59 Pial Arteriovenous Fistulas
Abstract
Pial arterivenous fistulas (PAVFs) are uncommon lesions. They are most commonly seen in the pediatric or young adult population. The natural history of PVAFs is unfavorable with a mortality rate more than 50%. In children, these lesions can quickly cause brain atrophy, white matter calcifications, and delayed myelinization. Factors that predict bad outcome include young age, complex multihole lesion, preintervention chronic heart failure, and preintervention intracerebral hemorrhage. All PAVFs should be treated. Radiological evaluation includes MRI, MRA, and digital subtraction angiography. Treatment modalities include endovascular embolizaztion and microsurgical resection. Regardless of the modality of treatment, understanding the anatomy is the key to their disconnection.
Introduction
Cerebral pial arteriovenous fistulas (PAVFs) are rare and complex vascular malformations. These lesions are often considered a subset of cerebral arteriovenous malformations (AVMs). The somewhat limited literature addressing these lesions consists predominately of retrospective analyses from larger high-volume neurovascular referral centers, with several case reports and small cases series also published. PAVF prevalence is therefore difficult to establish with high confidence, as these retrospective reviews depend largely on imaging and clinical notes, often with the diagnosis of PAVF assigned retrospectively. However, several of the larger case series indicate that PAVFs represent between 1 and 7% of brain AVM referrals. This would suggest prevalence on the order of 0.1 to 1/100,000, with these lesions being over-represented in neonates, infants, and children. With recent improvements in digital subtraction angiography (DSA), including superselective angiography, we have been better able to characterize PAVFs as a discrete subset of AVM. Although much less common in adults, there are several reviews from tertiary referral centers describing PAVFs in young adults older than 20 years. A single case report from 2015 describes an infratentorial PAVF in a 73-year-old, perhaps the oldest reported patient with this type of lesion.
It is worth noting that PAVFs are also frequently referred to as nongalenic pial arteriovenous fistulas (NGAVFs). NGAVF is particularly a useful terminology in children, efficiently distinguishing these lesions from classic midline vein of Galen malformations involving the embryological median prosencephalic vein of Markowski. The distinction between the PAVF and NGAVF nomenclature is controversial, as some investigators distinguish NGAVFs as a further subset of PAVFs in that the NGAVFs congenital lesions always associated with a venous varix. For the purposes of this discussion, we will consider the two distinctions equally. The angioarchitectures and clinical manifestations are similar as are the approaches to diagnosis, decisions to treat, treatment modalities, and outcomes.
Major controversies in decision making addressed in this chapter include:
Whether or not treatment is indicated for PVAFs.
Differential diagnosis between PVAFs, AVMs, and dural arteriovenous fistulas (DAVFs).
Open versus endovascular treatment for ruptured and unruptured PAVFs.
Clinical presentation and management differences among different age groups (neonates, children, adults).
Whether to Treat
Due to the rarity of PAVFs, the natural history and risk of bleeding have not been well established. However, the natural history of PVAFs is unfavorable in patients with multiple feeding arteries. If untreated, mortality rate can reach as high as 63%. In neonates and children, PVAFs can quickly damage the brain tissue surrounding the fistula orifice, resulting in subependymal or cortical atrophy, white matter calcifications, and delayed myelinization ( 1 , 2 in algorithm ). Therefore, early intervention is essential for optimal neurological and cognitive development ( 4, 8 in algorithm ). Adult PAVFs can cause high-flow occlusive venopathy in a major sinus within a relatively short time period; similarly, aggressive treatment is recommended ( 3, 5, 9, 11, 13 in algorithm ).
Pathophysiology
The causes are not quite clear. Generally, they are classified as congenital or acquired.
Congenital causes: During embryological stages, shunts between arteries and veins are common, but they regress with the development of capillary network and vessel wall maturation. If fistulous connections between pial arteries and veins persist, PAVFs form. They can accompany certain congenital diseases. High incidence of RASA 1 mutations in children with PAVFs has been found.
Traumatic/Iatrogenic PAVF: Formation of PAVFs has been reported after traumatic brain injury and neurosurgical procedure (intracranial aneurysm clip, ventriculoperitoneal shunt, revascularization for moyamoya, radiosurgery). The cause or relation is unknown.
