Residency · Residency · Neurosurgery
Dural Arteriovenous Fistulae
Overview
Dural arteriovenous fistulae (dAVFs) are acquired vascular malformations with abnormal arteriovenous shunting within the dura mater. They account for 10-15% of all intracranial vascular malformations. Arterial supply comes from meningeal arteries (middle meningeal, occipital, ascending pharyngeal), draining into dural venous sinuses or cortical veins. dAVFs are distinct from parenchymal AVMs in being dural-based, acquired, and having a different natural history. The most common locations are the transverse-sigmoid sinus junction, cavernous sinus, superior sagittal sinus, and tentorium.
Etiology and Pathophysiology
dAVFs are acquired lesions, not congenital. The proposed mechanism holds that dural sinus thrombosis leads to venous hypertension, which triggers angiogenesis and opening of physiological microshunts within the dura. Risk factors include prior dural sinus thrombosis, craniotomy, trauma, infection, and hypercoagulable states. Venous hypertension is the key driver of symptoms and hemorrhagic risk. Cortical venous reflux (CVR) is the critical determinant of aggressive behavior.
Classification Systems
Cognard Classification (1995)
Type I drains into a dural sinus with normal antegrade flow and is benign. Type IIa drains into a sinus with retrograde flow into the sinus and produces mild symptoms. Type IIb drains into a sinus with retrograde flow into cortical veins (CVR) and is aggressive. Type IIa+b has combined retrograde sinus and cortical venous drainage. Type III has direct cortical venous drainage without sinus involvement and is aggressive. Type IV is Type III with cortical venous ectasia (aneurysmal dilatation), carrying the highest risk. Type V has spinal perimedullary venous drainage and causes progressive myelopathy.
| Cognard Type | Drainage Pattern | CVR | Behavior | Annual Hemorrhage Risk |
|---|---|---|---|---|
| I | Sinus, antegrade flow | No | Benign | <2% |
| IIa | Sinus, retrograde flow | No | Mild symptoms | Low |
| IIb | Sinus, retrograde into cortical veins | Yes | Aggressive | ~7-8% |
| IIa+b | Retrograde sinus + cortical veins | Yes | Aggressive | ~7-8% |
| III | Direct cortical venous drainage (no sinus) | Yes | Aggressive | ~7-8% |
| IV | Cortical venous drainage + venous ectasia | Yes | Most aggressive | Highest |
| V | Spinal perimedullary venous drainage | — | Progressive myelopathy | — |
Borden Classification (2009)
Type I drains into a dural sinus or meningeal vein and is benign. Type II drains into a dural sinus with CVR and carries intermediate risk. Type III has direct cortical venous drainage only and is aggressive. This simpler system correlates Borden II and III with aggressive behavior.
| Borden Type | Drainage | CVR | Risk | Treatment |
|---|---|---|---|---|
| I | Dural sinus or meningeal vein | No | Benign | Observation or treat if symptomatic |
| II | Dural sinus with CVR | Yes | Intermediate-High | Treatment indicated |
| III | Direct cortical venous drainage only | Yes | Aggressive | Treatment indicated |
Clinical Presentation
Low-Risk (No CVR)
Pulsatile tinnitus is the most common presenting symptom overall. Orbital symptoms include chemosis, proptosis, and diplopia in cavernous sinus dAVFs. Headache and cranial bruit are also common.
High-Risk (With CVR)
Presentations include intracranial hemorrhage (intracerebral or subarachnoid), progressive neurological deficits from venous hypertension, seizures, papilledema and visual deterioration from venous congestion, and rarely progressive dementia from cortical venous congestion.
Specific Location Syndromes
Cavernous sinus dAVFs present with a red eye, proptosis, chemosis, cranial nerve palsies (III, IV, VI), and elevated intraocular pressure. They are often low-flow with a benign natural history. Transverse-sigmoid dAVFs cause pulsatile tinnitus and headache and may become aggressive if CVR develops. Tentorial dAVFs carry high hemorrhage risk and frequently present with hemorrhage. Anterior cranial fossa (ethmoidal) dAVFs almost always drain into cortical veins (Cognard III/IV), carry high hemorrhage risk, and are best treated surgically.
Natural History
Benign dAVFs (Borden I / Cognard I) carry an annual hemorrhage risk below 2% and may spontaneously thrombose. Aggressive dAVFs (with CVR) carry an annual hemorrhage risk of 7.4-8.1% per year and an annual neurological deficit risk of 6.9%. Once hemorrhage occurs, the rebleeding risk is approximately 35% within 2 weeks. Conversion from a benign to aggressive pattern can occur over time, warranting follow-up angiography.
Diagnostic Workup
CT/CTA may show dilated cortical veins, unexplained hemorrhage, or sinus abnormalities. MRI/MRA reveals flow voids in unexpected locations, dilated cortical veins, and venous congestion (T2 hyperintensity). Digital subtraction angiography is the gold standard and must include selective injection of all potential feeding arteries (both ECAs, both ICAs, both vertebral arteries). DSA evaluates arterial feeders, fistula location, venous drainage pattern, and the presence or absence of CVR. It is essential for classification and treatment planning.
