An arteriovenous malformation, or AVM, like that suffered by Sen. Tim Johnson (D-S.D.), is like the four-level interchange writ large -- a complicated, tangled mass of blood vessels that connect arteries to veins.
Present at birth, the tangles lie hidden and symptom-less in the brain for decades. But eventually, the constant pressure produced when the muscular arteries pump blood directly into the weaker, more fragile veins, causes the veins to spring small leaks.
Often the bleeding stops spontaneously, and the victim is not aware of anything other than perhaps a headache. Occasionally, however, the bleeding is more severe, and the patient, like Johnson, has a stroke.
The chances for survival and recovery depend on how soon the stroke is treated and where in the brain the AVM is located.
Johnson's stroke was detected quickly, when he suffered aphasia -- a difficulty understanding or expressing language and completing sentences -- during a conference call, and surgery was performed within hours.
But his surgeons have not yet revealed the precise location of the AVM, making it difficult for a physician not involved with Johnson's case to give a prognosis.
"We need to know where it was located, how big the blood clot was and what it was pressing on," said Dr. John Frazee, a neurosurgeon at UCLA's Geffen School of Medicine. "If it destroyed important tissue because it was deep in the brain, it could have a serious effect. If it was located peripherally, it might not do much damage at all."
Johnson is healthy, is not overweight and apparently does not have hypertension, increasing his odds of recovery, Frazee said.
AVMs affect about 300,000 Americans, according to the National Institutes of Health. About 12% of them suffer symptoms severe enough to be noticeable, and 1% die from bleeding in the brain.
Typically, the stroke produced by an AVM is less severe than that produced by an aneurysm, said Dr. Keith Siller, medical director of the Comprehensive Stroke Care Center at New York University Medical Center.
An aneurysm occurs in an artery, where the blood is under higher pressure, and a rupture can be devastating. But, Siller said, "an AVM is more of a lower-level lesion because a vein is bleeding and it is under lower pressure. Usually the prognosis is better and the disability is less than it is with an aneurysm."
There are several ways to attack an AVM, depending on where it is and the urgency of the situation. Often, interventional radiologists will begin by threading a thin catheter through an artery to the AVM, where they will use beads, glue or coils to block the artery, preventing blood from reaching the weakened or bleeding area.
Often that is enough to cure the problem. Sometimes, however, it is necessary to surgically remove the mass of tangled vessels to prevent future problems.
When the tangle is actively bleeding, the first recourse is to seal it surgically, removing clotted blood and easing pressure at the same time. But that is tricky, Siller said, because of the risk of damaging healthy brain tissue during the procedure.
Because there are no specific activities that trigger bleeding in an AVM, Frazee says a lifestyle change isn't needed. He does recommend patients avoid aspirin, ibuprofen and similar drugs because if bleeding does recur, they could make it worse.
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What is an AVM?
An arteriovenous malformation, or AVM, is a tangled mass of arteries and veins compromising blood flow and oxygen delivery.
Normal blood flow
Arteries carry oxygenated blood to capillary beds that slow blood flow and then feed into veins.
AVM blood flow
Arteries and veins merge with no capillary bed. If vessel walls weaken, bleeding may result.
Senator Tim Johnson's AVM
The senator's AVM occurred in the left side of the brain, which controls speech and right-side body strength.
Causes of AVM: Uncertain; AVMs develop in the womb or shortly after birth, but problems may not develop until later.
Who's affected: About 300,000 Americans have AVMs, about 12% of those will have severe symptoms; each year about 1% will die.
Symptoms: Headaches and seizures are among the most common, but include muscle weakness, dizziness and inability to speak or understand language, depending on the location of the AVM.
Sources: National Institute of Neurological Disorders and Stroke, Hyman-Newman Institute for Neurology and Neurosurgery, Columbia University Medical Center
Graphics reporting by Tom Reinken