UPDATE:  Bill Paxton's death certificate states the actor died last month from a stroke days after surgery to replace a heart valve and repair damage to his aorta, the Associated Press wrote on Monday. 

The actor Bill Paxton is said to have suffered a fatal stroke at age 61 while undergoing heart surgery, apparently to correct a damaged valve.

Details are scarce, but in an otherwise healthy man of that age, the risk of stroke during heart-valve surgery is less than 1 percent, according to a risk calculator from the Society for Thoracic Surgeons.

Paxton, whose death was announced Sunday in a statement from family representatives, said earlier this year that he had suffered valve damage from a case of rheumatic fever as a child.

Rheumatic fever can damage the heart's mitral valve, and less commonly the aortic valve, said Kimberly Campbell, a partner with Cardiology Consultants of Philadelphia.

This type of fever has become less common due to antibiotic therapy that is administered for strep throat, Campbell said. If strep isn't properly treated, it can lead to rheumatic fever.

Heart surgery is such a well-established field that many procedures, such as bypass and valve operations, are very low risk. But the chance of a stroke increases for patients with additional health problems, advanced age, and other factors, said Michael A. Acker, chief of cardiovascular surgery for Penn Medicine, the University of Pennsylvania's health system.
In the overall population of patients undergoing valve surgery, the risk of a permanent stroke is 6.6 percent, Campbell said.
The society defines permanent stroke this way: "any confirmed neurological deficit of abrupt onset caused by a disturbance in blood supply to the brain that did not resolve within 24 hours."
When rheumatic fever is severe enough to cause valve damage, the leaflets in the mitral valve can become stiff with scar tissue and are sometimes calcified, said Penn's Acker. That prevents the valve from opening easily, impairing the flow of oxygenated blood from the lungs, he said.
Mitral-valve surgery can involve a replacement or a repair, Acker said. In less severe cases, another option is a balloon procedure to open up the valve.

Paxton was known for his roles in Titanic, Aliens, and Apollo 13, among other popular films. Alongside Emily Blunt and Tom Cruise, he played a "Southern-fried sergeant" in 2014's Edge of Tomorrow.