Treatment
Indications
All dAVFs with cortical venous drainage (Cognard IIb or higher / Borden II or III) warrant treatment. Symptomatic Cognard I/IIa causing intolerable tinnitus or ophthalmic symptoms may be treated. Asymptomatic Cognard I lesions can be observed with periodic imaging.
Endovascular Treatment
Transarterial embolization accesses the fistula via feeding arterial pedicles and injects liquid embolic agent (Onyx/EVOH, NBCA/glue) to penetrate and occlude the fistulous point. Reaching the foot of the draining vein is necessary for cure. Multiple sessions may be needed, and there is risk of cranial nerve palsy especially if ECA-ICA anastomoses are present.
Transvenous embolization is particularly effective for cavernous sinus dAVFs. Access is gained via the inferior petrosal sinus or superior ophthalmic vein, and the cavernous sinus is packed with coils to eliminate the fistula. Cure rates exceed 80% for cavernous sinus dAVFs.
Surgical Treatment
Surgery is preferred for anterior cranial fossa (ethmoidal) dAVFs, where craniotomy is performed to identify and clip or coagulate the draining cortical vein at the fistula site. Simple disconnection of the arterialized cortical vein is usually curative. Surgery is also indicated when endovascular access is limited or dangerous, and may be combined with preoperative embolization for devascularization.
Stereotactic Radiosurgery
SRS plays an adjunctive role with slower onset of effect (latency of 1-3 years). Obliteration rates reach 50-70% at 3 years. It is not appropriate for dAVFs with CVR presenting with hemorrhage because the effect is too slow. It may be considered for residual low-grade dAVF after partial endovascular treatment.
Spinal Dural AVFs
Spinal dural AVFs are the most common spinal vascular malformation (type I spinal AVM), typically occurring in the thoracolumbar region of elderly men. The fistula is located in the dural nerve root sleeve, and arterialized perimedullary veins cause venous congestion of the spinal cord. Patients present with progressive myelopathy including ascending weakness, sensory changes, and bowel/bladder dysfunction. MRI shows T2 hyperintensity with cord edema and flow voids on the cord surface. Treatment involves surgical interruption of the draining vein at the fistula site (the gold standard) or endovascular embolization.
<image>Lateral projection digital subtraction angiography showing a transverse-sigmoid junction dural arteriovenous fistula with external carotid artery injection, demonstrating multiple meningeal arterial feeders from the middle meningeal and occipital arteries converging on the transverse-sigmoid junction with cortical venous reflux into temporal lobe cortical veins</image>
<image>Diagram illustrating the Cognard classification of dural arteriovenous fistulae, types I through V, showing the relationship between the fistula, dural sinus, direction of sinus flow (antegrade vs retrograde), presence or absence of cortical venous drainage, and venous ectasia for each type</image>
<image>MRI T2-weighted sagittal image of the thoracolumbar spine in a patient with a spinal dural arteriovenous fistula, demonstrating increased T2 signal within the spinal cord (edema from venous congestion) and serpentine flow voids along the dorsal surface of the cord representing dilated perimedullary veins</image>
Clinical Pearls
Pulsatile tinnitus in a middle-aged patient should raise suspicion for a dAVF, and CTA or MRA should be considered. The presence or absence of cortical venous reflux is the single most important factor determining hemorrhage risk and the need for treatment. Complete six-vessel DSA is mandatory for proper classification because incomplete angiography may miss feeders or mischaracterize drainage. Anterior cranial fossa (ethmoidal) dAVFs are best treated surgically due to dangerous ECA-ophthalmic artery anastomoses that limit safe embolization. Cavernous sinus dAVFs are frequently low-flow and may spontaneously thrombose; many are initially managed with manual carotid compression exercises. For spinal dAVFs, the key to cure is interrupting the intradural draining vein; simply ligating feeders is insufficient due to collateral recruitment.
References
- Cognard C, et al. "Cerebral Dural Arteriovenous Fistulas: Clinical and Angiographic Correlation with a Revised Classification of Venous Drainage." Radiology. 1995;194(3):671-680.
- Borden JA, Wu JK, Shucart WA. "A Proposed Classification for Spinal and Cranial Dural Arteriovenous Fistulous Malformations and Implications for Treatment." J Neurosurg. 1995;82(2):166-179.
- Gross BA, Du R. "The Natural History of Cerebral Dural Arteriovenous Fistulae." Neurosurgery. 2012;71(3):594-603.
- Gandhi D, et al. "Intracranial Dural Arteriovenous Fistulas: Classification, Imaging Findings, and Treatment." AJNR Am J Neuroradiol. 2012;33(6):1007-1013.
- Steinmetz MP, et al. "Outcome after the Treatment of Spinal Dural Arteriovenous Fistulae." J Neurosurg Spine. 2004;1(1):36-42.
