Barrow classified carotid-cavernous fistulas into direct (type A) and indirect (type B, C, and D) types based on angioarchitecture and the arterial feeder to the fistula [1, 2, 6]. Indirect CCF are dural fistulas of cavernous sinus. Exact aetiology is not known; however, they are seen in association with chronic hypertension, diabetes mellitus, cavernous sinus thrombosis, collagen tissue disease, atherosclerosis, and trauma [3, 7]. Association with hypertension and diabetes mellitus was present in our series in a significant proportion of cases. Several authors have found its occurrence more in post-menopausal women; however, in our series among eight female patients, only two were of post-menopausal age [3, 5, 7]. Occurrence of the dural fistulas in infancy raises the possibility of congenital origin of this disease [3]. One type C CCF in our series had simultaneous presence of multiple vascular anomalies like congenital ipsilateral orbital capillary haemangioma, intracranial cavernoma, and developmental venous anomaly in brain parenchyma. The patient had complete vision loss and progressive increase in proptosis for 18 months. The possibility of congenital aetiology of indirect CCF cannot be ruled out in this case. Trauma is another less common cause of indirect CCF. In two of our cases, there was temporal association of the development of this disease with a prior trauma from road traffic accidents. In cases of single-hole dural fistulas, history of trauma should be specifically asked about [3].
Clinically indirect CCF have a subacute or chronic course and mostly present with chemosis, proptosis, diplopia, or vision disturbances [3,4,5, 7]. Sometimes ocular bruit may be present. The degree of symptoms vary and there may be intermittent remission and relapse of the symptoms due to re-routing of the venous drainage. Sometimes ocular symptoms may resolve because of spontaneous thrombosis of ophthalmic veins. These cases should be followed-up closely and further work up should be done to rule out the possibility of re-routing of venous drainage to more dangerous cortical or deep cerebral veins, which put the patients at increased risk of intracranial haemorrhage or congestive brain pathologies [3, 7, 8]. Since this disease has a protracted clinical course and symptoms are largely non-specific, a high index of clinical suspicion is required to make an early diagnosis.
The natural course of indirect CCF is inconstant. The involved part of cavernous sinus may spontaneously thrombose resulting in resolution of the indirect CCF [3, 7]. Alternatively, in a significant proportion of cases, the symptoms progress leading to increased ocular pressure, congestive changes, and decrease in vision. These cases need urgent treatment as delay may result in irreversible damage to the vision or persistent cranial nerve palsy. Two of our cases, who had symptoms for more than 12 months and decreased vision at the time of presentation, did not show improvement in vision. Similarly in two other cases, 3rd and 6th cranial nerve palsy persisted even after embolization of the CCF.
Initial diagnostic work up includes non-invasive cross-sectional imaging with CT scan and MRI. Degree of proptosis, orbital oedema, engorged tortuous ophthalmic veins, and cavernous sinus enlargement are better depicted on MRI [3, 9]. MRI also helps in evaluation of oedema and congestive brain parenchymal changes in cases where there is suspicion of re-routing of venous drainage. Cross-sectional imaging further helps to rule out other mimics like neoplasms and infective/inflammatory pathologies which may present with similar ocular symptoms [3]. Digital subtraction angiography (DSA) is the gold standard for diagnosis, flow dynamics evaluation and treatment planning of CCF [4, 9]. DSA provides details of arterial feeders, site of fistulous connection, venous drainage pattern, venous re-routing, cortical venous reflux, and high-risk collaterals and in turn helps in determining the need of urgency of treatment [3, 4, 6, 7, 9].
Treatment options include conservative management, surgical management, radiosurgery, and endovascular embolization [10]. When symptoms are mild and high-risk features are absent on diagnostic DSA, the patient may be put under conservative management with manual compression therapy. The manual compression therapy may result in closure of the fistula in up to 34% of cases as reported in literature by various authors [3, 5, 7]. Surgery is not preferred and rarely performed. In the present era, the role of surgery is limited only to provide vascular access for endovascular management by surgically exposing ophthalmic veins, where other venous and arterial accesses had exhausted [3]. Stereotactic radiosurgery is another treatment method which appears effective; however, very delayed and variable response to treatment makes this therapy unsuitable for emergency cases [9, 11, 12].
Endovascular management is the main stay of treatment in symptomatic patients and performed by taking transvenous or trans-arterial access [3, 9]. The principle of treatment in any dural fistula is closure of the fistulous connection as well as the immediate draining veins close to the fistula. Since the feeders in cases of indirect CCF are small calibre meningeal arteries, mostly it is not possible to occlude the venous drainage along with the fistulous connection while performing embolization through the arterial route [13]. The delivery of embolization materials at the site of fistula and into the immediate draining vein is best achieved through transvenous access in dural type CCF. In a standard procedure, transvenous access is obtained through femoral vein puncture. After putting the vascular sheath, a guide/diagnostic catheter is advanced into the internal jugular vein and then ipsilateral inferior petrosal sinus (IPS) is super-selectively cannulated with a microcatheter to reach the involved cavernous sinus. Then, multiple coils are deployed to occlude the cavernous sinus near the fistula site. We used both bare platinum as well as fibred coils for embolization of the fistula. Complete closure of the CCF is ensured by taking in between angiograms by another diagnostic catheter already placed in the ECA of the side of the fistula. In 11 cases, where we treated the indirect CCF by taking transvenous access, the procedure was done in a single session and none of them showed recurrence or residual filling of the fistula.
