Q&A: What Happened to the Senator's Brain? Sen. Tim Johnson's (D-SD) emergency surgery was an attempt to fix a condition known as arteriovenous malformation. In the defect, called AVM, arteries end up delivering their fast-flowing blood directly into veins, instead of into tissue. A look at the condition's risks, treatment options and recovery expectations.

Q&A: What Happened to the Senator's Brain?

Q&A: What Happened to the Senator's Brain?

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An illustration depicting an arteriovenous malformation, or AVM, an abnormal collection of blood vessels. Normally, red oxygenated blood is pumped by the heart through arteries to the tissues, and blue, deoxygenated blood then passes back to the heart through the veins. In an AVM, the artery connects to a vein, preventing blood from reaching the tissue but also putting pressure on veins. Mediscan/Corbis hide caption

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Sen. Tim Johnson (D-SD) is recovering after surgery Thursday night to correct circulation problems in his brain, according to Adm. John Eisold, the U.S. Capitol physician.

Johnson's health has repercussions for control of the U.S. Senate, where Democrats are expected to take control in January with only a one-vote majority. If Johnson has to leave his Senate seat, South Dakota's Republican governor would appoint his replacement, likely a Republican, which would tip the Senate in the Republican Party's favor.

Doctors at George Washington University Hospital in Washington, D.C., where the surgery was performed, say Johnson suffered from a condition called arteriovenous malformation. Here, a look at the condition, how it's treated, and what recovery expectations are:

What is arteriovenous malformation (AVM)?

An AVM is a cluster of abnormally formed blood vessels. In medical images, it looks like a tangle of arteries and veins. About 300,000 people in the U.S. have these malformations, but most AVMs never cause any symptoms. The malformations can occur in various places around the body, however, those in the brain or spinal cord can cause the most widespread damage, because they affect the central nervous system.

AVMs disrupt the normal system used to provide oxygen to the brain. Ordinarily, arteries deliver oxygenated blood to the brain and veins return it to the heart and lungs. But in an AVM, blood that should be in an artery can flow through a vein. When that happens, part of the brain may not get enough oxygen. Also, veins are not meant to handle the high pressures and fast blood flow of arteries. So they may expand or even rupture, causing bleeding in the brain.

How do you treat it?

There are several different approaches. The idea is to get rid of the tangle. Surgeons can open up the brain and actually cut out the problem area. Another approach is radio-surgery, which uses gamma rays to eliminate the tangle. Or doctors can thread a small tube through the arteries and inject a glue-like substance that closes off the blood vessels.

The problem is that it may not be possible to eliminate the tangle of veins and arteries without disrupting blood flow to an important area of the brain. So not all AVMs can be safely eliminated.

What kind of recovery can be expected?

AVM is serious. A patient is most likely to do well if doctors can eliminate the malformation before it has caused brain damage. In this case, a patient can recover very quickly.

But if the AVM has already caused brain damage, recovery is similar to a stroke. It can take months or years. And the recovery may never be complete.

Also, all the procedures used to eliminate an AVM carry risks of their own. The surgery itself can cause brain damage or cause a stroke.