Arteriovenous Fistula
Arteriovenous fistulas are acquired lesions characterized by abnormal direct communication between an artery and a vein without flow through an intervening capillary bed. An arteriovenous fistula may be caused by trauma or degeneration. There are 2 forms, direct and indirect (dural). Direct carotid-cavernous fistulas are characterized by a connection between the internal carotid artery and the cavernous sinus; they typically occur after trauma that creates a tear or hole in a branch artery of the internal carotid within the cavernous sinus. They may also be caused iatrogenically, for example, during neurosurgical or neuroradiologic procedures.
Direct carotid-cavernous fistulas possess high blood flow and may produce characteristic tortuous epibulbar vessels, as well as pulsatile proptosis and an audible bruit. Ischemic ocular damage results from diversion of arterialized blood into the venous system, which causes venous outflow obstruction (Fig 5-6A). This in turn leads to elevated IOP, choroidal effusions, blood in the Schlemm canal, and nongranulomatous anterior uveitis. Ocular motility abnormalities can result from either congestion within the orbit or increased pressure in the cavernous sinus. The latter can cause compression of cranial nerves III, IV, or, most commonly, VI, with associated extraocular muscle palsies. CT scans may show diffuse enlargement of some or all of the extraocular muscles resulting from venous engorgement and a characteristically enlarged superior ophthalmic vein (Fig 5-6B, C).
Indirect (dural) carotid-cavernous fistulas are characterized by a connection between meningeal branches of the internal and/or external carotid artery and the cavernous sinus. These fistulas most commonly develop as a degenerative process in older patients with systemic hypertension, vascular disease, and/or atherosclerosis. Because dural fistulas generally have lower rates of blood flow than direct carotid-cavernous fistulas, their onset can be insidious, with only mild orbital congestion, proptosis, and pain. Arterialization of the conjunctival veins causes chronic red eye. Increased episcleral venous pressure results in asymmetric elevation of IOP on the affected side, and patients with chronic fistulas are at risk for glaucomatous damage to the optic nerve head.
Magnetic resonance angiography (MRA) may be helpful in diagnosing arteriovenous fistulas, with fewer associated adverse effects than conventional angiography (eg, stroke). However, conventional angiography possesses more sensitivity than MRA and remains the gold standard for diagnosis.
Management
The decision to treat an arteriovenous fistula is based on weighing the severity of symptoms against the risks associated with intervention. Because they are high-flow lesions, direct carotid-cavernous fistulas usually require intervention. Small indirect carotid-cavernous fistulas often close spontaneously and may initially be observed. However, because even these lesions may result in intracranial hemorrhage, some investigators have recommended more aggressive management of indirect fistulas.
Intervention typically involves an endovascular treatment (coils or glue) to block the fistula (Fig 5-7). Transvenous access is used to reach dural fistulas, while direct carotidcavernous fistulas are generally treated via a transarterial approach. Occasionally, a transvenous approach by transcutaneous canalization of the superior ophthalmic vein is employed for embolization, which may require an orbitotomy to directly access the vein.
See BCSC Section 5, Neuro-Ophthalmology, for additional discussion of carotidcavernous fistulas.
Stacey AW, Gemmete JJ, Kahana A. Management of orbital and periocular vascular anomalies. Opthalmic Plast Reconstr Surg. 2015;31(6):427–436.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.