The Borden Classification of dural arteriovenous malformations or fistulas, groups into three types based upon their venous drainage:

Type I: dural arterial supply drains anterograde into the venous sinus.

Type II: dural arterial supply drains into venous sinus. High pressure in sinus results in both anterograde drainage and retrograde drainage via subarachnoid veins.

Type III: dural arterial supply drains retrograde into subarachnoid veins.

see Borden type I intracranial dural arteriovenous fistula

The high pressure within a Type II dural AV fistula causes blood to flow in a retrograde fashion into subarachnoid veins which normally drain into the sinus. Typically this is because the sinus has outflow obstruction. Such draining veins form venous varices or aneurysms which can bleed. Type II fistulas need to be treated to prevent hemorrhage. The treatment may involve embolisation of the draining sinus as well as clipping or embolization of the draining veins.

Type III

Type III dural AV fistulas drain directly into subarachnoid veins. These veins can form aneurysms and bleed. Type III dural fistulas need to be treated to prevent hemorrhage. Treatment can be as simple as clipping the draining vein at the site of the dural sinus. If treatment involves embolization, it will only typically be effective if the glue traverses the actual fistula and enters, at least slightly, the draining vein.