Anatomical Considerations
Pial arteriovenous fistulas consist of a direct connection between a cerebral artery and a subpial cortical vein with no intervening angiomatous formation or nidus, distinguishing them from the conventionally defined AVM. Although there exist strong similarities in clinical manifestation and treatment algorithms between the vein of Galen malformations (VOGM) and PAVFs, the latter is distinguished by the lack of a direct communication with the embryonic median prosencephalic vein, as well as its location in the subpial meningeal space rather than in the subarachnoid space as seen in VOGM. Furthermore, PAVFs are distinct from DAVF in that the former demonstrate subpial fistulous connections without venous sinus or meningeal venous drainage, whereas in the DAVF, dural feeding arteries form fistulous connections within the dural sinus.
PAVF may demonstrate only a single arterial connection to a single draining cortical vein (▶ Fig. 59.1 ). Alternatively, PAVF can manifest as a large complex fistula with multiple arteriovenous connections (▶ Fig. 59.2 ). Almost universally the draining vein is engorged forming a varix. Although differentiated from nidus AVMs, the two entities not infrequently coexist. Because PAVFs often include very high-flow fistulas, both the arteries and veins are usually enlarged, obscuring a coexisting nidus AVM that may only become apparent after first treating the fistula (▶ Fig. 59.3 ). The complex multihole fistulas are more common in the neonatal period, whereas single-hole fistulas are more common in adolescents and adults. As pointed out in a 2015 review of NGAVF in adults, while adults more often demonstrate the single dominant fistula-type PAVF, the multiple-hole fistulas seen in neonates consist of several equally dominant fistulas. Additionally, several reviews have noted dilated persistent occipital sinus in neonates, further arguing for congenital nature or a process occurring in early stages of development in this group versus a probably distinct underlying process in adults who do not have an enlarged persistent occipital sinus. Interestingly, the high-flow shunt often results in proliferative angiopathy secondary to ischemic parenchyma. This may also manifest as proliferative dural collateralization to pial tissue in these ischemic areas. A 2012 review article on pediatric NGAVF also demonstrated de novo DAVFs on follow-up angiography in patients successfully treated for PAVF, perhaps also related to ischemia-induced angiogenesis.
Workup
Clinical Evaluation
As mentioned, PAVFs are much more common in the pediatric population than in adults and clinical evaluation varies between the two ( 1 , 2 in algorithm ). Moreover, within the pediatric population, clinical manifestations can be further differentiated between neonates, infants, and adolescents.
Neonates
Neonates present a uniquely challenging set of clinical considerations for the interventionist ( 2 in algorithm ). These patients most commonly present with symptom of high-output congestive heart failure followed by pulmonary hypertension, both secondary to massive arteriovenous shunting. The newborn heart cannot compensate for high-flow shunting which not infrequently progresses to profound heart failure and subsequently further systemic manifestations including liver failure, renal failure, cyanosis, anasarca, disseminated intravascular coagulation (DIC), or multiorgan failure, similar to VOGM patients. Complicating matters further, this cohort often harbors the most complex multihole fistulas which often require multiple procedures to treat. This results in inherently higher complication rates in a patient population with higher baseline morbidity and mortality. Whenever possible in the neonatal period, priority should be given to optimizing the cardiovascular status and delaying treatment for as long as clinically feasible. In those cases with medically refractory congestive heart failure or consumptive coagulopathy ( 4 in algorithm ), the next step in preparation for treatment should be thorough clinical exam and imaging to determine the extent of preexisting neurologic injury. In cases of profound irreversible existing brain injury, management should weigh toward medical abstention and goals of care should be addressed with the patient′s caregivers ( 6 in algorithm ).
For those patients whose cardiovascular status necessitates addressing the shunting lesion and who do not appear to have profound neurologic injury, intervention should be taken with the goal to diminish the arteriovenous shunting first, and not necessarily obtaining complete occlusion of the lesion. We start with a thorough multidisciplinary discussion involving the pediatric intensive care, pediatric neurology, neurointerventional radiology, and neurosurgical services prior to endovascular intervention ( 8, 10, 12 in algorithm ). Additionally, working with an anesthesiology team experienced in neonatal high-output heart failure is essential as cardiac reserve and fluid balance is tenuous at best.
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