Arteriovenous Malformations

Arteriovenous malformations (AVMs) are abnormal brain vascular lesions which connect arteries directly to veins, bypassing the normal capillary system. They are composed of abnormal blood vessels which shunt blood away from surrounding healthy brain tissue, causing neurological problems such as headaches, weakness and seizures. Most importantly, AVMs are prone to bleeding (hemorrhage); the mortality rate is approximately 30% per hemorrhage. Left untreated, AVMs bleed with a frequency of 2-4% per year. The risk of hemorrhage is cumulative, with young patients facing a high probability of experiencing an intracranial bleed in their lifetime.

Complete surgical removal of AVMs is the treatment of choice for patients who can safely undergo this procedure. Complete removal of the AVM immediately eliminates the risk of hemorrhage. However, many AVMs are located deep in inaccessible regions of the brain, making surgery difficult or not feasible. Additionally, patients with coexisting medical problems or who are elderly may be poor candidates for surgery. A variety of risks confront patients undergoing surgical removal of their AVMs, including infection, bleeding, seizures, deep vein thrombosis, pneumonia, and operative mortality.

Radiosurgery offers an excellent treatment alternative for patients with small AVMs; certain lesions may be too large for this treatment to be safe and effective. Radiosurgery can obliterate AVMs by causing the lining of abnormal blood vessels to progressively thicken in response to a high dose of radiation delivered to the core (nidus) of the lesion, until blood no longer flows through the AVM. This process typically takes one to three years to complete, which is a disadvantage compared with surgical removal, since the risk of hemorrhage persists after radiosurgery until obliteration of the AVM is complete. However, radiosurgery may be substantially safer than conventional surgery for inaccessible lesions or patients with coexisting medical problems. Approximately 10% of patients develop neurological symptoms related to radiosurgery, and half of these patients have their symptoms resolve completely, resulting in a permanent radiation-related complication rate of 5%.

Radiosurgery is successful in obliterating AVMs between 70 and 90% of the time. Results vary with the size of the lesion, with smaller AVMs having a higher obliteration rate than larger lesions. Smaller lesions typically can be treated with higher radiation doses than larger AVMs, leading to a higher success rate. However, if complete obliteration does not occur with the initial procedure, radiosurgery may be repeated 3-4 years later to the residual lesion. This subsequent procedure is successful in achieving complete obliteration of the AVM 60-70% of the time.

References
Pollock B: Patient Outcomes After Arteriovenous Malformation Radiosurgery. in Gamma Knife Brain Surgery. Prog Neurol Surg. Basel, Karger, 1998, vol 14, pp 51-59

Coffey R. et al: Stereotactic Radiosurgical Treatment of Cerebral Arteriovenous Malformations. Mayo Clin. Proc. 1995;70:214-222