Diagnosing Arteriovenous Fistulas

Dural arteriovenous fistulas (dAVFs) are rare and intricate vascular anomalies characterized by abnormal connections between arteries and veins within the protective membrane covering the brain, known as the dura mater. These anomalies can disrupt the normal blood flow within the brain and spinal cord, leading to a range of neurological symptoms.

The exact cause of dAVFs is not always clear, but they are thought to result from a combination of factors, including prior head trauma, blood clotting disorders, or congenital vascular abnormalities.

The clinical significance of dAVFs lies in their potential to cause severe health complications. Depending on their location and the specific blood vessels involved, dAVFs can lead to symptoms such as headaches, vision problems, tinnitus (ringing in the ears), and even life-threatening intracranial hemorrhages.

Timely diagnosis and treatment are crucial, as not all dAVFs are created equal, and their management can vary widely. From endovascular embolization to surgical intervention or radiosurgery, treatment is dependent on the specific characteristics of the fistula and the risks it poses to the patient.

Early detection and individualized treatment planning are essential to mitigate the potential complications associated with dAVFs. In this article, we will take a look into the process of diagnosing dAVFs. We hope to answer common questions or even concerns you may have.

The diagnosis of dAVF is done in a stepwise fashion, starting with initial symptoms of concern, and progressing through various diagnostic modalities until a clear clinical picture is established. The steps are outlined as follows: 

Patients with dAVFs may present with various neurological symptoms, including headaches, vision problems, seizures, or other neurological deficits.

A thorough medical history and physical examination are conducted to assess these symptoms and their potential underlying causes.

Once a collection of concerning symptoms is described, the care team may order non-invasive imaging studies; Magnetic resonance imaging (MRI) and computed tomography (CT) scans being the most commonly used methods.

These imaging studies help identify abnormalities in the blood vessels and surrounding structures.

Imaging abnormalities will then prompt further testing with angiography.

• Formal angiography, also known as digital subtraction angiography (DSA), is the gold standard for diagnosing dAVFs.
• Angiography is a minimally invasive technique in which a catheter is inserted into an artery, threaded through the blood vessels leading to the brain or spinal cord, and a contrast dye is injected near the target blood vessels.
• DSA is considered the gold standard for diagnosing and characterizing AVFs as it provides real-time, detailed images of the abnormal blood vessel connections.

• The best way to diagnose AVFs involves a combination of non-invasive imaging techniques, followed by invasive angiography when necessary.
• The use of classification systems like Borden and Cognard can further help in understanding the nature and severity of the AVF, aiding in the development of an appropriate treatment plan.

First developed in 1995, the Cognard classification for dAVFs is the most widely used method for classification. 

• Type I: Blood flow stays within the main blood channels (sinuses) and flows in the right direction, with no blood flowing back into smaller veins around the brain.
• Type IIa: Blood flows backward into the sinus but doesn’t affect nearby smaller veins around the brain.
• Type IIb: Blood flows in the right direction into the sinus but also leaks backward into the veins around the brain.
• Type IIa+b: Blood flows backward into both the sinus and nearby brain veins.
• Type III: Blood flows directly into the smaller veins of the brain rather than through the main channels, increasing the risk of bleeding (about 40%).
• Type IV: Blood flows directly into the smaller brain veins and may cause them to expand, leading to a higher bleeding risk (about 65%).
• Type V: Blood flow affects the veins around the spinal cord, which can cause worsening nerve-related symptoms and weakness.

This classification helps doctors understand the blood flow pattern and risks associated with different types of AVFs. Though this classification may seem complex, it is an important tool used to guide treatment planning with an appropriate balance of risk versus benefit. 

The Borden classification, also developed in 1995, is an additional method for grading dAVFs and their associated risks: 

• Type I: Blood drains into veins near the spine or main brain veins, flowing normally in the expected direction. This type is generally low-risk and has a good outcome.
• Type II: Blood drains into nearby veins but also flows backward into smaller veins around the brain. This type may be slightly riskier due to the backflow.
• Type III: Blood flows directly into smaller brain veins or into a blocked part of a larger vein. This type has a higher risk and is more complex.
• Each type can be further divided into two subtypes:
  • Type A: (single opening)
  • Type B: (multiple openings)

These details help doctors determine the treatment and risk level based on how blood is flowing through the veins.

On a computed tomography (CT) scan, an arteriovenous fistula typically appears as an area of increased density or abnormal contrast enhancement. It may manifest as a well-defined, localized collection of contrast material within the brain or spinal cord, often associated with dilated blood vessels.

The appearance can vary depending on the specific characteristics of the arteriovenous fistula and the surrounding anatomy. In some cases, there may be evidence of surrounding tissue damage or edema due to altered blood flow, which can provide additional clues to the presence of an AVF.

Cerebral angiography, on the other hand, offers a more detailed and dynamic view of arteriovenous fistulas. In angiography, a contrast dye is injected directly into the blood vessels through a catheter, and X-ray images are taken to capture real-time blood flow. Arteriovenous fistulas are typically visualized as a network of abnormal vessels or connections between arteries and veins.

The angiogram can reveal the precise location, size, and architecture of the fistula, as well as the direction of blood flow. This level of detail is crucial for accurately diagnosing and classifying arteriovenous fistulas, guiding treatment decisions, and assessing potential risks associated with these vascular anomalies.

The treatment options for dural arteriovenous fistulas (dAVFs) can be categorized into three main approaches: endovascular embolization, surgery, and radiosurgery. Endovascular embolization involves the insertion of a catheter into the blood vessels, followed by the injection of embolic agents or coils to block the abnormal blood flow and close off the fistula.

This minimally invasive technique is often the first choice for dAVFs, particularly for those with accessible and relatively straightforward anatomy. Surgical intervention may be necessary when the dAVF is complex or located in a challenging area, requiring open surgery to disconnect the abnormal connection between arteries and veins.

Radiosurgery, such as stereotactic radiosurgery or gamma knife therapy, offers a non-invasive option by delivering precise radiation to the dAVF to induce closure over time. The choice of treatment depends on the specific characteristics of the dAVF, its location, and the patient's overall health.

The primary goal of treatment lies in preventing potential complications, including bleeding into the brain or neurological deficits. Individualized treatment plans are formulated after careful consideration of these factors.

• dAVFs are rare and complex vascular anomalies with abnormal artery-vein connections in the brain's protective dura mater.
• Diagnosis of AVFs combines non-invasive imaging and, when needed, invasive angiography.
• Treatment options for dAVFs include endovascular embolization, surgery, radiosurgery, and observation.

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