What is the pathophysiology and classification of dural fistulas in adults, particularly in women with underlying conditions such as hypertension and atherosclerosis?

Pathophysiology and Classification of Dural Arteriovenous Fistulas

Dural arteriovenous fistulas (DAVFs) are acquired abnormal direct connections between meningeal arteries and dural venous sinuses or cortical veins within the dura mater, fundamentally distinguished from brain AVMs by their lack of a parenchymal nidus and their arterial supply from dural vessels rather than brain parenchyma. 1

Pathophysiology

Core Hemodynamic Mechanism

DAVFs create abnormal high-flow shunts between the high-pressure arterial system and low-pressure venous system without the normal intermediary capillary bed, resulting in direct arteriovenous communication within the dural layers. 1, 2

The pathophysiologic consequences depend critically on:

• Venous drainage patterns: The presence of cortical (leptomeningeal) venous reflux determines clinical aggressiveness, as arterial blood shunted directly into cortical veins rather than dural sinuses leads to venous hypertension in the brain parenchyma 1, 3

• Venous outflow obstruction: Venous stenosis or occlusion creates impedance to normal drainage, causing venous ectasia (indicating abnormal venous pressure) and reversal of normal venous flow direction away from the closest venous sinus 1, 4

• Venous sinus thrombosis: Often associated with DAVFs and thought to promote their development through venous hypertension; stenosis or thrombosis of dural sinuses worsens the venous hypertension 4, 3

Acquired Nature and Underlying Conditions

DAVFs are acquired lesions, not congenital, and are promoted by venous hypertension and venous sinus thrombosis. 4 In patients with hypertension and atherosclerosis (as in the expanded question context), these underlying vascular conditions may contribute to the venous pathology that predisposes to DAVF formation, though the exact mechanisms remain incompletely understood 2.

Clinical Manifestations by Pathophysiology

Symptoms directly correlate with fistula location and venous drainage patterns:

• Transverse/sigmoid sinus DAVFs: Pulsatile tinnitus from turbulent flow 1, 4
• Cavernous sinus DAVFs: Exophthalmos, chemosis, and cranial nerve palsies 1, 4
• DAVFs with cortical venous drainage: Seizures, focal neurological deficits, cognitive impairment, and dementia due to venous congestive encephalopathy 1, 3
• Increased intracranial pressure: Raised pressure within the superior sagittal sinus impedes cerebrospinal fluid reabsorption in arachnoid villi 3

Hemorrhage Risk

DAVFs with cortical venous drainage carry significant hemorrhage risk due to venous hypertension transmitted to fragile cortical veins, with venous ectasia further increasing bleeding risk. 1 Hemorrhagic complications can manifest as parenchymal hematoma, hemorrhagic infarction, or subarachnoid hemorrhage 3.

Classification Systems

Multiple classification systems exist for DAVFs, each emphasizing venous drainage patterns as the primary determinant of clinical risk:

Borden Classification

This system stratifies DAVFs based on venous drainage:

• Type I: Drainage directly into dural venous sinus or meningeal vein (benign behavior) 2
• Type II: Drainage into dural sinus with cortical venous reflux (intermediate risk) 2
• Type III: Direct drainage into cortical veins without dural sinus involvement (aggressive, high hemorrhage risk) 2

Cognard Classification

A more detailed system that further subdivides based on venous drainage direction and presence of venous ectasia:

• Type I: Antegrade drainage into dural sinus 2
• Type IIa: Retrograde drainage into sinus 2
• Type IIb: Antegrade drainage with cortical venous reflux 2
• Type IIa+b: Retrograde drainage into sinus with cortical venous reflux 2
• Type III: Direct cortical venous drainage without venous ectasia 2
• Type IV: Direct cortical venous drainage with venous ectasia (highest hemorrhage risk) 2
• Type V: Spinal perimedullary venous drainage 2

Djindjian and Merland Classification

An earlier system also based on venous drainage patterns 2.

Clinical Utility of Classification

The aggressiveness of the clinical course can be predicted through angiographic patterns of venous drainage, specifically the presence of cortical venous drainage, presence of venous ectasia, and aggressiveness of clinical presentation. 2 Higher-grade lesions (Borden Type II-III, Cognard Type IIb-V) warrant treatment due to hemorrhage risk and neurological complications, while lower-grade lesions may be observed conservatively 2, 5.

Anatomic Distribution

DAVFs constitute 10% to 15% of all intracranial AVMs and are most commonly located at:

• Transverse and sigmoid sinuses (most common) 6, 2
• Cavernous sinus 4
• Superior sagittal sinus 4
• Tentorial incisura 4
• Other locations: Ethmoidal region, petrosal sinuses, marginal sinuses 4

Key Pathophysiologic Distinctions from Brain AVMs

DAVFs differ fundamentally from brain parenchymal AVMs in arterial supply derivation (meningeal vs. brain arteries), lack of parenchymal nidus, acquired rather than congenital nature, and location within dura mater, making them often amenable to curative endovascular treatment. 6, 1 Unlike brain AVMs where up to 90% of pediatric cases historically died at surgery, up to 80% of certain DAVFs can now be palliated or cured by endovascular techniques with good to excellent functional outcomes 6.

Diagnostic Considerations

Catheter-directed arteriography remains the reference standard for identifying and characterizing DAVFs, though CTA and MRI/MRA serve as useful first-line noninvasive imaging modalities. 6 However, in one series of 89 patients with intracranial hemorrhage who had negative CTA and MRI/MRA, catheter arteriography identified 7 arteriovenous malformations and 3 dural arteriovenous fistulas, highlighting the superior sensitivity of invasive angiography 6.

DAVFs can present as a cause of rapidly progressive dementia and should be considered in the differential diagnosis, as they are detectable on MRI and MRA 6.