In cases where the IPS is occluded or could not be cannulated due to difficult anatomy and collateralization, other alternative venous access has to be taken to reach the cavernous sinus [14]. Major alternative route targets the superior ophthalmic vein and its drainage territory into facial vein. USG-guided percutaneous puncture of the facial vein, direct puncture of ophthalmic vein with or without surgical exposure of the same, and even direct cavernous sinus puncture based on anatomic landmarks and with certain manoeuvres are described in literature [5, 13,14,15,16,17,18]. Cases are reported in literature where the interventionists have successfully cannulated the cavernous sinus through unconventional routes like via superior petrosal sinus or basal venous plexus and inferior petro-clival veins to embolize the dural CCF [19, 20]. In one of our cases, we successfully recanalized the occluded IPS by the microwire to reach the cavernous sinus. Utmost care has to be exercised while trying to recanalize the occluded IPS, as there is always a potential risk of perforation on aggressive attempts leading to subarachnoid or subdural haemorrhage (SDH). The chances of successful recanalization of the IPS are more when duration of illness is less. In chronic cases where the occlusion is organized and fibrosis has taken place, it is not possible and also not advised to attempt for IPS recanalization due to increased risk of complications in the form of SDH [9, 21]. In one other case where we could not recanalize IPS, we approached the cavernous sinus through USG-guided direct puncture of the facial vein.
Trans-arterial access is taken when no suitable venous route is available and necessitates super-selective cannulation and embolization of ECA branches supplying the CCF [3, 7, 13]. Since the ICA branches to the fistula are usually too small to be selectively cannulated and there is also risk of migration of embolizing agents into the critical vessels supplying intracranial brain parenchyma, they are generally never embolized through arterial route. Thus, with a trans-arterial access, type C CCF may be treated in a single session; however, type D CCF may show residual filling of the fistula from tiny ICA branches. In these cases, repeat embolization is needed or manual compression therapy is advised to the patient and the patient is kept under follow-up. Since the ICA feeders to the fistula are small calibre arteries, there is a high chance of their occlusion by manual compression therapy on follow-up. We had to perform repeat embolization in two type D cases in our series where trans-arterial access was taken. All type D cases, treated with trans-arterial route, were advised and educated for manual compression therapy and all of them showed resolution of symptoms on follow-up.
Various embolizing agents are available including coils, poly-vinyl alcohol (PVA) particles and liquid agents like n-butyl cyanoacrylate (n-BCA — glue) and ethylene vinyl alcohol (EVOH) copolymers. Coils are preferred when the fistula site is accessed by transvenous route for their ease of use and excellent control during deployment [3, 4, 9, 10]. Initially, we were using detachable coils for transvenous embolization of indirect CCF. Later, we realized that fibred coils can be used safely in place of detachable coils without compromising the efficacy and outcome of the treatment. We also observed that increased thrombogenicity of fibred coils helps in achieving complete embolization of the indirect CCF in a single session. Similar observations were made by several authors while performing endovascular embolization with fibred coils. More thrombogenic fibred coils ensures better occlusion of the fistula, decreases the chances of recanalization, and helps in achieving complete embolization besides significantly reducing overall cost of the treatment [2, 7, 22]. Slight modification in the approach is required like use of large diameter microcatheter (at least 0.021″ internal diameter) for selective canulation of the IPS and optimum positioning of catheter tip in the cavernous sinus. We performed embolization using fibred coils in ten cases and achieved complete embolization of the indirect CCF in a single session. This helped us in significantly reducing the cost of the treatment.
PVA particles are preferred for embolizing small ECA branches supplying the fistula. With the advances in endovascular hardware, complete embolization of the dural CCF can be achieved even through trans-arterial route by using the liquid embolic agents like ethylene vinyl alcohol copolymers (EVOH, Onyx, Medtronics, USA) or similar materials. The liquid embolic agents should be used with caution while embolizing middle meningeal artery (MMA) supplying the fistula near the foramen spinosum, as there is a high risk of inadvertent embolization of cranial nerve vasa nervosa leading to facial nerve palsy [23]. Moreover, sometimes, a long compliant balloon needs to be kept in cavernous ICA to prevent reflux of liquid embolic agent into the ICA through tiny dural feeders. All these techniques need proper training and expertise to master and have been described in literature for embolization of dural fistula [4, 9, 10, 16, 21].
