other major kind of stroke.

Hemorrhagic strokes are also further classified by where they originate. When a hemorrhagic stroke happens inside the brain, it's called an intracerebral hemorrhage. When it happens in the subarachnoid space (between the brain and a lining surrounding it), it's called a subarachnoid hemorrhage.

The usual cause of intracerebral hemorrhage is high blood pressure. Another cause is an aneurysm — a weak spot in the wall of the artery that balloons out. When these balloons pop, a person has a stroke. This is the usual cause of subarachnoid aneurysms.

"Hemorrhagic stroke can be just as devastating as ischemic stroke," Siller says. "And there are other types of bleeding that affect the brain. Subdural hematomas and epidural hematomas are bleeding just outside the brain. Usually caused by a head injury, these are technically not considered strokes. But they do compress the brain and can cause similar disabilities."

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How Bad Is a Stroke?

Obviously, a stroke is never a good thing. But some are worse than others. Severity depends on two things: the part of the brain affected and the size of the affected brain area.

"The first distinction in stroke damage is the side of the brain affected," Siller says. "The left side is the dominant hemisphere, related to language function for most people. One of the things people fear the most is a stroke that leaves them unable to speak and to understand speech. When the stroke is on the right, usually patients are able to have normal conversations. There may be more subtle cognitive problems not immediately obvious, but usually right-side stroke patients can have a better recovery."

Sharon's stroke, Siller says, probably was quite large as it required many hours of surgery to drain the blood from his brain. And doctors did not announce which side of the brain was affected.

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Hole in Sharon's Heart

According to announcements by Sharon's doctors, the Israeli leader first suffered an ischemic stroke in mid-December. While looking for the cause of this fairly mild stroke, doctors found that Sharon had a tiny hole in his heart.

That sounds bad. But Siller says that one in four people has the same thing.

"There is naturally a hole in the heart while you are developing. Usually, at birth, it closes," Siller says. "It is called PFO: patent foramen ovale. About 25 percent of the population has that without any symptoms involved."

But if a person with a PFO has a stroke — and no other obvious risk for stroke — doctors often assume that the hole is allowing tiny blood clots to pass into the circulation and causing stroke. It's a very controversial area, Siller says, as there's no definitive proof that this really happens.

"We have the technology to close the hole, but also the controversy over whether to do it," Siller says.

Once a PFO is suspected of causing a stroke, doctors usually put the patient on blood thinners to prevent new clots from forming. Blood thinners (such as Coumadin) increase a person's risk of hemorrhagic stroke. So doctors sometimes decide to repair the hole in the heart so that a patient can stop taking these medicines.

"For closing the hole, you thread a catheter through the right side of the heart," Siller says. "And at the end of the catheter is a collapsed umbrella like in a cocktail drink. The catheter is inserted into the hole itself, and the umbrella is deployed. That is a permanent closure of the hole that was there since birth."

That's what Sharon's doctors were planning to do.

"The sad thing in Sharon's case is they went with the most conventional treatment, and everything was done properly," Siller says. "But unfortunately it was the blood thinner that probably created the new stroke."

Siller is quick to add that blood-thinning medications save people's lives. Though these important drugs can raise a person's hemorrhagic stroke risk, they also can cut ischemic stroke risk. And the drugs are an important part of treatment for atrial fibrillation, a life-threatening heart rhythm problem.

"People shouldn't walk away thinking, 'I don't want to take my Coumadin,'" Siller says.

Visit WebMD's Stroke Health Center

Best Treatment for Stroke

What's the best treatment for stroke? Siller is emphatic about stopping stroke before it happens.

"The real focus should be on prevention," he says. "People need to know their blood pressure, their cholesterol level, and whether they are prone to diabetes. People unfortunately often find out about these risks after having a stroke or heart attack. The treatments we give to prevent stroke are far more effective than the ones we give after a stroke. Being more active physically is far more effective at preventing stroke than blood thinners or fixing a hole in the heart."

Siller recommends that anyone worried about stroke should see their doctor, get evaluated for stroke risk, and make the needed changes in their lifestyle.

However, if a person has one of the five classic warning signs of stroke, they should immediately go to an emergency room. These warning signs are:

—Feeling weak, numb, or paralyzed in the face, arm, or leg

—Being unable to speak or understand simple phrases

—Blurry vision or loss of vision in one or both eyes

—Sudden, intense headache

—Dizziness or loss of balance or coordination — especially when another warning sign is present

Siller advises all stroke survivors to see a neurologist for a full evaluation and individualized treatment.

And though stroke is certainly life threatening, Dick Clark and others remind us that survival is the rule, not the exception.

"The majority of people do survive strokes. And the majority of survivors get through it and regain much function," Siller says.

Of course, recovering from stroke means hard work. It's called rehabilitation — rehab for short.

"Rehab is considered an expected part of stroke recovery," Siller says. "It is tailored based on what is needed."

There are four basic kinds of rehab:

—Speech therapy helps those with trouble talking.

—Physical therapy means getting the limbs moving, increasing power, and relearning how to walk.

—Occupational therapy is based on relearning the skills needed for daily care — for example, going up the stairs, putting one's clothes on, and using dining utensils.

—Cognitive therapy helps people who have lost the ability to speak or understand language.

"Most people benefit from at least some kind of rehabilitation therapy following stroke," Siller says.

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Keith A. Siller, MD, medical director, NYU comprehensive stroke care center, and assistant professor of psychiatry and neurology at the NYU School of Medicine. The National Stroke Association web site.