Dural Arteriovenous Malformations

Introduction

Dural arteriovenous malformations (DAVMs), also known as fistulas or shunts, are abnormal connections of dural arteries to dural veins or venous sinuses originating from within the dural leaflets. This is in contrast to the most common arteriovenous malformation where the abnormal connection resides within the brain parenchyma. DAVMs are typically located near or within the wall of a dural sinus that is frequently thrombosed. The dural fistula point, contained within the dural leaflets, drains directly to a dural sinus or via retrograde flow through cortical veins (leptomeningeal drainage). Dual arteriovenous fistulas can occur at any dural sinus with the most frequently observed location being at the cavernous or transverse sinus.

Historically DAVMs have been a confusing entity. This is in part due to multiple names given to them by different groups, and the difficulty in separating the primary abnormality, dural arteriovenous shunting, from other secondary changes. Some believe that DAVMs should be more appropriately named dural arteriovenous shunts, dural arteriovenous fistulas, or dural arteriovenous fistulous malformations. The DAVMs are distinguished from other intracranial vascular lesions by their nidus of arteriovenous shunting, housed wholly within the leaflets of the dura mater ( Fig. 70.1 ). They are typically located adjacent to a major dural sinus, and their arterial supply originates from dural (meningeal) arteries, pachymeningeal branches of cerebral arteries, and transosseous dural branches of scalp arteries. Venous drainage occurs through an adjacent dural sinus and/or other dural and leptomeningeal venous channels. While their etiology is controversial, the vast majority of DAVMs appear to be acquired lesions. DAVMs may remain asymptomatic or can manifest a wide range of symptoms including headache, tinnitus, bruit, cranial neuropathy, seizures, dementia, intracranial hypertension, or focal neurologic deficits from venous congestion. They may also present with life-threatening intracranial hemorrhage. While it remains difficult to prognosticate for a given lesion, the location (i.e., cavernous sinus, anterior cranial fossa, or tentorial incisura) and pattern of venous drainage (i.e., venous sinus versus retrograde leptomeningeal venous drainage [RLVD]) clearly influence clinical presentation. Lesions that present with hemorrhage are commonly associated with a significantly increased risk for additional morbidity and mortality. While many unruptured DAVMs are best treated expectantly, there are features that identify high-risk lesions and these may be definitively treated using a careful selection of endovascular, surgical, and/or radiosurgical techniques.

Clinical Presentation

DAVMs comprise 10%–15% of cranial vascular malformations; 6% of supratentorial and 35% of infratentorial vascular lesions are dural fistulas. Typical presentation is between 30 and 50 years of age with a female preponderance, but hemorrhage is more common in males. A few DAVMs present early in life and are thus thought to be congenital. These are usually associated with complex congenital anomalies, rare phakomatoses, or a vein of Galen malformation—a special form of DAVM. In these cases, there is often gross malformation of dural sinuses with atresia of venous outflow in the region of dura mater involved with the DAVM. The vast majority of DAVMs present later in life and are assumed to be acquired. ^, Lesions are often located in proximity to the major sinuses (50% transverse-sigmoid, 15% cavernous, 10% tentorial, 8% superior sagittal). Known or suspected etiologic factors include trauma, infection, hypercoagulable state, and neoplasm. Recent retrospective studies have further implicated the presence of a hypercoagulable state in patients with DAVMs. The most important aspect of these etiologic factors may be a shared propensity toward sinus thrombosis with a secondary alteration in venous hemodynamics (restricted outflow and venous hypertension) and development of overt shunting within arteriovenous connections that had involuted during development. Arteriovenous shunting is then thought to promote recruitment of pachymeningeal supply from dural, cerebral, and scalp arteries, into the pathologic dural leaflets. Eventually, the pathology progresses to involve changes on the venous side of the circulation (including retrograde leptomeningeal venous reflux of arterialized blood).

