An arteriovenous malformation (AVM) is an unusual formation of arteries and veins which are congenital in origin and occur throughout the body. When they occur within the brain they may cause symptoms such as seizure, headache or progressive neurological deficits. Most importantly they can spontaneously bleed resulting in a stroke with lasting neurological problems or even death.
An AVM may have several forms, such as a direct connection between an artery and vein, an AV fistula. Unusual formations of veins which bleed and cause seizures are cavernous angiomas. Abnormalities of very small vessels are capillary angiomas. The most important (and dangerous) are AVM’s which have both arterial and venous components. These AVM’s have a 3 to 4 percent chance of spontaneous hemorrhage each year. Roughly 10 percent of the hemorrhages will be fatal and about 15 percent of victims will suffer a continuing neurological deficit, such as weakness, sensory or visual loss, speech abnormality, etc.
The gold standard for diagnosis is a cerebral angiogram. The radiologist advances a catheter into the arteries which supply the brain and images the AVM nidus by injecting radio-opaque dye with serial x-rays of the skull. MR techniques can also identify AVM’s.
It is important to understand the natural history of AVM’s since this impacts the advise given to patients who harbor unruptured and asymptomatic malformations. Surgery can cure AVM’s, but not all AVM’s are amenable to open operation because of the risk. This risk is generally predictable from the size and position of the AVM. The age and general health of the patient also factor into the pre-operative equation. An important contribution to the estimation of pre-operative risk was the Spetzler-Martin grading system. This paradigm assigns points according to AVM size, position in eloquent brain (important functioning) and the presence of deep, draining veins. The higher the score, the greater the risk of post-operative problems.
Surgical excision of the AVM brings an immediate cure, but not all AVM’s can safely be removed. Gamma Knife radiosurgery can play a role in the obliteration of smaller AVM’s. This technique has the advantage that is non-invasive and accomplished in a single session. The disadvantage is that the effects of focused radiation occur over months and years, during which time the patient is still at risk for spontaneous hemorrhage. In general, AVM’s under 1 cm in diameter have a 90 percent chance of obliteration, while those under 2 cm have an 80 percent change of cure. Only about 50 percent of malformations 3 cm in size are obliterated.
Interventional radiology can reduce the blood supply to AVM’s and rarely cure them. This technique is performed by selectively filling the feeding arteries, or veins with clotting agents by a catheter during angiography. Endovascular therapy usually plays a supporting role, reducing the blood supply to aid open surgery or to reduce the size of the AVM before Gamma Knife radiosurgery.
Once an AVM has ruptured, surgery is indicated to prevent repeat hemorrhage. Most experts advise emergency operation for patients in danger of death from very large clots. Smaller, minimally symptomatic or asymptomatic bleeds can be managed medically, as the hematoma is gradually reabsorbed from the brain tissue. Early re-bleeding is rare in AVM’s (6 percent in the first 6 months after initial hemorrhage and there is no proof early surgery can reduce the eventual outcome in individuals with a deficit, since the brain damage has already occurred.
Case treated by Kenneth Ott, M.D., F.A.C.S., and David Hodgens, M.D., F.A.C.R., San Diego Gamma Knife Center, La Jolla, CA.