“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 hematoma, a collection of blood between the dura and the brain.
“You would hope it’s a subdural, a relatively benign process,” Dr. Langer said. “It’s common in the elderly, especially if they’re on blood thinners. It can occur from relatively minor head injuries. The elderly brain loses volume, and as it retracts, the bridging veins from the brain to the dura are under increasing tension, and minor trauma can cause them to ooze or leak.”
The senator’s staff has not disclosed whether he takes a blood thinner.
Such hematomas can develop over weeks and months with subtle symptoms if they press on the brain, or even no symptoms, and removing them is usually an elective procedure, not an emergency, he said.
He said the operation is relatively straightforward. Hematomas often have an oily consistency and are easily drained once the skull is opened. The piece of skull that is removed for the procedure is then put back in place and fastened with titanium plates.
“He would be able to return to being a senator in a relatively short period of time with no ill effects,” Dr. Langer said. “This is an assumption. But it sounds like something not life-threatening or even a career-threatening problem.”
Dr. Philip E. Stieg, the chairman of neurological surgery at Weill Cornell Medicine and the neurosurgeon in chief at NewYork-Presbyterian, said it seemed a good sign that Mr. McCain was able to go home so quickly.
“I think the one possibility that’s of concern is that melanomas are known to go to the brain and they can bleed,” Dr. Stieg said. “They’ll have to wait for the pathology to come back. The good news is that five centimeters is a sizable blood clot, but in the frontal lobe, it should be well tolerated and hopefully he won’t have any neurologic deficits.”