A dural arteriovenous fistula (dAVF) represents an abnormal, direct connection between an artery and a vein within the dura mater, the tough, protective membrane covering the brain and spinal cord. This condition bypasses the normal network of capillaries, forcing high-pressure arterial blood directly into the lower-pressure venous system. Dural arteriovenous fistulas are rare cerebrovascular conditions, accounting for about 10% to 15% of all intracranial vascular malformations and typically affecting adults between the ages of 40 and 60. While some dAVFs may remain asymptomatic, others can be serious, leading to potentially life-threatening neurological complications like bleeding in the brain.
Understanding the Anatomy and Malformation
The dura mater is the outermost of the three layers of membranes, known as the meninges, that surround the brain and spinal cord. Within this thick tissue are the dural venous sinuses, which are large channels responsible for collecting and draining blood from the brain back to the heart. In a healthy circulatory system, arteries and veins are separated by tiny capillaries that slow the blood flow and reduce pressure before it enters the veins.
A dAVF is characterized by the absence of this capillary network. Instead, high-pressure arterial blood flows directly into the low-pressure dural vein or sinus, a process called shunting. This forceful influx overloads the venous system, causing the veins to become engorged, tortuous, and unable to drain blood efficiently. The resulting increase in pressure, known as venous hypertension, is the underlying mechanism that causes most dAVF symptoms.
dAVFs are overwhelmingly acquired conditions that develop over time, though some vascular malformations are present from birth. The exact cause is often unknown, but dAVF formation is frequently associated with an event that causes a narrowing or blockage of a dural venous sinus, such as venous thrombosis or previous trauma. This blockage may trigger the body to create new blood vessels (neovascularization) to bypass the obstruction, which then connect abnormally to the veins, forming the fistula. The most common location for a dAVF is at the junction of the transverse and sigmoid sinuses at the base of the skull.
Symptoms and Clinical Manifestations
The clinical presentation of a dAVF can vary significantly, depending largely on the fistula’s location and the direction of the blood flow. If the high-pressure blood is shunted forward into the dural sinus without backing up into the brain’s delicate veins, the symptoms may be minor or absent. If the blood flow reverses and drains backward into the cortical veins of the brain, the condition is considered aggressive and carries a high risk of severe neurological injury.
One of the most frequent symptoms is pulsatile tinnitus, a rhythmic whooshing or humming sound synchronized with the heartbeat. This auditory phenomenon is caused by turbulent, high-velocity blood flow near the inner ear. Other common non-hemorrhagic symptoms include persistent headaches and various visual disturbances. These ocular symptoms can range from redness and swelling of the eye (chemosis), bulging of the eye (exophthalmos), or vision loss, particularly when the fistula involves the cavernous sinus.
More severe manifestations arise from increased venous pressure and include focal neurological deficits. Patients may experience seizure activity, progressive weakness or numbness in the limbs, or signs mimicking a stroke due to venous congestion and reduced blood flow to the brain tissue. The most feared complication is intracranial hemorrhage, or bleeding into the brain, which occurs when high pressure causes a fragile vein to rupture, potentially leading to permanent disability or death. Retrograde drainage into the cortical veins significantly elevates the risk of this life-threatening event.
Establishing the Diagnosis and Risk Profile
Identifying a dAVF begins with a high suspicion based on the patient’s symptoms, especially pulsatile tinnitus. Initial screening typically involves non-invasive imaging methods like magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA), or computed tomography (CT) and CT angiography (CTA). These scans can reveal signs of the condition, such as engorged veins, swelling in the brain, or signs of past bleeding. However, these methods are often insufficient to fully characterize the complex nature of the fistula.
The definitive diagnostic procedure, considered the gold standard, is digital subtraction angiography (DSA). This invasive test involves guiding a catheter through a blood vessel (usually in the groin) up to the vessels supplying the brain, and injecting a contrast dye while taking real-time X-ray images. DSA provides a detailed, dynamic view of the blood flow, allowing physicians to precisely locate the fistula, identify all arterial feeders, and map the pattern of venous drainage. DSA evaluation often requires injecting contrast into the internal carotid, external carotid, and vertebral arteries on both sides.
The DSA results are used to classify the fistula, determining the risk profile and guiding intervention. Classification systems, such as the Borden or Cognard scales, categorize dAVFs primarily based on the presence and direction of venous drainage. Low-risk fistulas typically drain forward into a dural sinus, avoiding the brain’s cortical veins. In contrast, high-risk, or “aggressive,” fistulas are those that drain backward into the cortical veins, as this retrograde flow significantly increases the risk of intracranial hemorrhage and non-hemorrhagic neurological deficits.
Comprehensive Treatment Strategies
Treatment for a dAVF is tailored to the fistula’s risk profile and the severity of the patient’s symptoms. For low-risk fistulas (those without cortical venous reflux) that are asymptomatic or cause only mild, manageable symptoms like non-disabling pulsatile tinnitus, a conservative management approach may be chosen. This involves closely monitoring the patient with regular clinical assessments and imaging to watch for any progression or change in the venous drainage pattern.
For high-risk fistulas, or low-risk fistulas causing severe or progressive symptoms, intervention is necessary. The most common primary treatment is endovascular embolization, a minimally invasive procedure performed by a neurointerventional specialist. A catheter is guided through the blood vessels to the fistula, and a liquid embolic agent (such as specialized glue or coils) is delivered to permanently block the connection. This approach can be performed either transarterially, by accessing the feeding arteries, or transvenously, by entering the draining vein or sinus.
If endovascular techniques are not feasible or fail to fully close the connection, surgical disconnection remains a viable option. Open surgery involves a neurosurgeon accessing the fistula directly, typically through a craniotomy, to sever the abnormal arterial supply or clip the draining vein. Another treatment modality is stereotactic radiosurgery, which uses highly focused beams of radiation to damage the walls of the abnormal vessels, causing them to thrombose and close. This method may take months or years to achieve complete obliteration.