eMedicine Specialties > Radiology > Brain/Spine

Brain, Arteriovenous Malformation

Robert A Koenigsberg, DO, MSc, FAOCR, Professor, Director of Neuroradiology, Program Director, Diagnostic Radiology and Neuroradiology Training Programs, Department of Radiology, Hahnemann University Hospital, Drexel University College of Medicine
Jeffrey R Wasserman, DO, Diagnostic Radiologist, Manatee Memorial Hospital and Lakewood Ranch Medical Center; Bernadette R Diegnan, Drexel University College of Medicine

Updated: Dec 31, 2008

Introduction

A CT scan of the head that demonstrates a left occipital arteriovenous malformation (AVM), with multiple calcified phleboliths and numerous hyperattenuating vascular channels.

A sagittal T1-weighted MRI demonstrating a large occipital arteriovenous malformation (AVM) with parasagittal flow voids.

A diffusion-weighted MRI showing a lack of signal intensity associated with an arteriovenous malformation (AVM).

A lateral left carotid angiogram demonstrating a mixed pial-dural arteriovenous malformation (AVM). Arterial and occipital arterial feeders extend to the nidus via distal branches of the middle cerebral artery.

Arteriovenous malformation (AVM) of the brain. A CT scan of the posterior fossa demonstrating a hemorrhage in the fourth ventricle, with extension to the left cerebellum.

Background

An arteriovenous malformation is a tangled cluster of vessels, typically located in the supratentorial part of the brain, in which arteries connect directly to veins without any intervening capillary bed. The lesion may be compact, containing a core of tightly packed venous loops, or it may be diffuse, with anomalous vessels dispersed among normal brain parenchyma. In 77% of cases the core, or nidus, of a compact arteriovenous malformation is 2-6 cm in diameter.¹

Arteriovenous malformations account for approximately 11% of cerebrovascular malformations; venous angiomas, a more common type of cerebrovascular malformation, account for 64% of cases. Arteriovenous malformations are more likely than other types of vascular malformations to be clinically symptomatic. Arteriovenous malformations typically involve the brain, but occasionally, they are associated with the spinal cord and its dura.^2,3,4,5

Categorization of arteriovenous malformations

Arteriovenous malformations are categorized by their blood supply. Pial or parenchymal arteriovenous malformations are supplied by the internal carotid or vertebral circulation, whereas dural arteriovenous malformations are supplied by the external carotid circulation. Mixed arteriovenous malformations are supplied by both. Pial arteriovenous malformations, which are almost exclusively congenital, are the most common kind. Dural arteriovenous malformations are relatively uncommon and are theorized to be secondary to trauma, surgery, thrombosis of an adjacent venous sinus, or venoocclusive disease. Mixed arteriovenous malformations usually occur when the lesion is large enough to recruit blood vessels from both the internal and external carotid arteries. A pediatric variant of arteriovenous malformation is the vein of Galen aneurysm, in which an arteriovenous malformation drains to and dilates the great vein of Galen.

Pial arteriovenous malformations tend to be asymptomatic until the second, third, or fourth decade of life. They most commonly manifest as spontaneous hemorrhage or seizure. Other clinical signs include headache and transient or progressive neurologic deficit. Dural arteriovenous malformations typically feature pulsatile tinnitus, cranial bruits, headaches, or hemifacial spasm. Infants with a vein of Galen malformation may present with hydrocephalus or severe congestive heart failure.

Saccular aneurysms occur in association with arteriovenous malformations in 6-20% of patients. The preferred site for an arteriovenous malformation–associated aneurysm is a feeding artery. Venous and intranidal aneurysms occur less frequently. When aneurysms and arteriovenous malformations occur together, they can cause intracranial or subarachnoid hemorrhage; however, intracranial hemorrhage is more likely to stem from an arteriovenous malformation.^6,7,8

Spetzler and Martin grading system

The Spetzler and Martin grading system attempts to predict the risk of surgical morbidity and mortality by assigning points to an arteriovenous malformation on the basis of its size, the eloquence of the adjacent brain, and the pattern of venous drainage. The grading system includes 5 possible points. If the arteriovenous malformation is small (<3 cm), 1 point is assigned; if medium in size (3-6 cm), 2 points are assigned; or if large (>6 cm), 3 points.

An eloquent brain region is one in which injury to that region results in a disabling neurologic deficit; in the Spetzler-Martin grading system, 0 points are assigned for a noneloquent region, and 1 point is assigned for involvement of an eloquent brain region.

Lastly, an additional point can be assigned if the arteriovenous malformation drains into the deep venous system.

The grade of a lesion is determined by summing the points given in each of the 3 categories. Surgical treatment of a grade I arteriovenous malformation, therefore, presents little risk of morbidity and mortality. By contrast, a grade V lesion is associated with significant risk. A grade VI arteriovenous malformation is described as an inoperable arteriovenous malformation that is associated with a totally disabling deficit or death.⁹

Related eMedicine topics:

Arteriovenous Malformations (Neurosurgery)

Arteriovenous Malformations (Vascular Surgery)

Intracranial Arteriovenous Malformation

Presentation

Demographics

• Most studies have shown a slightly increased preponderance of pial arteriovenous malformations (AVMs) in men.
• Dural AVMs of the anterior cranial fossa occur more frequently in men than in women. Other types of dural AVMs occur more commonly in women.
• No clear correlation exists between race and the prevalence of AVMs; however, the 1:7 ratio of intracranial vascular malformations to intracranial saccular aneurysms in the overall population has been shown to increase to 1:4 in people of Asian descent.
• The incidence of AVMs is estimated to be 0.04-0.52%; the incidence in the United States is the same as that seen worldwide.
  

Natural history and presentation

AVMs can be found throughout the central nervous system (CNS). They may be microscopic or large enough to involve an entire hemisphere of the brain. Grossly, angiographically invisible AVMs are termed cryptic vascular malformations, a name suggesting that the lesions are completely thrombosed. Most AVMs are small (2-4 cm in 42% of cases) or moderately sized (4-6 cm in 35% of cases).

