A fistula is the irregular connection between an artery and a vein. 

Dural Arteriovenous Fistula (dAVF) of the Brain and Spine

A dural arteriovenous fistula (dAVF) is the irregular connection between an artery and a vein in the tough covering over the brain (dura). The condition is rare and often related to an injury. Symptoms vary and, as with AVMs, dAVFs may be discovered when undergoing testing for another condition. This is a very rare condition, so care by experts in dural arteriovenous fistulas is vital. Treatments may include endovascular embolization and microsurgical resection.

Cavernous Carotid Fistulas (CCFs)

Cavernous-carotid fistulas (CCFs) are abnormal connections between the carotid artery and the large cavernous sinus vein, located behind the eye. Some forms of CCFs happen spontaneously with no known cause. Other forms result from injury, a ruptured aneurysm or a congenital vascular disease. Symptoms may include bulging eyes, deteriorating vision, and ringing in the ears. Surgical treatment may include minimally invasive endovascular embolization.