Kevin Lamarque / Reuters U.S. Secretary of State Hillary Clinton delivers a speech in Dublin on Dec. 6, 2012

UPDATED: Dec. 31, 2012, 7 p.m.

Hillary Clinton’s doctors released a statement announcing that the Secretary of State experienced a blood clot “between the brain and the skull behind the right ear.” In such cases, says Dr. Geoff Manley, professor of neurological surgery at the University of California, San Francisco, doctors turn to blood-thinning medications to ensure that blood flow out of the brain is not interrupted; such obstruction can cause permanent swelling and damage to brain tissue.

Secretary of State Hillary Clinton was hospitalized Sunday night for a blood clot that a State Department press release says is related to a concussion she suffered in early December. While the statement does not specify how life threatening the clot may be, the treatment she is receiving — blood-thinning drugs known as anticoagulants — provides some clues.

In general, following a concussion, during which the brain is rattled inside the skull, tiny blood vessels crisscrossing the surface of the brain can rupture and bleed, leading to clots as the body attempts to stem the flow of blood. Vessels that are nestled in the folds of brain tissue can also tear during a blow to the head and lead to clotting. But in both of these cases, says Dr. Lee Schwamm, vice chairman of the Department of Neurology and director of the Stroke Services at Massachusetts General Hospital, blood thinners would be the last treatment doctors would recommend since the drugs might increase the risk of additional bleeding and swelling in the brain.

That’s not the case if the clot occurs in the space between the brain and the skull, says David Hovda, director of the UCLA Brain Injury Research Center. In those circumstances, doctors would be more concerned about ensuring that any clogged vessels are unobstructed so blood that has circulated through the brain and is on its way back to the heart is flowing freely. In older patients, this space is slightly more expansive than that among younger people since the brain tends to shrink gradually with age.

Schwamm, who has not treated Clinton but has treated patients with concussions and clots, says there are two other, relatively rare scenarios in which the concussion itself might lead to a need for blood thinners. Clinton apparently hit her head after a fall while dehydrated from a stomach virus, and if her head were twisted on impact, small tears, or dissections, might have shredded the arteries that feed blood to the brain, leading to clots. Blockages in these arteries can prevent the brain from receiving the oxygen-rich blood it needs, leading to a stroke, so doctors often use blood-thinning drugs to prevent clots from growing. The other possibility, Schwamm says, is that the clot formed in the veins that drain blood from the brain, so the danger of having that flow interrupted and backing up in the brain would also require treatment with anticoagulants.

But these types of clots are relatively rare, affecting less than 1 in 100,000 patients hospitalized for clot-related problems. More likely, says Schwamm, is that the concussion led more indirectly to the clot. Patients recovering from such a fall often stick to a period of bed rest and immobility that can trigger a clot outside of the brain. “If there is postconcussive syndrome, you have persistent headaches, dizziness, nausea and you feel poorly, so people end up lying in bed, which is a setup for a clot developing in the leg,” says Schwamm.

Clinton apparently has a history of blood clots. She was treated for one that formed behind the right knee in 1998. Her extensive travel schedule — she is the most-traveled Secretary of State in U.S. history — may also put her at increased risk. Lengthy plane rides with limited mobility are known to increase the chances of clot formation, especially for those who may be predisposed to them, either due to their genetic makeup or medications they may be taking, such as oral contraceptives or hormone treatments that boost the risk of clots.

Clots that form anywhere in the body can be dangerous. After disintegrating, they send pieces of debris coursing through the circulatory system until they eventually reach small enough vessels, typically in the lungs, where they can block flow and cause an embolism. Depending on how large the clot is, the symptoms can range from shortness of breath to more severe pain in the chest.

That’s why it’s important for people who experience trauma to the head, including a fall, to seek immediate medical care, says Manley. “My prediction is that things are going be O.K. here [with Clinton],” he says. “But it’s hard to predict. To me, this highlights two ongoing issues, which is that we haven’t studied the problem of traumatic brain injury well enough to answer these questions. And people really need to be more aware of the seriousness of brain injury. We have made great progress with stroke, heart disease and cancer in making people aware that they need to seek out medical care. But they don’t always seek care if they fall or slip and hit their head.”

Blood-thinning drugs are the best treatment for counteracting the more serious consequences of clots since they essentially stop the blockages from getting any larger. That allows the body’s own clot-busting mechanisms to liquefy the obstruction, dissolving it away over time. “Her chances of embolism if she is on an anticoagulant are very, very low,” says Schwamm. “That treatment is quite effective.” According to the State Department, doctors plan to monitor Clinton for another 48 hours in the hospital.