Arteriovenous malformations (AVMs) are blood vessel defects that occur before birth when the fetus is growing in the uterus (prenatal development). The blood vessels appear as a tangled mass of arteries and veins. They do not possess the capillary (very fine blood vessels) bed that normally exists in the common area where the arteries and veins lie in close proximity (artery-vein interface). An arteriovenous malformation may hemorrhage, or bleed, leading to serious complications that can be life-threatening.
AVMs represent an abnormal interface between arteries and veins. Normally, arteries carry oxygenated blood to the body's tissues through progressively smaller blood vessels. The smallest are capillaries, which form a web of blood vessels (the capillary bed) through the body's tissues. The arterial blood moves through tissues by these tiny pathways, exchanging its load of oxygen and nutrients for carbon dioxide and other waste products produced by the body cells (cellular wastes). The blood is carried away by progressively larger blood vessels, the veins. AVMs lack a capillary bed, and arterial blood is moved (shunted) directly from the arteries into the veins.
AVMs can occur anywhere in the body and have been found in the arms, hands, legs, feet, lungs, heart, liver, and kidneys. However, 50% of these malformations are located in the brain, brainstem, and spinal cord. Owing to the possibility of hemorrhaging, such AVMs carry the risk of stroke, paralysis, and the loss of speech, memory, or vision. An AVM that hemorrhages can be fatal.
Approximately three of every 100,000 people have a cerebral (brain) AVM and roughly 40–80% of them will experience some bleeding from the abnormal blood vessels at some point. The annual risk of an AVM bleeding is estimated at about 1–4%. After age 55, the risk of bleeding decreases. Pre-existing high blood pressure or intense physical activity do not seem to be associated with AVM hemorrhage, but pregnancy and labor could cause a rupture or breaking open of a blood vessel. An AVM hemorrhage is not as dangerous as an aneurysmal rupture (an aneurysm is a swollen, blood-filled vessel where the pressure of the blood causes the wall to bulge outward). There is about a 10% fatality rate associated with AVM hemorrhage, compared to a 50% fatality rate for ruptured aneurysms.
Although AVMs are congenital defects, meaning a person is born with them, they are rarely discovered before age 20. A genetic link has been suggested for some AVMs, but studies have been inconclusive. The majority of AVMs are discovered in people ages 20–40. Medical researchers estimate that the malformations are created during days 45–60 of fetal development. Another theory suggests that AVMs are primitive structures that are left over after fetal blood-circulating systems developed.
However they form, AVMs have blood vessels that are abnormally fragile. The arteries that feed into the malformation are unusually swollen and thin walled. They lack the usual amount of smooth muscle tissue and elastin, a fibrous connective tissue. These blood vessels commonly accumulate deposits of calcium salts and hyaline. The venous part of the malformation receives blood directly from the artery. Without the intervening capillary bed, the veins receive blood at a higher pressure than they were designed to handle; this part of the malformation is also swollen (dilated) and thin walled. There is a measurable risk of an aneurysm forming near an AVM, increasing the threat of hemorrhage, brain damage, and death. Approximately 10–15% of AVMs are accompanied by saccular aneurysms, a type of aneurysm that looks like a small sac attached to the outer wall of the blood vessel.
Although the malformation itself lacks capillaries, there is often an abnormal proliferation of capillaries next to the defect. These blood vessels feed into the malformation, causing it to grow larger in some cases. As the AVM receives more blood through this "steal," adjacent brain tissue does not receive enough. These areas show abnormal nerve cell growth, cell death, and deposits of calcium (calcification). Nerve cells within the malformation may demonstrate abnormal growth and are believed to be nonfunctional. This may lead to progressive neurological deficits, or seizures, or both.
Julia Barrett, The Gale Group Inc., Gale, Detroit,