Dural Arteriovenous Fistulas

Dural arteriovenous fistulas (dAVFs) are exceptionally rare vascular anomalies formed by an abnormal connection between arteries within the dura mater and veins that normally drain the brain.  Numerous arteries and veins can participate in the formation of these lesions.  Ultimately, however, there is a single point of connection between the arterial and venous systems.  Angiographically, dAVFs can appear complex because arteries are recruited from branches of both the external and internal carotid artery.  Venous drainage can occur through the large dural venous sinuses and retrogradely (that is, opposite normal flow) through the cortical cerebral veins. This latter pattern of drainage is associated with a higher risk of the dAVF rupturing and hemorrhaging.  Ethmoidal and tentorial locations also predict higher-risk dAVFs.  These lesions are most frequently located in the transverse and sigmoid sinuses. 

Basic Pathophysiology and Signs and Symptoms

The pathophysiology of dural arteriovenous fistulas is believed to derive from venous thrombosis of a dural sinus. Subsequent collateral revascularization permits an abnormal fistulous connection to form between arteries and veins residing in the walls of a dural sinus or involving an adjacent cortical vein.  Arterialized veins contribute to venous hypertension, which can produce symptoms such as pulse synchronous bruit (sound from the blood vessel), tinnitus (ringing in the ears), headache, visual impairment, and papilledema (swelling of the optic disk indicative of increased intracranial pressure).  In high-risk dAVFs, the aberrant blood flow through the fistula and arterialized veins can lead to rupture and subsequent brain hemorrhage.

Grading Scale

Grading scales based on the anatomy of venous outflow have been developed to predict the risk of brain hemorrhage.  According to the Borden-Shucart grading system, type I dural arteriovenous fistulas exhibit antegrade drainage through a venous sinus; type II dAVFs exhibit antegrade venous sinus and retrograde cortical venous drainage; and type III dAVFs exhibit only retrograde cortical venous drainage.  The University of California, San Francisco system categorizes fistulas as grade 1, no venous restriction with antegrade flow; grade 2, ante- and retrograde venous drainage with or without cortical venous drainage; grade 3, solely retrograde and cortical venous drainage; and grade 4, cortical venous drainage only. Evaluation of their natural history indicates that high rates of visual disturbances and hemorrhage in the central nervous system are associated with Grade 3 and 4 dAVFs.

Treatment

The grade of the dural arteriovenous fistula and a patient's symptoms factor into the treatment strategy.  Typically, Borden grade I lesions can be followed without treatment unless the associated symptoms affect a patient's quality of life significantly.  In that case, stereotactic radiosurgery may be considered.  Endovascular treatment, possibly combined with microsurgical treatment, is usually recommended for grade II and III dAVFs, which have an increased risk of hemorrhage.

At Barrow, the usual treatment paradigm includes a thorough catheter-based angiographic evaluation with possible endovascular intervention during the same setting.  Endovascular surgery involves both transarterial and transvenous approaches to the site of the fistulous connection.  Typically, large feeding arteries are embolized transarterially to reduce blood flow through the fistula.  Then transvenous routes to the fistula can permit coil embolization of the fistulous point.  Sometimes, transarterial routes are large enough to permit glue embolization of the fistula as well.  If the fistula cannot be obliterated by endovascular methods alone, embolization is followed by microsurgical obliteration.  Postoperative angiography is used to confirm successful obliteration.