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.