Ninety percent of AVMs are supratentorial, and they tend to occur at watershed areas (straddling more than 1 vascular territory); the remaining 10% are infratentorial. Seventy percent of supratentorial AVMs are purely pial, with no meningeal or dural vascular supply. The remainder of lesions are purely dural or a pial-dural mix. Approximately one half of posterior fossa AVMs are purely dural or a pial-dural mix.

Pial AVMs lie within the brain parenchyma, and they derive blood from the cerebral arteries, namely, the anterior cerebral artery (ACA), middle cerebral artery (MCA), or posterior cerebral artery (PCA). The rapid shunting of blood that is typical of pial AVMs is visualized as an abnormal tangle of blood vessels with early, and frequently rapid, venous drainage, which is uniquely demonstrated by catheter angiography. Most AVMs involving the ACA or its branches are purely pial.

Dural AVMs are almost always infratentorial. They most frequently drain into the transverse and sigmoid sinuses in the posterior fossa, but they may also involve the cavernous sinus, inferior petrosal sinus, superior sagittal sinus, or other areas of the brain or spinal venous system. The occipital artery and meningeal branches of the external carotid artery are the vessels that most commonly supply dural AVM components. Tentorial and small dural branches of the internal carotid artery and vertebral arteries may also contribute. Dural AVMs may be classified according to the sinus involved. Furthermore, dural AVMs may be associated with venous outlet stenosis or obstruction.^2,3,4,5

The pathogenesis of areteriovenous malformations (AVMs) is not well understood. Because these malformations characteristically lack the capillary bed that normally intervenes between arteries and veins, investigation into the pathogenesis of AVMs has focused on, among other things, the molecular differences between arteries and veins, capillary bed morphogenesis, and inherited disorders of vasculogenesis.¹

A major recent discovery demonstrates that the endothelial cell population is not homogeneous, as was previously believed. On the contrary, arterial and venous endothelial cells express different receptors from the onset of angiogenesis. Ephrin-B2 is found on arterial cells but not venous endothelial cells, whereas ephrin-B4 is found on venous cells but not arterial endothelial cells. Angiogenesis is impaired in ephrin-B2 knockout mice. That the endothelial cells lining arteries and veins are molecularly distinct suggests a mechanism for defective vasculogenesis and angiogenesis.¹⁰

In addition, the role of angiopoietins and their tyrosine kinase receptors is being explored. Angiopoietin 1 (ang-1) and its ligand, tie-2, may be crucial for vascular remodeling in the embryo. Ang-1 apparently controls the recruitment of pericytes and smooth muscle cell precursors to the blood vessel wall. Upregulation of tie-2 has been demonstrated in AVM vasculature. The improper recruitment of periendothelial cells can contribute to dysvasculogenesis of the capillary bed.

The gene coding for endoglin (CD105), which is a transforming growth factor b–binding endothelial cell receptor, has been implicated in the pathogenesis of hereditary hemorrhagic telangiectasia (HHT) type 1. HHT, also termed Osler-Rendu-Weber disease, is an autosomal dominant disorder that causes AVMs in the brain, skin, and viscera. Analogously, its variant, HHT type 2, is caused by mutations in the gene coding for activin receptorlike kinase (ALK-1).

Mutations in Flt-1, a tyrosine kinase receptor for vascular endothelial growth factor (VEGF), can result in thin-walled vessels of abnormally large diameter. Because the proper morphogenesis of the capillary bed probably depends on signaling between arteries and veins, any distortion of vessel anatomy or function may be expected to impair the process. Immunohistochemistry has demonstrated increased expression of VEGF in the vasculature of AVMs and the surrounding brain parenchyma.^2,3,4,5,11

Symptoms

The symptoms of an AVM may include headache, weakness, numbness, visual problems, or, most often, the abrupt onset of stroke. Usually, AVMs are initially clinically silent, subsequently becoming symptomatic in the second, third, or fourth decade of life. Spontaneous rupture with hemorrhage is the presenting symptom in 30-55% of pial AVMs.

Pial AVMs are present from birth, but they are usually asymptomatic until the second, third, or fourth decade, when hemorrhage, seizure, or other symptoms occur. More than 95% of patients develop symptoms before age 70 years.

Dural AVMs, which most commonly involve the transverse, sigmoid, and cavernous sinuses, are believed to be acquired and develop during adulthood. Patients with dural AVMs of the anterior cranial fossa may be congenital, and they may present with hemorrhage from a ruptured venous aneurysm.

Morphologic features

Morphologically, AVMs may be either compact or diffuse. Compact AVMs are characterized by a nidus formed by tightly packed entangled venous loops that are interconnected by small venules. Small amounts of nonfunctional brain tissue are found between the loops. When located supratentorially, compact AVMs are often wedge-shaped, and they extend through both the gray matter and white matter. Typically, the base of the wedge is parallel to the meninges, with the vertex pointing toward the ventricles or deep brain.¹

The venous loops of a compact AVM are attached to shunting arterioles, communicating venules, and draining veins. The microscopic shunting arterioles are the terminal branches of angiographically visible feeding arteries. A feeding artery is by definition an artery that transfers arterial blood to the AVM core. That the feeding arteries send branches both to the AVM and to normal brain tissue can significantly confound treatment.^2,3,4,5

Feeding arteries and vessels

There are 3 types of feeding arteries. The circumferential feeding artery extends around the nidus and sends branches to both small arterioles connected to the nidus and normal brain capillaries. Penetrating feeding arteries bisect the AVM core and send branches to it. Final feeding arteries either connect directly to an AVM loop or branch to shunting arterioles.¹

After arterial blood has circulated through the AVM nidus, it is drained by collecting veins, which then feed into larger veins; they may be either superficial or deep. The larger veins ultimately join to connect to a major draining vein. Major draining veins course through the sulcus and are connected to the neighboring cortical veins by numerous venules. The distal end of a major draining vein is connected to large hemispheric veins, which drain into the venous sinuses.¹

AVMs are currently believed to be hemodynamically compartmentalized; each compartment has its own feeding arteries and draining veins. For example, a large AVM in the sensorimotor area may have a lateral compartment supplied by MCA branches, a medial compartment supplied by ACA branches, and a posterior compartment supplied by PCA branches. The number of compartments in an AVM is proportional to its size. An AVM smaller than 3 cm in diameter is likely to have 1 compartment, whereas an AVM 3-4 cm in diameter may have 2 compartments, and an AVM larger than 4 cm in diameter typically has at least 3 compartments.

