Doctors treating Beau Biden, the 41-year-old son of Joe Biden, and Delaware's attorney general, have not yet revealed the underlying cause or the nature of the "mild stroke" that he experienced on Tuesday, but neurologists said that such strokes can arise from any of a number of underlying factors.
Dr. Timothy Gardner, medical director of the Center for Heart and Vascular Surgery at Christiana Care Health System and the doctor who treated Biden, said he likely will have a full recovery.
"[He is] fully alert, in stable condition and has full motor and speech skills," Gardner said in a statement.
Hospital officials said Biden was communicating with his wife and parents, who were with him. On Tuesday evening he was transferred to Thomas Jefferson University Hospital in Philadelphia for "further observation and examination," according to the vice president's office.
Biden's doctors have not yet revealed whether his stroke was hemorrhagic (due to a blood vessel bursting in his brain) or ischemic (brought about by a clot that blocked the blood supply to part of his brain). The distinction is important, as treatments for the two kinds of stroke differ greatly.
Still, some doctors say that even when more details are available, the exact cause of a stroke can be difficult to pinpoint.
ABC News senior health and medical editor Dr. Richard Besser spoke with Dr. Anthony Furlan, co-director of the Neurological Institute at Case Western Reserve University in Cleveland, Ohio. Furlan said that, indeed, in most cases of stroke in patients younger than 50, the exact cause remains a mystery.
"Often after a stroke, they will find a [heart-related cause] that was undetected," Furlan said.
One such heart problem is known as patent foramen ovale -- a small hole in the heart that allows clots to pass from one side of the heart to the other. Sometimes these clots can enter the brain and bring on a stroke.
Biden's Stroke Not Likely Related to Father's Aneurysm
Then there is the fact Joe Biden had two separate surgeries in 1988 to repair brain aneurysms, a bulging of the blood vessel that can lead to hemorrhagic stroke. However, doctors were quick to point out that this does not necessarily mean the vice president's son has the same ailment.
"If there were a strong suspicion that this was an aneurysm, it's unlikely the institution would label it a stroke," said Dr. Steven Gianotta, chairman of the department of neurological surgery at the USC Keck School of Medicine in Los Angeles.
"Despite a father with an aneurysm, the most likely cause of a stroke in a young person like this would be a dissection of either the carotid or vertebral artery," he said. "This is a spontaneous tear in the inner lining of the artery that can cause a partial or total blockage, or shower blood clots up to the brain."
Furlan, too, said that it is unlikely that the stroke had to do with an aneurysm.
"Based on the press reports, [there was] no evidence of bleeding," he said, adding that the stroke is "probably unrelated to his father's aneurysm."
Possible causes, Furlan said, would be high blood pressure, diabetes, high cholesterol or smoking status, all of which could lead to the artery-hardening condition atherosclerosis, which in turn makes stroke more likely.
Like Biden, Other Younger Adults Can Experience Stroke
Studies show that about 795,000 people suffer a new or recurrent stroke each year. About 610,000 of these are first attacks and 185,000 are recurrent attacks. About 137,000 Americans die from stroke each year, according to the Centers for Disease Control and Prevention, making it the third leading cause of death in the United States behind heart disease and cancer.
While the condition is most often associated with older people, young adults under 45, like Biden, are not immune to stroke.
"Many people are shocked and surprised to hear that someone who is 41 has had a stroke," said Dr. Carolyn Brockington, director of the stroke section at St. Luke's-Roosevelt Hospital in New York. "The fact that someone who is 41 has had a stroke is not unheard of."
Strokes Can Strike Younger People, Doctors Warn
Indeed, an estimated 10 percent to 15 percent of strokes -- most of them ischemic rather than hemorrhagic -- occur in those under 45, according to Dr. Aneesh Singhal, associate professor of neurology at Harvard Medical School, who practices at Massachusetts General Hospital in Boston.
Even when stroke does not kill, it can cause disabilities such as paralysis, speech and, especially among young people, emotional problems.
Because less is known about this disease among young people, the American Academy of Neurology has just received a grant to study stroke in that population, Singhal said.
"The risk is still very low, but young men and women need to be aware," said Singhal. "Stroke is the leading cause of disability worldwide. It can happen in the young, and one of the real problems is they don't recognize the symptoms and it is often overlooked in the emergency department when they visit."
Just last year, a Wayne State University study revealed that young adults arriving in hospital emergency rooms after a stroke are often misdiagnosed.
Researchers found eight of 57 stroke patients were incorrectly diagnosed with conditions including vertigo, migraine, alcohol intoxication, seizure, an inner ear disorder or other conditions.
The first hours are critical in treatment. In the case of an ischemic stroke, the most common among the young, doctors can administer the clot-busting drug tissue plasminogen activator, or tPA, the only U.S. government-approved treatment for acute stroke. It must be given within three hours of the onset of symptoms to reduce permanent disability.
It is as yet unclear whether Biden received tPA or not. Furlan said the clot-buster is rarely administered to patients experiencing a very mild stroke or whose stroke occurred more than four hours before treatment.
Determining Cause of Biden's Stroke Is Crucial, Doctors Say
Doctors said that now that Biden appears to be stable, however, the most important course of action is to determine the underlying cause of the stroke.
"If that problem has not been solved, then the risk of having another stroke is certainly present," Brockington said.
"The mild symptoms suggest that full effort will focus on rapidly identifying the cause and intervening to avoid a second event," said Dr. George Newman, chairman of the department of neurosensory sciences at Albert Einstein Medical Center in Philadelphia. "Rehab therapies will follow if needed."
Biden was elected as Delaware's attorney general in 2006. He recently served in Iraq for one year as a captain with the Delaware Army National Guard. He was considered a front-runner to fill his father's former U.S. Senate seat this November, but in January he announced he had decided against a run for higher office.