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Aneurysms, AVMs, and dAVFs: Diagnosis and Treatment Bradford T March M.D, Mahesh V Jayaraman M.D

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S0037-198X(13)00069-2 http://dx.doi.org/10.1053/j.ro.2013.10.004 YSROE50447

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Seminar in Roentgenology

Cite this article as: Bradford T March M.D, Mahesh V Jayaraman M.D, Aneurysms, AVMs, and dAVFs: Diagnosis and Treatment, Seminar in Roentgenology, http://dx.doi.org/ 10.1053/j.ro.2013.10.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Aneurysms, AVMs, and dAVFs: Diagnosis and Treatment Bradford T. March M.D., Mahesh V. Jayaraman M.D. Departments of Diagnostic Imaging (BTM), and Neurosurgery (MVJ, BTM) Warren Alpert School of Medicine at Brown University Rhode Island Hospital Address correspondence to: Mahesh V. Jayaraman M.D. Email: [email protected] 3rd Floor, Main Building, Room 377 593 Eddy Street Providence, RI 02903 (401) 444-5184 (401) 444-5017 (fax) Introduction Despite the fact that intracranial aneurysms and vascular malformations account for a small overall percentage of hemorrhagic strokes, their rupture is potentially devastating and often fatal. Imaging plays a crucial role in the accurate identification and treatment planning of these lesions. This article provides a review of the imaging findings of cerebral aneurysms, arteriovenous malformations and dural arteriovenous fistulae. For the sake of length, we will not discuss other cerebral vascular malformations such as cavernous malformations or capillary telangiectasias. Basic pathologic and clinical concepts for each lesion will be discussed, with a focus towards the diagnostic imaging work-up. We will also briefly discuss treatment options for these lesions. Intracranial Aneurysms Saccular Aneurysms Etiology and Pathology

Saccular aneurysms (SAs) are acquired focal arterial outpouchings affecting only part of the parent artery circumference, and are commonly called “berry” aneurysms due to their sac- or berry-like appearance. Autopsy and angiographic studies estimate the prevalence of unruptured intracranial aneurysms at 3.6 – 6.0% of the general population age over 30 years of age.1 These lesions are thought to develop due to increased wall shear stress from abnormal vascular hemodynamics. Over time, this leads to progressive remodeling and breakdown of the internal elastic lamina and tunica media, weakening the arterial wall and creating a favorable environment for aneurysm formation.2 Consequently, most SA walls are comprised of only a thin layer of intima and adventitia. While truly congenital SAs are rare, certain vascular anomalies which increase hemodynamic stress (ex. bicuspid aortic valves, aortic coarctation, a persistent trigeminal artery, and arteriovenous malformations) and inherited connective tissue disorders that weaken the extracellular matrix of the arterial wall (ex. Marfan and Ehlers-Danlos syndromes, and autosomal dominant polycystic kidney disease) predispose to aneurysm development.3 Most aneurysms (90%) arise from the anterior circulation, with 33% of all SAs arising from the anterior communicating artery (Acomm), 33% from the posterior communication artery (Pcomm), and 20% from the MCA bifurcation/trifurcation.3 The remaining 10% arise from the posterior circulation, most commonly at the basilar artery bifurcation or PICA origin. SAs vary in size from small (2.5 cm). Most SAs are solitary; however 15-20% of patients have multiple lesions, with 75% of those having only two lesions.3 Presentation

