The vote to repeal Obamacare and replace it with Trumpcare was initially stalled following Sen. John McCain's unexpected surgery to remove a blood clot. The news of McCain's surgery became more serious as it was revealed the Senator from Arizona had a procedure called a craniotomy.

According to a statement from the senator’s office, a 5-centimeter blood clot was removed “from above his left eye.” Surgeons at Mayo Clinic Hospital in Phoenix performed “a minimally invasive craniotomy with an eyebrow incision.”

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This all happened after McCain went in for a routine annual physical, his office said.

What Is The Definition of Craniotomy?

A craniotomy has the sound of a scary procedure to remove your skull, but it actually involves taking out a small piece of the bone so that doctors can access the brain, Johns Hopkins explains. That portion of the bone, although moved out of the way, remains attached and is replaced afterward — this distinguishes it from a craniectomy, in which the piece of the skull that is taken out is fully removed, and can be replaced at a later date if at all. The section of bone that is moved aside in a craniotomy is called a bone flap.

To get at the bone, the surgeons will also create a skin flap, to move that upper layer out of the way, but will replace it afterward as well.

The surgery could be used to remove a tumor or blood clot, among other applications. It is sometimes used to treat an aneurysm, which is a bulging and weakened section of artery wall that could be fatal if it bursts.

In the case of McCain, his office noted that the craniotomy involved an incision above his eyebrow. According to Johns Hopkins, this is officially called a “supra-orbital craniotomy,” with the word “orbital” referring to the cavity in the skull that contains the eyeball, the orbit — although most people know it as an eye socket. When doctors use this type of incision to access the brain, they are often removing a tumor, and are taking this route because “a tumor is very large or close to the optic nerves or vital arteries.” It can also be used to remove a cyst.

McCain’s staff said he was having a blood clot removed, but there’s a chance that could indicate something deeper. The senator has the skin cancer melanoma in his medical history, including a surgery several years ago to remove a tumor. Although doctors don’t often find the underlying cancer while investigating the source of blood clots, “cancer can make blood clot more easily,” a Harvard Health blog post explains. Doctors might look for indications of cancer in McCain’s clot.

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Indeed, his office said that “tissue pathology reports are pending within the next several days.”

A blood clot could also be the result of a stroke or a head injury, something that isn’t a long-term health issue.

“He’s an 80-year-old man,” Dr. Steven Kalkanis, chairman of the department of neurosurgery at Henry Ford Health System, told Time. “It doesn’t mean his health is in jeopardy.”

For now, McCain’s office said the surgery went well.

“The senator is resting comfortably at home and is in good condition. … Once the pathology information is available, further care will be discussed between doctors and the family.”