Arteriovenous Malformations

Arteriovenous malformations (AVMs) are abnormal tangles of arteries and veins without an associated capillary bed. The mass of the AVM, referred to as the nidus, is devoid of neural tissue.   Most AVMs are congenital, but they can enlarge over time.  These lesions can exist anywhere in central nervous system. 

Signs and Symptoms

Arteriovenous malformations most often become symptomatic with brain hemorrhage, but headaches, focal neurological deficits, seizures, even stroke-like symptoms can also occur.  Data on the natural history of AVMs indicate that hemorrhage rate is 2 to 4% per year.  Because these lesions are present from birth, patients tend to develop symptoms at a younger age than patients with aneurysmal hemorrhage.  The mortality rate associated with an AVM-related hemorrhage is about 10%. The rate of neurological morbidity ranges from 30 to 50%.  As the brain heals over time, neurological deficits caused by hemorrhage of an AVM can often resolve.  

Grade

Arteriovenous malformations are categorized according to the Spetzler-Martin grading system. This system considers the size of an AVM, its location near important cortical tissue, and the presence or absence of deep venous drainage.  (See illustrations of grading system.)  Low-grade AVMs (grades I, II, and III) usually warrant treatment.  High-grade AVMs (grades IV and V) seldom warrant treatment because the associated complications are often worse than the natural course of the disease.  If, however, a high-grade AVM hemorrhages repeatedly or if a patient's neurological condition progressively deteriorates, treatment of high-grade AVMs can be considered.

Aneurysms Associated with Arteriovenous Malformations

Ten to 20% of arteriovenous malformations are associated with aneurysms.  These aneurysms can occur along the feeding (flow-related) arteries that lead to the AVM, along dysplastic arteries within the nidus of the AVM itself (nidal aneurysm), or along the circle of Willis.  The rupture rate of aneurysms associated with AVMs is thought to be significantly higher than that of patients with an aneurysm and no AVM.  Hence, aggressive treatment of aneurysms associated with an AVM is usually recommended, irrespective of the grade of the AVM. 

Treatment

Treatment options for arteriovenous malformations include endovascular embolization, microsurgical resection, stereotactic radiosurgery, or a combination of these treatments.  Microsurgical resection involves a craniotomy to access the AVM followed by microscopic and stereotactic magnetic resonance (MR) imaging guidance to dissect the lesion.  Endovascular embolization requires access to the arterial system via the common femoral artery, followed by guidance of catheters and microcatheters inside of the arteries.  The specific arterial branches that feed the AVM are selectively microcatheterized, permitting selective microangiography of the feeding pedicle.  Embolic agents (materials that cause clotting) can be deposited into the nidus of the AVM, progressively obliterating the lesion.  Metal coils, N-butyl-2-cyanoacrylate (NBCA), and Onyx glue are materials that can be used to embolize AVMs.  At Barrow, AVM embolization is typically followed by microsurgical resection during the same hospitalization. Stereotactic radiosurgery can be performed using Gamma Knife radiosurgery or Cyberknife radiosurgery, both of which are available at Barrow.  These methods use a focused beam of radiation so that the size and shape of the AVM can be treated precisely while exposure of the adjacent normal structures to radiation is minimized.  Ultimately, the mode or modes of AVM treatment are dictated by the patient's clinical history and by the Spetzler-Martin grade and location of the AVM.