Intracranial aneurysms are typically clinically silent until they rupture into the subarachnoid space, with often catastrophic consequences. Nontraumatic SAH (ntSAH) accounts for approximately 3-5% of all strokes, and aneurysmal rupture is implicated in 80% of these cases.4 Annual incidence has been estimated at 6 to 8 per 100,000 persons in developed nations, with between 16,000 and 30,000 new cases in the United States each year.4 Despite recent advances in diagnosis and treatment, aSAH remains fatal or severely disabling in over two-thirds of all patients, and 50% of patients admitted with SAH either die in hospital or are discharged to long term care facilities.5 Approximately 75% of patients present with a “thunderclap” headache, often described as the “worst headache of my life,” that peaks within minutes. Peak age at presentation is 40-60 years, and is approximately 1.6 times more common in women than men.6 Modifiable risk factors associated with an increased risk of aSAH include hypertension, smoking, and BMI under 25.7 The most commonly used grading scales for aSAH are the clinically based Hunt and Hess and World Federation of Neurological Sciences scales (Table 1). These serve as predictors of clinical outcome and are often used to guide treatment options.8,9 Imaging Imaging findings generally depend on whether the SA is ruptured or unruptured, and whether or not there is intra-aneurysmal thrombosis. NECT remains an important screening tool in the evaluation of severe, acute headache in the emergency setting, with a sensitivity of 98% for SAH within 12 hours of symptom onset.10 Because most aneurysms arise from the circle of Willis (COW) or middle cerebral artery (MCA) bifurcation, hyperdense blood typically fills the suprasellar cistern and sylvian fissures. Vertebrobasilar aneurysms may fill the fourth ventricle,

prepontine cistern, and foramen magnum. Intraventricular hemorrhage (IVH) may also be present, and is associated with poorer long-term outcomes. Parenchymal hematomas are rare, but can be seen with lobulated Acomm SAs that rupture superiorly, producing a “flame” shaped frontal cortex hematoma (Figures 1,2) or with posteriorly oriented MCA bifurcation aneurysms that rupture into the anterior temporal lobe. Posterior circulation aneurysms such as those arising from the posterior inferior cerebellar artery can produce focal hemorrhage in the Foramen of Luschka (Figure 3). Small, unruptured SAs are usually occult on NECT, however larger aneurysms may appear slightly hyperdense or display a rim of mural calcifications outlining portions of their wall. MR findings are highly variable depending on individual aneurysm characteristics and the age of associated hemorrhage if present. Approximately half of all patent SAs demonstrate “flow voids,” or flow related loss of signal, on T1 and T2WI.3 Larger aneurysms may contain heterogeneous signal from slow or turbulent flow within the aneurysm sac. Laminated clot with different signal intensities and “blooming” on susceptibility-weighted imaging (GRE) is common if intra-aneurysmal thrombosis is present. Pulsation artifact in the phase encoding direction can be diagnostic for aneurysm (Figure 4). FLAIR sequence images are best for identifying SAH, appearing as hyperintense CSF in the sulci or cisterns. Other causes of hyperintense CSF on FLAIR include meningeal infection or neoplasm, and artifact due to supplemental oxygen or pulsation.3 Once ntSAH has been diagnosed on NECT, angiography is indicated to evaluate for aneurysm presence and location. While DSA remains the reference gold standard for the detection of intracranial aneurysms, multidetector CTA has gained increasing acceptance as a fast, accurate, and noninvasive tool for evaluation of ntSAH. When a ruptured aneurysm is to

blame, CTA has a 95% sensitivity for the detection of lesions greater than 2 mm in size.11 In addition, CTA usually provides enough information to triage patients to either endovascular coiling or surgical clipping, without the prior use of DSA. The differential diagnosis of ntSAH is important to keep in mind, especially in cases where angiography fails to reveal an aneurysm. Traumatic SAH typically occurs in proximity to cortical contusions and is therefore focal and peripheral in distribution, as opposed to the diffuse pattern seen in aSAH. Perimesencephalic nonaneursymal SAH (pnSAH), the most common cause of nonaneurysmal nontraumatic SAH, has a characteristic distribution localized to the prepontine, interpeduncular, and/or ambient cisterns. Although this is a clinically benign and self limited entity, thought to represent venous rupture, vascular imaging is still indicated as this SAH pattern can be mimicked by a ruptured verbrobasilar aneurysm.12 Convexity SAH (cSAH) is localized to the superficial sulci over the cerebral convexities, a pattern very different from typical aSAH. A wide range of potential etiologies exist for cSAH, including reversible cerebral vasoconstriction syndrome (which often presents with a “thunderclap” headache mimicking aSAH), vasculitis, cortical vein thrombosis, vascular malformations, and cerebral amyloid angiopathy in the elderly.4,13 Treatment Treatment options depend on patient presentation (ruptured vs unruptured), aneurysm anatomy, and operator experience. Unruptured anterior circulation aneurysms (except for Pcomm aneurysms) less than 7mm in size in patients without personal or family history of aSAH have a favorable natural history and many can be managed with serial imaging observation.14 Ruptured aneurysms are generally treated emergently. The traditional gold standard of aneurysm occlusion

