Pathology of Dural Arteriovenous Fistulas in Children

Pathophysiology

• Venous outflow obstruction: DAVFs identified during infancy might be considered congenital secondary to venous obstruction or venous hypertension during the fetal period. DAVFs identified later in childhood might be acquired secondary to sinus thrombosis (17), trauma, or surgery. In both cases, obstruction of the natural venous outflow causes collateral circulation through occult anastomotic vessels, which leads to DAVF formation (5).
• Progressive venous hypertension recruits additional vessels: After an abnormal AV 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 pressures cascade disease progression (5).
• 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 (18).
• Serious consequences: The most serious manifestations of DAVF include ICH due to rupture, venous infarction, seizure, and intracranial hypertension (19).

Anatomical and Clinical Classification

• Anatomical characteristics: DAVFs have a predilection for the occipital-suboccipital region. Their most common locations are the sigmoid-transverse, sagittal, and cavernous sinuses (a somewhat distinct pathology due to the close approximation with the internal carotid artery) (20).
• DSM: Consider DSM when evaluating a newborn with a lesion of the dural sinuses. DSMs are a congenital/neonatal pathology that usually involves the posterior sinuses (sagittal, transverse, jugular bulb, and torcular region) (20). Those affecting the posterior region tend to be low-flow shunts, while those involving the jugular are high-flow sigmoid sinus DAVFs. In a DSM-type 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 contralateral normal venous vessels provide the necessary drainage for the brain.
• Lasjaunias classification: In 1996, Lasjaunias et al suggested a distinction between infantile-type dural AV shunt and an adult-type dural AV shunt (20). However, aside from presentation in the first few years of life, there is not a clear distinction in the anatomic configuration and no evidence of biological difference. Lasjaunias et al. characterized adult-type dural AV shunts as those in the cavernous venous plexus. However, this classification appears somewhat convoluted. It has the appeal of some prognostic information, but advances in endovascular techniques have changed the treatment viability. Therefore, it may be more suitable to consider the effect of the venous pathophysiology and the potential risks of disordered venous flow.

Venous Drainage Classification

DAVFs have been classified according to the degree of blood flow 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.

• Classification by number of fistulas: Borden et al categorized these DAVFs into three types, each of which may be subclassified based on the number of fistulas (21).
• Classification by direction of flow: Cognard et al provided another classification system  based on venous drainage patterns; this system also considers the direction of flow and presence of venous ectasia (22).

Molecular/Genetic Pathology

• VEGF: VEGF has been implicated in the development of DAVFs (23).

Histopathology

• Thick vessel walls: Thickened walls of the artery and vein with smooth muscle cells may be observed and likely indicate high-pressure blood flow (5).
• Stratified elastic lamina: The internal elastic lamina of dural arteries in DAVF may be stratified. This may be seen in pial AVMs as well (5).