Arteriovenous Malformations (AVM)
Genesis and Natural History of AVM
AVM is a developmental vascular abnormality characterized by a tangle of vessels shunting blood directly from arterial to the venous system (Figure 1). Capillaries, the microscopic vascular channels that are typically interposed between an artery and vein, are not present in an AVM. Capillaries are necessary in human physiology because they allow for the delivery and exchange of oxygen and carbon dioxide to and from tissue. A natural consequence of blood flow through a capillary system, furthermore, is a reduction of blood pressure from an arterial mean of 80-90mm Hg to venous pressure which hovers at 3-8mm Hg. The alteration in normal cerebral blood flow found in AVM-pressurized blood flow in thin walled venous channels has clinically significant implications including hemorrhage, brain swelling and seizures.
Arterial phase cerebral angiogram demonstrating a large left parietal AVM. Nidus or the center of the AVM is indicated along with feeding arteries and draining vein.
Clinical Presentation and Treatment Considerations
AVMs are often identified incidentally before the patient has developed any symptoms. In this circumstance, the decision to offer treatment depends on a careful consideration of the risks imposed by the natural history of AVM counterbalanced with the risks of treatment. The overall the annualized risk of internal bleeding from a ruptured AVM range from 2-3% and influenced by the details of its vascular anatomy, its location in the brain, size and history of previous rupture. These factors together with patient age and general medical status contribute to the calculus of management.
Diagnosis and Treatment of Ruptured AVM
AVM rupture is a serious event and is heralded by headache, nausea, vomiting, paralysis, loss of consciousness and other neurological symptoms. Diagnosis of intra-cerebral hemorrhage is established and well characterized with head CT and often the pattern of bleeding suggests the presence of an AVM. Further studies including CT angiography, MR and MR angiography support the diagnosis and provide localizing anatomical information. A definitive understanding of the anatomy of AVM is obtained from cerebral angiography. An AVM is a complex 3 dimensional structure and often involves multiple feeding arteries together with unpredictably positioned draining veins. Angiography provides anatomical clarity as well as an understanding of the nature of the pathophysiology of its blood flow.
Treatment of the patient depends first on assuring that there is no ongoing internal bleeding and second by ameliorating the deleterious effect of bleeding and brain swelling. Craniotomy or craniectomy to remove the coagulum is sometimes performed in more critically ill patients to prevent injury to as yet uninjured brain tissue. In more mild cases, hemorrhage and brain swelling may improve with supportive care only. After the patient is stabilized, further interventions are offered to prevent rebleeding and obliterate the AVM. Treatment options include craniotomy for resection, endovascular embolization (Figure 2,3), radiosurgery (Figure 4 Gamma Knife), or combined/staged treatments. The anatomy, size, and location of the AVM are but some of the factors considered by the neurovascular team in formulating the plan of care.
Lateral late arterial phase cerebral angiogram showing a small posterior parietal AVM with indicated arterial supply, nidus and draining vein.
Same patient as above with before and after images superimposed showing embolic material filling the nidus and draining vein.
Lateral cerebral angiogram showing small AVM encircled by white line. Right: One year post gamma knife angiogram showing absence of filling (white line indicates estimated location of target) proving obliteration of AVM.