Massachusetts Sen. Edward Kennedy remained in the hospital Monday, awaiting test results that could explain why the 76-year-old Democrat suffered a seizure over the weekend.
An aide told NBC News that Kennedy would undergo more tests on Monday. It was unclear whether the results would be available Monday or on Tuesday.
On Sunday, Kennedy was described as resting while watching the Boston Red Sox and Boston Celtics games on television.
Kennedy's wife, Vicki, was seen walking into the hospital early Sunday morning. The aide said his daughter, stepdaughters and sister Jean Kennedy Smith also visited during the day.
It was a smaller crowd than on Saturday, when members of his family, including Kennedy's sons and niece Caroline Kennedy, and Sen. John Kerry went to the hospital.
'Not in any immediate danger'
Kennedy was flown Saturday morning to Massachusetts General from the emergency room of Cape Cod Hospital, near his home in Hyannisport. Doctors originally suspected he had suffered a stroke, but his physician, Dr. Larry Ronan, later said it had been a seizure and that the senator was "not in any immediate danger."
In October, Kennedy had surgery to remove a blockage in his left carotid artery, which supplies blood to the face and brain. After the surgery, the 76-year-old resumed his busy schedule on Capitol Hill and across Massachusetts.
The doctor who operated on Kennedy said at the time that surgery is reserved for those with more than 70 percent blockage, and Kennedy had "a very high-grade blockage."
Distinguishing between a seizure and a transient ischemic attack, TIA, often called a mini-stroke, can sometimes be difficult.
Seizures are little electrical storms in the brain. They tend to be brief; an occasional one can happen to anyone even without a prior history of seizures, especially if there has been some prior brain trauma.
A stroke is either ischemic — a clog in a blood vessel — or hemorrhagic, bleeding in the brain. Kennedy had the carotid artery surgery to try to prevent the ischemic type.
A stroke kills brain tissue; how much depends on how big it is and how long it lasts.
‘At high risk’
"Sen. Kennedy was at high risk because he had surgery for an artery in his neck," said Dr. Wendy Wright, an assistant professor at The Emory Clinic, Departments of Neurology and Neurosurgery.
But she said there are a lot of things that can cause seizures, such as an infection or medication.
"Certain medications are known to cause seizures. A stroke can cause a seizure, a brain tumor or a head injury, or something in the brain itself," Wright said. "Common symptoms that we know about are falling on the ground, shaking and having confusion."
Kennedy is the second-longest serving member of the Senate and a dominant figure in national Democratic Party politics. He was elected in 1962, filling out the term won by his brother, John F. Kennedy, who was elected president in 1960.
His eldest brother, Joseph, was killed in a World War II airplane crash, while President Kennedy was assassinated in 1963 and a third brother, Robert, was assassinated while campaigning for president in 1968.
Kennedy is active for his age, maintaining an aggressive schedule.
He has been vocal in both his opposition to the Iraq war and support for Democratic presidential contender Barack Obama, who is trying to become the first senator elected to the White House since John F. Kennedy.
Kennedy made several campaign appearances for the Illinois senator in February, and most recently another in April.
Recently, he and Caroline Kennedy awarded the annual "Profiles in Courage" award commemorating President Kennedy. And Friday, he attended a ribbon cutting at the New Bedford Whaling National Historical Park.
He was preparing to host the annual Best Buddies Challenge event on Saturday afternoon, a fundraiser for the Best Buddies organization founded by Anthony Kennedy Shriver that helps people with intellectual disabilities.
The event attracted celebrities, including New England Patriots quarterback Tom Brady and Olympian Carl Lewis.