Carotid Cavernous Fistula (CCF)

CCF constitute pathologic vascular shunts between the carotid arterial system and the cavernous sinus, or rarely, directly to an ophthalmic or orbital vein.³ CCF are categorized as direct and indirect. Direct CCF demonstrate direct vascular communication between the ICA and the cavernous sinus.¹¹ Indirect CCF involve dural vascular connections between branches of the ICA (e.g., meningohypophyseal artery) or external carotid artery (e.g., middle meningeal and/or ascending pharyngeal artery) and the cavernous sinus. Although indirect CCF are considered a subtype of DAVF, they will be discussed separately here.

The notion of “high-flow” and “low-flow” fistulae is often discussed but is incorrect. Fistulae, by definition, are high-flow vascular lesions, and stratification based on “high- or low-flow” fistula is not considered appropriate. However, direct fistulae may have higher flows relative to those that are indirect, although both still represent high-flow lesions. Intraorbital fistulae have also been observed and consist of fistulization between the ophthalmic artery and the ophthalmic vein or one of its branches.^11–13

Many CCF are direct and are associated with traumatic blunt or perforating injury.¹⁴ Bilateral CCF have been reported in 1% to 2% of posttraumatic cases.¹⁵ Arterial tear secondary to skull-base fractures and shear wall tension resulting from traumatic positioning or sudden increase in pressure during injury are proposed mechanisms.¹⁶ Iatrogenic fistulae have also been reported following carotid endarterectomy, neuroendovascular procedures, and transsphenoidal pituitary and paranasal sinus surgery.^17–20

Nontraumatic (i.e., spontaneous) CCF are less frequent. Spontaneous cavernous ICA aneurysm rupture²¹ and inherited connective tissue disorders predisposing to aneurysm formation and subsequent rupture have both been implicated with CCF. In particular, patients with fibromuscular dysplasia,²² pseudoxanthoma elasticum,²³ and Ehlers-Danlos syndrome²⁴ have increased risk of arterial dissection and may rupture from “minor” trauma, such as coughing or low-impact motor vehicle accidents. However, similar to DAVF, the majority of nontraumatic CCF are idiopathic.

CCF

Direct CCF are estimated to represent approximately 75% of posttraumatic cavernous sinus fistulae.¹⁴ They occur most in males between the age of 20 and 30 years, likely relating to increased rates of traumatic events in this cohort.²⁵ CCF signs and symptoms are related to increased blood flow and volume in the involved cavernous sinus(es), resulting in increased venous pressures and congestion. Arterialization of the cavernous sinus results in flow reversal in the orbital veins and, in severe cases, reflux into the cortical veins.²⁹ Symptom onset is often rapid when associated with trauma (Box 26.2).⁹

Funduscopic examination often demonstrates venous stasis retinopathy, dilated retinal veins, and retinal hemorrhages. Severe proptosis and chemosis may result in keratopathy and corneal ulceration. Cavernous sinus or cortical draining vein rupture, resulting in intracranial parenchymal and/or subarachnoid hemorrhage, may occur in approximately 5% with this type of CCF.

Indirect CCF have a more diverse presentation. An incidence of 0.29 per 100,000 people per year has been reported, and this type of fistula is common in females between the ages of 50 and 60 years.³⁰ Given the indirect nature of the vascular shunts, vascular pressures are typically reduced, resulting in a more insidious onset. In contrast to direct CCF, objective or subjective bruits are uncommon, as is involvement of cranial nerves V and VII.

Anteriorly directed venous outflow is the most common pattern of venous drainage seen associated with indirect CCF, characterized by retrograde flow in the SOV, often draining into the facial veins. Anterior drainage often results in orbital edema. These patients may present with signs of increased intraocular pressures and are at increased risk of ischemic optic neuropathy.³¹

Posterior venous drainage usually occurs via the inferior and superior petrosal sinuses. In this type of venous drainage, edema and proptosis are often absent (termed “white-eyed” painful diplopia).^31–33

Mixed venous drainage is also common. Although cortical venous drainage can occur in isolation, it is more frequently associated with anterior or posterior drainage. Cortical vein involvement is varied but more frequently involves veins communicating with, or in close proximity to, the cavernous sinus, such as the Sylvian vein or the basal vein of Rosenthal.^31,33 This pattern of drainage results in increased venous pressures in the cortical veins and can restrict normal cerebral drainage. As such, neurologic examination is important to rule out intracranial complications, such as hemorrhage, venous congestion-related ischemia or infarction, or seizure. Signs and symptoms for each pattern of venous drainage are summarized in Table 26.1.

Ultrasonography

Orbital sonographic evaluation is often not the initial imaging modality utilized in the workup of orbital pathology; however, it can play an important role. Although conditions with similar presentation to DAVF and CCF, such as intraocular tumors, scleritis, and myositis, can be excluded by using this modality, the role of sonography in the diagnosis and management of DAVF and CCF is limited. Orbital color Doppler sonography can detect flow reversal or thrombosis of the SOV.¹¹ Arterialization and low resistance flow in the SOV on Doppler sonography can be seen in both DAVF and CCF. Arterialization of the ophthalmic veins should also be assessed in relation to carotid flow velocities, which have been found to be abnormal in the presence of a direct fistula.³⁷

Computed Tomography (CT)

Orbital CT is the initial imaging modality in most institutions. It allows for rapid and fairly comprehensive assessment of the structures of the orbit. On routine contrast-enhanced CT, an ectatic SOV is seen in 86% to 100% of orbital vascular fistulae.³⁸ Orbital CT findings that can be seen in DAVF and CCF are summarized in Box 26.3. CT is also useful in evaluating patients with abrupt onset of symptoms, identifying skull-base and calvarial fractures and intracranial hemorrhage. CT angiography (CTA) permits evaluation of the arterial and venous structures.³⁹ Given the typical flow characteristics of DAVF and CCF, respectively, early enhancement of the cavernous sinus is observed in both, although classically more prominent in direct CCF, whereas enhancement will be more delayed in indirect DAVF.⁴⁰ Direct CCF also results in increased cavernous sinus pressures with possible sinus expansion. Dural venous sinus thrombosis can also be detected by using this modality. Although CTA with dynamic arterial and Valsalva-augmented venous phase technique is a powerful tool for assessing a variety of orbital vascular pathologies, its use in high flow lesions is limited.⁴¹

CTA also helps with treatment planning, providing the spatial information of the surrounding parenchyma and calvarial anatomy, as well as serving as a guide to possible endovascular routes of access for closure of the fistula (see later). The recently developed four-dimensional CTA (4D-CTA; time-resolved) imaging provides dynamic information and may become a valuable adjunct to conventional CTA in the diagnosis, treatment planning, and follow-up of patients with cranial DAVF.^42,43 However, the radiation dose of 4D-CTA is substantially higher compared with conventional CTA^44,45 (Fig. 26.4).