An AVM is a tangle of abnormally formed blood vessels (arteries and veins), where blood flows directly from the arterial system to the venous system without passing through a capillary system. The result is an innate propensity to bleed. An AVM can occur anywhere in the body, but brain and spinal AVMs present substantial risks when they bleed.
Some 12% percent of people with AVMs will experience symptoms varying in severity. AVMs can irritate the surrounding brain and cause seizures or headaches. The most common symptom is brain hemorrhage. Any of the following symptoms may occur:
AVMs are usually diagnosed through a combination of magnetic resonance imaging (MRI), magnetic resonance angiography (MRA) and cerebral catheter angiography. These tests may need to be repeated to analyze a change in the size of the AVM, recent bleeding or the appearance of new lesions.
The recommendation for surgery is typically elective, except in the case of large, life-threatening blood accumulations (hematomas) caused by bleeding of the AVM. If surgical removal is necessary, the neurosurgeon will perform a craniotomy and use microsurgical techniques to gain access to the AVM. The use of an operating microscope and image-guided surgical navigation (also known as computer-assisted or frameless stereotaxy) helps enable safer surgery with as little disruption as possible to normal brain activity. Once the skull is opened, the AVM will be closed off with special clips and ultimately removed. The skull bone is then secured back in place with miniplates, and the incision in the scalp closed.
Stereotactic radiosurgery is a minimally invasive treatment that uses computer guidance to concentrate radiation to the malformed vessels of the brain. This radiation causes abnormal vessels to close off. Unfortunately, stereotactic radiosurgery is usually limited to lesions less than 3.5 cm in diameter and may take up to two years to completely obliterate the lesion. For this reason it is not ideally suited to AVMs that have already bled, unless they are surgically inaccessible. Because ionizing radiation is harmful to normal tissue as well as AVM vessels, it must be used judiciously. It is unavoidable to expose normal brain tissue to ionizing radiation during treatment, as it must be traversed by the beam, independently of how accurately the AVM is targeted.
Endovascular embolization uses specially designed microcatheters, which are guided directly into the AVM via angiography. The lesion is blocked from the inside using the process of embolization, which occludes the abnormal blood vessels in the AVM. Once the catheter reaches the core of the AVM, liquid glue or particles can be injected to close off portions of the AVM or its feeding arteries. Materials used include fast-drying biologically inert glues, polyvinyl alcohol particles and fibered titanium coils. Although this method may be effective in reducing the size of an AVM, it is rarely able to completely eliminate it. Neuroendovascular therapy can make subsequent surgical removal of an AVM safer or can reduce the size of an AVM to a size that may inevitably improve the outcome of stereotactic radiosurgery. This procedure is also associated with substantial risk, since the path taken by such embolic materials can be difficult to predict, and blockage of normal vessels or of the outflow of the AVM may occur. The former may result in stroke and the latter in bleeding from the AVM. These procedures are therefore also used judiciously and with ample clinical judgment.
Patient outcome depends on the size and location of the AVM and severity of the bleeding, as well as the extent of neurological symptoms. Many patients undergoing microsurgery make an excellent and quick recovery after several days of hospitalization. Following or during surgery, an angiogram is performed to assure complete removal of the AVM. If the AVM is completely removed, the patient is considered cured.