The origin of these vascular lesions has not been settled—several mechanisms may be involved. Most evidence suggests that at least some of them, unlike conventional cerebral AVMs and aneurysms, are not developmental in origin. The best-defined examples of acquired fistulas are those that arise adjacent to a venous sinus thrombosis or in association with a vascular atresia, most often of the transverse sigmoid sinus or adjacent to the cavernous sinus. However, it is not always clear whether the abnormality of the venous sinus is the cause or the result of the dural fistula. In a number of cases, a dural fistula has appeared after a forceful head injury, often in a region remote from the site of impact. Another small group that is more clearly developmental is associated with the Klippel-Trenaunay or Osler-Weber-Rendu syndromes, diseases in which a conjunction with AVMs is well known. (In the first of these, they may also be an associated enlargement of the limb containing the malformation.) Usually, these causes can be excluded by physical examination and an absence of family history and the largest group remains idiopathic.
A major obstacle to understanding of dural fistula is the varied ways in which this lesion presents itself clinically. Most commonly, there is a fluctuating ischemic-like deficit appropriate to the cerebral or spinal location underlying the lesion or at some distance from it in the case of spinal lesions. The spinal presentations are complex and discussed in Chap. 42. Subdural hemorrhage is an infrequent but dramatic mode of presentation, sometimes creating a large and fatal clot; another syndrome is a cerebral–subarachnoid hemorrhage, although this occurs with not nearly the same frequency or severity as bleeding from brain AVMs. Indeed, the risk of bleeding from dural fistulas and the evolution of these lesions is far less precisely known than it is for cerebral AVMs. It appears that the dural lesions most at risk of bleeding are those located in the anterior cranial fossa and those at the tentorial incisura. Seizures are uncommon. Yet another special syndrome linked to dural AVMs, although it may occur also with high-flow cerebral malformations, is of headache, vomiting, and papilledema—namely, pseudotumor cerebri (see Chap. 29). A rare syndrome is that of thalamic venous congestion causing a dementing syndrome. Whether the increased intracranial pressure is the cause or the result of the fistula is unsettled, but relief of venous insufficiency may result in regression of fistulae. A cranial bruit, audible to either the examiner or patient, is infrequent with fistula but may be sought. In small children, the high-flow lesions may shunt so much blood as to cause congestive heart failure, similar to arteriovenous malformations of the vein of Galen.
Treatment is by surgical extirpation or endovascular embolization, at times a painstaking procedure because of the multitude of potential feeding vessels. Surgery seems preferable for the smaller lesions and embolization for larger and inaccessible ones. Slowed flow in a venous sinus that is draining a malformation risks venous thrombosis but the advisability of anticoagulation in this circumstance remains uncertain.