has been surgical clipping across the aneurysm neck. Over the last two decades, however, endovascular occlusion using a varity of techniques has gained increasing popularity. This is in part due to the International Subarachnoid Aneurysm Trial (ISAT), an international multi-center trial which randomized patients whose clinical and aneurysmal features would have been amenable to either surgical clipping or endovascular coiling. In that study, patients who were treated with coiling had lower rates of death or disability both at one year, and at longer followup periods.15,16 Currently, endovascular therapy is considered a first line option for many patients with favorable aneurysm geometry. The most commonly used endovascular technique is occlusion of the aneurysm lumen using detachable coils. While large studies have confirmed the high technical success rate of endovascular coiling, up to 20% of aneurysms display some degree of luminal recanalization on follow up imaging, requiring retreatment in 10% of cases. While surgical clipping may be preferable for wide-neck SAs, newer endovascular techniques such as balloon assisted and stent assisted endovascular coiling have expanded the role of endovascular management for these lesions. Flow diversion using intravascular stents has recently emerged as a new and promising technique for endovascular occlusion. A reduction in blood flow within the aneurysm lumen is created by bridging the aneurysm neck with a low porosity stent, leading to complete luminal thrombosis. In addition, the stent theoretically provides a scaffold for neo-endothelialization and closure across the aneurysm neck. Adjacent side branches of the parent artery are thought to remain patent due to flow demand. Currently these devices are primarily used to treat larger wide-necked aneurysms for which conventional endovascular or surgical therapy may not lead to

an optimal result. Since dual antiplatelet therapy is recommended to reduce the risk of in-stent thrombosis, the application of this technique to ruptured aneurysms has been limited.17,18 Imaging Follow Up Imaging follow-up is important in aSAH to evaluate for the known complication of cerebral vasospasm (CVS). Delayed cerebral ischemia due to vasospasm is one of the most common causes of death in patients who survive the initial hemorrhage. Patients with aSAH receive frequent neurological assessments, which may be combined with transcranial Doppler (TCD) or CT/MR perfusion imaging to identify those at risk for this complication. In patients with high TCD velocities or perfusion abnormalities, CT or MR angiography may be used to screen for vessel narrowing, with eventual use of DSA for definitive diagnosis and potential treatment.19 Up to 70% of aSAH patients develop vessel narrowing on follow up angiography, with clinical symptoms developing in 46% of those cases.20 Studies investigating the efficacy of nicardipine and clazosentan infusion, as well as angioplasty, to treat vasospam related to aSAH demonstrated angiographic improvement in the degree of arterial narrowing but no benefit in preventing cerebral ischemia or clinical symptoms.21 Mycotic and Oncotic Aneurysms Mycotic and oncotic aneurysms are rare pseudoaneurysm (PA) subtypes that result from either infectious (mycotic) or neoplastic (oncotic) breakdown of the vascular wall. Either a remote origin (typically cardiogenic emboli) or direct invasion from a local source may be implicated. Endocarditis is the most common cause of mycotic aneurysms and atrial myxoma is the most common cause of oncotic aneurysms. Arterial rupture leads to formation of a paravascular hematoma, which then cavitates and communicates directly with the arterial lumen via the

