When an abnormal connection happens between either of the carotid arteries and the veins just behind the eye, it is called a carotid cavernous fistula.
Because the carotid arteries have a greater pressure of blood flow than do the veins, the rush of blood prevents the veins that serve the eyes from draining properly.
The blood pressure starts to build up in the eye, causing it to bulge. The eye turns red. As the condition gets worse, a person may start losing his or her vision. Without treatment, the sight can be lost in that eye.