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Background
A carotid-cavernous fistula (CCF) is an acquired abnormal arteriovenous (AV) connection between the carotid artery and the cavernous sinus (CS). CCFs can be divided into direct and indirect subtypes. A direct CCF is a direct AV fistula between the cavernous segment of the internal carotid artery (ICA) and the CS. Direct CCFs tend to be high flow and cause symptoms such as chemosis, proptosis, ophthalmoplegia, ophthalmalgia, among others. An indirect CCF is an AV connection between either the non-cavernous segment ICA or the external carotid artery (ECA) which tends to be low flow and asymptomatic. CCFs can arise after there is intimal damage to the carotid artery, most commonly from trauma, aneurysm rupture, or atherosclerosis. Diagnosis of CCFs is heavily dependent on computed tomographic angiography (CTA), magnetic resonance imaging (MRI), and catheter angiography. Indication for treatment is based on the degree of symptoms as higher flow lesions are unlikely to resolve spontaneously. Treatment options include carotid compression, endovascular embolization (transarterial vs transvenous), or surgical ligation. Untreated CCFs carry a risk of progressive increased flow which can lead to permanent damage to the eye and vision loss.

Educational Goals / Teaching Points
The goal of this exhibit is to review the subtypes of CCFs as well as their common clinical presentations and imaging findings, discuss the differential considerations of these imaging findings, discuss the treatment options of CCFs, and present three cases of CCFs which were treated endovascularly.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
CCFs are rarely visualized directly on CTA or MRI and often require catheter angiography or dynamic CTA for definitive diagnosis. On CTA secondary findings include engorgement of the CS and the veins which drain into the CS, most commonly the superior ophthalmic veins (SOVs). On MRI, the CS and SOV may have prominent flow voids on T2-weighted sequences that represent abnormal arterialized flow. Additionally, enlargement and enhancement of the extraocular muscles can be seen on both CTA and MRI. Catheter angiography is pursued for lesions that cannot be clearly diagnosed on CTA or MRI and intend to be treated. This is especially important to delineate the dominant outflow from the CS, which will in-turn determine if a transarterial or transvenous approach is more optimal for endovascular treatment. Dynamic CTA is used less frequently but can help elucidate lower flow lesions in settings where catheter angiography is unavailable or undesired.

Conclusion
CCFs can arise for a variety of reasons, most commonly after trauma. A high clinical suspicion for CS pathology, specifically CCF, should lead the clinician to order the appropriate initial imaging of CTA or MRI (preferably MRI of the orbits). Secondary findings on imaging, i.e. enlarged SOVs, are not specific for CCF but should be a key indicator to the radiologist of CS pathology. High-flow, direct CCFs tend to cause symptoms and should be treated.