Overview of Dural Arteriovenous Fistula

Dural arteriovenous fistulas (dAVFs) are uncommon, yet significant vascular disorders that occur within the dura mater, the protective membrane that surrounds the brain and spinal cord. These abnormal connections form between the dural arteries and veins, bypassing the capillary system which can lead to a range of symptoms depending on their location and the pattern of venous drainage.

Dural AVFs are also categorized as a type of arteriovenous malformation (AVM). Dural AVFs typically have a more direct connection between an artery and a vein, and are more likely to be acquired, versus congenital (a patient is born with the condition). 

The most common cause of dAVFs is thought to be related to trauma, surgery, or the presence of a thrombosis (blood clot) in the dural venous sinuses. Symptoms can vary widely, from headaches and pulsatile tinnitus (a whoosing or ringing in the ears) to more severe complications such as seizures or stroke.

The severity of a dAVF is largely determined by its venous drainage pattern. Fistulas draining into cortical veins carrying a higher risk of bleeding and subsequent neurological damage.

Treatment options range from conservative monitoring to more aggressive interventions like endovascular embolization or surgical disconnection. In this article, we will provide an overview of the complexities surrounding dAVFs, aiming to shed light on their causes, symptoms, and diagnosis.

The most common cause of dAVFs is thought to be related to an acquired condition rather than a congenital one. Many patients with dAVFs have a history of head trauma, surgery, or a blood clot in the brain (dural venous thrombosis).

In some cases, the cause remains unknown, known as an idiopathic dAVF. One theory of disease development is that inflammatory processes associated with central nervous system trauma can lead to maladaptive new blood vessel formation (angiogenesis).

There are several types of arteriovenous fistulas, but when it comes to dAVFs, they are typically classified based on their location and the pattern of venous drainage. The Borden and Cognard systems are commonly used to classify dAVFs.

These classifications help predict the behavior of the lesion and guide treatment decisions. For example, dAVFs with cortical venous drainage (Borden types II and III) are considered more aggressive and have a higher risk of bleeding.

As mentioned, dAVF suspected causes can include trauma, infection, surgery, or conditions that lead to blood clots in the veins of the brain.

In some cases, dAVFs may form spontaneously without a clear cause.

Factors like high blood pressure and venous hypertension (high blood pressure in the veins) have also been associated with the development of dAVFs.

Brain fistulas, including dAVFs, are relatively uncommon. They account for approximately 10-15% of all intracranial vascular malformations. While they can occur at any age, they are more frequently diagnosed in adults between 50 and 60 years of age. Men experience dAVF with more frequency than women. 

The seriousness of a fistula depends on its location, type, and venous drainage pattern. dAVFs with cortical venous drainage are considered serious because they carry a higher risk of bleeding.

This bleeding can lead to stroke or other neurological deficits. Symptoms can range from headaches and ringing in the ears to more severe effects like seizures or hemorrhage.

A dural fistula behind the ear typically refers to a dAVF located in the region of the transverse and sigmoid sinuses. These are among the most common locations for dAVFs.

Patients may experience symptoms such as a pulsating noise in the ear (pulsatile tinnitus), headache, or in some cases, more severe symptoms if the fistula leads to increased pressure on the brain or hemorrhage.

The symptoms of a dAVF can vary greatly depending on the location and severity of the fistula. One of the most common symptoms is a headache, which can range from mild to severe. Patients often describe these headaches as different from any other type of headache they've experienced before.

Another frequent symptom is a whooshing sound in the head, known as pulsatile tinnitus. This sound is synchronous with the patient's heartbeat and can be quite distressing. It occurs due to the abnormal blood flow caused by the fistula.

Some individuals with a dAVF may also experience neurological deficits, such as weakness or numbness in certain parts of the body, vision changes, or even seizures. These symptoms arise when the fistula places pressure on the surrounding brain tissue or when it leads to a reduction in blood flow to certain areas of the brain.

Apart from the symptoms experienced by the patient, there are signs that healthcare providers look for when diagnosing a dAVF. One such sign is the presence of a bruit, which is an abnormal “whooshing” sound heard through a stethoscope placed over the skull.