In contrast to the vessels of compact AVMs, those of diffuse AVMs are dispersed among normal brain tissue. Diffuse AVMs are typically found in the basal ganglia or thalamus. Blood flow through an AVM is proportional to the number of compartments and to AVM volume. For example, the rate of flow may be 285 mL/min for a 2-cm lesion and 800-1000 mL/min for a 4- to 5-cm lesion.

The vessels themselves are enlarged and dilated as a result of passive congestion secondary to increased pressure in the arterial core. Bright-red blood under relatively high pressure is often seen in the veins of an AVM, owing to arteriovenous (AV) shunting. Occasionally, aneurysms develop in AVM vessels, which is consistent with the altered hemodynamic stress characteristic of the lesions. Aneurysms may, therefore, develop along feeding arterial pedicles or along venous drainage pathways. Typically, the latter occurs proximal to a venous stenosis.^2,3,4,5

Histologic features

Histologically, distinguishing between the arteries and the arterialized veins of an AVM can be difficult because the wall thickness of each can vary unpredictably. Both feeding arteries and draining veins may be attenuated in some places and thickened by intimal hypertrophy in others. Greatly attenuated arterial or venous walls may be the source of hemorrhage. Within the vessel, atherosclerosis and thrombosis are common, presumably because of the unusually high volume of blood flow and the tortuosity and angulation of the vessels.

Regional blood flow in the area immediately surrounding an AVM may be reduced to approximately 81% of normal. This is referred to as the steal phenomenon. Accordingly, the neighboring gyri and underlying parenchyma are often discolored, infarcted, and atrophic, which results from chronic ischemia. Other typical features include gliosis, russet-colored pigmentation resulting from the presence of hemosiderin-laden macrophages after prior hemorrhage, and scattered foci of calcification. Overlying meninges may be thickened and fibrotic. Because AVMs are congenital lesions that replace normal brain tissue rather than displace it, they are not typically associated with mass effect unless hemorrhage has occurred; however, some AVMs do demonstrate mass effect, edema, and ischemic changes.

Despite the hypoperfusion seen in the normal brain parenchyma surrounding the AVM, total cerebral blood flow may be increased by as much as 50-100%. AVMs tend to exhibit slow progressive growth over many years because the shunted blood continually seeks adjacent vessels. Normal vasculature may be involved in the process.^2,3,4,5,6

Risk

The cumulative lifetime risk that an intracranial AVM will eventually bleed is estimated to be 50%. Hemorrhage is more likely to be intracerebral (parenchymal) or intraventricular rather than purely subarachnoid. Overall, a history of hemorrhage is the best clinical predictor of future bleeding. Pial AVMs are more likely to bleed than dural AVMs are.^2,3,4,5,6

• The risk of spontaneous intracranial hemorrhage is 2-4% per year. Each episode has a 10-15% rate of mortality and a 20-30% rate of permanent neurologic deficit.
• In the year after the first hemorrhage, the risk of rebleeding is 6%; thereafter, it decreases to 2-4%. Overall, hemorrhage is implicated in 29% of patient deaths.
• The risks associated with a residual AVM of any size are widely believed to be equivalent to the risks associated with untreated lesions; however, this statement is controversial.

Treatment

Relevant factors in the decision to treat an arteriovenous malformation include patient and family preferences, Spetzler-Martin grade, lesion site and angioarchitecture, clinical presentation, neurologic status, patient age, past medical history, and pregnancy. The treatment of intracranial AVMs typically involves 1 or more techniques, including embolization, direct surgical or microsurgical resection, or radiosurgery. Usually, small or medium-sized AVMs located in noncritical areas of the brain can be successfully removed with conventional microsurgery. In contrast, large AVMs or AVMs in eloquent cerebral locations usually require staged, multimodal treatment.

Embolization

Occasionally, embolization is the only procedure performed, particularly when surgery is inadvisable or refused by the patient. The long-term goal of embolization is to reduce the risk of cerebral hemorrhage, which ultimately reduces the risk of overall morbidity and mortality from AVM rupture. AVM-related headaches may often improve after embolization alone, particularly when AV shunting ceases. When combined with surgery, embolization is performed to reduce the volume of the AVM nidus before surgical resection.

Smaller lesions have lower complication rates at surgery; therefore, they may not need preoperative embolization. Rarely, embolization may follow microsurgery, radiosurgery, or both (eg, when partial microsurgical resection yields a residual AVM that is too large for treatment with radiosurgery alone). A delay between embolization and surgical resection of 2-4 days up to weeks may be beneficial, thereby permitting vascular adaptation without allowing time for a major collateral supply to the AVM to mature.

Primarily, the agents used for AVM embolization include liquid adhesives, particles, and alcohol. Because particles alone are not usually considered a permanent embolic agent, polyvinyl alcohol (PVA) is typically used only in the preoperative setting. PVA particles have been extensively used in this capacity for more than 20 years. Advantages include their relative ease of use and favorable short-term histotoxicity. Among the shortcomings are an inability to fully permeate the AVM nidus and the need to use microcatheters that can be directed by guidewires, which have additional attendant risks. During embolization of a large AVM, a staged increase in particle size may be necessary. When flow in the AVM nidus has slowed significantly, final blockage can be achieved by using small microcoils.

Unlike particulate embolization, glue embolization with cyanoacrylates may allow a permanent and complete cure of AVM, although long-term follow-up studies have yet to demonstrate this definitively (see Images below and Images 21-22 in Multimedia). Cyanoacrylates are a family of rapidly polymerizing adhesives, of which isobutyl-2-cyanoacrylate (IBCA) and n-butyl-2-cyanoacrylate (NBCA) are 2 members. Currently, many practitioners regard NBCA as the ideal embolic agent. Absolute alcohol can both obliterate aberrant vessels and penetrate capillary walls, causing destruction of normal cells. Accordingly, the use of glue or alcohol requires extreme care in the placement of the microcatheter tip.

A lateral left carotid angiogram in a patient with seizures and an unruptured left parasagittal arteriovenous malformation (AVM).