rupture site. These lesions are particularly prone to rupture and rebleeding, as they are contained by only a fragile network of clotted blood. Mycotic and oncotic PAs generally occur distal to the COW. NECT often reveals a parenchymal hematoma which may obscure the underlying PA. Unexplained enlargement or abnormal/delayed evolution of an existing parenchymal hematoma can suggest the presence of an occult PA. The appearance on MR depends on the age of the associated hematoma and the PA is rarely visualized discretely (Figure 5). The appearance of the aneurysm sac on DSA varies considerably, and may appear globular, fusiform, or irregular and “neckless.” The PA lumen commonly displays delayed filling and emptying of contrast. Differentiating between SAs and PAs on imaging can occasionally be challenging. In these cases, clinical history, aneurysm location and the presence or absence of parenchymal hemorrhage are helpful clues.22 Open surgery with evacuation of the hematoma and trapping or sacrifice of the parent artery with or without bypass grafting is the traditional treatment approach. Increasingly, endovascular techniques have emerged as a viable option. These include both coil embolization of the parent artery and the use of liquid embolic agents such as Onyx (ev3, Irvine, CA) or TruFill nBCA (Codman Neurovascular, Raynham, MA). Blood Blister Aneurysm Blood blister aneurysms (BBAs) are rare, wide-necked, shallow bulges that arise from nonbranching sites of cerebral arteries. These lesions likely arise from a combination of hemodynamic stress and atherosclerosis, and are often covered by only a thin cap of fibrous tissue. Therefore, like PAs they are particularly prone to rupture.

BBAs are notoriously subtle lesions that are frequently diagnosed using DSA after negative CT or MR angiography for ntSAH. Slight arterial bulging in only one projection is typical. By far the most common location is the dorsal wall of the supraclinoid ICA (Figure 6).23 Given their friable nature and lack of a definable neck, these lesions are particularly difficult to treat. Attempts at surgical clipping or endovascular coiling frequently lead to perforation of the aneurysm wall. In the future, flow diverting stents may offer another therapeutic option. Similarly, dissecting aneurysms of the vertebrobasilar system can present with devastating intracranial hemorrhage and can often have a similar appearance (Figure 7) Arteriovenous Malformations Etiology and Pathology Brain arteriovenous malformations (AVMs) are rare congenital defects of vascular development, characterized by direct arterial to venous shunting via a complex tangle of abnormal, thin walled vessels called a “nidus.” Without the resistance of intervening capillaries and arterioles, draining veins are subjected to arterial pressure resulting in increased blood flow, tortuosity, and enlargement. Most intracranial AVMs are parenchymal lesions or “pial AVMs,” with an abnormal connection existing between a pial artery and a cortical draining vein. Given their rarity, accurate statistics are difficult to obtain; however prevalence has been estimated as high as 0.2% of the general population.24 Most pial AVMs are supratentorial (85%) and located within the cerebral hemispheres. The overwhelming majority are sporadic and solitary (98%), though multiple AVMs are associated with certain syndromes such as hereditary hemorrhagic telangiectasia (HHT) and cerebrofacial arteriovenous metameric syndrome (CAMS).3 High velocity flow and increased

hemodynamic stress within the arteries feeding an AVM predisposes to the formation of saccular aneurysms either on the feeding artery or along more traditional Circle of Willis sites.25 Presentation and Natural History Despite their rarity, AVMs are potentially devastating lesions. Approximately half of all AVMs come to clinical attention after rupture into the surrounding brain parenchyma, presenting with severe headache and stroke like symptoms. The remainder initially present with either a seizure or focal neurologic deficit. Given the increasing availability of MRI, a small but significant number of these lesions are discovered incidentally. Peak age at presentation is 20-40 years; however 25% of patients are symptomatic by age 15.24 The Spetzler-Martin grading system for brain AVMs, developed in 1986, has been widely accepted by clinicians as a simple guide to estimating the surgical risk associated with AVM treatment. Points are assigned to a lesion based on size, eloquence of surrounding brain, and venous drainage pattern (Table 2). Venous drainage is considered superficial if it involves only cortical veins and convexity dural sinuses, and deep if it courses into the vein of Galen. AVM locations considered eloquent include the sensorimotor cortex, visual cortex, language areas, hypothalamus, internal capsule, brainstem, cerebral peduncle, and deep cerebellar nuclei. The final AVM grade is the sum of the points, ranging from I to V. The relation between lesion grading and treatment planning is discussed below.26 Predicting AVM hemorrhage is a challenging task. While the average annual AVM rupture rate is estimated at 2-4% cumulative, this varies widely depending on certain risk factors. Imaging features associated with an increased risk of rupture help guide lesion grading and treatment options and are important to note during work-up. These include imaging evidence of

