There are both right and left transverse venous sinuses. "If one is blocked by thrombosis, the other one can take over as a detour," he said. "Thrombosis of the other nonduplicated sinuses -- the superior saggital or straight sinuses - is usually much more dangerous."
Dr. Keith Black, head of neurosurgery Cedars Sinai Medical Center in Los Angeles, said he was "surprised" by Clinton's diagnosis. "If I were her treating physician, you would have to ask, why did she develop a thrombosis. A very unusual event following a concussion without a skull fracture."
He speculated that Clinton may have had a "predisposition to clot formation." She had a deep vein thrombosis, a clot in the leg, in 1998.
"It is likely she will be evaluated for a clotting abnormality," he said.
Treatment of this condition consists of reversing the cause when known, controlling seizures if present, controlling the blood pressure in the head, and anticoagulation. Anticoagulation is the most essential.
The doctors all agreed her prognosis was good, providing the clot does not expand, a likelihood of about 1 percent, according to Black. Over time the clot will either reabsorb or scar down.
A worst case scenarios would be that the vein would occlude and she could get a venous infarct or stroke in the temporal lobe, the area responsible for speech.
But Greenwald is convinced Clinton's excellent health care team "caught it early."
"Assuming it was a follow-up MRI, that probably would not have happened for regular people," he said. "I suspect a lot of other people have poorer outcomes. I would call her lucky."
Clinton and her doctors say that with regular monitoring, she will have a "full recovery."
"Knowing Hillary Clinton, she can do plenty of stuff from home and at her bedside," said Greenwald. "It's not a situation where you want to throw her on a long plane ride to the Middle East, though. She will need to be followed closely medically. But she'll be back to herself in no time."