A lateral left carotid angiogram obtained after a successful staged embolization with a liquid adhesive that shows lack of arterial venous shunting.

Inadvertent occlusion of nontarget vessels is a potential complication of any embolization procedure. Normal chronically hypoperfused vessels in the parenchyma surrounding the AVM may not be present during preembolization testing, but they remain at risk for inadvertent occlusion.

Normal perfusion pressure breakthrough (NPPB) is postulated to be another possible complication of AVM embolization resulting in cerebral hemorrhage, although the validity of the assertion is under debate. According to the theory, the small hemispheric vessels adjacent to an AVM must remain maximally dilated in order to divert blood from the lesion to normal brain parenchyma. Over time, the chronic dilatation leads to a loss of autoregulation, possibly at the arteriolar level.¹²

When aberrant vessels of the AVM are successfully occluded during embolization, surrounding capillary beds may not be able to withstand the sudden exposure to normal perfusion pressure. Edema and or frank hemorrhage may result. Findings from some series suggest that NPPB may account for serious clinical complications in 1-3% of patients. Positive risk factors for NPPB are believed to include size, high flow, scanty filling of adjacent normal vasculature, steal from the vertebrobasilar or contralateral carotid circulations, excessive contribution to the nidus from the external carotid artery, and clinical findings of progressive or fluctuating neurologic deficit.¹²

A large vein that drains both an AVM and normal structures may undergo thrombosis following occlusion by embolization of an AVM blood supply because arterial occlusion results in decreased venous blood flow. Venous infarct may occur in the normal territory.¹³

Radiosurgery

Focused radiosurgery may be performed alone or in conjunction with other treatment modalities. The details are beyond the scope of this review; however, the advantages of radiosurgery include a high obliteration rate (particularly with small or medium-sized lesions), a low morbidity rate, and the lack of a need for general anesthesia. The radiosurgical obliteration rate is significantly lower with larger AVMs. Other disadvantages to radiosurgery include the 1- to 4-year latency period required for complete obliteration to occur, as well as the risk of radiation-related white-matter changes or vasculopathy.¹³

Microsurgical resection

Microsurgical resection is the treatment of choice for small AVMs in noncritical areas of the brain. For AVMs in critical regions, microsurgery may provide immediate and permanent cure of the lesion and relief of symptoms. Microsurgery also poses risks related to the use of general anesthesia as well as the risk of creating 1 or more new neurologic deficits. Potentially curative embolization has, therefore, gained in popularity in the definitive treatment of AVMs, both small and large.

Patient age is another important consideration in the decision to perform surgery. Because the incidence of hemorrhage is highest in younger age groups, conservative treatment is often recommended for elderly patients. Other risk factors include the size and location of the AVM. The Spetzler-Martin system is, therefore, commonly used to predict the overall surgical outcome.¹³

Preferred Examination

Imaging of arteriovenous malformations

The first imaging study that is performed in patients with a suspected AVM is usually a CT or MRI scan. These studies are good for depicting AVMs and they are relatively noninvasive, only requiring an injection of contrast material into a small vein.

Overall, arteriovenous malformations are best imaged by using magnetic resonance imaging (MRI), which can uniquely show these lesions as a tangle of vascular channels that appear as flow voids. Nonenhanced computed tomography (CT) is superior for visualizing the small foci of calcification often associated with arteriovenous malformations, and it may also delineate hyperattenuating serpiginous vessels the constitute the nidus.

Nonenhanced CT scanning is valuable for demonstrating the extent of acute hemorrhage and hydrocephalus. Contrast-enhanced CT shows enhancement of the typical vascular channels. Magnetic resonance angiography (MRA) or CT angiography (CTA) may be adequate for initial or follow-up evaluation of an arteriovenous malformation. Demonstration of the aneurysms sometimes found on arteriovenous malformation feeding arteries may be accomplished by means of MRA, CTA, or catheter angiography.^14,15,16

Computed tomography

CT scanning of the brain is the imaging test for evaluating acute headache or other acute mental status changes suggestive of acute cerebral hemorrhage. Detection of a lobar hemorrhage can suggest an underlying mass or AVM. Cerebral CT scanning can be used to identify areas of acute hemorrhage, and the results can suggest a vascular malformation, particularly with the judicious use of contrast material. Furthermore, CT scanning can uniquely demonstrate vascular calcifications associated with AVMs.¹⁵

Magnetic resonance imaging

MRI can help identify and characterize AVMs of the CNS, including the brain and spinal cord, without the use of radiation or invasive techniques. MRI is the examination of choice in patients with chronic headaches, seizure disorders of unknown etiology, and pulsatile tinnitus (among other conditions).

MRI typically follows CT scanning in the acute setting of neurologic illness when an underlying vascular lesion, such as an AVM, is suggested. MRI scans can demonstrate areas of parenchymal AVM involvement, showing both dilated feeding arteries and enlarged draining veins.

MRA and venography can further supplement conventional MRI in demonstrating in a near angiographic fashion the anatomy and microarchitecture of an AVM. MRI is the study of choice in the detection of vascular malformations of the spinal cord and spinal dura.^14,15,16

Angiography

Catheter angiography remains the criterion standard for characterization and delineation of brain and spinal AVMs. Angiography is a dynamic real-time study that not only demonstrates the presence or absence of an AVM, but also shows vascular transit time. Diagnostic angiography is uniquely able to delineate the size and number of feeding arteries, and it can define the pial, dural, or mixed origin of the AVM.

Angiography can be used to measure the size of the AVM and judge the compactness of the nidus. Furthermore, angiography can be used to evaluate the venous drainage pattern (superficial, deep, or mixed). In addition, angiography frequently depicts associated risk factors for hemorrhage, including aneurysms and venous stenosis. Planning an angiography is a vital step in both interventional neuroradiologic and neurosurgical evaluation of patients with AVM.¹⁵

Angiography can reveal certain features that are believed to correlate with an increased risk of hemorrhage. These features include the presence of associated intranidal, remote, or pedicular aneurysms; central or deep venous drainage; stenosis of a draining vein; and a periventricular or intraventricular location. Natural-history studies have shown that a small nidus is another positive risk factor for hemorrhage; however, whether this risk is overestimated is unclear because small unruptured AVMs are asymptomatic and often go undiagnosed.

