Head trauma can cause blood in a venous sinus to clot, but it almost always has to be severe enough to cause a skull fracture, said Aaron S. Dumont, director of cerebrovascular surgery at Thomas Jefferson University in Philadelphia.
Its probably a coincidence, he said of Clintons fainting spell and the clot. He noted, however, that her history of a blood clot in the leg may indicate a predisposition to clotting.