Experts Optimistic About Senator's Recovery

The purpose of the operation would have been to reduce the pressure by taking out the clotted blood and to stop the bleeding. Some or all of the AVM might also have been removed, though that is often hard to do.

The surgery is difficult and dangerous. If the bleeding is coming from a weakened vein "downstream" of the defect, the surgeon cannot simply tie off that area. High-pressure blood will continue to flow in and will have an even smaller route out -- raising the pressure in the malformation even higher.

"If he ligates the vein first, it just basically explodes in his face," said William O. Bank, chief of neuro-interventional radiology at Washington Hospital Center.

Instead, the surgeon must cut off the flow in the arteries feeding the malformation. There may be many; if the bleeding is extensive, they may be hard to find; and getting to them may require touching the already highly unstable defect.

"It becomes very technically difficult," Bank said.

If the bleeding can be stopped, most surgeons will leave definitive treatment of the AVM for another day, once the patient has recovered. A neuroradiologist will thread a catheter from an artery in the thigh into the brain and release a dye that will fill the AVM and outline its structure.

The radiologist can then inject a gluelike substance that shuts down blood flow in the arteries to the AVM. Alternatively, a surgeon can cut the malformation out. AVMs can also be cauterized from outside the skull by crossing beams of gamma radiation and causing pinpoint burns in the blood vessels, which then form scar tissue and close off.

An AVM has about a 10 percent chance of bleeding a second time in the six months after a first hemorrhage. After that, the risk falls to what it is before the first bleed -- about 3 to 4 percent a year.