This sound is caused by the turbulent blood flow through the fistula. In some cases, dAVF can lead to more serious complications, such as bleeding in the brain (hemorrhage). This can present as a sudden, severe headache, often described as the worst headache of one's life, along with nausea, vomiting, and an altered level of consciousness.

Patients may also present with cranial nerve deficits, which can cause symptoms such as double vision, facial numbness, or difficulty swallowing. These occur when the fistula affects the nerves that control these functions.

It's important to note that not all dAVFs will cause symptoms. In fact, some may be discovered incidentally during imaging studies for unrelated issues.

However, when symptoms are present, they should not be ignored, as dAVFs can lead to serious health risks if left untreated.

Most people with dAVFs are diagnosed after they start experiencing symptoms, which can vary widely depending on the location and severity of the fistula. Common symptoms include a pulsating ringing in the ears (tinnitus), headaches, nausea, vomiting, seizures, and neurological deficits such as weakness or numbness.

In some cases, dAVFs can lead to more serious complications like bleeding in the brain or stroke-like symptoms. If bleeding inside the head occurs, it can be fatal. 

When a patient presents with symptoms that could be related to a dAVF, a healthcare provider will typically begin with a thorough medical history and physical examination. If a dAVF is suspected, imaging studies are the next step to confirm the diagnosis.

The gold standard for diagnosing dAVFs is a formal angiogram, also known as digital subtraction angiography (DSA). This minimally invasive procedure provides a detailed view of the blood vessels in the brain and can identify the abnormal connections characteristic of dAVFs.

During a DSA, a catheter is inserted into an artery in the wrist or groin and navigated to the blood vessels of the brain. A contrast dye is then injected, and X-ray images are taken to visualize blood flow and detect any abnormalities.

While DSA is the most definitive way to diagnose a dAVF, it is not typically the first imaging test used to discover a dAVF. Noninvasive imaging studies such as CT angiograms (CTA) and magnetic resonance angiograms (MRA) are often performed first. These tests can provide valuable information and may suggest the presence of a dAVF, prompting further investigation with DSA.

On a CT angiogram, a dAVF may appear as abnormal clusters of vessels within the dura mater. The CT images can show dilated blood vessels, a sign of increased blood flow due to the fistula.

Sometimes, the CT angiogram can reveal a very bright, dense dural sinus, which may indicate a blood clot (thrombosis), or enlarged vascular channels in the skull, which are signs of long-standing dAVFs.

The diagnostic cerebral angiogram provides a much more detailed view of the blood vessels and can show the exact location and characteristics of the dAVF. On a DSA, a dAVF is identified by the premature appearance of veins or venous sinuses during the arterial phase of the blood flow cycle.

This means that veins, which normally fill with blood after the arteries, are filling at the same time as the arteries due to the abnormal connection. The angiogram can also show the direction of blood flow in the veins, whether it is anterograde (normal direction) or retrograde (reversed), which can indicate important information to your care team regarding the risk of complications.

The angiogram will also reveal the "nidus" of the dAVF, which is the central point where the arteries and veins connect. This is a critical piece of information for planning treatment, as the goal is to disconnect the nidus to cure the dAVF.

• Dural arteriovenous fistulas (dAVFs) are rare, abnormal connections between the arteries and veins in the dura mater, the protective covering over the brain and spinal cord.
• Symptoms can vary but often include pulsatile tinnitus (a whooshing sound in the ears), headaches, vision changes, and neurological issues like weakness or numbness.
• In severe cases, dAVFs can cause bleeding in the brain or stroke-like symptoms.
• The most accurate way to diagnose a dAVF is through a diagnostic cerebral angiogram, or digital subtraction angiography (DSA).
• Early diagnosis of a dAVF is important to prevent complications such as brain hemorrhage or permanent neurological damage.

• American Association of Neurological Surgeons (AANS): Cerebrovascular Imaging
• NeuroAngio.org: Spinal dAVF
• National Institute of Neurological Disorders and Stroke (NINDS)
• MedlinePlus