Sen. John McCain had brain surgery on Friday to remove a blood clot that was above his left eye.

The surgery led the Senate to postpone a planned vote on the healthcare bill to allow the senator time to recover. McCain is recovering in Arizona this week, the office of the senator said in a statement.

But some experts think it might take more time for McCain to recover.

"Usually, a blood clot in this area would be a very concerning issue," Dr. Nrupen Baxi, a neurosurgery professor at the Albert Einstein College of Medicine in New York City told The New York Times. "The recovery time from a craniotomy is usually a few weeks."

Here's what we know

The Mayo Clinic in Arizona said in a statement that McCain had the procedure to remove the blood clot after a routine checkup. The surgeons removed the 5-centimeter blood clot by entering through his left eyebrow. The senator, who is 80, is back at home and "in good spirits," the Clinic's statement said.

Blood clots are instances where blood hardens into a solid. They can occur in veins around the body, and are more likely to form after an injury.

People with obesity, cancer, or liver or kidney disease are all at risk for blood clots, according to the U.S. National Library of Medicine.

The surgical procedure that McCain received to remove the blood clot is called a craniotomy. In order to get into the brain, surgeons have to open up the skull, often removing a piece of bone in the process.

Once the clot is removed, that bone is replaced and the head is stitched back up. Here's an illustration of what that looks like from the side of the head (not the same place where McCain had his surgery).

U.S. National Library of Medicine

What's still unknown

The cause of McCain's blood clot still isn't clear. Results of tests that could determine the clot's origin are pending, the Mayo Clinic said.

One possible cause could have to do with McCain's medical history. The senator has a history of melanoma, which is considered the most serious form of skin cancer. In 2000, McCain had a melanoma-related growth removed from his left temple (not far from where the blood clot was) according to Discover Magazine.

Other possible causes could have been a fall, a stroke, or changes in the brain that come with aging, the Times reported.

Once doctors figure out the cause of the clot, their conclusion could determine how serious the senator's condition is. But doctors not associated with McCain's case seem optimistic.

"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," Dr. Philip Stieg, the chairman of neurological surgery at Weill Cornell Medicine told The Times.

This article originally appeared in Business Insider.