Factors that are known to reduce the risk of hemorrhage include peripheral or mixed venous drainage and angiomatous changes. Angiomatous change is the development of an anomalous transcortical supply to the AVM in response to chronic ischemia of the brain parenchyma adjacent to the lesion.

High flow within the AV shunt is believed to induce significant hypotension in the lesion's feeding arteries. This has been postulated to cause ischemic symptoms in some patients (such as seizure or prolonged or transient focal neurologic deficit). Further study is needed to clarify the matter. Seizures may also result from gliosis of the margins surrounding the lesion, ischemia, or mass effect. Occasionally, seizures do not correspond to the site of the malformation.

An unruptured AVM may cause headaches, mimicking migraine or cluster headaches. Headaches are usually ipsilateral to the lesion, and they are believed to stem from hydrocephalus, stretching of the dura, or increased pressure in the dural sinuses. In 16% of patients, headache may be the only presenting symptom.^2,3,4,5,7,15

Limitations of Techniques

Computed tomography

CT scanning is an excellent examination for detecting cerebral hemorrhage, but it can miss an underlying AVM. AVMs are typically isoattenuating relative to normal parenchyma and, therefore, can be overlooked, particularly if a contrast agent is not administered. In an emergency setting, the administration of an iodinated contrast agent is typically deferred in favor of patient stabilization. Contrast-enhanced CT scanning also poses an inherent risk of radiation and, because of its cost, MRI may be a better screening examination for AVM in the general population. Contrast-enhanced CT scanning is performed to detect cerebral AVM, however, when MRI is contraindicated or otherwise not feasible.¹⁵

Magnetic resonance imaging

MRI is excellent for demonstrating the AVM nidus and abnormal flow voids typical of an AVM; however, in acute cerebral hemorrhage, compressed AVMs may no longer demonstrate flow and may, therefore, be overlooked. This may lead to the need for serial MRI studies to search for an underlying cause of cerebral hemorrhage not shown on a single MRI study. MRI can cause underestimation of the number of feeding arteries and associated aneurysms, which might also be missed. Furthermore, MRI can have a relatively poor sensitivity in detecting dural malformations. Gadolinium-based contrast material may be needed to demonstrate abnormal vascular channels.¹⁵

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. 

NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Angiography

Diagnostic angiography is the criterion standard for the evaluation of AVMs; however, it is invasive and carries risks related to catheter placement, contrast agents, and their injection. Specific neurangiographic risks include stroke, arterial dissection, reactions to the contrast material, and renal insufficiency and/or failure (among others). Nevertheless, modern cerebral angiography remains a safe and reliable method for AVM analysis, with an overall complication rate of less than 1%. Spinal angiography can be tedious and is associated with the risk of spinal cord infarction.¹⁵

Differential Diagnoses

Brain, Aneurysm
Brain, Capillary Telangiectasia
Brain, Cavernous Angiomas
Brain, Hypertensive Hemorrhage
Brain, MRI Appearance of Hemorrhage
Brain, Stroke

Other Problems to Be Considered

• AVMs can be clinically differentiated from saccular aneurysms. A patient with an AVM is likely to present with a history of seizures and/or unilateral headache, as well as a history of subarachnoid hemorrhage (particularly if it is mild). Because vascular malformations are under low pressure, blood tends to well up rather than spurt out of the lesion during a rupture. Bleeding is often parenchymal or mixed over a hemisphere, away from the circle of Willis. In contrast, rupture of a saccular aneurysm usually involves massive arterial bleeding from the base of the brain, accompanied by arterial spasm. Consequently, patients bleeding from an aneurysm may be more seriously ill than those bleeding from an AVM.
• A headache indistinguishable from migraine may occur ipsilateral to an unruptured AVM. The coexistence of migraine and seizures is particularly suggestive of a vascular malformation. Dural AVMs of the sigmoid or cavernous sinus also may produce migrainelike episodic headaches or pulsatile tinnitus.⁷
• Large AVMs are easily diagnosed with conventional angiography; however, smaller lesions may mimic malignant vascular brain tumors with AV shunting. AVMs may be distinguished by the more uniform caliber of their vessels, the characteristic dilatation of proximal arterial trunks, and the potentially huge, tortuous, and redundant draining veins. Although early venous filling and drainage typically results from vascular AVM, neoplasms (eg, glioblastoma multiforme, cerebral infarction, and an inflammatory mass) may also cause it.
• On CT scans, an AVM that appears as a noncalcified mass or a calcified and hyperattenuating focal mass must be distinguished from other calcified masses, such as tuberous sclerosis, colloid cysts, neoplasms, and aneurysms.
• Possible causes (in addition to vascular malformation) for parenchymal hematomas seen on CT scans include trauma; coagulopathy; hypertension; other vascular pathologies, such as aneurysm, amyloid angiopathy, or vasculitis; vascular occlusion, as from a venous infarct or embolic stroke with reperfusion hemorrhage; infection; and neoplasm.
• If abnormal intracranial calcifications are seen, the differential diagnosis includes congenital or developmental diseases; infection; endocrine or metabolic causes, such as hypercalcemia, hypoparathyroidism, lead encephalopathy, and carbon monoxide intoxication; other vascular causes, including aneurysm and atherosclerosis; hematoma resulting from trauma; neoplasm; and iatrogenic causes, such as radiation therapy and chemotherapy.
• Dark areas on T2-weighted MRIs can be caused by rapid blood flow, as from an AVM, aneurysm, or neoplasm; dense calcification, as from an AVM, infection, or neoplasm; or a variety of other causes not associated with vascular malformations. These include the presence of air, minerals or metals, hemorrhage, and mucinous or dense proteinaceous material.
• Ring-enhancing lesions may result from a thrombosed vascular malformation or aneurysm, high-grade astrocytoma, primary lymphoma, metastasis, subacute infarct, resolving hematoma, abscess or fungal granuloma, demyelination, and radiation necrosis.^2,3,4,5,15

Radiography

Findings

Plain radiography is not a modern modality used for imaging cerebral AVMs. Nevertheless, abnormally dilated vascular channels can be seen on plain skull images. Further abnormal intracranial calcifications associated with AVMs can also be detected; these are suggestive of an AVM. These findings should prompt the clinician to order cross-sectional imaging.^{15,17,18,19,20}

Degree of Confidence

The degree of confidence is poor, since impressions on the calvarium can be seen normally.

