Arteriovenous Malformations (AVMs) are an uncommon developmental anomaly of the intracranial vasculature, which arise embryologically but usually do not progress into the symptomatic phase until the third or fourth decade of life. AVMs are tangled masses of vessels connecting arteries to veins without an intervening capillary bed region.
True AVMs contain one or more enlarged feeding arteries and have enlarged draining veins. Under normal circumstances, oxygenated blood arrives to the brain through arteries, which branch until the blood reaches very small vessels called capillaries. The oxygen is removed in the capillaries for utilization by the brain. The deoxygenated blood then enters the venous system to drain out of the brain. In the troublesome case of AVMs, the blood flows directly to the vein from the artery, thereby depriving that region of the brain its vital oxygen source.
Aside from providing the brain opportunity to acquire oxygen, the capillary connection has the vital role of slowing down the blood flow. Without the capillaries to slow the blood flow, the veins near AVMs are placed under enough pressure to cause concern for rupture and subsequent brain hemorrhage. The risk of hemorrhage from a previously ruptured AVM is estimated to be as high as 17 percent in the first year. The AVM scenario is further complicated by the "steal" phenomenon, in which blood supply preferentially seeks the AVM and normal brain parenchyma is relatively undersupplied.
Over time, arteriovenous malformations can produce focal neurologic deficit, headaches, seizures (25 percent), or intracranial hemorrhage (50 percent). AVMs have a tendency to hemorrhage at a rate estimated in a study from Finland at 4 percent annually with an annual mortality rate of 1 percent and a mean interval between hemorrhagic events of 7.7 years.
There are 10 to15 new cases of AVMs per million people per year. The average age at diagnosis is 31.2 years. However, approximately 8 to 20 percent of AVMs occur in children and adolescents. Considering that AVMs are dynamic lesions known to potentially enlarge if left untreated, younger patients are often considered candidates for definitive treatment.
It is important to map completely the anatomy of AVMs. The AVM consists of feeding arteries, which are usually dilated, and a cluster of entangled vascular loops connected to abundant vascular channels where the arterial blood is shunted, terminating in an enlarged draining vein or veins. The diagnosis of AVM can be made on CT or magnetic resonance (MR). With CT, the tangled vessels in the brain parenchyma are high density without contrast and have a serpentine, punctuate or an irregular configuration. With MR, curvilinear flow voids secondary to fast flow are observed on most pulse sequences, and dilated feeding arteries can also be noted. MR angiography is useful for mapping the AVM. The definitive study is cerebral angiography.
Treatment of an arteriovenous malformation depends on its size, location and angioarchitecture. The goal of AVM treatment is to eliminate the threat of intracranial hemorrhage, while striving to preserve neurologic function and minimize complications. The decision to treat a patient with an AVM requires balancing the natural history of the disease, particularly the risk of hemorrhage, against the risk of surgery.
The SPETZLER-MARTIN CLASSIFICATION SCHEME is commonly, though not universally, recognized as the standard for assessing the surgical risk associated with a patient's AVM. The scale assigns points of risk based on the size of the nidus, the associated venous drainage, and the eloquence of the surrounding brain (i.e. the known functionality). A low SPETZLER-MARTIN CLASSIFICATION rating is a mark of a better prognosis with microsurgical treatment than a high rating.
|Graded Feature||Points Assigned|
|Size of AVM|
|Small (<3 cm)||0|
|Medium (3-6 cm)||0|
|Large (>6 cm)||0|
|Eloquence of adjacent brain|
|Pattern of Venous Drainage|
When it is determined that treatment is indicated, the next step is selecting the treatment modality. Treatment options consist of endovascular therapy, microsurgery and stereotactic radiosurgery.
Endovascular embolization is often used in conjunction with another form of treatment. For example, endovascular therapy may be utilized to slow the flow in an AVM prior to surgical removal of the AVM. In large AVMs, embolization may be utilized to divert blood flow away from the AVM and instead encourage flow toward the regular brain tissue. In the instance of a small AVM, endovascular embolization may be used as the sole means of therapy.
Endovascular therapy is a minimally invasive treatment approach. A flexible catheter is inserted into the femoral artery at the groin and threaded through the arterial system to reach the cerebral arteries involved in the AVM. A glue-like substance is injected into the arterial vessels feeding the AVM, via the catheter. The role of the glue is to reduce the blood flow into the AVM.
Surgery removes the nidus and eliminates arteriovenous shunting. The most viable AVMs for surgical intervention are small AVMs located superficially in non-eloquent areas. The neurosurgeon approaches the AVM by removing a section of the skull. The AVM is sealed off with clips, then cut out or removed with a laser. Surgical treatment of an AVM is intended to minimize the risk of future hemorrhage and neurological deterioration. Surgery is often preceded by endovascular embolization with glue to slow blood flow, thereby improving the safety of surgical resection.
Radiosurgery may serve as the primary treatment for an AVM, or as a secondary intervention after embolization. Deep AVMs and those located in critical structures are often best approached by sterotactic radiosurgery, whereby a high-dose of radiation is delivered to a selective region of the brain. Over the course of one to three years, the vessels of the AVM form scar tissue, which clots the blood vessels and shuts down the AVM.