An arteriovenous malformation (AVM) is a congenital, abnormal tangle of blood vessels occurring within any area of the brain that commonly presents with spontaneous hemorrhage, seizure, or intense headache. Only one or two persons per 100,000 are diagnosed annually with a brain AVM. Most AVMs do not cause any symptoms until the third decade of life.
Because of the variability inherent in each AVM and in each patient, treatment is highly individualized. Treatment — by conventional surgery, stereotactic radiosurgery, endovascular embolization, or a combination of these methods — is most effective when performed by an experienced team of neurosurgeons, neurologists, and interventional neuroradiologists who have training in the latest techniques, access to the most advanced equipment, and an appreciation of a collaborative approach to care that assures each patient receives the best treatment for an optimal outcome.
Michael J. Link, M.D., a neurosurgeon at Mayo Clinic in Rochester, Minnesota, comments, "The cerebrovascular team at Mayo Clinic has experience treating thousands of patients with AVMs and a variation of AVM, dural arteriovenous fistula. Given this broad and deep base of experience, we believe we are well poised to respond fully to the uniqueness of each case."
Adds Mayo Clinic interventional neuroradiologist Harry Cloft, M.D., Ph.D., "The treatment for any particular AVM has to be carefully individualized to minimize risk to the patient and maximize the chance for completely obliterating the lesion. Surgery, radiosurgery, or embolization, alone or in combination, may be advisable, depending on the location and size of the AVM and the general health of the patient. Each of these variables requires thorough and expert evaluation, which is why a multidisciplinary treatment model works best for AVMs."
About 50 percent of patients with AVMs come to medical attention because of a spontaneous hemorrhage. The hemorrhage is most commonly in the brain surrounding the AVM and may result in headache, unilateral weakness or numbness, trouble with speech or alteration of consciousness, depending on the size and location in the brain where the hemorrhage occurs. Between 80 percent and 90 percent of patients who experience a hemorrhage from an AVM survive the initial rupture.
The second most common presentation of an AVM is a seizure, which occurs in about 25 percent of cases. All types of seizures have been reported in association with AVMs, and most are well controlled with anticonvulsant medication.
Other common presenting symptoms include headache not associated with hemorrhage and progressive neurologic deficit. The headaches are believed to be due to stretching of the covering of the brain and venous channels, which have many pain sensing fibers. (The brain itself has no pain sensing fibers.) Although the headaches may be similar to migraine headaches, migraine sufferers are not at increased risk of AVM.
Progressive neurologic deficits may occur in association with an AVM because the brain does not have time to extract adequate oxygen from the fast-flowing blood. The malformation is therefore stealing blood from the surrounding brain, which can result in symptoms that mimic a stroke. As in the case of hemorrhage, these symptoms depend on where the malformation is located in the brain.
A hemorrhage from an AVM is usually detected by obtaining a CT scan when the patient comes to the hospital. Calcification within the AVM may also be demonstrated on a CT scan. After intravenous administration of contrast, the nidus may light up and become apparent. MRI and magnetic resonance angiography are even more sensitive in demonstrating the AVM vessels. MRI also provides essential anatomic information about where in the brain the AVM is located, which is important for treatment planning and risk assessment.
Angiography remains the gold standard in demonstrating the AVM, the feeding arteries and the draining veins. Angiography is performed by radiologists. A catheter is inserted into a leg artery and routed to the vessels that supply the brain or spinal cord. Contrast dye is then injected as radiographic pictures are taken. This imaging technique provides a detailed road map of the AVM and other blood vessels. Angiography is usually done using local anesthesia, with minimal pain and only a small risk. It is crucial in deciding on therapy for an AVM.
The three main forms of intervention to treat an AVM are surgical removal, stereotactic radiosurgery and endovascular embolization.
The size and location of the AVM largely determine how safely surgery can be performed. The many advances in surgical technique in the past 20 years have improved results. Many AVMs can safely and effectively be removed surgically. These improvements include:
Stereotactic radiosurgery involves delivering high-dose radiation to a defined area, such as an AVM nidus. Radiosurgery is usually performed as an outpatient procedure and does not require general anesthesia or an incision. The Leksell Gamma Knife® uses cobalt radiation sources to accomplish this precise delivery of radiation. The radiation causes the AVM vessels to slowly close during the ensuing one to three years until the AVM is completely obliterated. This form of treatment works best for small AVMs and AVMs that have not recently caused a life-threatening hemorrhage.
In endovascular embolization, performed like an angiogram, a catheter is introduced into the leg artery and threaded through the vascular system to the brain arteries. The catheter is positioned in one of the feeding arteries to the AVM. Instead of injecting dye to take a picture of the AVM, small particles or a glue-like substance are injected to occlude the vessel and reduce the blood flow into the AVM. Injection is often done before surgery to reduce the chance of a bleeding complication during the operation or to reduce the overall size of the AVM to make it more responsive to stereotactic radiosurgery.
In some large AVMs that are considered inoperable, embolization may reduce the stroke-like symptoms caused by the stealing phenomenon by diverting blood back to the normal brain. In rare cases, endovascular embolization may completely obliterate or cure the AVM, eliminating the need for any other therapy.
The risk of hemorrhage from an AVM is between 2 percent and 4 percent. About 10 percent to 20 percent of patients die from their AVM-related brain hemorrhage. The risk of severe permanent neurologic problems is 2 percent to 3 percent. Notes Dr. Link, "While the day-to-day risk of hemorrhage or other problems is small, over time the risk adds up. Therefore treatment is usually recommended for otherwise healthy young people discovered to have AVMs."