Intraosseous AVF of the Mastoid Emissary Canal: Combined Endovascular Treatment

A 70-year-old woman was treated for a first-of-its-kind intraosseous arteriovenous fistula (AVF) located in the mastoid emissary canal (MEC), according to a recent clinical report. Doctors achieved complete obliteration of the fistula using a dual-stage endovascular approach involving transvenous coil embolization and transarterial Onyx injection.

What is an intraosseous AVF of the mastoid emissary canal?

An intraosseous arteriovenous fistula (AVF) is an abnormal connection between an artery and a vein that occurs within the bone. In this specific case, the fistula formed in the mastoid emissary canal (MEC), a bony channel in the temporal bone that connects the sigmoid sinus (an intracranial vein) to the extracranial venous system.

According to the case report, the patient presented with progressive left-sided pulsatile tinnitus. Digital subtraction angiography (DSA) revealed the fistula was supplied by the left occipital and ascending pharyngeal arteries. The blood flowed retrogradely into the sigmoid and transverse sinuses and antegrade into subcutaneous veins.

This lesion differs from conventional dural arteriovenous fistulas (DAVFs). While DAVFs occur in the dura mater, this shunt was strictly localized within the short osseous segment of the MEC. The authors note this represents a rare interface between intraosseous and dural vascular malformations.

How was this rare vascular lesion treated?

The surgical team used a combined endovascular strategy because the fistula’s location posed a high risk of complications. The shunt pouch was directly connected to a patent sigmoid sinus. If doctors used transarterial embolization (TAE) alone, the liquid embolic material could have migrated into the sinus, potentially causing a blockage.

The procedure followed a two-step protocol:

• Step 1: Transvenous Coil Embolization. Surgeons placed six detachable coils in the shunt pouch. These coils didn’t close the fistula but created a mechanical barrier to protect the sigmoid sinus.
• Step 2: Transarterial Embolization (TAE). Using the occipital artery as a feeder, doctors injected 0.30 mL of Onyx (a liquid embolic agent). The previously placed coils prevented the Onyx from leaking into the sinus.

Final angiography confirmed the complete obliteration of the AVF. The patient’s pulsatile tinnitus resolved immediately, and a six-month follow-up MRA showed no recurrence.

Why does this case change the understanding of AVFs?

This case highlights a “continuum” between different types of fistulas. For years, clinicians have categorized these as either dural or intraosseous. However, this MEC fistula shares traits of both: it has a dural arterial supply but resides in a bony canal.

The report compares this to condylar AVFs, which form near the hypoglossal canal. Unlike condylar AVFs, which are largely extracranial and don’t involve true dural sinuses, the MEC AVF drains directly into a functional dural venous sinus. This makes the MEC variant more complex to treat due to the risk of sinus thrombosis.

What are the future trends for treating skull-base AVFs?

Experts suggest that similar lesions are likely underreported. Because many of these fistulas have “benign” drainage patterns and mild symptoms, they’re often managed conservatively, leading to a publication bias.

Future diagnostic trends will likely involve more frequent use of 3D rotational angiography and multiplanar reconstruction (MPR) to pinpoint the exact “shunt point” within bony canals. The report suggests that identifying whether a shunt is strictly intraosseous or dural will dictate whether a surgeon uses a transvenous, transarterial, or combined approach.

As liquid embolic agents like Onyx become more refined, the combined strategy used in this case—creating a venous “dam” before arterial filling—may become the standard for high-flow fistulas located near critical venous sinuses.

Frequently Asked Questions

What is the difference between a DAVF and an intraosseous AVF?

A dural AVF (DAVF) occurs within the dura mater, the outermost membrane of the brain. An intraosseous AVF occurs within the bone itself, such as the mastoid emissary canal or the hypoglossal canal.

Can these fistulas be treated without surgery?

Yes. According to the case report, the patient was initially managed conservatively because her symptoms were mild and there was no cortical venous reflux. Intervention is usually reserved for intolerable symptoms, such as severe pulsatile tinnitus.

What is Onyx and how is it used?

Onyx is a liquid embolic agent used in endovascular surgery to plug abnormal blood vessels. It’s injected via a catheter to permanently block the flow of blood through a fistula.