Explaining the medicine behind John McCain's surgery

— -- The Republican push for a vote on a replacement health care plan has been delayed once again, this time because a key vote in the Senate is missing.

Sen. John McCain, R-Ariz., underwent surgery last week to remove a blood clot from above his left eye.

The Senator’s office released a statement on July 15 stating that the surgery came "following a routine annual physical," during which the clot was discovered.

A two-inch blood clot was removed in a "minimally invasive craniotomy with an eyebrow incision."

In this procedure, doctors cut the skin, remove a small piece of the skull and remove the blood clot in the front of the brain near the pituitary gland. They then replace the skull segment, may hold it in place with titanium plates and sew the patient up at the eyebrow hairline to leave the smallest scar possible.This approach is less painful than an open craniotomy and has faster recovery.

What kind of clot did McCain have?

That’s the unanswered question. Craniotomies are used on clots that range from acute and serious to chronic and benign.

The brain has a tough membrane around it called the dura.

If the clot is inside the dura, between the membrane and the surface of the brain, that’s a subdural hematoma, which can be chronic and slow growing. This kind of clot is relatively common in older patients and happens more frequently to those on blood thinners including aspirin and Plavix. It is not publicly known if McCain, 80, is on any such medication.

If the clot is between the skull and the dura, it’s putting pressure on the brain from the outside and can quickly progress and compress the brain. This type of bleed would be an emergency and unlikely here, since sources say the diagnosis was made after a routine examination. Spontaneous bleeds can also occur inside the spongy tissue coating the brain -- a subarachnoid bleed -- or within the brain tissue itself.

Dr. Amir Dehdashti, director of cerebrovascular neurosurgery research at North Shore University Hospital and associate professor of neurosurgery at Hofstra North Shore-LIJ School of Medicine told ABC News the circumstances surrounding the surgery indicate the most promising scenario.

He said the bleed was most likely in the frontal lobe based on the approach and that's "a good place because it can expand and not cause many neurologic symptoms."

What could have caused the clot?

Clots in the brain occur when blood from a vein or an artery pools and thickens. Arterial bleeds are fast and can quickly compress the tissues around it, while venous bleeds can be slow and more chronic.

Bleeds can occur from trauma, like a blow to the head or whiplash, or from non-traumatic causes. High blood pressure, aneurysm rupture, blood thinners or tumors can cause those bleeds.

It is fairly uncommon for brain bleeds to be diagnosed at a routine check up; they’re more commonly diagnosed in an emergency room or when a patient visits their doctor with a specific complaint.

"If the bleed was spontaneous and was found at a routine exam it is reassuring," Dehdashti told ABC News. "If it was not spontaneous, then the prognosis depends on the underlying cause, for example if its related to a tumor."

Doctors have not publicly disclosed the type of clot that McCain had, and that distinction could make a significant difference when it comes to recovery times.

The statement released by McCain’s office states the senator is “resting comfortably at home and is in good condition.”

McCain's medical history

The former Republican presidential candidate had two malignant melanomas removed in 2000 – skin cancers taken from his left temple and left upper arm.

Melanoma is aggressive, and can spread to the brain, which may be why McCain’s doctors scanned him even if he hadn’t had symptoms.

"I think the one possibility that’s of concern is that melanomas are known to go to the brain and they can bleed,” Dr. Philip E. Stieg, the chairman of neurological surgery at Weill Cornell Medicine, told The New York Times.

Recovery time

There is no official word from his office on how long McCain will be recovering from the craniotomy. A family friend told ABC that he had heard McCain would need three weeks to recover from the surgery and some on Capitol Hill have indicated that they had planned to delay Senate proceedings, like the vote on the revised health care bill, for one week. Some medical experts say an 80-year-old would likely need at least two to three weeks for recovery.

"Best scenario is that there is no underlying cause and it is removed and he is recovering at home with a few days to a week or two for recovery," Dehdashti said. "Within two weeks he should be functional, not necessarily in the office, but could discuss important issues."

However long it may take, based on the limited information released about the surgery and his condition, McCain seems to have a good prognosis.

"Overall the likelihood of him improving with what has been done -— in the sense that he gets back to his prediagnosis functional -- is high given very limited info," said Dehdashti.