“Usually, a blood clot in this area would be a very concerning issue,” said Dr. Nrupen Baxi, an assistant professor of neurosurgery at the Albert Einstein College of Medicine in New York City.
He added, “The recovery time from a craniotomy is usually a few weeks — at least a week or two.”
A statement from the Mayo Clinic Hospital said that the senator was recovering well and in good spirits at home, and that tissue pathology reports would come back in several days.
But many questions have been left unanswered, including whether Mr. McCain had symptoms that prompted doctors to look for the clot. In June, his somewhat confused questioning of James B. Comey, the former F.B.I. director, led to concerns about his mental status, which he later jokingly dismissed by saying he had stayed up too late watching baseball the night before.
“Usually, a blood clot like this is discovered when patients have symptoms, whether it’s a seizure or headaches or weakness or speech difficulties,” Dr. Baxi said. “Generally, it’s not found on a routine physical because doctors would not know to look for it.”
The cause of the clot has not been disclosed. The possibilities include a fall or a blow to the head, a stroke or certain brain changes associated with aging. Mr. McCain is 80.
He also has a history of melanoma, an aggressive skin cancer that can spread to the brain and cause bleeding. That cancer history could have prompted Mr. McCain’s doctors to scan his brain even in the absence of symptoms, some doctors said. The pending pathology reports are expected to help explain what caused the bleeding.
The clot could have been in one of several locations: between the skull and the dura, the membrane that covers the brain; between the dura and surface of the brain; or inside the brain itself.
Dr. David J. Langer, the chairman of neurosurgery at Lenox Hill Hospital in Manhattan, said a likely diagnosis was a subdural…