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What is Dural arteriovenous fistula and explain in detail?
A dural arteriovenous fistula (DAVF) or malformation is an abnormal direct connection (fistula) between a meningeal artery and a meningeal vein or dural venous sinus.

Signs and symptoms
The most common signs/symptoms of DAVFs are:
Pulsatile tinnitus
Occipital bruit
Headache
Visual impairment
PapilledemaPulsatile tinnitus is the most common symptom in patients, and it is associated with transverse-sigmoid sinus DAVFs. Carotid-cavernous DAVFs, on the other hand, are more closely associated with pulsatile exophthalmos. DAVFs may also be asymptomatic (e.g. cavernous sinus DAVFs).

Location
Most commonly found adjacent to dural sinuses in the following locations:
Transverse (lateral) sinus, left-sided slightly more common than right
Intratentorial
From the posterior cavernous sinus, usually draining to the transverse or sigmoid sinuses
Vertebral artery (posterior meningeal branch)

Causes
It is still unclear whether DAVFs are congenital or acquired. Current evidence supports transverse-sigmoid sinus junction dural malformations are acquired defects, occurring in response to thrombosis and collateral revascularization of a venous sinus.

Diagnosis
Cerebral angiography is the diagnostic standard. MRIs are typically normal but can identify venous hypertension as a result of arterial-venous shunting.

Classification
Borden Classification
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 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.

Type I
Type I dural arteriovenous fistulas are supplied by meningeal arteries and drain into a meningeal vein or dural venous sinus. The flow within the draining vein or venous sinus is anterograde.

Type Ia – simple dural arteriovenous fistulas have a single meningeal arterial supply
Type Ib – more complex arteriovenous fistulas are supplied by multiple meningeal arteriesThe distinction between Types Ia and Ib is somewhat specious as there is a rich system of meningeal arterial collaterals. Type I dural fistulas are often asymptomatic, do not have a high risk of bleeding and do not necessarily need to be treated.

Type II
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 embolization 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