False Positives/Negatives

Plain films of the skull are not considered diagnostic for the detection of AVMs of the CNS.

Computed Tomography

Findings

CT scanning of AVMs of the brain can show an isoattenuating-to-hyperattenuating hemispheric mass, as well as detect an accompanying abnormal vascular supply. In the absence of hemorrhage, nonenhanced CT scanning can demonstrate small foci of calcification in as many as 30% of patients (see Image below and Image 1 in Multimedia). Other possible findings include cystic cavities representing previous hemorrhage and hypoattenuation of surrounding parenchyma representing encephalomalacia, cerebral atrophy, or gliosis.

A CT scan of the head that demonstrates a left occipital arteriovenous malformation (AVM), with multiple calcified phleboliths and numerous hyperattenuating vascular channels.

In the hyperacute stage of hemorrhage, a pial AVM appears as a hyperattenuating parenchymal lesion on nonenhanced CT scans because CT attenuation values and blood hemoglobin concentrations are directly proportional. Attenuation increases in the acute stage as a result of clot formation and the concomitant increase in hemoglobin concentration. The hyperattenuating region may be surrounded by a rim of hypoattenuation caused by extruded serum and edema.

Because the attenuation of a hematoma decreases with time, the ruptured hemorrhagic component of an AVM evolves through a stage of isoattenuation that progresses to normal brain parenchyma. Nonenhanced lesions viewed during the isoattenuating phase may therefore appear almost normal or may shine through, appearing minimally abnormal. If intravenous contrast material is administered during this stage, vascular enhancement may be seen, as well as nonspecific or ringlike areas of enhancement.

An AVM in the chronic stage of intracerebral hemorrhage appears as a hypoattenuating area relative to normal brain tissue. In general, AVM enhancement that is not contiguous with the site of hemorrhage points to an associated aneurysm or venous varix.

Dural AVMs can be visualized by CT scanning.

In an emergency setting, CT scanning can show a presenting cerebral or extra-axial hemorrhage. CT scans may show secondary signst that infer the presence of a dural AVM (ie, abnormal enlarged dural sinuses or draining cerebral veins). Typically, these are best appreciated using contrast imaging. Unfortunately, the dural malformation nidus is typically poorly demonstrated on CT scans alone.¹⁵

Degree of Confidence

The degree of confidence is moderate with CT. Typically, an additional study, such as MRI or catheter angiography, is necessary to confirm the presence of an AVM; however, this is not always needed.

False Positives/Negatives

False-positive CT results may occur with lesions demonstrating enhancement or calcifications. Tumor neovascularity occasionally mimics an AVM, particularly that of a neovascular glioblastoma multiforme. In addition, a wide variety of CNS abnormalities are associated with CNS calcifications, which can lead to false-positive results.

False-negative results may occur if an AVM is isoattenuating relative to regional parenchyma. Some lesions may be detectable only if iodinated contrast is administered. Furthermore, an AVM may be overlooked if it is compressed by an adjacent parenchymal hemorrhage. Lastly, vascular AVMs may be misconstrued as cerebral hemorrhage because of the presence of large hyperattenuating vessels. Contrast-enhanced CT scanning or supplemental MRI or MRA can help clarify difficult cases.

Magnetic Resonance Imaging

Findings

MRI findings

On MRI, a typical unruptured AVM appears as a tightly packed or loose tangle of vessels (see Image below and Image 2 in Multimedia).

Arteriovenous malformation (AVM) of the brain. An axial T2-weighted MRI showing numerous flow voids corresponding to the CT findings (not shown). Note the mass effect on the lateral ventricle despite the lack of a mass or hemorrhage.

MRI scans can show the lesion size and, usually, the primary supply of the AVM and its venous drainage. MRI can further demonstrate associated aneurysms on arterial feeders as well as associated sequelae, such as mass effect, edema, or ischemic changes.

Vascular steal in the brain or spinal cord adjacent to the lesion may be visualized as a region of abnormally reduced signal intensity on T1-weighted images and increased signal intensity on T2-weighted, proton density—weighted, and short-tau inversion recovery (STIR) images.

MRI is particularly well-suited to document AVM rupture. The appearance of the lesion depends on the stage of the hematoma.

An acute hemorrhage appears isointense on T1-weighted images and hypointense on T2-weighted images because of the presence of deoxyhemoglobin in extravasated but unlysed erythrocytes. A subacute intraparenchymal hemorrhage appears hyperintense on both T1- and T2-weighted imaging, which is consistent with the presence of methemoglobin. Chronic hematoma is characterized by a central hyperintense core surrounded by a ring of hypointensity resulting from the presence of hemosiderin deposits in macrophages in the surrounding brain. Hemosiderin is mildly hypointense on T1-weighted images and markedly hypointense on T2-weighted images.

MRI is an excellent preoperative planning tool for delineating the relationship between an AVM nidus and critical brain structures. In particular, the relationship between hemispheric AVMs and eloquent brain regions can be clarified, particularly with functional MRI. Associated aneurysms may be seen within a hematoma as a flow void. Unfortunately, the sensitivity of MRI for detecting aneurysms smaller than 1-2 cm is low.¹⁵

Postoperative MRI

Postoperative MRI is useful for studying the effect of surgery on the adjacent brain; however, documentation of complete obliteration of the nidus is best performed with conventional angiography because MRI may fail to depict small amounts of residual nidus or persistent AV shunting. MRI can show the extent of nidal, arterial, or venous thrombosis following embolization. T2-weighted imaging is particularly useful for the detection of embolic complications.¹⁵

Magnetic resonance angiography

MRA is a noninvasive alternative to conventional angiography. Certain lesions hidden on conventional angiograms may be identified only on MRI scans because of their ability to depict hemosiderin deposits or other evidence of blood breakdown. Blood-breakdown products appear in a time-dependent manner after intracranial hemorrhage.