prior hemorrhage (best identified on MR), deep brain location, deep venous drainage (i.e. towards the vein of Galen), saccular aneurysm of a feeding artery or anywhere in the circle of Willis, intranidal ectasia, and draining vein stenosis.24,27 Although controversial, higher SpetzlerMartin Grade28 and larger nidal size29 have also been implicated as having higher risk of hemorrhage. Imaging Lesion size varies from microscopic to giant, occupying most of a cerebral hemisphere. The typical AVM nidus is intermediate in size, ranging between 2 and 6 cm in diameter, with a compact ovoid or pyramidal shape and no associated mass effect (Figures 8,9). Often the broadest surface of the lesion lies near the cortex, with an apex pointing towards the ventricles. There is no normal brain parenchyma interspersed between the vessels of an AVM nidus, and varying degrees of gliosis, ischemia, dystrophic calcification, and products hemorrhage commonly lie within the adjacent brain. Diagnostic criteria include (1) the presence of a nidus within the brain parenchyma and (2) early venous drainage, best identified using DSA. On NECT, AVMs typically exhibit numerous, compact, well-delineated and slightly hyperdense serpentine vessels, often with associated calcification. These vessels enhance strongly with the addition of contrast, resembling a “bag of worms.” The appearance on MR varies with vascular hemodynamics and the presence and age of associated hemorrhage. Given that most AVMs are high flow lesions, a tightly packed “honeycomb” of flow voids is usually present on T1 and T2WI. Associated draining veins may display linear enhancement. Any brain parenchyma found within the nidus is typically gliotic and hyperintense on T2WI and FLAIR.

Susceptibility artifact can be seen adjacent to the lesion on GRE and is indicative of prior hemorrhage (Figure 10).3,30 DSA is the gold standard for the diagnosis and characterization of brain AVMs, and is generally indicated in all cases to identify subtle features often missed on cross sectional imaging. Increased flow and hemodynamic stress results in enlargement and tortuosity of the feeding arteries, which may display stenosis and/or thrombosis. Flow-induced “pedicle” aneurysms are seen in 10-15% of cases. The nidus appears as a tightly packed tangle of small enhancing vessels. Approximately half contain at least one ectactic vessel, commonly called an “intranidal aneurysm.” Early opacification of draining veins during the mid- to late-arterial phase indicates arteriovenous shunting and is a hallmark for the diagnosis of AVM. Under arterial pressures, these veins are often enlarged and tortuous (Figure 11). Stenosis of a draining vein may increase intranidal pressure and lead to AVM hemorrhage.31 The main differential diagnosis of brain AVM is cerebral proliferative angiopathy (CPA), a rare vascular disorder characterized by diffusely increased angiogenesis often involving an entire cerebral lobe or hemisphere. These are less aggressive lesions, which rarely present with intracranial hemorrhage. MR reveals numerous enlarged vessels interspersed with normal brain parenchyma, and DSA fails to show a well circumscribed nidus.32 A pial AV fistula is a rare vascular malformation involving a single dilated pial artery connecting directly to an enlarged and often aneurysmal cortical draining vein without an intervening capillary network or nidus. These are generally brain surface, subcortical, or subependymal lesions.33 Treatment

Treatment options vary with individual lesion characteristics. Low grade (Spetzler-Matin I-III), superficial AVMs in noneloquent brain are typically treated surgically, with high rates of technical success and low morbidity. High grade (Spetzler-Martin III-V), larger, deeper lesions with deep venous drainage are associated with high rates of surgical morbidity and mortality, however, and alternative or multimodality options are often indicated. Stereotactic radiosurgery (SRS) may be used to treat small, deep AVMs, with a reported cure rate of 65%-80% for lesions

Aneurysms, arteriovenous malformations, and dural arteriovenous fistulas: diagnosis and treatment.

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