The condition can cause permanent brain damage, coma or death if not detected and treated in time. In Clinton's case, however, it appears to have caused few if any symptoms and to be responding to treatment with blood-thinning drugs.
"In all other aspects of her recovery, the secretary is making excellent progress and we are confident she will make a full recovery," her physicians, Lisa Bardack of the Mount Kisco (N.Y.) Medical Group and Gigi El-Bayoumi of George Washington University, said in the statement. "She is in good spirits, engaging with her doctors, her family, and her staff."
Almost three weeks ago, Clinton, who is 65, reportedly became severely dehydrated with an intestinal infection. She fainted, fell and hit her head, suffering a concussion. Whether those events are related to the blood clot is unknown, with experts offering divergent opinions.
Cerebral venous thrombosis the general term for Clinton's condition is rare. It occurs in about four out of every million adults per year. It is somewhat more common in children. Head trauma, pregnancy, cancer, brain infection, autoimmune diseases and inborn clotting abnormalities are all predisposing factors. Some experts believe severe dehydration may be a factor as well.
In 30 to 40 percent of cases, however, no cause is found.
Clinton has had at least one previous blood clot, in her right leg in 1998. She was treated with blood-thinning drugs for several months. In her memoir she attributed the clot to "my nonstop flying around the country." Airplane flights lasting longer than six hours appear to be a slight risk factor for developing leg clots, known as deep venous thromboses (DVTs).
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 Dumont, director of cerebrovascular surgery at Thomas Jefferson University in Philadelphia.
"It's probably a coincidence," he said of Clinton's fainting spell and the clot. He noted, however, that her history of a blood clot in the leg may indicate a predisposition to clotting.
But Gregory Piazza, a cardiologist and vascular medicine specialist at Brigham and Women's Hospital in Boston, said he thinks a fall serious enough to cause a concussion could have caused the clot. It is also possible that a clot could have caused the fall. "Either could be a possibility in this case," he said.
Whether Clinton got an MRI scan after her fall 21/2 weeks ago, and what it found, hasn't been revealed.
The conventional treatment is an anticoagulant drug for at least six months. With Clinton's history of a previous clot, however, it could be longer.
The usual drug is warfarin, which requires that the blood be tested periodically to make sure it is not excessively thinned. If it is, that problem can be reversed.
There are newer oral anticoagulants that don't require monitoring, but they are not easily reversed.
"They probably wouldn't try a brand-new drug that we have limited experience with," Piazza said. "They'll probably be conservative."
As a result of her fall and concussion, Clinton did not testify before Congress on Dec. 20 about the Sept. 11 attack on a U.S. diplomatic compound in Benghazi, Libya. She also did not appear at the White House on Dec. 21, when President Barack Obama introduced Sen. John Kerry, D-Mass., as his nominee to succeed her.
The medical setback could affect Clinton's plans to testify to Congress on the Libya attack, which she had rescheduled for January, or even the timing of Kerry's confirmation hearing. Speeding it up could foreclose the promised Benghazi testimony.
A State Department official who spoke on the condition of anonymity to discuss Clinton's medical status in greater detail said she is unlikely to return to Washington this week, as once planned, and is unlikely to travel overseas again as secretary. The long flights and sleep deprivation common to her travel schedule would be too difficult because of her new medical restrictions, the official said.
Clinton remains committed to testifying about the Benghazi report, assuming the House and Senate committees can schedule the sessions, the official said. The congressional schedule is in flux because of the "fiscal cliff" negotiations, and one or both houses may remain out of session nearly until Inauguration Day. That would make Clinton's testimony a tight fit, but possible, the official said.
The official said Clinton has been making official phone calls and is generally feeling well, if a bit cooped up. The fall Clinton suffered more than two weeks ago was not more serious than was first thought, the official said.