A carotid-cavernous fistula is an abnormal short-circuit between the carotid artery and the veins behind the eye. High-pressure arterial blood rushes into the low-pressure vein, so eye and face changes show up fast.

Pulsing eye is the headline. You feel or see your eyeball bounce with your heartbeat, especially when you lie flat.

Red eye follows. The white part becomes brick-red from engorged veins, usually on one side only.

Bulging eye (proptosis) appears. The eyeball pushes forward, making blinking hard or contact lenses uncomfortable.

Ringing in the head is common. A whooshing or humming sound keeps time with your pulse and may wake you at night.

Vision can blur or double. The high pressure distorts the optic nerve or shifts the eye out of alignment.

Late alarms include sudden vision loss, facial numbness, or severe eye pain—signs the pressure is damaging the optic nerve.