RENEE MONTAGNE, host:

We have an update now on Senator Tim Johnson. The South Dakota Democrat underwent emergency surgery last night for circulation problems in his brain. The U.S. capital physician said the operation was successful and that Senator Johnson is recovering. His health has repercussions for control of the U.S. Senate, where Democrats are expected to take control in January with a one-vote majority.

NPR's Jon Hamilton joins us now to discuss what's known about Senator Johnson's condition. Good morning, John.

JON HAMILTON: Good morning, Renee.

MONTAGNE: What are you hearing at this hour?

HAMILTON: Well, the latest we're hearing is that the Senator is in critical but stable condition. The other information that's new is they said that the - they have surgery because of bleeding in the brain. Now, earlier they had said - his office had said that the senator did not have a stroke, but now they're talking about a surgery to deal with damage from bleeding in the brain. That sounds like an awful lot like a stroke to most people. And all of this is because of a condition known as an arteriovenous malformation, or an AVM.

MONTAGNE: Explain to us what AVM is.

HAMILTON: Well, an AVM is, if you see pictures of it in medical textbooks, it looks like this tangle in the brain, this tangle of blood vessels, of the arteries and veins. And what happens is that ordinarily you have a blood brought into the brain from arteries, and it comes out of the brain again, in the veins.

But in this tangle, that all gets mixed up, so that you have veins trying to act like arteries, and it's a mess and it can cause bleeding and it can cause a lack of blood flow to the brain, which can keep enough oxygen from getting there, and it's a real bad thing.

MONTAGNE: Now, Senator Johnson had surgery. How do doctors treat this condition?

HAMILTON: Well, there's several different methods that you can use to go after these AVMs. It's not clear whether the surgery that he had was for the AVM itself specifically, or for the blood that had got into the brain; if very much blood gets into the brain, you need to go there and get rid of it so that it doesn't cause swelling and put pressure on the structures in the brain.

But the way that doctors go after AVMs is, one, you can go in and literally sort of inject this glue into these arteries, which causes them to close up, so you don't have the blood flowing through the them, you don't have bleeding. You can also use radiation, which does the same thing, but much more slowly; it can take months or years. And finally you can go in surgically and literally cut out the affected area.

MONTAGNE: What kind of recovery can be expected and how long would it take for this sort of condition?

HAMILTON: It all depends on how much brain damage there has been. Now, it's - we're thinking about this as being very much like a stroke. I mean, there's bleeding in the brain in this case. If he had very minor damage, he could be back up from - and they were able to fix the AVM, he'd be back up and around in a few days. But it sounds like - that whatever happened is more serious than that, and if, for instance, he had paralysis or clearly a part of the brain that affects speech was involved in this - because he had trouble speaking when the first symptoms appeared - and if there was any kind of damage to those areas of the brain, you could be talking about months, years, or maybe never, to recover from it.

MONTAGNE: Jon, thanks very much.

HAMILTON: You're welcome.

MONTAGNE: NPR's Jon Hamilton.

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