MRA offers several advantages over conventional angiography. For example, because of its ability to image all vessels in a given volume nonselectively, an AVM with multiple feeding arteries can be imaged noninvasively in a single study. In addition, 2-dimensional (2D) and 3-dimensional (3D) phase-contrast MRA can be used to examine the direction, rate, and quantity of blood flow. Another advantage of MRA is the ability to retrospectively examine images in any plane.

3D time-of-flight (TOF) angiography may be used to image the fast-flow components of AVMs. Flip angles of approximately 15º and a repetition time (TR) of 40 ms are usually adequate for saturating the stationary background tissues while allowing the visualization of fully magnetized inflowing blood. Slower-flowing components of the AVM tend to be visualized poorly without the use of an MRI contrast agent because the vessels become more saturated as they course through the imaging volume. This is not entirely undesirable, as it allows an unobstructed view of the feeding arteries and nidus by effectively suppressing overlying venous structures.

The arterial supply may be identified by means of 3D TOF, phase-contrast slab, or 3D phase-contrast acquisitions. Visualization of vessels with angiomatous change may require phase-contrast slab angiograms encoded for low flow velocities, such as velocity encoding (V enc) = 20 cm/s. Otherwise, imaging with V enc of 80-100 cm/s typically demonstrates the arterial supply. Complex flow in the AVM nidus is best seen on 3D TOF acquisitions, using small voxel size, partial echo sampling, and a short echo time (TE).^14,15,16,21

Degree of Confidence

The degree of confidence is high with MRI. MRI scans of vascular malformations of the brain are unique and typically diagnostic of cerebral or spinal AVMs, with a high degree of confidence. MRI findings may prompt catheter angiography for confirmation and preoperative or postoperative AVM treatment.

False Positives/Negatives

False-positive results may occur when other types of CNS vascular malformations are encountered; these include cavernous angiomas, venous angiomas, and capillary telangiectasias. Lesions are associated with a lower risk of rupture, but they can mimic the appearance of an AVM; however, they lack characteristic AV shunting. Nevertheless, false-positive findings may prompt catheter angiography for clarification. MRI scans can also show abnormally enlarged arteries (atriomegaly), a finding which is suggestive of an underlying malformation when none is present.

False-negative MRI findings of CNS AVMs can occasionally occur as a result of a small AVM or an inconspicuous location. AVMs may be overlooked or not apparent if they are compressed by an adjacent hematoma. AVMs can also be missed if they are indistinguishable from the flow void of an adjacent normal vessel.

Ultrasonography

Findings

Ultrasonography is not typically used for evaluating cerebral AVMs. Ultrasonography may play an adjunctive role during open neurosurgery for the purposes of AVM localization.¹⁵

Nuclear Imaging

Findings

Isotopic cerebral blood flow studies have largely been supplanted by modern CT scanning, MRI, and digital subtraction angiography of the brain for the evaluation of AVMs.²¹

Single-photon emission CT (SPECT) and positron emission tomography (PET) of the brain are useful for imaging ischemic penumbra surrounding a vascular lesion. Furthermore, these studies may be helpful in the functional imaging of normal parenchyma surrounding a vascular malformation. This discussion is currently beyond the scope of this article.¹⁵

Angiography

Findings

Conventional cerebral angiography is the criterion standard for the evaluation of AVMs (see Images below and Images 6-10 in Multimedia). The study should include both internal carotid arteries and both vertebral arteries, with sequential evaluation of the arterial, capillary, and venous phases. External carotid arteries should be evaluated for dural contributions. The goal of the study should be to identify the number and location of feeding arteries, the angiographic location and size of the nidus, the shunt type of the lesion (eg, high flow vs low flow), and the pattern of venous drainage (eg, superficial, deep, or mixed).

A lateral left carotid angiogram demonstrating a mixed pial-dural arteriovenous malformation (AVM). Arterial and occipital arterial feeders extend to the nidus via distal branches of the middle cerebral artery.

Arteriovenous malformation (AVM) of the brain. An anteroposterior right carotid angiogram showing left anterior cerebral artery supply secondary to vascular steal. Note that the left anterior cerebral artery does not opacify with an ipsilateral carotid injection of contrast material (see also Image 6).

Arteriovenous malformation (AVM) of the brain. A lateral left vertebral angiogram demonstrating a huge left posterior cerebral artery feeder to the nidus.

Arteriovenous malformation (AVM) of the brain. An anteroposterior left vertebral angiogram.

Arteriovenous malformation (AVM) of the brain. The venous phase of a vertebral angiogram that demonstrates numerous superficial and deep draining veins.

Dural malformations typically have slower flow or AV shunting, and they are supplied by dural vessels, such as the meningeal branches or occipital arteries of the external carotid arteries, or the meningeal branches of the internal carotid or vertebral arteries.

Catheter angiography can usually be used to map all malformation feeders (pial, dural, or mixed), and it can be used to accurately access the size of the nidus.

Spetzler and Martin proposed a commonly used classification scheme to predict the surgical outcome. This scheme is typically applied to the angiographic data described above. In brief, grade I lesions are small, superficial, and located in noneloquent areas of the brain, whereas grade V AVMs are large, deep, and found in functionally critical locations. Inoperable lesions are assigned to grade VI.^9,15,21

Degree of Confidence

With angiography, the degree of confidence is high. The presence of abnormal CNS vascularity is usually best accessed by using catheter angiography, which is considered the criterion standard for AVM detection. Nevertheless, catheter angiography is a useful adjunct to cross-sectional imaging in the overall assessment of CNS AVMs, and each test provides complementary information.

False Positives/Negatives

Angiographic false-positive findings are unusual but can occur in the presence of an early draining vein. This vein can be seen in a variety of disorders (most typically, in stroke). Abnormal neovascularity and abnormal venous drainage can also be seen in CNS neoplasms, particularly vascular glioblastomas and hemangioblastomas.

False-negative results can occur after an acute hemorrhage, when an AVM may become angiographically occult or compressed by an adjacent hematoma. Partially or totally thrombosed lesions may show less-pronounced or absent AV shunting, or they may appear largely normal, with a vascular-shift mass effect stagnant flow; this can further lead to a false-negative result.²²

Multimedia

Media file 1: A CT scan of the head that demonstrates a left occipital arteriovenous malformation (AVM), with multiple calcified phleboliths and numerous hyperattenuating vascular channels.

