Caroticocavernous Fistula

Caroticocavernous fistulas (also known as carotid-cavernous fistulas) can be classified as direct (high-flow) or indirect (low-flow). High-flow fistulas are caused by direct communication between the internal carotid artery and the cavernous sinus and are usually due to trauma or rupture of an aneurysm along the cavernous internal carotid artery. Indirect fistulas are caused by communication between dural branches of the external and/or internal carotid arteries and are thought to be due to revascularization of a cavernous sinus thrombosis.

Caroticocavernous fistulas can also be divided into types A-D. Type A is the direct/high-flow type previously described and is more common in young males, while types B-D are more common in women older than 50 years. A type B caroticocavernous fistula is a dural shunt between intracavernous branches of the internal carotid artery and the cavernous sinus. A type C caroticocavernous fistula is a dural shunt between meningeal branches of the external carotid artery and the cavernous sinus. Finally, a type D fistula is a combination of types B and C: dural shunts between internal and external carotid artery branches and the cavernous sinus.

Caroticocavernous fistulas result in a pulsating exophthalmos, orbital bruit, and glaucoma. On CT and MR, there is enlargement of the extraocular muscles, proptosis, and dilatation of the superior ophthalmic vein. When these findings are unilateral, as is usually the case, we can differentiate their cause from diffuse cerebral edema, in which case the findings are bilateral. However, a caroticocavernous fistula can also result in bilateral superior ophthalmic vein dilatation (as in the case shown below). In addition, the cavernous sinus may bulge laterally.

Angiography will show communication between the internal carotid artery and the cavernous sinus in cases of direct caroticocavernous fistulas. Due to increased pressure in the cavernous sinus, there may also be filling of other venous structures that drain into the cavernous sinuses bilaterally, such as the ophthalmic veins and the petrosal sinuses.

The maximum-intensity projection from a CT cerebral angiogram shows dilatation of the superior ophthalmic veins and engorgement of the cavernous sinuses. Digital subtraction angiogram from a right internal carotid artery injection shows filling of the right and left cavernous sinuses and retrograde opacification of the left superior ophthalmic vein as well as the right sphenoparietal sinus and a temporal draining vein.

During angiography the contralateral internal carotid artery must also be injected to determine the extent of collateral flow, which is particularly important if the ipsilateral internal carotid artery is to be coiled.