updated 12/14/2006 1:58:10 PM ET 2006-12-14T18:58:10

Envision a snarled ball of red and blue yarn, packed too tightly in the center to pull apart. The bleeding in Sen. Tim Johnson’s brain resulted from a condition not too different — where arteries and veins grew so knotted together that eventually at least one bursts.

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It’s called an arteriovenous malformation, or AVM. They can occur anywhere, but about 300,000 Americans are believed to have one in the brain or spinal cord. Only about 12 percent of them, roughly 36,000, will notice symptoms such as headaches or seizures.

The greatest danger from these brain tangles is bleeding. AVMs account for about 2 percent of the nation’s hemorrhagic, or bleeding, strokes each year, according to the National Institutes of Health. And the condition claims about 3,000 lives a year.

It’s a condition people are born with, but symptoms rarely arise until they’re grown. What happens: As the arteries and veins grow, often abnormally large, the tangle becomes tighter. The sheer size and pressure of the mass can cause headaches or other symptoms that may come and go.

Then there’s bleeding. A trickle deep inside the mass may never be detected, not severe enough to cause lasting damage. But a larger bleed is a potential disaster that must be stopped.

Johnson’s stroke-like symptoms sent him to the hospital Wednesday, where doctors were able to monitor him until determining that his AVM was deteriorating enough to require a late-night surgical fix.

Doctors haven’t disclosed how they drained the blood that had accumulated and, in their words, stabilized the AVM.

Conventional brain surgery can remove at least part of the AVM, depending on where it’s located and how much surrounding healthy brain tissue surgeons must go through to reach it.

Also, catheters can be used to snake their way up to the AVM and plug blood flow. It’s a procedure called endovascular embolization that often is used when the AVM is too deep in the brain for regular surgery, or is particularly large and needs shrinking before a second procedure, either surgery or radiation, to try to get rid of it.

Regardless, AVM surgery is risky; the NIH puts the chances of serious complications or death at 8 percent.

Dr. John Eisold, the Capitol physician, said it was “premature to determine whether further surgery will be required, or to assess any long-term prognosis.”

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