Media file 2: Arteriovenous malformation (AVM) of the brain. An axial T2-weighted MRI showing numerous flow voids corresponding to the CT findings (not shown). Note the mass effect on the lateral ventricle despite the lack of a mass or hemorrhage.

Media file 3: A sagittal T1-weighted MRI demonstrating a large occipital arteriovenous malformation (AVM) with parasagittal flow voids.

Media file 4: A T2-weighted coronal MRI of an arteriovenous malformation (AVM) of the brain.

Media file 5: A diffusion-weighted MRI showing a lack of signal intensity associated with an arteriovenous malformation (AVM).

Media file 6: A lateral left carotid angiogram demonstrating a mixed pial-dural arteriovenous malformation (AVM). Arterial and occipital arterial feeders extend to the nidus via distal branches of the middle cerebral artery.

Media file 7: Arteriovenous malformation (AVM) of the brain. An anteroposterior right carotid angiogram showing left anterior cerebral artery supply secondary to vascular steal. Note that the left anterior cerebral artery does not opacify with an ipsilateral carotid injection of contrast material (see also Image 6).

Media file 8: Arteriovenous malformation (AVM) of the brain. A lateral left vertebral angiogram demonstrating a huge left posterior cerebral artery feeder to the nidus.

Media file 9: Arteriovenous malformation (AVM) of the brain. An anteroposterior left vertebral angiogram.

Media file 10: Arteriovenous malformation (AVM) of the brain. The venous phase of a vertebral angiogram that demonstrates numerous superficial and deep draining veins.

Media file 11: A sagittal T1-weighted MRI showing a small, right parietal arteriovenous malformation (AVM) with superficial venous drainage.

Media file 12: Arteriovenous malformation (AVM) of the brain. A T2-weighted axial MRI showing flow voids.

Media file 13: Arteriovenous malformation (AVM) of the brain. A diagnostic angiogram obtained with a carotid injection that shows the distal anterior cerebral artery (pericallosal artery) feeder, with superficial venous drainage to the superior sagittal sinus.

Media file 14: Arteriovenous malformation (AVM) of the brain. A vertebral-injection image showing the collateral arterial supply stemming from the distal posterior cerebral artery vessels.

Media file 15: Arteriovenous malformation (AVM) of the brain. a venous-phase image demonstrating large, superficial draining veins.

Media file 16: Arteriovenous malformation (AVM) of the brain. A CT scan of the posterior fossa demonstrating a hemorrhage in the fourth ventricle, with extension to the left cerebellum.

Media file 17: Arteriovenous malformation (AVM) of the brain. An axial T2-weighted MRI that shows abnormal vascular flow voids along the left cerebellar tentorium.

Media file 18: Arteriovenous malformation (AVM) of the brain. A left carotid angiogram that demonstrates feeders to the dural malformation via the posterior division of the middle meningeal artery and the occipital arteries.

Media file 19: Arteriovenous malformation (AVM) of the brain. A left vertebral angiogram demonstrating small dural arterial feeders via posterior meningeal arteries, which arise directly from the vertebral artery.

Media file 20: Arteriovenous malformation (AVM) of the brain. A venous-phase image demonstrating a venous aneurysm that projects inferiorly from the cerebellar tentorium; it is the presumed cause of the patient's hematoma.

Media file 21: A lateral left carotid angiogram in a patient with seizures and an unruptured left parasagittal arteriovenous malformation (AVM).

Media file 22: A lateral left carotid angiogram obtained after a successful staged embolization with a liquid adhesive that shows lack of arterial venous shunting.

References

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Keywords

AVM, arteriovenous malformation, AV malformation, arteriovenous malformations, vascular malformation, AVM brain, arterial venous malformation, arteriovenous aneurysm, arteriovenous angioma, cerebrovascular malformations, pial AVMs, parenchymal AVMs, dural AVMs, vein-of-Galen aneurysm

Contributor Information and Disclosures

Author

Robert A Koenigsberg, DO, MSc, FAOCR, Professor, Director of Neuroradiology, Program Director, Diagnostic Radiology and Neuroradiology Training Programs, Department of Radiology, Hahnemann University Hospital, Drexel University College of Medicine
Robert A Koenigsberg, DO, MSc, FAOCR is a member of the following medical societies: American Osteopathic Association, American Society of Neuroradiology, Radiological Society of North America, and Society of NeuroInterventional Surgery
Disclosure: Nothing to disclose.

Coauthor(s)

Jeffrey R Wasserman, DO, Diagnostic Radiologist, Manatee Memorial Hospital and Lakewood Ranch Medical Center
Jeffrey R Wasserman, DO is a member of the following medical societies: American Medical Association
Disclosure: Nothing to disclose.

Bernadette R Diegnan, Drexel University College of Medicine
Disclosure: Nothing to disclose.

Medical Editor

Lucien M Levy, MD, PhD, Director of Neuroradiology, Professor of Radiology, Department of Radiology, George Washington University Medical Center
Lucien M Levy, MD, PhD is a member of the following medical societies: American Cancer Society, American College of Radiology, American Heart Association, American Medical Association, American Roentgen Ray Society, American Society of Neuroradiology, and Radiological Society of North America
Disclosure: Nothing to disclose.

Pharmacy Editor

Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand
Disclosure: Nothing to disclose.

CME Editor

Robert M Krasny, MD, Resolution Imaging Medical Corporation
Robert M Krasny, MD is a member of the following medical societies: American Roentgen Ray Society and Radiological Society of North America
Disclosure: Nothing to disclose.

Chief Editor

James G Smirniotopoulos, MD, Professor of Radiology, Neurology, and Biomedical Informatics, Chairman, Department of Radiology and Radiological Sciences, Uniformed Services University of the Health Sciences
James G Smirniotopoulos, MD is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, American Society of Head and Neck Radiology, American Society of Neuroradiology, American Society of Pediatric Neuroradiology, Association of University Radiologists, and Radiological Society of North America
Disclosure: Nothing to disclose.

Tina Maiorano, BS, is gratefully acknowledged for contributions made to this article.

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