Condition:
Arteriovenous Malformation (AVM)
Contents:
What is an Arteriovenous Malformation (AVM)?
How does an AVM develop?
What are the symptoms of AVMs?
How is an AVM diagnosed?
What are the treatment options for AVMs?
What are the advantages of stereotactic radiosurgery for AVM?
What are the disadvantages of stereotactic radiosurgery for AVM?
How is CyberKnife® radiosurgery different from other radiosurgery systems?
What to expect after radiosurgery for an AVM
What are possible complications of AVM radiosurgery?
References
What is an Arteriovenous Malformation (AVM)?
An Arteriovenous Malformation (AVM) is an abnormal cluster of blood vessels within the brain or spine. In most parts of the body, arteries carry blood away from the heart to the various organs.
Within the organs, arteries branch into very small vessels called capillaries, which carry the blood’s nutrients and oxygen to the tissue itself. Having nourished the organs, blood is drained back to the heart through veins. When an AVM is present, the arteries carry blood directly to veins, without any interposing capillaries.
The blood that flows through an AVM does not carry oxygen or nutrients to the brain or spinal cord, but instead represents a high flow “short circuit” of blood often referred to as a shunt. The pathologic blood flow that passes through an AVM will often cause certain specific patient symptoms.
How does an AVM develop?
Most researchers believe that nearly all AVMs are present from the time of birth (congenital). “AVM-like structures” are present within the developing fetal brain, but involute and disappear prior to birth.
When fetal AVMs fail to disappear in the neurologic tissue of an individual, that patient is born with a brain or spinal AVM. Although there are other theories regarding AVM formation, this theory is the one that is most widely accepted by clinical neuroscientists.
In rare instances, AVMs can develop as a result of trauma to the head. This specific type of AVM is termed a dural fistula. Generally, physicians can differentiate between an AVM and a dural fistula.
What are the symptoms of AVMs?
The most devastating symptom of an AVM is hemorrhage or bleeding into the brain or spine. Such bleeding is unpredictable and episodic. Patients with AVMs have a 3 to 4% risk per year of hemorrhage from their AVM.
When a patient suffers such a bleed, there is a 10% chance of dying, and a 20% chance of permanent neurologic problems (a stroke); a bleed from an AVM is generally thought to be self-limiting, lasting from mere seconds to a few minutes. Given the odds of an AVM bleeding, affected patients have a relatively significant risk of “hemorrhagic” stroke.
Other symptoms which can be caused by AVMs include seizures, headaches, or a gradual onset of neurologic deficits. In some instances, AVMs can be discovered after routine evaluation for other neurologic disorders, and are therefore “incidental” findings. Given the significant risk of bleeding, some type of treatment is recommended in most patients diagnosed with AVMs.
How is an AVM diagnosed?
The typical diagnosis of an AVM is made with either a CT or MRI scan, followed by a cerebral angiogram. Both CT and MRI show the approximate location of the AVM within the brain or spine, and provide some estimate of the size.
Knowing the location of an AVM is critical in determining the risks of treatment. A cerebral angiogram is a procedure during which a catheter is threaded through the groin into the major blood vessels of the brain. By simultaneously taking x-rays and injecting dye, it is possible to obtain detailed pictures of the blood vessels in the brain, and in particular, those vessels that supply blood to the AVM.
In addition, this study characterizes the veins that are draining blood away from the AVM. Knowledge of the blood vessels that supply and drain an AVM is valuable in selecting the best treatment for a given patient.
What are treatment options for AVMs?
In rare circumstances, such as in elderly patients for example, it may be reasonable to not treat an AVM patient and simply observe him or her over time. For nearly all other patients there are three treatment options: surgery, endovascular therapy, and stereotactic radiosurgery(1). Surgical resection involves operating on the brain to physically remove the AVM.
If an AVM can be completely removed, the patient is cured. However, surgery on the brain is obviously invasive and can be associated with risks such as stroke, infection, anesthetic complication, or neurologic deficits.
Endovascular therapy involves depositing glue within the vessels of the AVM, using the angiographic techniques described above. During an angiogram, the small angiographic catheter is used to both identify the vessels of the AVM and inject glue into the lesion. While this technique is less invasive than open surgery, most AVMs can be only partially obliterated with endovascular therapy.
Such partially treated AVMs still can bleed and cause a stroke. As a result, most neurosurgeons believe that a patient with a partially treated “embolized” AVM is not protected against future problems. Endovascular therapy is most typically used to decrease the size of AVMs prior to either surgical resection or stereotactic radiosurgery.
Stereotactic radiosurgery can eliminate an AVM by using large numbers of very accurately targeted cross-fired beams of radiation to destroy the abnormal shunt vessels. Over time, the radiation causes the blood vessels of the AVM to narrow and eventually close off completely(2). Once the AVM is closed, no more blood can flow through it and the patient no longer is at risk for hemorrhage or stroke.
