Pathology of Dural Arteriovenous Fistulas in Children

Pathophysiology

• Venous outflow obstruction: Those that present during infancy might be considered congenital secondary to venous obstruction or venous hypertension during the fetal period. DAVFs that present later in childhood might be acquired secondary to sinus thrombosis(18), trauma or surgery. In both cases, obstructions of the natural venous outflow cause collateral circulation through crack-like vessels that lead to DAVF formation(10).
• Abnormal vessels and thrombus: Abnormal arteries and veins are found inside the dura and an organized sinus thrombus sometimes exists in the sinus.
• Arteriovenous fistulas: Fistulous connections between arteries and veins occur inside or in continuity with the dura mater and not in the sinus itself.
• Progressive venous hypertension recruits additional vessels: After such an abnormal arteriovenous fistula has formed in the wall of the venous sinus, thickening and steno-occlusive sinus lesions can induce the fistula to recruit arterial blood from numerous dural and, later, pial arteries. Increased sinus pressure stimulates disease progression, and thus a vicious cycle might ensue(10).
• Multifocal, complex anatomy: As opposed to their adult counterparts, children tend to have a more aggressive clinical course and a greater incidence of multifocality and complexity.

Anatomic-Clinical Classification

DAVFs usually affect the occipital-suboccipital region. Their most common locations are sigmoid-transverse, sagittal, and cavernous sinuses(14).

• Dural sinus malformation: Usually involves the posterior sinuses (sagittal, transverse, jugular bulb and torcular region), which typically occur in neonates. Those affecting the posterior region tend to be low-flow shunts, while those involving the jugular are high-flow sigmoid sinus DAVFs. In this type of DAVF the prognosis depends on the location—midline malformations (those involving the torcula) have a worse prognosis than those located laterally because in the latter cases the normal venous vessels provide the necessary drainage for the brain.
• Infantile type dural arteriovenous shunts: High flow, low pressure malformations that generally present in the first few years of life. Sinuses are large and patent with no lakes. The long-term prognosis is poor.
• Adult type dural arteriovenous shunts: These can present in all age groups and are almost always located in the cavernous venous plexus.

Venous Drainage Classification

Other authors have classified DAVFs according to the degree of blood reflow into the leptomeningeal veins. Data used for these classifications are mostly from adult patients and based on the idea that the venous drainage pattern determines the clinical presentation of cranial DAVFs. Borden (2) categorized these malformations into the following types:

• Type I: Those that drain directly into the dural venous sinus or meningeal vein. Flow is in a normal direction and their course is typically benign.
• Type II: Those that drain into the venous sinuses with retrograde drainage into the subarachnoid veins. They usually present with symptoms related to venous hypertension or hemorrhage.
• Type III: Those that drain into the subarachnoid veins. The sinus may be patent or occluded; however, when the sinus is patent, the arterialized vein and the sinus are not in communication at the point of the fistula. They also present with hemorrhage or other manifestations of venous hypertension.
• Subclassification: The above three types of malformations can be subclassified into subtype a, simple fistula, or subtype b, multiple fistulae.

Molecular/Genetic Pathology

Vascular endothelial growth factor (VEGF): VEGF has been implicated in the development of these lesions(27).

Histopathology

• Thick vessel walls: Thickened walls of the artery and vein, with smooth muscle cells, probably indicate high pressure blood flow, sustained over a long period of time.
• Stratified elastic lamina: In addition, the internal elastica lamina of dural arteries in DAVF cases tended to be stratified, as is usually seen in arteries of pial AVMs.