Why Clinton's clot didn't cause stroke

BUFFALO, N.Y. (WIVB) - Secretary of State Hillary Clinton has been released from the hospital after receiving treatment for a blood clot.

Last month Clinton was being treated for a concussion when doctors discovered a blood clot in her head. Why did the clot form and why did it not cause a stroke?

Most of the time, when we hear about a blocked blood vessel in the head, it's accompanied by serious brain damage - a stroke. But that's because the vast majority of blockages are in arteries that carry blood to the brain. The area of the brain that's deprived of blood may be destroyed.

But Clinton's blockage was in a vein, not an artery. Veins drain blood from the brain back toward the heart. The blood could be drained by a different route.

That's why a vein blockage might not be symptomatic. Those clots will typically be removed by the body's natural defense mechanisms. But there is a risk - if the blockage enlarges, it will cause swelling and will damage the brain.

Clinton's blockage was found in a scan that was done as a follow-up to her recovery from a concussion. She was treated with drugs called anti-coagulants to prevent the clot from enlarging and to keep any new ones from forming.

It takes a few days to determine that blood clotting has been altered correctly by the medication, and that's why she stayed in the hospital. She'll be on that medication for months to keep new blockages from forming while her body removes the old ones. That's standard treatment and we know it works.

It's very rare for that to happen as a result of a concussion. It's likely that she was dehydrated from her original stomach problem and the dehydration contributed to the clot forming. And some people have a tendency to clot - called hypercoagulability. She may be one of them, because she had a previous episode in her leg. If that's the case, that can be managed successfully, too.