What are the advantages of stereotactic radiosurgery for AVM?
Compared to surgical removal, the primary advantage of stereotactic radiosurgery is that radiosurgery is noninvasive and does not carry the risks of conventional open brain surgery.
Furthermore, some AVMs are located in areas of the brain that cannot be safely approached with conventional brain operations, and in these cases, radiosurgery may be the only feasible option for the patient. Radiosurgery can be used in conjunction with other therapies to cure the AVM. For example, radiosurgery can be used to eliminate a persistent AVM remnant after partial surgical resection or endovascular therapy.
One of the very attractive aspects of radiosurgery for AVMs is that treatment is an outpatient procedure, and does not require any recovery period before a patient can resume a normal lifestyle.
What are the disadvantages of stereotactic radiosurgery for AVM?
There are three main disadvantages to treatment of AVM with radiosurgery. First, the radiation can take up to three years to obliterate the AVM; radiation is a slow process that causes gradual shrinkage of the AVM. During this three-year period, the patient is still at risk for another bleed from their AVM (at the 3 to 4% per year rate).
It is possible for a patient to have a significant neurologic deficit as a result of an AVM hemorrhage after radiosurgery, even if the AVM is in the process of obliterating. Second, not all AVMs disappear after radiosurgery. Small AVMs, those 3 cm in diameter or less, have an 80% chance of disappearing with a single radiosurgery treatment.
Larger AVMs have a lower obliteration rate. As a result, it is not uncommon to have only a portion of an AVM disappear after radiosurgery. In such situations, the patient is often treated with a second course of stereotactic radiosurgery.
Finally, even if an AVM is cured with radiosurgery, not all of a patient’s symptoms may necessarily disappear. Some patients who presented with headaches or seizures prior to their AVM treatment, continue to have these symptoms even after their AVM is cured(3). In such cases, the symptoms are believed to result from scar tissue that develops around the AVM, during both the process of development and disappearance.
How is CyberKnife radiosurgery different from other radiosurgery systems?
Most radiosurgery systems require the use of a metal frame for accurate targeting. The metal frame is attached to a patient’s head with four screws that penetrate the scalp and anchor to the skull.
Although local anesthesia (“novacaine”) is used, a patient inevitably experiences some pain. Instead, the CyberKnife uses a frameless system for targeting and thereby avoids the discomfort of a stereotactic frame. Despite being less painful, the accuracy of targeting the radiation beams is comparable, if not superior, to that of other radiosurgical instruments.
Furthermore, other radiosurgery technologies limit treatment to AVMs of the brain. Since CyberKnife radiosurgery is frameless, AVMs of the spinal cord can also be treated safely using this technique.
What to expect after radiosurgery for an AVM
Radiosurgery for AVMs is performed on an outpatient basis for most patients. During the actual treatment, a patient does not feel anything unusual.
After treatment, patients are followed by clinical examination and through MRI scans. Most typically an MRI is performed every 6 to 12 months to monitor shrinkage of the AVM vessels and to look for any radiation side-effects in the adjacent normal brain.
Approximately three years after radiosurgery, an angiogram is required to confirm complete AVM obliteration. Generally, MRI scans alone are not adequate to determine whether an AVM has completely disappeared after radiosurgery.
What are possible complications of AVM radiosurgery?
The main potential complication of AVM radiosurgery is the risk that the radiation can injure the normal surrounding brain as the AVM is obliterating. This process is called radiation edema or, in severe cases, radiation necrosis.
The likelihood of radiation injury to the brain is low, and in most instances, is believed to be much lower than the risk of leaving an AVM untreated with the possibility of hemorrhage(4).
In most patients, the symptoms of radiation edema will significantly improve with oral steroid medications, which reduce swelling.
References
- Chang SD, Marcellus M, Marks MP, Levy RP, Do HM, Steinberg GK. Multimodality treatment of giant intracranial arteriovenous malformations. Neurosurgery 2003;53:1-14.
- Chang SD, Shuster DL, Steinberg GK, Levy RP, Frankel KA. Stereotactic radiosurgery of arteriovenous malformations: Pathologic changes in resected tissue. Clinical Neuropathology 1997;16:111-116.
- Chang SD, Dodd RL, Steinberg GK. The resolution of medically intractable epilepsy following treatment of intracranial AVMs. J Neurosurg 2001;94:172A.
- Chang SD, Adler JR. Management of the radiosurgery patient: Causes and treatment of adverse sequalae. In: Meyer J, editor. Radiation Injury: Advancements in Management and Prevention. New York: Karger Medical; 1999. p. 155-165.