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
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.
Causes and symptoms
About half of all patients with AVMs first come to medical attention because of hemorrhage; small AVMs are most likely to hemorrhage. If a hemorrhage occurs, it produces a sudden, severe headache. The headache may be focused in one specific area or it may be more general. It can also be mistaken for a migraine in some cases. The headache may be accompanied by other symptoms such as
Almost half of AVM patients first present with seizures. A person may experience decreased, double, or blurred vision. About 25% of patients begin with a progressive neurological deficit such as loss of vision, weakness, or cognitive changes, depending on the exact location of the AVM. Larger AVMs are more likely to cause seizures and progressive deficits than smaller ones. Large AVMs exert pressure against brain tissue, cause abnormal development in the surrounding brain tissue, and slow down or block blood flow. Hydrocephalus, a swelling of brain tissue caused by accumulated fluids, may develop.
Additional warning signs of a bleeding AVM are impaired speech or smell, fainting, facial paralysis, drooping eyelid, dizziness, and ringing or buzzing in the ears.
About 65% of AVM patients have a mild learning disability present long before coming to medical attention for the AVM. There may also be a history of headaches or migraines.
Based on the clinical symptoms such as severe headache or neurological problems, and after a complete neurologic exam, a computed tomography (CT) scan of the head will be done. In some cases, a whooshing sound from arteries in the neck or over the eye or jaw (called a bruit) can be heard with a stethoscope. The CT scan will reveal whether there has been bleeding in the brain and can identify AVMs larger than 1 in (2.5 cm). Magnetic resonance imaging (MRI) is also used to identify an AVM. A lumbar puncture, or spinal tap, may follow the MRI or CT scan. A lumbar puncture involves removing a small amount of cerebrospinal fluid from the lower part of the spine. Blood cells or blood breakdown products in the cerebrospinal fluid indicate bleeding.
To pinpoint where the blood is coming from, a cerebral angiography is done. This procedure uses x rays to map out the blood vessels in the brain, including the vessels that feed into the malformation. The information gained from angiography complements the MRI and helps distinguish the precise location of the AVM. During angiography, an anesthetic may be introduced into the AVM area to determine the precise function of the surrounding region. The patient will be given a variety of tests of language comprehension, speech production, sensation, and other tasks, depending on the precise location of the AVM. These results help determine the risk of treatment.
Neurosurgeons consider several factors before deciding on a treatment option. There is some debate over whether or not to treat AVMs that have not ruptured and are not causing any symptoms. The risks and benefits of proceeding with treatment need to be measured on an individual basis, taking into account factors such as the person's age and general health, as well as the AVM's size and location. In older patients at low risk for future hemorrhage, or for those in whom the AVM is located very close to critical brain areas, the doctor and patient may decide that treating symptoms alone is the best course. Antiseizure medications, pain relievers for headaches, and migraine medications may provide adequate symptom control for many patients.
To treat the AVM directly, several options are available. These treatment options may be used alone or in combination.
Removing the AVM is the surest way of preventing it from causing future problems. Both small and large AVMs can be handled in surgery. Surgery is recommended for superficial AVMs (those close to the surface), but may be too
Radiation is particularly useful to treat small (under 1 in [2.5 cm]) malformations that are deep within the brain. Ionizing radiation is directed at the malformation, destroying the AVM without damaging the surrounding tissue. Radiation treatment is accomplished in a single session, and it is not necessary to open the skull. However, the radiation takes months to exert its complete effect, and success can only be measured over the course of the following two years. A year after the procedure, 50–75% of treated AVMs are completely blocked; two years after radiation treatment, the percentage increases to 85–95%.
Embolization involves plugging up access to the malformation. This technique does not require opening the skull to expose the brain and can be used to treat deep AVMs. Using x-ray images as a guide, a catheter is threaded through the artery in the thigh (femoral artery) to the affected area. The patient remains awake during the procedure and medications can be administered to prevent discomfort. A device is inserted through the catheter into the AVM, and released there to block the blood supply to the malformation. The device may be metal spheres, an adhesive, a hardening polymer, or other such substance.
There may be a mild headache or nausea associated with the procedure, but patients may resume normal activities after leaving the hospital. At least two or three embolization procedures are usually necessary at intervals of 2–6 weeks. At least a three-day hospital stay is associated with each embolization. Embolization rarely provides complete blockage, and may be used prior to one or the other types of treatment.
Recovery and rehabilitation
Recovery and rehabilitation vary with each form of treatment. In general, successful treatment leads to reduction in the risk for cerebral hemorrhage and improvement of symptoms caused by the AVM. Surgical complications, including hemorrhage, infection, and treatment of too large an area, make recovery longer and more difficult, and may leave the patient with permanent neurologic deficits.
Clinical trials of surgical techniques for treatment of AVMs are conducted in large medical centers.
Approximately 10% of AVM cases are fatal. Seizures and neurological changes may be permanent in another 10–30% cases of AVM rupture. If an AVM bleeds once, it is about 20% likely to bleed again in the next year. As time passes from the initial hemorrhage, the risk for further bleeding drops to about 3–4%. If the AVM has not bled, it is possible, but not guaranteed, that it never will. Untreated AVMs can grow larger over time and rarely go away by themselves. Once an AVM is removed and a person has recovered from the procedure, there should be no further symptoms associated with that malformation.
The Official Patient's Sourcebook on Arteriovenous Malformations: A Revised and Updated Directory for the Internet Age. San Diego: Icon Health Publications, 2002.
Steig, P., H. H. Batjer, and D. Samson. Intracranial Arteriovenous Malformations. New York: Macel Dekker, 2003.