Clinical manifestations of DAVMs are variable, ranging from minor symptoms to catastrophic hemorrhages, and are related to the lesion location, arterial supply, and venous drainage. Symptoms can be absent, slowly progressive or have sudden onset. The more benign symptoms of pain, tinnitus, or bruit are related to arteriovenous shunting and vascular congestion in the affected dural leaflets and/or adjacent venous sinus. Serious neurologic sequelae from DAVMs are associated with RLVD. Venous hypertension may cause a myriad of related focal neurologic deficits or seizures (non-hemorrhagic neurologic deficit [NHND]), depending on the region of the brain affected. Arterialized leptomeningeal veins may become frankly variceal and are prone to hemorrhage. Hemorrhage may also occur within congested brain tissue, again a result of RLVD, and may be remote from the fistulous arteriovenous shunting.

Neuro-ophthalmic manifestations of DAVMs include visual and gaze abnormalities caused by venous hypertension, as well as orbital or ocular venous hypertension with resulting orbital crowding, venous stasis retinopathy, and possibly glaucoma. Some cranial DAVMs may present with symptoms of increased intracranial pressure (ICP), including papilledema. ^, Headaches associated with DAVMs are multifactorial, often referred from regions of dural congestion, and/or as a result of elevated ICP.

DAVMs may also result in altered cerebrospinal fluid (CSF) hydrodynamics. Dilated venous structures may act as mass lesions, obstructing CSF circulation resulting in hydrocephalus. Additionally, dural venous hypertension may result in decreased absorption of CSF with secondary intracranial hypertension and possibly papilledema. This phenomenon appears to be more common with high-flow lesions draining into large dural venous sinuses, in the setting of concomitant dural sinus outflow obstruction. Lesions at the anterior cranial fossa or the tentorial incisura rarely drain into a patent dural venous sinus and are more frequently associated with RLVD. They are more likely to cause serious clinical sequelae from RLVD, including focal neurologic deficits and hemorrhage. Hemorrhage in DAVMs has not been reported in the absence of RLVD in any published case with carefully documented diagnostic studies. Hemorrhage from DAVMs is often associated with rupture of arterialized venous structures in the leptomeningeal circulation. The prognosis of a first hemorrhage from DAVMs is ominous and is associated with a greater than 30% mortality or serious disability.

Pathophysiology and Lesion Evolution

DAVMs are usually acquired, and the exact nature of lesion genesis is frequently unknown. ^, It is thought to result from altered angiogenesis within the dura following an inciting event such as thrombosis, trauma, surgery, or chronic infection. It is hypothesized that an initial event precipitates venous sinus thrombosis that leads to opening up of occult arteriovenous vascular channels in the vicinity of the affected sinus. Cases of angiographically proven dural sinus thrombosis in which DAVMs subsequently developed in relation to the obstructed sinus have been documented. Initial microshunts then proliferate, helped by venous hypertension, maturing into clinically significant arteriovenous fistulae. The degree of progression or involution determines the significance of the abnormality. DAVMs cause decreased regional cerebral blood flow in cortical regions where there is RLVD, and this positively predicts the development of hemorrhage, focal neurologic deficits, and/or seizures.

The development of DAVMs following trauma and surgery is well known. ^, They have also been reported in association with chronic infection, vascular diseases, and tumors, all considered to be related to hypercoagulable state. Many cases of DAVM have no clear etiologic trigger. They may be identified at anatomic sites distinct from the presumed inciting event. It is inferred that development of a DAVM in these diverse settings would probably require a common mechanism, as well as a possible anatomic or genetic predisposition. ^, Recent experimental work suggests that the diverse clinical associations of DAVM may be explained by the development of venous obstruction and hypertension with aberrant angiogenesis. This has been confirmed clinically by the reports of fistulas in patients with hypercoagulable states on expanded screening. ^, When examining patients for laboratory or clinical manifestations of hypercoagulability, one retrospective review of 17 patients found an association in 88%. Expanded hypercoagulable screening from this study includes the laboratory and clinical factors listed in Box 70.1 . Additionally, in a cohort of 116 patients with DAVM, the frequency of mutations associated with thrombophilia was higher than that in the general population.